Realization of the Low Background Neutrino Detector Double Chooz: from the Development of a High-Purity Liquid & Gas Handling Concept to ﬁrst Neutrino Data

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Realization of the low background neutrino detector Double Chooz: From the development of a high-purity liquid & gas handling concept to ﬁrst neutrino data

Dissertation of Patrick Pfahler TECHNISCHE UNIVERSITAT¨ MUNCHEN¨

Physik Department Lehrstuhl f¨urexperimentelle Astroteilchenphysik / E15 Univ.-Prof. Dr. Lothar Oberauer

Realization of the low background neutrino detector Double Chooz: From the development of high-purity liquid- & gas handling concept to ﬁrst neutrino data

Dipl. Phys. (Univ.) Patrick Pfahler

VollständigerAbdruck der von der FakultätfürPhysik der Technischen UniversitätMünchen zur Erlangung des akademischen Grades eines

Doktors des Naturwissenschaften (Dr. rer. nat)

genehmigten Dissertation.

Vorsitzender: Univ.-Prof. Dr. Alejandro Ibarra Pr¨uferder Dissertation: 1. Univ.-Prof. Dr. Lothar Oberauer 2. Priv.-Doz. Dr. Andreas Ulrich

Die Dissertation wurde am 3.12.2012 bei der Technischen UniversitätMünchen eingereicht und durch die FakultätfürPhysik am 17.12.2012 angenommen.

Contents

Contents i

Introduction 1

I The Neutrino Disappearance Experiment Double Chooz 5

1 Neutrino Oscillation and Flavor Mixing 6 1.1 PMNS Matrix ...... 6 1.2 Flavor Mixing and Neutrino Oscillations ...... 7 1.2.1 Survival Probability of Reactor Neutrinos ...... 9 1.2.2 Neutrino Masses and Mass Hierarchy ...... 12

2 Reactor Neutrinos 14 2.1 Neutrino Production in Nuclear Power Cores ...... 14 2.2 Energy Spectrum of Reactor neutrinos ...... 15 2.3 Neutrino Flux Approximation ...... 16

3 The Double Chooz Experiment 19 3.1 The Double Chooz Collaboration ...... 19 3.2 Experimental Site: Commercial Nuclear Power Plant in Chooz ...... 20 3.3 Physics Program and Experimental Concept ...... 21 3.4 Signal ...... 23 3.4.1 The Inverse Beta Decay (IBD) ...... 23 3.4.2 Signature of the IBD ...... 24 3.4.3 Expected Signal ...... 27 3.5 Detector Design ...... 29 3.5.1 Neutrino Target (NT) ...... 29 3.5.2 Gamma Catcher (GC) ...... 30 3.5.3 Buﬀer (BF) ...... 31 3.5.4 Inner Muon Veto (IV) ...... 31 3.5.5 Passive Steel Shielding ...... 31 3.5.6 Outer Muon Veto (OV) ...... 31 3.5.7 Detector Liquids ...... 32 3.5.8 Detector Readout System ...... 32 3.5.9 Detector Calibration System ...... 33 3.6 Background ...... 35 3.6.1 Accidental Background ...... 35 3.6.2 Correlated Background ...... 36 3.6.3 Artiﬁcial Background ...... 36 3.7 Neutrino Selection ...... 37

i CONTENTS

3.7.1 Pre-Selection Cuts for the Neutrino Search ...... 37 3.7.2 Neutrino Selection Cuts ...... 37

II Development and Production of two Detector Liquids 39

4 Hardware Installations for Detector Liquid Production 41 4.1 Liquid Storage Area (LSA) ...... 42 4.2 Liquid Handling System ...... 43 4.2.1 Pumping Stations ...... 46 4.2.2 Storage Tanks for Buﬀer and Muon Veto ...... 50 4.2.3 Monitoring- and Safety-Systems ...... 53 4.3 Gas Handling System ...... 56 4.3.1 Liquid Nitrogen Plant and Gas Filter Station ...... 58 4.3.2 High Pressure Nitrogen ...... 59 4.3.3 Low Pressure Nitrogen ...... 60 4.3.4 Low Pressure Ventilation ...... 60 4.4 Trunk Line System (TLS) ...... 61

5 Material Selection for the Detector Liquid Production 64 5.1 Organic Liquid Scintillators and Requirements for Double Chooz ...... 64 5.1.1 Scintillating Mechanism and Stokes Shift ...... 64 5.1.2 Requirements for Double Chooz ...... 68 5.2 Component Selection for the Muon Veto Scintillator ...... 69 5.2.1 Scintillating Solvent ...... 69 5.2.2 Non-scintillating Dilution ...... 71 5.2.3 Wavelength Shifter ...... 73 5.3 Component Selection for the Buﬀer Liquid ...... 75 5.3.1 Non-scintillating Mineral Oils ...... 75 5.4 Selected Components for Muon Veto Scintillator and Buﬀer Liquid ...... 77

6 Detector Liquid Production 78 6.1 Composition of Muon Veto and Buﬀer ...... 78 6.2 Preparation of the LSA ...... 78 6.3 Parallel Production of the Muon Veto Scintillator and Buﬀer Liquid ...... 79 6.3.1 Master Solution ...... 79 6.3.2 Mixing Process ...... 80

III Filling and Handling of the Double Chooz Far Detector 83

7 Hardware Installations for the Filling and Handling of the DC far Detector 84 7.1 Liquid Handling System ...... 85 7.1.1 Detector Fluid Operating System (DFOS) ...... 85 7.1.2 DFOS Main Operation Modes ...... 93 7.1.3 Expansion Tank Operating System (XTOS) ...... 94 7.2 Gas Handling System ...... 98 7.2.1 Nitrogen Supply System ...... 99 7.2.2 Consumers ...... 104 7.2.3 Ventilation System ...... 106 7.3 Detector Monitoring System (DMS) ...... 112 7.3.1 Liquid Level Monitoring Systems ...... 112 7.3.2 Gas Pressure Monitoring System (GPM) ...... 124

ii CONTENTS

8 Detector Filling 127 8.1 Preparations for Filling ...... 128 8.1.1 Filling Team ...... 128 8.1.2 Detector Flushing ...... 128 8.1.3 DFOS Cleaning ...... 129 8.2 Detector Filling ...... 129 8.2.1 Filling Sequence ...... 132

IV Performance and Results 147

9 Quality of the Produced Detector Liquids 148 9.1 Muon Veto Scintillator ...... 149 9.1.1 Transparency, Light Yield and Density ...... 149 9.1.2 Radio Purity ...... 151 9.2 Buﬀer Liquid ...... 151 9.2.1 Transparency, Light Yield and Density ...... 151 9.2.2 Radio Purity ...... 153 9.3 Performance of the Liquid- and Gas Handling Systems in the LSA ...... 153

10 Accuracy and Performance of Detector Filling and Handling 156 10.1 Detector Filling Process ...... 156 10.1.1 Performance of the Filling Systems ...... 157 10.2 Detector Handling ...... 162 10.2.1 Performance of XTOS ...... 162 10.2.2 Performance of the Gas Handling System ...... 165

11 Detector Performance & Results from 2012 168 11.1 Cosmogenic Muons in the Inner Muon Veto ...... 169 11.2 Cosmogenic Muons in the Inner Detector ...... 170 11.3 First Neutrino Data ...... 177 11.4 First Result for Θ13 ...... 180

Summary & Outlook 183 11.5 Detector Liquid Production ...... 186 11.6 Liquid Transfer from the Surface to the Underground Laboratory ...... 187 11.7 Filling of the Double Chooz Far Detector ...... 188 11.8 Detector Handling During Data Taking ...... 190 11.9 Conclusion and Outlook ...... 191

V Appendix 193

A Double Chooz Experiment 194 A.1 Detector Design ...... 194

B Surface Installations 197 B.1 Liquid Storage Area ...... 197 B.1.1 Pumping Station ...... 197 B.1.2 Storage Tank Instrumentation ...... 198 B.1.3 Gas Handling System ...... 200 B.2 Trunk Line System ...... 201

iii CONTENTS

C Scintillator Production 204 C.1 Liquid Composition ...... 204

D Underground Installations 206 D.1 Liquid Handling System ...... 206 D.1.1 DFOS Instrumentation of MU, BF, and GC ...... 208 D.1.2 DFOS Valve Identiﬁcation ...... 213 D.1.3 DFOS P&ID’s ...... 213 D.1.4 DFOS Instrumentation of Target Module ...... 217 D.1.5 Programmable Logic Controller, DFOS-PLC ...... 219 D.1.6 DFOS Connections ...... 222 D.1.7 Expansion Tank Operating System (XTOS) ...... 224 D.2 Gas Handling System ...... 228 D.3 Monitoring System ...... 232 D.3.1 Liquid Level Monitoring System ...... 232 D.3.2 Gas Pressure Monitoring ...... 232

E Detector Filling 234 E.1 Filling Modes ...... 235

F Results 240

List of Figures 241

List of Tables 245

Glossary 246

Bibliography 250

Acknowledgement 256

iv Abstract Neutrino physics is one of the most vivid fields in particle physics. Within this field, neutrino oscillations are of special interest as they allow to determine driving oscillation parameters, which are collected as mixing angles in the leptonic mixing matrix. The exact knowledge of these parameters is the main key for the investigation of new physics beyond the currently known Standard Model of particle physics. The Double Chooz experiment is one of three reactor disappearance experiments currently taking data, which recently succeeded to discover a non-zero value for the last neutrino mixing angle Θ13. As successor of the CHOOZ experiment, Double Chooz will use two detectors with improved design, each of them now composed of four concentrically nested detector vessels each filled with different detector liquid. The integrity of this multi-layered structure and the quality of the used detector liquids are essential for the success of the experiment. Within this frame, the here presented work describes the production of two detector liquids, the filling and handling of the Double Chooz far detector and the installation of all necessary hardware components therefore. In order to meet the strict requirements existing for the detector liquids, all components were individually selected in an extensive material selection process at TUM, which compared samples from different companies for their key properties: density, transparency, light yield and radio purity. Based on these measurements, the composition of muon veto scintillator and buffer liquid were determined. For the production of the detector liquids, a simple surface building close to the far detector site was upgraded into a large-scale storage and mixing facility, which allowed to separately, mix, handle and store 90 m3 of muon veto scintillator and 110 m3 of buffer liquid. For the muon veto scintillator, a master-solution composed of 4800 l LAB, 180 kg PPO and 1.8 kg of bis/MSB was produced and, together with all other ingredients of muon veto and buffer, delivered to the experiment, where they were mixed and tuned in due consideration of the individual requirements of the different liquids. For the filling and handling of the DC-far detector, the underground laboratory was equipped with a comprehensive liquid-handling, gas-handling and monitoring-system, which provides all necessary functions to flush, fill, operate and empty the detector safely. Us- ing these systems, the DC-far detector was flushed and filled in accordance with an especially developed sequence, which considered critical filling points and avoided unnecessary stress on the different detector vessels. By the means of this, the far detector of Double Chooz could be filled without damaging the detector vessels. In addition, it could be demonstrated that the quality and cleanliness of the detector liquids were maintained during filling. As a result of this, Double Chooz was able to acquire first neutrino data and to publish its first result of Θ13 with 2 sin (2Θ13) = 0.109 ± 0.030(stat.) ± 0.025(syst.). Zusammenfassung Die Neutrinophysik ist fur¨ die Teilchenphysik von besonderer Bedeutung, nicht nur weil die Neutrinophysik in den letzten Dekaden die meisten Erfolge verzeichnen konnte, sondern weil es die Untersuchung von Neutrinooszillationen erlaubt, auch neue und unbekannte Physik jen- seits des Standardmodels zu untersuchen. Vorraussetzung hierfur¨ ist jedoch die exakte Bestim- mung der verschiedenen Oszillationsparameter, die als drei Mischungswinkel in der leptonischen Mischungsmatrix zusammengefasst sind. Das Double-Chooz-Experiment ist eines von drei der- zeit laufenden Neutrino-Oszillationsexperimenten, die mit Hilfe von Reaktorneutrinos erfolgreich einen ersten Wert fur¨ den letzten und bis dato unbekannten Neutrino-Mischungs-Winkel Θ13 be- stimmen konnten. Als Nachfolger des CHOOZ-Experimentes benutzt Double Chooz zwei identi- sche Detektoren, jeweils bestehend aus vier konzentrisch ineinandander liegenden Behältern, die mit unterschiedlichen Detektorflussigkeiten¨ gefullt¨ sind. Die Integrität dieser mehrschichtigen Struktur und die Qualität der benutzten Detektorflussigkeiten¨ sind grundlegend fur¨ den Er- folg des Double-Chooz-Experiments. In diesem Zusammenhang beschreibt die hier präsentierte Arbeit die Produktion zweier Detektorflussigkeiten,¨ das Fullen¨ und Betreiben des ersten De- tektors sowie die Entwicklung und Installation aller hierfur¨ benötigten Systeme. Um die stren- gen Qualitätsanforderungen an die Detektorflussigkeiten¨ zu erfullen,¨ wurden alle Bestandteile in einem umfassenden Materialselektionsprozess einzeln ausgewählt und Proben verschiedener Anbieter auf ihre Schlusseleigenschaften¨ wie Dichte, Transparenz, Lichtausbeute und radioche- mische Reinheit hin untersucht. Basierend auf diesen Messungen wurden die Zusammensetzun- gen von Myon-Veto-Szintillator und Bufferflussigkeit¨ bestimmt. Fur¨ die Produktion der De- tektorflussigkeiten¨ wurde ein einfaches Gebäude in der Nähe des Experimentstandorts in eine hochreine, großvolumige Lager- und Mischanlage umgewandelt, die es ermöglichte, 90 m3 Myon- Veto-Szintillator und 110 m3 Bufferflussigkeit¨ getrennt voneinander herzustellen. Fur¨ den Myon- Veto-Szintillator wurde eine Master Solution, bestehend aus 4800 l LAB, 180 kg PPO und 1,8 kg bis/MSB hergestellt und, zusammen mit allen anderen Flussigkeiten¨ fur¨ Myon Veto und Buffer, zum Standort des Experiments transportiert. Dort wurden alle Bestandteile unter Einbeziehung der individuellen Anforderungen an die verschiedenen Detektorflussigkeiten¨ in eigens dafur¨ entwickelten Systemen gemischt und feinabgestimmt. Fur¨ das Fullen¨ und den Betrieb des ersten Detektors von Double Chooz wurde das Untergrundlabor mit einem umfassenden Flussigkeits-,¨ Gas-handling und Monitoring system ausgestattet, welche zusammen alle erforderlichen Funktio- nen fur¨ das sichere Fullen,¨ Leeren und den störungsfreien Betrieb des Detektors gewährleisteten. Mit Hilfe dieser Systeme folgte das Fullen,¨ im Anschluss an das Spulen¨ mit Stickstoff, einem speziell hierfur¨ entwickelten sequenziellen Ablauf, der kritische Fullabschnitte¨ berucksichtigte¨ und unnötigen Stress auf die verschiedenen Behälter vermied. Also Folge der erfolgreichen Um- setzung aller Systeme und Abläufe konnte der ersten Detektor ende 2010 erfolgreich bef¨llt und ohne Schaden in Betrieb genommen werden. Zusätzlich konnte gezeigt werden, dass die Qualität und Reinheit der Detekorflussigkeiten¨ durch den Fullprozess¨ nicht beeinträchtigt wurde. Basie- rend auf dieser Arbeit war es der Double Chooz Kollaboration möglich, erste Daten zu erheben 2 und den ersten reaktorbasierten Wert fur¨ Θ13 mit sin (2Θ13) = 0.109 ± 0.030(stat.) ± 0.025(syst.) zu veröffentlichen. Introduction

[1] First proposed by Wolfgang Pauli [2] in 1930, who required a third particle in order to explain the continuous energy spectrum of the electrons emitted by the beta-decay, neutrinos are nowadays well established and part of the standard model of particle physics. Belonging to the group of leptons, neutrinos exist in three different flavors (νe, νµ, ντ ), named after their charged family partners e, µ, τ. Of all elementary particles, neutrinos are the ones most difficult to detect, as they have (almost) no mass, no electrical charge and interact only weakly with matter. Therefore neutrinos travel nearly with the speed of light and are neither deflected by magnetic fields nor influenced by matter due to the extremely small cross section for weak −43 2 interactions (σ ∼10 cm )[3]. Although difficult to detect, these properties make neutrinos to ideal messengers for elementary- and astro-particle physics. Produced in our sun, neutrinos allow for the first time to “look” directly into the center of a star and to observe fusion processes as they happen. Hence, the observation of solar neutrinos provides important information about the structure, evolution or energy production in our sun and allows to test the theoretical predictions provided by the standard solar model (SSM) [4]. The first successful solar neutrino experiment was realized in the late 1960’s by R. Davis, who installed 615 tons of perchloroethylene (C2Cl4) in a large tank deep underground in the Home- stake gold mine in South Dakota. The Homestake-Chlorine experiment [5] (1969-1989) ob- 37 37 served the absorption of νe on Cl, which lead to the formation of the instable isotope Ar (τ1~2 = 34.8 days) and the emission of an electron.

37 37 − νe + Cl → Ar + e (threshold 814 keV) Extracting the argon atoms from the detector and analyzing their decay in a proportional counter allowed to measure the solar neutrino flux and to compare it with the predictions of the SSM. The result, however, was surprising, as the observed neutrino flux was a factor of three below the prediction [6]. This deficit, also known as the solar neutrino problem, motivated the proposal of other experiments with a higher sensitivity, as SAGE [7] (1989-2012), GALLEX [8] (1992- 1997) and its successor GNO [9] (1998-2003), which used a similar detection method. These radiochemical experiments were based on the neutrino absorption on 71Ga and its transformation into 71Ge, what provides a significantly lower detection threshold of 233 keV and made these experiments sensitive to the most abundant solar neutrino type produced by the pp-chain [4]. 71 71 − νe + Ga → Ge + e (threshold 233 keV)

Although these experiments were able to measure a higher neutrino flux and therefore to confirm the leading energy production process (pp-cycle) predicted by SSM, the solar neutrino problem remained, as all of these experiments measured the same deficit between the expectation and the observation. A possible solution for the solar neutrino problem was presented by Pontecorvo, who suggested that the observed neutrino deficit might be the result of undetected, rather than missing neutrinos [10]. He reported the possibility that solar neutrinos were able to change

1 Introduction

their flavor from νe → νµ,τ , which he described as neutrino oscillations. As the experimental techniques were only sensitive to νe, this flavor change could have been one of the possible explanations for the solar neutrino problem. As will be shown in chapter 1 neutrino oscillation can be mathematically expressed as transition probability between two flavor eigenstates α and β, which is described by

3 2 L 2 ∗ ∗ −i(∆mij ) Pα→β(U, L, E, ∆mij) = Q UαiUβiUαjUβje 2E , (1) i,j=1 where the amplitude is given by the elements of the leptonic mixing matrix (U), which contains three different mixing angles and an additional CP violating δ-phase (Θ12, Θ23, Θ13, δCP ). The second and oscillating term is driven by the squared mass difference of the oscillating neutrinos 2 (∆mij), the traveled distance (L) and the neutrino energy (E). After 20 years of uncertainty, whether neutrino oscillation caused the observed deficit or not, a great advance could be realized by new experiments and the first employment of a real time detection channel as the elastic scattering (ES), as well as neutral-(NC) or charged-current- interactions (CC), which lead to the dissociation or transformation of nuclei: − − 1. νx + e = νx + e (ES, sensitive to νx)

2. νx + d = p + n + νx (NC, Qth = 1.44 MeV, sensitive to νx)

− 3. νe + d = p + p + e (CC, Qth = 2.22 MeV, sensitive to νe).

The first real time measuring experiment, which exploited one of these new detection channels, was the Kamiokande-experiment [11] in Japan, which succeeded in 1987 to measure the direction of neutrinos by using the directional correlation between incoming neutrino and recoiled lepton (e−, µ−). Kamiokande employed a 3000 t water-Cherenkov detector equipped with 1000 photomultiplier tubes (PMTs) and its successor, the Super-Kamiokande (SK) [12] (1998), even 50.000 tons of water and 30.000 PMTs. The recoiled leptons produced characteristic Cherenkov- rings, whose shape allow to distinguish between solar-νe’s and the higher energetic νµ’s produced in the atmosphere. This allowed to study solar as well atmospheric neutrinos independently. The comparison of atmospheric neutrinos provided strong evidence for atmospheric oscillation 2 (νµ → ντ ) [13], which allowed to narrow down Θ23 and ∆m32. The solar neutrino data allowed to exclude a small solar mixing angle, which indicated a fundamental difference between the mixing of quarks and leptons. Although these observations were in accordance with neutrino oscillations, alternative explanations as the neutrino decay [14] could yet not be excluded. In 2001, the solar neutrino problem could finally be resolved by the SNO experiment [15, 16], which used a 1000 t heavy water (D2O) Cherenkov detector equipped with 9700 PMTs, surrounded by several kilotons of normal water in a copper mine in Sudbury, Canada. The idea was to measure the solar neutrinos independently of their respective flavor. Using heavy water, SNO was sensitive to all three detection channels (ES, NC and CC), which could be used to compare the interaction rate of νe (CC-channel) with the interaction rate of all neutrino flavors, observed by the NC-channel. The outcome was definite and showed a CC/CN-ratio of 0.301 [16] in accordance with predictions of the solar standard model. Thus, SNO solved the solar neutrino problem and provided compelling evidence for neutrino oscillations, as well as the influence of matter on the oscillation probability described by the MSW-effect [17, 18]. Based on these data, 2/3 of the produced νe’s undergo a flavor change still within the sun (ν1 → ν2) and are later (on their way to earth) subdued to vacuum-oscillations. Combining the oscillation data from

2 Introduction the different experiments allowed to further precise the other solar- and atmospheric-oscillation 2 parameters as well as to determine the sign of ∆m12 to be positive (see table 1.1). Although neutrino oscillation solved the solar neutrino problem, the confirmation of its cause had two major implications: firstly, neutrino oscillations require massive neutrinos and, moreover, three different mass eigenstates. Secondly, flavor change violates the lepton family number. Both facts contradict the standard model of particle physics and prove evidently the existence of yet unknown physics, resulting in the need to refine the standard model. This gives rise to a whole set of new questions regarding:

the absolute mass scale of neutrinos (mν), 2 the hierarchy of the diﬀerent mass eigenstates (ν1 < ν2 < ν3) or (ν3 < ν1 < ν2) for ∆m12 > 0,

the existence of CP-violation in the leptonic sector (δCP -phase) and

the nature of the neutrino itself (Majorana-(ν = ν¯) or Dirac-(ν ≠ ν¯) particles).

The investigation of this new physics requires a precise knowledge of Θ13 as well as of all other oscillation parameters, as only a complete mixing matrix will allow to disentangle the small inﬂuence of mass hierarchy or CP-violation on the oscillation-pattern. Within this frame, reactor neutrinos play a major role as they provide ideal conditions to investigate the mixing angle Θ13. Nuclear power plants produce exclusivelyν ¯e’s with low energies between 0-8 MeV [19], which are emitted from a well deﬁned position. Providing CPT-invariance1, this pure and low energetic anti-neutrino sample can be used to observe the disappearance ofν ¯e’s in (¯νe → ν¯µ, τ )-oscillations, which is dominated by Θ13 on short baselines (<10 km).

The first attempt to measure the last mixing angle Θ13 was realized in 1997 by the CHOOZ- experiment (1993-1998) [20], which measured the disappearance of anti-neutrinos (¯νe → ν¯µ, τ ) emitted by a nuclear power plant in Chooz, France. The experiment used a 112 ton liquid scintillator detector installed in a shallow depth underground laboratory about 1050 m from the two power cores. The detector itself had a multi layered design and was composed of three different detector vessels, filled with 5 tons of gadolinium doped target scintillator, 17 tons of gamma catcher scintillator as well as a 90 ton water Cherenkov muon veto. Measuringν ¯e’s, CHOOZ could use the inverse beta decay (IBD) as real time detection channel and exploit the direct correlation between the neutrino energy and the kinetic energy of the positron

+ IBD: ν¯e + p → n + e (Qth = 1.8 MeV, sensitive toν ¯e), which allowed to carry out neutrino spectroscopy. As reactor neutrinos have only low energies − − − (Eν¯ < 8 MeV [19]), the IBD can only produce e but not the heavier µ or τ , because of which CHOOZ is only sensitive toν ¯e’s but not to the appearingν ¯µ, τ ’s. This electronic-channel of the IBD leads to a delayed coincidence, composed of the prompt positron signal and the delayed capture of the neutron on gadolinium, which can be used to identify the IBD and reduce background events efficiently. Located close to the oscillation maximum (for 2 MeV ν’s), CHOOZ compared the measured flux to a calculated expectation based on a neutrino spectrum, which was measured at the former Bugey4-experiment [21]. After a data taking phase of one year, the detector of CHOOZ suffered from a degradation of the target scintillator, which led to a premature end of the experiment. The analysis of the available data showed that the experiment was dominated by background events and systematics, no sign of oscillations could be found, 2 2 −3 2 providing only an upper limit for Θ13 of sin (2Θ13) < 0.18 at ∆m = 2.43 ⋅ 10 eV [20].

Still motivated by the search for Θ13, the here presented thesis is related to the successor- experiment of CHOOZ, named Double Chooz, which anticipates to increase the sensitivity for

1 CPT-invariance implies, that the survival probability of νe’s is the same as forν ¯e’s, what allows to use both for the investigation of the driving oscillation parameters.

3 Introduction

Θ13 in a two staged approach. In the first phase, Double Chooz repeats the measurement of CHOOZ using a new detector with improved design, now including: a bigger target volume, a newly added buffer vessel, newly developed detector liquids and highly increased radiopurity requirements. This new design provides a higher neutrino statistics and, at the same time, a significantly reduced number of background events. In the second phase, Double Chooz will eliminate the reactor based uncertainties (2 - 3 %) with the installation of an identical detector only 400 m from the two power cores. The relative comparison between near and far detector will then allow a clean measurement of Θ13. The Double Chooz experiment is currently in phase one and takes data since April of 2011. An analysis of the first 220 days of data (see chapter 11) already 2 indicates a non-zero value for Θ13, corresponding to sin (2Θ13)=0.109±0.030(stat.)±0.025(syst.), excluding the non-oscillation hypothesis now with 2.9 σ [22].

Apart from Double Chooz, also two other experiments aim for a precise measurement of Θ13, the Daya Bay experiment in China [23] and the RENO experiment in South Korea [24]. Both experiments recently published similar values for Θ13 (see [25, 26] or section 11.4) in accordance with Double Chooz. As part of the Double Chooz collaboration, the author of this thesis had, in cooperation with other colleagues, the responsibility for the production of two detector liquids, the filling of the far detector and the development and realization of all necessary systems needed therefor. This work will provide an insight into the mentioned tasks and all related information. The here presented thesis is divided into four parts: The first part is dedicated to the Double Chooz experiment and will shortly revise the theoretical framework of neutrino oscillations and the production process ofν ¯e in nuclear power plants in chapters 1 and 2. Chapter 3 will then be dedicated to the presentation of the Double Chooz- experiment and the design of the far detector. The second part describes the development and on-site production of the muon veto scintillator and the buffer liquid for the two outer detector layers of the Double Chooz far detector. In chapter 4, the instrumentation of a large scale on-site mixing facility is presented, whereas the material selection process for the different detector liquids is described in chapter 5. The production process of the two detector liquids is finally presented in chapter 6. The third part presents the development and realization of a complete liquid- and gas-handling- concept for the Double Chooz far detector and its use during the filling and handling of the detector. In detail, this part presents the instrumentation of the far underground laboratory composed of comprehensive liquid-handling, gas-handling and monitoring systems in chapter 7 and in chapter 8 the realization of the developed filling procedure. The last part of this thesis will be used to summarize the achieved results, presenting the quality of the detector liquids in chapter 9, the accuracy of the filling process in chapter 10 and the first results obtained by the Double Chooz experiment in chapter 11.

4 Part I

The Neutrino Disappearance Experiment Double Chooz

5 Chapter 1

Neutrino Oscillation and Flavor Mixing

1.1 PMNS Matrix

In the current view, the three neutrino ﬂavors (α = e, µ, τ) can be described as three orthogonal eigenstates of the weak interaction, called ﬂavor-eigenstates. Each of them is a superposition of three orthogonal mass-eigenstates (i = 1, 2, 3) [27, 28] and can be mathematically expressed by: ∗ Sνα⟩ = Q Uαi Sνi⟩ respectively Sν¯α⟩ = Q Uαi Sνi⟩ . (1.1) i i

The entanglement between mass and ﬂavor eigenstates is described by Uαi, an unitary 3 × 3- matrix called PMNS1-matrix:

⎛ Ue1 Ue2 Ue3 ⎞ Uαi = ⎜ Uµ1 Uµ2 Uµ3 ⎟ . (1.2) ⎝ Uτ1 Uτ2 Uτ3 ⎠

The ﬂavor-mixing can be described by a rotation about three axes, represented by a rotation- π matrix with (3+1) parameters, which consist of three rotation angles (Θ12, Θ13, Θ23 < 2 ) and a δ-phase, describing possible CP-violations. The phase would be zero, or ±π, for CP-conservation and between −π < δ < 0 and 0 < δ < +π for a violation of CP [27, 3]. Apart from that, two additional Majorana-phases (α1, α2) exist, which are zero for Dirac-particles and non-zero for the Majorana case [27, 3]. The full matrix including all mentioned parameters is presented below.

i i α1 α2 −iδ ⎛ c12c13e 2 s12c13e 2 s13e ⎞ i i c cos Θ −iδ α1 −iδ α2 ij = ( ij) Uαi = ⎜ −(s12c23 + c12s23s13e )e 2 (c12c23 − s12s23s13e )e 2 s23c13 ⎟ ⎜ i i ⎟ sij sin Θij −iδ α1 −iδ α2 = ( ) ⎝ (s12s23 − c12c23s13e )e 2 −(c12s23 + s12c23s13e )e 2 c23c13 ⎠ (1.3)

Uαi can be parametrized in the following chain of matrices, disentangling the oscillation para-

1Acronym for Pontecorvo-Maki-Nakagawa-Sakata, the group of physicists, who developed the concept of neutrino oscillation [29] and formulated a neutrino-mixing matrix.

6 Neutrino Oscillation and Flavor Mixing meters:

i iδ α1 ⎛ 1 0 0 ⎞ ⎛ c13 0 −s13e ⎞ ⎛ c12 −s12 0 ⎞ ⎛ e 2 0 0 ⎞ i α Uαi = ⎜ 0 c23 −s23 ⎟ ⎜ 0 1 0 ⎟ ⎜ s12 c12 0 ⎟ ⎜ 0 e 2 2 0 ⎟ . (1.4) iδ ⎜ ⎟ ⎝ 0 s23 c23 ⎠ ⎝ s13e 0 c13 ⎠ ⎝ 0 0 1 ⎠ ⎝ 0 0 1 ⎠ ´¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¸¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¶ ´¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¸¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¶ ´¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¸¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¶ ´¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¸¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¶ atmospheric interference solar Majorana−phases

The names “atmospheric” and “solar” represent the kind of neutrinos that were investigated in order to reveal the oscillation parameters Θ23 and Θ12. The “interference” part, connecting the atmospheric and solar sector, includes the mixing angle Θ13, which is additionally entangled with the CP-violating phase δ. This entanglement indicates, that the experimental access to measure a possible CP-violation depends on a non-zero value of Θ13. In the past decades, neutrino oscillations have been under strong investigation by many experiments, what allowed to determine some of the oscillation properties as the mixing angles 2 2 Θ12,Θ23 and the mass-squared-diﬀerences ∆m12 and ∆m23. A former attempt to measure the mixing angle θ13 by CHOOZ [30] solely allowed to determine an upper limit for Θ13. Minos [31] and the younger T2K-experiment [32] were able to measure upper and lower limits for Θ13, but didn’t reach the 3 − σ−level (see [33] for Minos and [32] for T2K). Only recently, during the preparation of this thesis, Double Chooz [34, 22], Daya Bay [26] and RENO [25] published results on Θ13. The currently known values are summarized in table 1.1.

2 2 ij ∆mji sin (Θij) Experiment +0.22 −5 2 +0.018 12 7.58−0.26 ⋅ 10 eV 0.306−0.0015 KamLAND [35], SNO [36] +0.12 −3 2 +0.007 13 2.35−0.09 ⋅ 10 eV 0.021−0.008 Minos [37] DC[34, 22], DB [26], RENO [25], +0.12 −3 2 +0.08 23 2.35−0.09 ⋅ 10 eV 0.42−0.03 Minos [37]

Table 1.1: Currently known best ﬁt values for the oscillation parameters Θ12, Θ13, Θ23, as well as their 2 2 2 mass-squared diﬀerences ∆m21, ∆m31, ∆m32. Values from [38]

Using the current values of table 1.1 and the assumption (δCP , α1,2=0), allows to obtain the 2 current leptonic neutrino-mixing-matrix Uαi as well as SUαiS , which presents the composition probability (P) of the different mass and flavor eigenstates. The composition of the different mass eigenstates is illustrated in figure 1.4, which shows the hierarchy of the three neutrino mass eigenstates.

0.67 0.30 0.03 2 ⎛ ⎞ P = SUαiS = ⎜ 0.24 0.27 0.49 ⎟ (1.5) ⎝ 0.09 0.42 0.49 ⎠

1.2 Flavor Mixing and Neutrino Oscillations

The three orthogonal neutrino ﬂavor-eigenstates Sνα⟩, with α = e, µ, τ, can be expressed as a superposition of three orthogonal mass-eigenstates Sνi⟩, with (i = 1, 2, 3) [27, 3].

3 Sνα⟩ = Q Uαi Sνi⟩ (1.6) i=1 A propagating mass eigenstate is described by Hamilton mechanics, leading to a dependency on energy (E) and time (t): −iEit Sνi(t)⟩ = e Sνi⟩ , (1.7)

7 Neutrino Oscillation and Flavor Mixing

where Ei is the energy of the i-th mass eigenstate. The propagation of a neutrino ﬂavor-eigenstate α in time can thus be expressed by combining eq. 1.6 and eq. 1.7:

3 −iEit Sνα(t)⟩ = Q Uαie Sνi⟩ . (1.8) i=1

The transition amplitude (A) for a ﬂavor change να→νβ, at a time t, can be found by projecting νβ onto να(t) and using ⟨νiSνj⟩ = δij what leads to:

3 3 ∗ −iEit ∗ −iEit A(α → β; (E, t)) ≡ aνβSνα(t)f = Q UαiUβjδije = Q UαiUβie . (1.9) i,j=1 i=1

2 The transition probability (P ) for an oscillation να→νβ is given by P = SAS :

3 2 3 2 ∗ −iEit ∗ ∗ −i(Ei−Ej )t P (α → β) = SA(α → β; t)S = WQ UαiUβie W = Q UαiUβiUαjUβje . (1.10) i=1 i,j=1

The first part of eq. 1.10 describes the influence of the PMNS-matrix-elements (Uαi,Uβi) on the oscillation probability. This part is invariant and only defined by the oscillation parameters (E −E )t Θ13,Θ12,Θ23 (and a possible δ-phase for CP-violation). The second argument e i j depends on the neutrino energy and the traveled time and can be interpreted as oscillation frequency, also referred to as phase difference. For the next steps, it is helpful to rewrite the phase difference by assuming, that neutrinos have higher momentum than mass, and that all neutrino mass-eigenstates have the same momentum (more information regarding light ray and equal momentum assumption can be found in [3], page 253).

2 2 ¼ m <

2 2 mi − mj L=t L E E t t ∆m2 . (1.12) ( i − j) ≈ 2E → ij 2E

This allows to formulate a general form of the oscillation probability P(α → β):

3 2 L ∗ ∗ −i(∆mij ) Pα→β(L, E) = Q UαiUβiUαjUβje 2E . (1.13) i,j=1

The matrix elements Uαi are composed of real and imaginary arguments [27, 3], which allows to re-write eq. 1.13 into a sum of real- and imaginary parts of Uαi of which the latter include the CP-violating phase δ. This leads to a general the transition probability for a ﬂavor change Pα→β, which is shown in eq. 1.14.

∗ ∗ 2 2 L Pα→β(L, E) = δαβ − 4 Q Re(UαkUβkUαjUβj) sin (∆mij ) k>j 4E (1.14) ∗ ∗ 2 L +2 Q Im(UαkUβkUαjUβj) sin(∆mij ) . k>j 2E

8 Neutrino Oscillation and Flavor Mixing

1.2.1 Survival Probability of Reactor Neutrinos

For the special case of P(α → α), and thus the survival probability for a given ﬂavor, the matrix elements of the mixing matrix Uαi are only real arguments, simplifying eq.1.14 to

∗ ∗ 2 2 L Pα→α(L, E) = 1 − 4 Q Re(UαkUαkUαjUαj) sin (∆mij ) (1.15) k>j 4E

2 2 2 2 L = 1 − 4 Q SUαkS SUαjS sin (∆mij ) . (1.16) k>j 4E

Without the imaginary arguments in Uαi and therefore without the CP-violating phase δ, the survival probability is not affected by a possible CP-violation. In consequence, all experiments, which measure the survival probability (as Double Chooz, Daya Bay and RENO) are not effected by CP-violation and can therefore measure Θ13 independent from the influence of a δ-phase. Here, reactor experiments have an inherent advantage compared to accelerator experiments, which measure an appearance of neutrinos and are therefore subdued to CP-violating effects [27, 3]. However, once Θ13 is precisely known, accelerator experiments will be able to measure the influence of CP-violation in the leptonic sector. Such a CP-violating δ-phase in the leptonic mixing matrix would lead to a different oscillation probability for neutrinos than for anti-neutrinos (P (να → νβ) ≠ P (ν¯α → ν¯β)) [3, 27]. Future accelerator experiments will be able to produce both neutrino- as well as anti-neutrino-beams and have therefore a good chance to find a CP-violation in the leptonic sector. The survival probability for reactor neutrinos can explicitly be written as [3]:

st nd rd Pν¯e−ν¯e = 1 + 2 + 3 (1.17) ∆m2 L 1st 1 sin2 2Θ sin2 31 (1.18) = − ( 13) 4E ∆m2 L 2nd cos4 Θ sin2 2Θ sin2 21 (1.19) = − ( 13) ( 12) 4E 1 ∆m2 L ∆m2 L ∆m2 L 3rd sin2 2Θ sin2 Θ cos 31 21 cos 31 . (1.20) = + 2 ( 13) ( 12) 2E − 2E − 2E

The survival probability is composed of three different parts, each composed of an amplitude 2 2 st (defined by the mixing angles) and a sin -oscillation, which depends on (∆m L~E). The 1 - 2 nd 2 part of eq. 1.17 is dominated by Θ13 and ∆m13, the 2 -part is dominated by Θ12 and ∆m12 rd (the Θ13-contribution is only minor due to cos(Θ13) ≈ 1) and the 3 -part is an interference term, which has only minor effect on base lines below 5 km. Figure 1.1 compares the three individual contributions of 1st-, 2nd- and 3rd-part on the survival probability. For a better comparison, each contribution starts at 1.00. The comparison uses 3 MeV-neutrinos and the oscillation parameters summarized in table 1.1. The important part for Double Chooz is the 2 first one (dominated by Θ13 and ∆m13) and is shown in blue. The curve indicates the first minimum between 1.5 km from the cores (a short baseline) and an amplitude that corresponds 2 nd to sin (2Θ13). The 2 part is shown in red, indicating long baseline oscillations (≥ 50 km) 2 and an amplitude that corresponds to sin (2Θ12). The interference part is shown in green and shows a larger influence on longer base lines than on short base lines, because of which this interference can be neglected in Double Chooz. As can be seen dominates Θ13 the oscillation pattern at short baselines because of which the survival probability for reactor neutrinos on short baselines can be approximated by only the first part of eq. 1.17. Figure 1.2 presents the sum of all three contributions and therefore the full survival probability as given in equation 1.17. Obvious are the two oscillation maxima defined by the first and second part. The small value of

9 Neutrino Oscillation and Flavor Mixing

Figure 1.1: Comparison of the 1st, 2nd and 3rd part of eq.1.17 and their individual influence on the survival probability for 3 MeV-neutrinos. For a better comparison all parts start at 1.00. The blue curve shows the first oscillation minimum already at about 1.5 km from the source, indicating oscillations on 2 short base lines which are produced by the large value of ∆m13. The amplitude of the blue curve is 2 defined by Θ13. The red curve indicates the same behavior, however dominated by a smaller ∆m12 and a bigger Θ12, which leads to long baseline oscillations and a larger amplitude, and finally almost a full 2 conversion of ν¯e into ν¯µ,τ at the first oscillation maximum (L0 = Eν ~∆m12). The third part, depicted in green, describes an interference between the oscillation parameters and has only influence at longer baselines.

2 −5 2 2 −3 2 ∆m21 ∝ 10 eV leads to long baseline oscillations, whereas the bigger ∆m31 ∝ 10 eV leads to short baseline oscillations forν ¯e → ν¯µ,τ . Also prominent to see are the two different oscillation 2 2 amplitudes depending on sin (2Θ13) and sin (2Θ12). The big mixing angle of the atmospheric ○ ○ sector Θ12 = 33.5 leads to a large conversion ofν ¯e intoν ¯µ,τ , while the smaller Θ13 = 8.3 leads only to a significantly smaller conversion (∼10%). Due to the small effect Θ13 is more difficult to detect and requires sensitive experiments, which allow resolve such a small variation with the necessary accuracy. In the case of Double Chooz the design goal was to limit the systematical and the statistical errors below one percent, what could successfully be realized (compare with section 11.4) Comparing the two blue graphs of figures 1.1 and 1.2 visualizes, that the survival probability st (on small baselines ≤10 km) can be well approximated by only the 1 -part of equation 1.17. The survival probability for reactor neutrinos in the vicinity of the power plant can therefore be simplified to: 2 2 2 ∆m31L P − 1 sin 2Θ sin (1.21) ν¯e ν¯e = − ( 13) 4E ≈ ´¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¸¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¶ amplitude ´¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¸¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¶ frequency 2 2 2 2 ∆m31[eV ]L[m] ≈ 1 − sin (2Θ13) sin 1.27 (1.22) 4E[MeV ] An important consequence of this formula is, that neutrino oscillation needs not only massive 2 neutrinos, but also at least two different mass eigenstates in order to produce the ∆mij ≠ 0, which is necessary for the oscillation. This simple form allows to determine the position of the survival probability minimum, in dependence of L/E and given ∆m2. The survival probability is

10 Neutrino Oscillation and Flavor Mixing

Figure 1.2: Survival probability of ν¯e’s for 3 MeV neutrinos and the current oscillation parameters given in table tab:CurrentValuesForTheOscillationParameters. The curve represents the sum of all three parts of the oscillation formula presented in equation 1.17. The amplitude of short baseline oscillations is 2 defined by sin (2Θ13), while the first oscillation minimum is defined by L0. Comparing the blue curves of figures 1.2 and 1.1 at short baselines (1-5 km) indicates, that the survival probability of reactor neutrinos can be approximated by the first part only.

2 2 2 minimal, if the oscillating-term (sin (∆m L~E)) is maximal. Given that sin (x) = 1 (maximal) π for n 2 , the minimum of the survival probability is described by:

2 2 2 π ! 2 πL ! 2 ⎛1.27∆mijL⎞ πL ! 1.27∆mijL sin n = sin = sin Ô⇒ = . 2 2L0 ⎝ Eν ⎠ 2L0 Eν

This allows to define the oscillation length L0, which represents the first minimum of the survival probability, depending only on the neutrino energy (Eν) and the squared mass difference 2 (∆mij). πEν Eν[MeV ] L0 [m] = = 1.23 (1.23) 2.54∆m2 ∆m2[eV 2]

2 −3 2 Using a ∆m13=2.4⋅10 eV and neutrino energies between 2 and 10 MeV, different values for L0 are summarized in table 1.2. In addition presented are the individual survival probabilities for the near and far detector at 400 m and 1050 m and their difference, which indicates the maximal measurable disappearance effect for the detector setup in Double Chooz. The individual survival probability curves for 2, 3, 4 and 5 MeV neutrinos are presented in figure 1.3 together with the positions of the near and far detector. As can be seen varies the survival probability for a fixed position with the neutrino energy. Due to this dependance and the defined position of the far detector, Double Chooz is most sensitive for 2-MeV-neutrino oscillation and less for higher energies. Based on the observable energy spectrum of reactor neutrinos, which is also shown in figure 1.3 and which peaks around 3 MeV, the Double Chooz far detector is almost in an ideal position. This energy dependent behavior of neutrino oscillations, will lead to a distortion of the observable energy spectrum at the lower energy region. Searching for neutrino oscillations, this distortion is an unmistakable evidence of oscillation and far more convincingly than just

11 Neutrino Oscillation and Flavor Mixing the observation of a reduced counting rate (rate-analysis). Consequently, neutrino oscillation experiments beneﬁt from a good energy resolution, as this will allow to resolve the distortion in the energy spectrum providing the possibility of an additional shape analysis. This, however, requires a well understood experimental setup and a suﬃcient energy resolution.

Survival probability forν ¯e’s between 2 and 10 MeV

Neutrino Energy Eν MeV 2 3 4 5 6 7 8 9 10

First Minimum (L0) m 1025 1537 2050 2563 3075 3588 4100 4613 5125 Surv. Prob.@400 m % 96.6 98.4 99.1 99.4 99.5 99.7 99.7 99.8 99.8 Surv. Prob.@1050 m % 90.0 92.3 94.8 96.4 97.4 98.0 98.4 98.7 99.0 Disappearance Eﬀect % 6.6 6.0 4.2 2.9 2.1 1.6 1.2 1.0 0.8

Table 1.2: Oscillation length (L0), the survival probabilities for the position of the near (400 m) and far (1050 m) detector of Double Chooz. Additionally indicated are the diﬀerences between near and far detector indicating the maximal measurable disappearance eﬀect for neutrino energies between 2 and 10 MeV.

Figure 1.3: (left): Diﬀerent survival probability curves for reactor neutrinos with 2, 3, 4 and 5 MeV. As can be seen depends the survival probability for a ﬁxed position on the neutrino energy. The two vertical lines indicated the positions of the near and the far detector of Double Chooz. Based on these positions, the Double Chooz is most sensitive to oscillations of 2 MeV-neutrinos and less sensitive for higher energies as indicated in table 1.2. This energy dependance will lead to a spectra distortion of the observed neutrino spectrum in the lower energy region. (right): a) Observable energy spectrum of the IBD in a liquid scintillator detector (without spectral distortion) b) energy spectrum for reactor neutrinos −43 2 c) energy dependent cross-section (σ(E)∼ 10 cm ) for the IBD. Plot taken from [3]

1.2.2 Neutrino Masses and Mass Hierarchy

Although the observation of oscillations implies the existence of mass-differences between the different neutrinos, their absolute mass scale could not be determined so far. The determination of the absolute mass scale is fundamental and subject of various different experiments, which investigate the β-spectrum of tritium decays or the neutrino less double beta decay. In addition, provides the observation and analysis of cosmological data to limit the total mass of all neutrinos mass eigenstates (∑i mi). Currently the strongest limit on the absolute neutrino mass is provided by the tritium decay experiments Troitsk and Mainz, which provide mβ ≤ 2.1 eV [39] and mβ ≤ 2.3 eV [40] respectively. Furthermore allowed the observation and analysis ob cosmological data to limit sum of all neutrino masses down to ∑i mi ≤ 0.5 eV [41]. The future tritium decay

12 Neutrino Oscillation and Flavor Mixing experiments KATRIN [42] plans to improve this measurement by a factor of ten aiming for an sensitivity of mβ ≤ 0.2 eV [43], future cosmological observations might be even sensitive to −3 ∑i mi ≤ (6 ⋅ 10 − 0.1) eV [44]. The oscillation of solar neutrinos in combination with the 2 MSW-effect [18], however, allowed to determine the sign of ∆m21 to be positive, and thus that 2 m1 < m2 . The sign of the other squared mass-difference ∆m23 is still unknown, what provides two different scenarios:

st 1 ∶ ν1 < ν2 < ν3 called “normal” hierarchy

nd 2 ∶ ν3 < ν1 < ν2 called “inverted” hierarchy Both scenarios are depicted in figure 1.4. The used colors indicate the different flavor eigenstates, out of which the different mass-eigenstates are composed. Experiments studying the decay-width of the Z-Boson (mZ ≈ 91 GeV, [38]) jus- Hierarchy tify the current picture of three active mass- Normal Inverted eigenstates with mν < 45 GeV in the Standard Model. However, a recent revision of neu-

3 trino ﬂux predictions from nuclear reactors [45] started a new discussion about the existence of a fourth and heavier mass-eigenstate 2.3 10 ( 1 eV). This fourth mass-eigenstate could > 0 ∼ explain an observed discrepancy between prediction and observation of neutrino-ﬂux from > 0 7.6 10 nuclear power cores by introducing a new

1 1 (sterile) oscillation channel [46, 45]. These new heavy neutrinos supposedly do not interact via weak forces, for which they are 2.3 10 called “sterile” and do not participate in in- < 0 teractions described by the SM. Due to the

3 big mass-difference between sterile and normal neutrinos (of about ∼ 1 eV), the oscilla- = 0 tion length would be less then < 10 m, because of which current experiments are not able to resolve the question regarding a fourth neutrino mass eigenstate and the existence of Figure 1.4: Mass-hierarchy for the three neutrino sterile neutrinos. A possibility to confirm this 2 mass-eigenstates; Due to the unknown sign of ∆m32 oscillation channel would require a neutrino exist two possible configurations for the three mass- flux comparison on ultra short baselines as an- eigenstates, either ν1 < ν2 < ν3 called normal hierar- ticipated by the currently assembled Nucifer- chy or ν3 < ν1 < ν2 called inverted hierarchy. The dif- experiment [47]. ferent colors indicate the different flavor-eigenstates, which compose each mass-eigenstate.

13 Chapter 2

Reactor Neutrinos

2.1 Neutrino Production in Nuclear Power Cores

As already mentioned in the last chapter, nuclear power plants are ideal sources for disappearance experiments like Double Chooz, not only because disappearance experiments are independent from CP-violation, but because of the very high, low-energetic flux of pure electron-anti- neutrinos (ν ¯e), which is produced by fission processes in nuclear power cores. Each fission process of 235U, 238U, 239Pu, 241Pu (the main fission isotopes) leads to the release of nuclear binding energy, the emission of high-energetic neutrons and produces two instable fission-fragments. Figure 2.1 exemplarily depicts the fission of 235U, as well as the subsequent following processes.

Figure 2.1: Schematic view of 235U-fission and subsequent following processes. The neutron-rich and therefore instable fission products (144Ba or 89Kr) decrease their n-excess with multiple β−-decays, what − leads to the production of (in average) 6 e ’s and ν¯e ’s per fission. The emitted neutron, on the other hand, sustains the chain reaction or produces heavier elements as 239Pu. Picture from [48]

14 Reactor Neutrinos

The high-energetic neutrons are moderated and interact in different ways. Some produce heavier elements, like 239Pu, some lose their energy in collisions and some produce a new fission process (on average one, in order to maintain a controlled chain-reaction). In general, the fission leaves two fragments, a lighter one with an atomic mass of around 90, and a heavier one with a mass of about 140 (in figure 2.1, 89Kr and 144Ba). Both of these fragments are neutron-rich and thus unstable. Consequently, they undergo multiple β−-decays to reduce their n-excess

− n → p + e + ν¯e. This chain of β−-decays leads in average to production and emission of about 6 electrons and 6ν ¯e ’s per fission. The total energy released by each fission process depends on the fissioned isotope. Table 2.1 summarizes the averaged energy release per fission for the four main fission isotopes 235U, 238U, 239Pu and 241Pu.

Isotope aEfissf (MeV) 235 U 201.92±0.46 238 U 205.52±0.96 239 Pu 209.99±0.60 241 Pu 213.60±0.65

Table 2.1: Mean values for the energy release per ﬁssion of the four main ﬁssion isotopes [22].

Investigating the fission of Uranium and measuring the kinetic energy of the different constituent parts (fission fragments, neutrons, electrons, neutrinos and gamma emissions), allowed to find an energy distribution of [49, 50, 51]:

Kinetic Energy of the neutrinos ≈ 12MeV , Kinetic Energy of the ﬁssion fragments ≈ 167MeV , Kinetic Energy of the neutrons ≈ 5MeV , Kinetic Energy of the electrons ≈ 8MeV , Gamma Ray emissions ≈ 10MeV .

2.2 Energy Spectrum of Reactor neutrinos

Considering the production process of reactor neutrinos, their possible energy is limited. The n-rich fission products reduce their n-excess via multiple β-decays, what finally leads to the production of electron anti-neutrinosν ¯e. As this is a three-body-decay, the available energy is unequally shared between the electron, the neutrino and the respective fission product. Neglect- ing the recoil energy of the fission product, the energy relation between neutrino and electron is Eν = E(β−decay) -Ee.

The 235U-fission, depicted in Figure 2.1, presents one possible pair of fission fragments. Each of these fragments has its individual decay-chain leading to an individual beta-spectrum. Combin- ing this spectrum with all other spectra that can follow the fission of 235U, allows to determine the β-spectrum for 235U. This combined spectrum can be defined theoretically (if all possible fission products and their beta-branches are known) or measured experimentally, what allows to verify the prediction. The good agreement between predicted and measured electron spectrum (few percent level) shows the fundamental understanding of the processes and justifies the second step to convert the electron spectrum into a neutrino spectrum [19]. This approach

15 Reactor Neutrinos considers, that the total energy of each β-decay has to be shared between electron and antineutrino. Knowing the total energy of the β-decay, as well as the energy of the emitted electron, allows to deduce the residual energy for the neutrino. On reactor level, this means to have a very detailed (theoretical & experimental) knowledge about the β-spectra of 235U, 238U, 239Pu and 241Pu and to combine them weighted by their abundance in the reactor fuel. Knowing the theoretical β-spectrum of a reactor, as well as the actual measured β-spectrum, allows then to determine the resulting neutrino spectrum

Stotal(Eν¯) = S αkSk(Eν¯)d(Eν¯) = Q αkSk(Eν¯),

th where Stotal(Eν¯) is the sum over all individual neutrino spectra of the k -isotope, weighted by th 235 their abundance factor αk of the k -isotope. The constant “Burn-up” of U however changes the isotope composition and thus has to be considered for the prediction of any spectrum. Figure 2.2 shows the initial reactor fuel composition and its development over one year (full duty cycle). th Shown is αk, the abundance of the k -fission-isotope in percent. As can be seen, the constant fission leads to the burn-up of 235U and the production of 239Pu and 241Pu, while the amount of 238U remains constant as only of fast neutrons would be able to induce the fission of 238U. Figure 2.2 shows the expected reactor neutrino spectrum Stotal(Eν¯), found by simulations using the simulation software MURE. Further information regarding the simulation of reactors and correlated spectra can be found in [19] and [30].

Figure 2.2: (left): Energy spectrum of reactor neutrinos simulated in course of the Double Choz experiment. Shown is the number of neutrinos per ﬁssion, and MeV as function of energy. Plot from [52]; (right): Reactor fuel composition and development over one year, also referred to as the burn-up eﬀect. The plot shows abundance αk of the k-th isotope in %. Plot from [53].

2.3 Neutrino Flux Approximation

The determination of the total ﬁssion rate within a nuclear power core, depends on many diﬀerent variables (fuel composition, history of the power levels, neutron transport, geometry of the core etc.), because of which these simulations are done with specialized software tools as MURE1

1MURE is a 3D full core simulation which uses Monte Carlo techniques to model the neutron transport in the core.

16 Reactor Neutrinos or DRAGON2. A detailed description about reactor simulations and the its conversion into a neutrino spectrum can be found in [19], shall however not be discussed here. In order to provide an idea about the immense neutrino flux emitted by nuclear power plants, the neutrino flux can be roughly approximate, which will be briefly presented in the following. Assuming that the entire energy of nuclear fission is heating up the reactor core, the emitted flux can be approximated by comparing the thermal power output of nuclear power plants Pth with the heating-increment, that is released per fission aEfissf. In order to do this approximation, it has to be assumed that reactor power Pth and aEfissf are constant over time. When this is given, the fission rate ⟨F ⟩ can be approximated by dividing thermal power output Pth by the energy released per fission aEfissf. Once the number of fissions is known, it can be multiplied by the number of neutrinos, which are produced per fission in average. Important for the approximation is the effective value of aEfissf. Reactor fuel is a composition of the four main fission isotopes 235U, 238U, 239Pu and 241Pu. Each of these isotopes contributes to an individual energy per fission aEfiss,Isotopef. In order to account for the real reactor fuel composition, which is indicated in table 2.2, the energy release per fission aEfiss,reactorf will be determined as weighted sum, considering the individual amount of each isotope. This, however, neglects the constantly changing fuel-composition, also referred to as burn-up effect, which is indicated in figure 2.2 for the time period of one year.

Element Unit 235U 238U 239Pu 241Pu

aEfiss,isotopef MeV 201.92 205.52 209.99 213.6 Composition % 49.6 8.7 35.1 6.6

Table 2.2: Reactor fuel composition about 250 days after the start of a burning cycle and the energy release per ﬁssion for the four main ﬁssion isotopes [22].

Using the composition and energy contributions shown in table 2.2, leads to a weighted sum and aEfiss,reactorf of 205.8 MeV. Using this number allows to approximate the ﬁssion rate of a nuclear power plant. In case of Double Chooz, the two currently running power cores in Chooz B (B1 & B2) have a maximal thermal power of 8.5 GW. MeV P 8.5 GW 8.5 6.24 1021 th,ChoozB@100% ≈ ≅ ⋅ ⋅ s . 21 ⟨Pth⟩ 8.5 ⋅ 6.24 10 MeV ~s 20 −1 F = = ≈ 2.6 ⋅ 10 s aEfiss,reactorf 205.8MeV Using ⟨nν¯⟩ ≈ 6 allows to approximate the number of anti-neutrinos emitted per second: 1 N n F 6 2.6 1020 1.5 1021 . ν¯ = ⟨ ν¯⟩⟨ ⟩ = ⋅ ⋅ ≈ ⋅ s This rate is isotropically distributed over the full solid angle. The neutrino ﬂux Φ per cm2 can therefore be approximated in dependence of the distance, which is

21 ν¯ 20 Nν¯ 1.5 ⋅ 10 1.2 10 1 Φ r s ⋅ . ( ) = 4πr2 = 12.56r2 = r2 s cm2 Applying this approximation to Double Chooz, the neutrino ﬂux per cm2 at the positions of near detector (ND: 400 m) and far detector (FD: 1050 m) would be: 1 ND Φ 400 m 7.5 1010 and ∶ ( ) = ⋅ s cm2 2DRAGON is a 2D simulation which models the individual fuel assemblies, which solves the neutron transport equation in the core.

17 Reactor Neutrinos

1 FD Φ 1050 m 1.1 1010 . ∶ ( ) = ⋅ s cm2 Only 25% of this flux has enough energy to induce an inverse beta decay in the detector [3]. Based on this neutrino flux values the measurable event rate in the near and far detector can be approximated by: Nν(r) = Φ(r) ⋅ σIBD ⋅ Nprotons ⋅ Det where Φ(r) is the neutrino flux at a certain distance, σIBD the cross section for the inverse beta decay, Nprotons the number of target protons and Det an overall detection efficiency. Using −43 2 29 the approximated neutrino flux, σIBD=5.25⋅10 cm [22] and Nprotons=6.74⋅10 [54] allows to approximate the neutrino rate to:

−1 Nν(ND) = 588 × Det d

−1 Nν(FD) = 84 × Det d

Using a data taking efficiency of 71% (see figure 11.1) as well as a neutron detection efficiency of 91.85% as presented in [22] leads to an Det of about 65% which results in an expected rate of −1 Nν(ND) = 383 d −1 Nν(FD) = 57 d Comparing this rough approximation with the actual measured neutrino rate in the DC-far detector (42 d−1, compare with fig. 11.12) shows that the used simplifications are reasonable.

18 Chapter 3

The Double Chooz Experiment

3.1 The Double Chooz Collaboration

The Double Chooz collaboration is an international working group that shares the scientiﬁc eﬀort between 183 scientists from 36 Institutes and 8 nations. France, as host of the experiment, takes, apart from its working packages, additionally care of organizational issues of the experiment, allocating the spokesman1, project manager2 and leading engineers3. Table 3.1 summarizes the composition of the Double Chooz collaboration and breaks down the number of involved countries, institutes and scientists.

Nation USA Germany Japan France Brazil Russia Spain UK Sum Institutes 12 5 7 5 3 2 1 1 36 Scientists 54 37 25 25 21 7 5 5 183 Fraction 30% 20% 14% 14% 12% 4% 3% 3% 100%

Table 3.1: Composition of the Double Chooz Collaboration

Figure 3.1: Double Chooz Collaboration in front of the nuclear power plant in Chooz. Picture from DC-coll.

1Prof. Dr. Herv´ede Kerret(APC) 2F. Ardellier (CEA), Z. Sun (CEA); currently: C. Veyssiere (CEA) 3P. Perrin (CEA), L. Scola (CEA)

19 The Double Chooz Experiment

3.2 Experimental Site: Commercial Nuclear Power Plant in Chooz

Over the past decades, France relied on the production of nuclear energy, due to which the “électricitéde France” (EDF) operates 59 nuclear reactors in 19 facilities. One of these facilities is the “Centre Nucléairede Production d’Electricité(CNPE) de Chooz”. The power plant is situated in the French Ardennes, near the village of Chooz and close to the Belgium border. As can be seen in figure 3.2, the “CNPE de Chooz” consists of two separate facilities, the currently running facility “Chooz B” and the already shut down underground facility “Chooz A”, which was operated between 1960 and 1991, and is dismantled since 2008 [30],[55]. “Chooz B” combines

Figure 3.2: Nuclear power plant in the French Ardennes close to the village of Chooz: The picture shows the two reactor blocks of Chooz B in the foreground, framed by the Meuse-river. The currently dismantled facility of Chooz A is indicated by surface buildings on the right. The village of Chooz can be found in the background along the riverbanks. two reactor-blocks, which began their operation in 1996 (B1) and 2000 (B2), respectively [30]. Each block is equipped with a N4-type pressurized water reactor (PWR), representing the latest and most powerful generation of water-cooled and -moderated power cores. Each reactor is equipped with 110 t of Uranium, symmetrically arranged in 205 fuel elements forming the core [30, 55]. The controlled fission of one power core produces a thermal energy of 4.25 GWth and an electrical power of 1.45 GWe, which corresponds to an efficiency of 34.1%. Nuclear power plants are ideal sources for neutrino disappearance experiments as Double Chooz, as they emit a high and pure flux of electron anti-neutrinos with low energies (compare with chapter 2).

20 The Double Chooz Experiment

3.3 Physics Program and Experimental Concept

Physics Program

The Double Chooz experiment aims for a high precision measurement of the recently found mixing angle Θ13 as part of the neutrino mixing matrix [34, 23, 24]. The completion of the leptonic mixing matrix clears the way for future precision experiments, searching for CP-violation in the leptonic sector or the neutrino mass hierarchy. In addition, Double Chooz supports the International Atomic Energy Agency (IAEA) in investigating the possibility to monitor nuclear power cores with neutrino detectors as part of their non-proliferation efforts. As neutrinos cannot be shielded, but carry information about the fissile materials, as well as the fission rate, neutrino detectors could be used to monitor the power-level and the amount and composition of fissile materials within nuclear power plants. The implementation of such an independent tool would be a significant improvement to the international efforts of non-proliferation and help to keep track of fissile materials.

Experimental Concept

In order to measure the disappearance effect of reactor neutrinos one has to know the number of actually emitted neutrinos and the number of neutrinos at a certain distance form the power core. The truly emitted neutrino flux can either be calculated (simulated) or it can be measured using a second detector. The Double Chooz experiment is realizing both scenarios in two different experimental phases. During the first phase, Double Chooz repeats the CHOOZ-experiment using only one detector with a distance of 1050 m from the reactors. For the first phase of Double Chooz, the expected ν-signal for the far detector has to be obtained by calculation, which is based on a actually measured neutrino spectrum from the former Bugey-4-experiment [21]. Using reactor evolution simulations, this spectrum is adapted to the present situation in Chooz B and finally used for the prediction of a no-oscillation-signal. The far detector is then used to measure the oscillated and therefore reduced neutrino flux, which can then be compared to the expectation. This approach, was already used by the CHOOZ-experiment, suffered however from the necessary assumptions regarding the neutrino production process in nuclear power cores, which lead to systematic errors in the range of 2 - 3 %. In the second phase Double Chooz will improve this situation by employing a second, near detector with only an averaged distance of 400 m from the cores. This near detector is supposed to measure the actual emitted neutrino flux before oscillation-effects have to be considered (see figure 3.4). Using this unoscillated ν-flux, the expected neutrino signal for the position of the far detector can be predicted completely independent from simulations or any reactor based assumptions. Hence, the employment of a second detector is a significant improvement and will allow a relative measurement, which will reduce the systematic uncertainties of the experiment significantly. In order to facilitate the later analysis of the two different oscillation base lines, both detectors will be installed on the iso-flux-line. The line on which the neutrino-flux-ratio of both reactors is equal for both detectors. Figure 3.3 provides an overview of the facility in Chooz and the relative positions two underground lab, which are used to host near- and far-detector. In addition Double Chooz improved the experimental situation by employing a new detector design. Both detectors are equally designed and are composed of four concentrically nested vessels, each filled with different newly developed detector liquids. The detector is optimized to reduce the disturbing influence of background events using several layers of as passive shielding

21 The Double Chooz Experiment

Figure 3.3: Overview scheme of CNPE de Chooz: Indicated are the two power cores B1 (purple) and B2 (yellow), as well as the location of the near and far detector (red circles). The individual base lines are shown in black. The Double Chooz detector setup comprises two underground laboratories, a far one, which has already been used for the Chooz experiment, and a near one, which is currently excavated. The “far lab” is affiliated to Chooz A and lays on the far side of the river, providing an average distance of 1050 m from the two cores. It is 17 m below ground and was additionally driven into a hill, what provides an additional rock overburden leading to a passive shielding of 300 m.w.e. The new “near lab” will be excavated below a smaller geological elevation just outside of Chooz B, which leads to an averaged distance of 400 m and a rock overburden corresponding to 120 m of water. Both underground labs are equipped with a drivable entrance tunnel, connecting the labs with necessary surface installations. and active muon vetos. A detailed presentation of the detector design will be presented later in this chapter. Most important is the inner most neutrino target, which holds 8 tons of Gd-doped liquid scintillator. With its high neutron cross-section, the gadolinium compound facilitates the identification of a neutrino exploiting the distinct coincidence signal of the inverse beta decay (see next section). Using the IBD each of the two detectors is able to identify single neutrino events and to measure their initial energy. Being able to do neutrino spectroscopy, Double Chooz is able to measure not only the neutrino rate but also the energy depending distortion of their energy spectrum, which is an unmistakeable sign for the existence of neutrino oscillation. Due to the energy dependance of the survival probability and the fixed position of the far detector, Double Chooz is most sensitive to the oscillation of 2-MeV-neutrinos, which have their survival probability minimum at about 1025 m from the neutrino source. As presented in table 1.2 the maximal measurable disappearance effect for 2-MeV neutrinos is 6.6 % (comparing near and far detector) and even less for higher energies. In consequence, the far detector will not only observe a lesser neutrino rate but also a spectral distortion around 2 MeV, where the oscillation effect is maximal. A plot indicating the expected signal is presented in figure 3.8. Figure 3.4 presents exemplarily this survival probability as function of distance for 3 MeV-Neutrinos, 2 −3 2 ○ m31 = 2.4 × 10 eV and Θ13 = 8.3 ).

22 The Double Chooz Experiment

SurvivalProbabilityH%L 1.00 L 0 = 1.23 Δm13 / Eν E = 3 MeV 0.98

0.96 13 2

0.94 sin (2 θ )

0.92 ND FD LHmL 150 200 300 500 700 1000 15002000

2 −3 2 ○ Figure 3.4: Survival probability of ν¯e’s, assuming (E = 3 MeV, ∆m31 = 2.4 × 10 eV and Θ13 = 8.3 ). At 2 L0, the oscillation amplitude is maximal and sin (2Θ13) converts about 10 % of the emitted ν¯es into ν¯µ τ to which the detector is insensitive4 The vertical lines represent the positions, the horizontal lines the individual survival probability of near- (ND) and far-detector (FD). The diﬀerence between the latter indicates the maximal measurable disappearance eﬀect of 6 %.

3.4 Signal

3.4.1 The Inverse Beta Decay (IBD)

The inverse beta decay describes the neutrino induced conversion of a proton into a neutron [27, 3]. As the neutron is heavier than the proton the incoming anti neutrino has to provide at least the energy Qth to initialize the conversion, which is given by:

2 2 2 Qth = mnc − mpc + me+ c = (939.56 − 938.27 + 0.51) MeV . = 1.80 MeV

The inverse beta decay is the main detection channel for reactor neutrinos. Table 3.2 summarizes the Qth-values for IBDs, which can be induced by the diﬀerent neutrino ﬂavors.

Channel IBD with ν¯ Energy Threshold

+ Electronic ν¯e + p → e + n Qth,e = 1.80 MeV

+ Muonic ν¯µ + p → µ + n Qth,µ = 106.94 MeV

+ Tauonic ν¯τ + p → τ + n Qth,τ = 1778.28 MeV

Table 3.2: Inverse beta decay channels, including their necessary energy threshold Qth [27, 3].

Comparing the diﬀerent channels and the corresponding energy thresholds Qth of table 3.2 with the energy spectrum of reactor neutrinos (0 < Eν¯e < 12MeV ), illustrates that reactor neutrinos can only induce the electronic IBD-channel. This characteristic makes them ideal for reactor

23 The Double Chooz Experiment

disappearance experiments, as all detected IBDs are the result ofν ¯e, but notν ¯µ,τ , which allows a clean measurement of the oscillation eﬀect.

3.4.2 Signature of the IBD

As indicated earlier, an IBD-event in the target (Gd- doped liquid scintillator) leads to a delayed coincidence signal that allows to identify not only a neutrino interaction, but also to measure the initial neutrino energy. The signal is composed of a prompt event and a delayed event. The prompt event is the result of the positron, while the delayed event is produced by the capture of the neutron in the scintillator. The capture time τ depends on the capturing nuclei, what can be used to distinguish the n-captures on hydrogen, carbon or gadolin- Figure 3.5: An IBD-event produces a de- ium. In the following, the nature of both events shall layed coincidence composed of prompt and be discussed in more detail. delayed event.

Prompt Signal

The energy of the prompt signal is resulting from two contributions. The ﬁrst one comes from the deceleration of the positron, which produces a variable signal depending on the initial neutrino energy. The second one comes from the subsequent annihilation, which adds 2×511 keV to the prompt signal. Both parts together lead to a prompt signal, which corresponds to the initial neutrino energy: the correlation between neutrino energy and visible energy in the detector can be explained by the energy balance of the IBD, which is given by [3, 56, 27]:

Eν + mν + mp = mn + me+ + En + Ee+ , (3.1) where (mν, Eν) are the mass and kinetic energy of the incoming neutrino, and (mp) the proton mass on one side, while (mn, me+ ,En,Ee+ ) describe the masses and energies of the outgoing neutron and positron. The visible energy of the prompt signal is produced by deceleration (Ee+ ) and subsequent annihilation of the positron:

Evis,e+ = Ee+ + me+ + me− .

Neglecting the neutrino mass (mν ≈ 0) and using, that the dominant part of the neutrino energy is, for kinematic reasons, transferred to the lighter positron (En ≈ 0), allows to write eq. 3.1 as:

Eν ≈ mn − mp + me+ + Ee+ Eν ≈ mn − mp + Evis,e+ − me− Evis,e+ ≈ Eν − 0.8MeV .

Measuring the energy of the prompt signal consequently allows to measure the initial neutrino energy and to perform neutrino spectroscopy.

The characteristic energy spectrum of the prompt signal, observable in the detector, is shown in ﬁgure 3.6 , curve (a). It is the convolution of the cross-section for the IBD shown in graph (c) and the neutrino rate shown in graph (b). As shown in ﬁgure 2.2, the reactor energy spectrum

24 The Double Chooz Experiment

. Figure 3.6: a) Observable energy spectrum for the IBD in liquid scintillator detectors, b) energy spectrum for reactor neutrinos, c) energy dependent cross-section for the IBD. Plot taken from [3], with kind permission of Oxford University Press. decreases almost exponentially to higher energies, while the cross section for an IBD increases parabolic with energy and is described by [3]:

2π2 E p τ −1 σ 9.56 10−44 e e n cm2 . IBD = 5 ≅ × 2 τnmef MeV 886s

Considering the minimal energy Qth required to induce the electronic channel of the IBD and the energy spectrum of nuclear power plant, implies, that only 25% of the emitted neutrinos can be detected at all, the other 75% are below the detection threshold of 1.8 MeV [57]. Neutrinos with higher energies result from β-decays with large Q-value. Since these β-decays are relatively fast, the intensity of the neutrino ﬂux is closely related in time with the thermal power of the reactor. The observed interaction rate is therefore proportional to the current power level of the reactor and allows an online monitoring of the power core as anticipated by the IAEA.

Delayed Signal

The delayed signal of the IBD-signature is produced by the free neutron, which is thermalized and captured on the surrounding nuclei. The mean capture time, as well as the energy released upon a neutron capture, depends on the capturing nuclei. Liquid scintillators are mainly composed of hydrogen, carbon and in case of the target scintillator, also gadolinium, which has a extremely high neutron capture cross-section. Table 3.3 summarizes the mean capture time (τn−capture), energy release (Eγ−emission), n-capture cross-section (σn−absorption), as well as the natural abundances of hydrogen, carbon and gadolinium isotopes. Because of the bigger cross-section, gadolinium has a much shorter neutron capture time. Comparing these values with hydrogen and carbon, indicates the doping with gadolinium has signiﬁcant advantages. Firstly, the high-energetic γ-signal, which is following a neutron capture,

25 The Double Chooz Experiment

Isotope nat. abundance σn−absorption Eγ−emission τn−capture AX (%) (barn) (MeV) (µs) 1 H 99.98 ≈0.33 ≈2.22 ≈ 220 12 C 98.93 ≈0.0035 ≈4.94 ≈ 180 154 Gd 2.18 ≈ 60 ≈6.43 155 Gd 14.80 ≈ 61000 ≈8.53 156 Gd 20.47 ≈ 2 ≈6.36 ≈ 30 157 Gd 15.65 ≈ 254000 ≈7.93 158 Gd 24.84 ≈ 2.3 ≈5.94 160 Gd 21.86 ≈ 1.5 ≈5.63 Table 3.3: Natural abundance of H, C and Gd in percent, the absorption cross-section for thermal neutrons (σn−absorption), the emitted γ-energy (Eγ−emission) for n-capture in MeV and the capture time (τn−capture). Values from [58] allows to identify the delayed signal far above the energy levels of natural radioactivity. Sec- ondly, the short neutron capture time decreases the chance for accidentally found coincidences. Figure 3.7 presents the signature of the inverse beta decay, indicating the prompt signal in blue (marking t=0) and the three diﬀerent delayed signals in red, as they would occur upon a neutron capture on hydrogen, carbon or gadolinium, respectively.

Energy [MeV]

12 prompt e+ -signal [0-12]MeV 8 delayed Gd-signal E~8.3 MeV delayed 12C-signal E~4.4 MeV 3 Energy spectrum of delayed natural radioactiviy 1 [0-3MeV] H-signal E~2.2 MeV

τ = 0 30 180 220 μs Figure 3.7: The IBD produces a distinct signature, used to identify a neutrino interaction in liquid scintillators. The signature is composed of the prompt energy deposition and annihilation of the positron, depicted in blue, which is followed by a delayed neutron-capture, depicted in red. The mean capture time τ, as well as the energy Eγ−emission of the second signal depend on the capturing nuclei, both allow to tune the coincidence parameters, which increase the detection eﬃciency, as well as the suppression of background events. The here presented capture times τ are no ﬁxed capture times, as indicated for a better illustration, but statistical mean values.

As positron and neutron are produced in the same vertex, the delayed signal is in close vicinity to the prompt signal. Although this would allow a spatial cut, Double Chooz uses only the two main characteristics, energy and timing, for the identiﬁcation of the IBD.

26 The Double Chooz Experiment

3.4.3 Expected Signal

Measuring neutrino rate and neutrino energy spectrum in both detectors will allow, after calibration and normalization, a direct comparison between near and far detector. The calibration is realized with different sources and techniques, what will be introduced later in this chapter. As the detectors will have different baselines (Ln,Lf ), as well as a slightly different neutrino detection efficiency (n, f ), or a slightly different number of target protons (Nn,Nf ), the signals will have to be normalized to be comparable. After normalization, the ratio (NIBD,f /NIBD,n) should be equal to one, in the case for no oscillation, and less than one, for a positive oscillation signal. As the oscillation is energy-dependent, the ratio varies with energy. For the far detector signal with Lf =1050 m, the oscillation signal is most dominant at 2 MeV, because of which the energy spectrum is expected to show a dip around 2 MeV. The ratio is defined by:

2 NIBD,f Nf Ln n Psur(E, 1050 m) = , (3.2) NIBD,n Np Lf f Psur(E, 400 m) where NIBD is the number of detected IBD-events in the near and far detector, and Psur(E,Ln) the corresponding survival probability. Figure 3.8 presents two simulated energy spectra, as they 2 2 −3 2 could be measured by near and far detector, assuming sin (2Θ13) = 0.1 and ∆m31 = 2.5⋅10 eV . The simulated oscillation effect of 10 % would lead to a reduction of the neutrino rate and a distortion in the energy spectrum as result of a higher oscillation probability for a certain energy. The top plot of figure 3.8 presents the normalized and therefore comparable spectra of the prompt signal. The expected near detector signal is shown in green and the oscillated far detector spectrum is superimposed in blue. The plot in the lower left shows the ratio of far and near detector spectra, as described in equation 3.2. The energy spectrum indicates a 10 % disappearance effect for neutrino energies around 2 MeV. The plot in the lower right shows a subtraction of both far and near detector spectra, indicating the spectral distortion around 2 MeV.

27 The Double Chooz Experiment

2 2 −3 2 Figure 3.8: Simulated detector response assuming sin (2Θ13) = 0.1 and ∆m = 2.5 ⋅ 10 eV after three years of data taking. (top): Simulated energy spectra for the prompt signal of near and far detector in direct comparison already corrected for reduced ν-rate due to distance; (center): Ratio between far and near spectrum indicating the characteristic spectral distortion at lower energies (error bars are only statistical); (bottom): Spectrum diﬀerence between near and far detector (normalized to the far detector statistics); Plot taken from [55].

28 The Double Chooz Experiment

3.5 Detector Design

Double Chooz uses a multi layered detector design composed of four concentrically nested vessels. The detector has a total volume of 233 m3, subdivided into four compartments filled with 90 m3 of muon veto scintillator, 110 m3 non-scintillating buffer liquid, 22.5 m3 gamma catcher scintillator and, most important, 10.3 m3 of gadolinium doted target scintillator. The two innermost vessels, used for gamma catcher and target, are optimized for physics and made of transparent acrylics. Both vessels are thin walled, include a long and thin neck and are additionally designed to be stiff (see figure 3.9). Consequently, these vessels are fragile and quickly endangered by already little mechanical stress. These scintillating volumes, kept in transparent vessels, are set up in a bigger steel vessel (buffer), which is filled with a non-scintillating mineral oil. These three vessels compose the inner detector (ID), which is completely encased by the outermost vessel (inner muon veto (IV)), which uses another liquid scintillator to identify muons and muon-induced background. The muon veto tank is, in addition, completely surrounded by a 15 cm steel layer, which is supposed to shield the inner vessels from external radioactivity. Furthermore, this setup is covered by the outer muon veto (OV), which is an individual detector module made of plastic scintillator stripes. An overview of this detector setup is presented in figure 3.9 and indicates the two detector parts: The inner detector (ID), comprising neutrino target (NT), gamma catcher (GC) and buffer (BF), and the outer detector, composed of the inner- and outer-muon veto. While the ID searches for the signature of the inverse beta decay, the OD is used to identify cosmogenic muons and correlated backgrounds. In the following, the most important detector parts (detector layers, read-out and calibration system) shall be shortly summarized to provide a better overview about the Double Chooz detector. Table 3.4 summarizes the main dimensions of the different detector vessels, further information about the detector vessels, as well as all other parts and systems of the detector, are summarized in [55].

Vessel Height [mm] Ø[mm] Wall Material Vol. [m3] Layer [mm] Muon Veto 6873 6470 12 mm steel 90 570 Buﬀer 5679 5516 3 mm st. steel 100 1050 Gamma Catcher 3572 3416 12 mm acryl 22.5 550 Target 2458 2300 8 mm acryl 10 2300

Table 3.4: Summary of detector dimensions, used materials and volume ﬁlled with scintillator, what leads to diﬀerent layers of active and passive detector liquids. Values from [60].

3.5.1 Neutrino Target (NT)

The main purpose of the neutrino target is the detection of the inverse beta decay and therefore the measurement of the energy deposition of prompt and delayed signal. The target vessel (red outlines in figure 3.9) is a very fragile acrylic cylinder with conical bottom and top-lid. The top-lid includes a long chimney (ø=15 cm, l=4 m), which exits the detector and connects directly to the Glove Box (GB), providing an access point for calibration. The neutrino target has a volume of 10.3 m3, UV-transparent acrylic walls and stiff geometry, which defines the volume of the vessel and helps to determine the number of protons in the target. Both features, the fragile walls on one side and the stiff geometry on the other, imply a very high risk of fracturing the acrylic vessels, because of which all monitoring, as well as detector handling systems, have to work flawless. The target scintillator is a composition of PXE and Dodecan, PPO and bis/MSB used as wavelength shifter, as well as a gadolinium complex in order to increase the neutron capture rate. The composition of the target scintillator, as well as all other detector liquids, can be found in table 3.5. The gadolinium has a very high neutron capture cross-section and

29 The Double Chooz Experiment

Figure 3.9: Vertical cut through the DC-far-detector, presenting the individual vessels and detector layers. The onion shell structure can be divided into two main parts, the inner detector (ID), searching for the neutrino interactions, and the outer detector (OD), used to identify muons. A technical drawing can be found in the appendix A.1. Picture from [59]. provides a mono energetic γs with about 8 MeV, which improves the detection eﬃciency and allows to suppress background events signiﬁcantly.

3.5.2 Gamma Catcher (GC)

The gamma catcher vessel (blue outlines in figure 3.9) is filled with 22.5 m3 un-doped scintillator, what generates a 56 cm thick layer of undoped liquid scintillator. This second layer of scintillator increases the energy collection efficiency of the detector and is supposed to capture all gamma emissions originating escaping from the target. The UV-transparent acrylic vessel is equal in shape and material to the target vessel, the only differences are bigger dimensions and a shorter neck, that does not exit the detector. The composition of the scintillator is summarized in table 3.5. The light yields of target and gamma catcher are matched, in order to produce a homogeneous detector response.

30 The Double Chooz Experiment

3.5.3 Buﬀer (BF)

The buffer-volume is one of the improvements of Double Chooz, in comparison to the design of Chooz. It is filled with 110 m3 of a non-scintillating and highly transparent mixture of mineral oil, which generates a 105 cm thick layer of passive detector liquid around the scintillating volumes. This passive layer is used as transparent shield against internal radioactivity, mostly coming from the detector materials itself, as the PMTs. The buffer-vessel (green outlines in figure 3.9) is a 3 mm thick stainless steel tank with conical bottom and top, which separates inner- from outer detector. The entire inner surface is equipped with 390 10-inch photo multiplier tubes (PMTs), all facing towards the active region and homogeneously covering about 19% of the inner surface. The buffer-liquid is a non-scintillating mixture of mineral oil and n-paraffine, as can be seen in table 3.5.

3.5.4 Inner Muon Veto (IV)

The inner muon veto (yellow outlines in figure 3.9) is filled with 90 m3, what generates a 57 cm thick layer of undoped liquid scintillator. The muon veto is supposed to identify cosmogenic muons, which pass through the inner detector. Identifying those muons allows not only to veto them, but also to estimate the contribution of correlated background produced by spallation processes in or near the detector. The muon veto scintillator is based on LAB and n-paraffine and uses PPO and bis/MSB as wavelength shifter, the exact composition can also be found in table 3.5. The volume is equipped with 78 encased 8-inch PMTs, facing in different directions, allowing to cover only 1.8% of the inner surface area. In order to increase the detection efficiency, the inner surface is covered with white paint and a highly UV-reflective foil. The muon veto vessel is a massive steel tank, which retains the entire detector liquids. The 12 mm thick steel tank has a removable top-lid which comforts the central chimney and allows to seal the detector hermetically. The top side of the tank is dominated by the central chimney and a ring of 48 flanges, which allow to feed through all necessary detector connections.

3.5.5 Passive Steel Shielding

Between the muon veto vessel and the detector pit walls, a 15 cm thick layer of demagnetized steel shields the inner detector parts from external radioactivity coming from the surrounding rock. The top layer of the shielding is supported by a rail system and splits at the center. One half of this shielding is able to move away from the center, granting access to chimney and muon veto top-lid. Although near and far detector are supposed to have an identical design, the passive shielding is diﬀerent for near- and far-detector. Using the already existing underground laboratory for the far detector restricted the available space for the shielding and necessitated the usage of steel (expensive) in order to provide suﬃcient shielding at the available space. In the bigger designed near lab, the available space is not an issue, because of which the near lab provides a 1 m thick water shielding surrounding the veto-vessel.

3.5.6 Outer Muon Veto (OV)

The outer muon veto (light green area in ﬁgure 3.9) is not surrounding the detector, but spanning 2 a wide area above it. It covers in total 90 m (7 m×12.8 m) and exceeds the area above the detector by almost 3 m on each side. Covering a bigger area allows to identify also those muons, which passed nearby the detector, which are a source of additional background. The OV is composed of three modules, two main modules mounted on the shielding and comforting the

31 The Double Chooz Experiment

2 central target chimney, and a smaller module (5×5 m ) mounted to the ceiling above the chimney to cover the blind-spot chimney. With an energy deposition of 2 MeV/cm, each passing muon produces an 80 MeV-signal in the 40 cm thick OV-modules, what leads to a very high detection efficiency (>99%). The cross-layered setup of the main modules and the parallel mounted third module, which covers the blind spot in the above the chimney, provide very good tracking capability. An schematic overview of the different OV-modules in correlation with the detector setup is shown in figure 3.10.

Side view 5m 7m Glove Box 2.5m

Detector Hall 12.8m

Detector

Figure 3.10: Overview drawing of the detector, indicating the position and dimensions of the outer veto modules. Picture from [61]

3.5.7 Detector Liquids

Double Chooz is filled with 4 different detector liquids, three liquid scintillators and one non- scintillating buffer liquid. Each liquid serves a different purpose and is thus differently composed, nevertheless these different liquids have to work together and suit the requirements of the detector. One of these requirements is the density of the four liquids. Any deviation from equality (ρ=0.804 g/cm3@15 C○) above the per-mill level would lead to devastating buoyancy forces, which thereupon would lead to the rupture of the acrylic-vessels and a subsequent following mixture of the liquids. Table 3.5 presents a detailed overview of the individual detector liquids and their composition. The given volumes correspond to the necessary amounts, as they have been produced for the far detector.

3.5.8 Detector Readout System

The data acquisition system of Double Chooz reads out the photo multiplier tubes installed in IV and Buffer. The muon veto is equipped with 78 PMTs (8-inch, R1408 formerly used in the IMB- experiment), while the inner detector is monitored by 390 PMTs (10-inch, R7081-MOD-ASSY) from Hamamatsu [63]. Provided with a voltage of about 1.5 kV, each PMT shows a gain of about 7 10 and a single-PE signal of < 10 mV [64]. Both, signal and HV, are conducted by the same coax-cable (Teflon-Coated, RG303) and have to be separated once the cable left the detector. The separation of signal and HV is done outside of the detector by a custom-made high-pass filter, also referred to as splitter. The splitter-box conducts the signal to the custom made “Front-End”electronics (FEE), which serves several purposes: it matches the signal dynamics between the output of the splitter and the input of the FADC electronics, it performs signal pre-amplification or signal-clipping when necessary (high energetic events), it stabilizes the baseline and reduces noise. The FEE provides signals to two FADC-systems (ν-FADC, µ-FADC), as well as a trigger system. The ν-FADC (customized CAEN-V1721, max.rate: 500 MHz) is optimized for neutrino interaction signals between single-PE and 15 MeV, while the µ-FADC (custom made, max.rate: 125 MHz) is optimized for higher energies >40 MeV (too high for ν- FADC) [64]. Upon each trigger, both FADCs record and digitize an interval of 256 ns, which is

32 The Double Chooz Experiment

Detector Liquid Composition Amount Ingredient CAS-Number 3 Neutrino Target 80 %vol 8 m n-dodecane 112-40-3 3 3 10.3 m NT-LS 20 %vol 2 m PXE, Phenylxylyethane 6196-95-8 4.5 g/l 45 kg Gd-(thd)3 14768-15-1 0.5 %wt. 5 kg Oxolane, THF 1099-99-9 7 g/l 7 kg PPO 92-71-7 20 mg/l 0.2 kg bis-MSB 13280-61-0

3 Gamma Catcher 66 %vol. 14.8 m Mineral oil / Ondina 909 8042-47-5 3 3 22.5 m GC-LS 30 %vol 6.75 m n-dodecane 112-40-3 3 4 %vol. 0.9 m PXE, Phenylxylyethane 6196-95-8 2 g/l 45 kg PPO 92-71-7 20 mg/l 0.4 kg bis-MSB 13280-61-0

3 Buﬀer Liquid 53.5 %vol. 53.5 m Mineral oil / Ondina 917 232-455-8 3 3 110 m BF-Oil 46.5 %vol. 46.5 m n-paraﬃne (C10-C13) 265-233-4

3 Muon Veto 50 %vol. 45 m n-paraﬃne (C10-C13) 265-233-4 3 3 90 m MU-LS 50 %vol. 45 m LAB, linear alkyl benzene 265-233-4 2 g/l 180 kg PPO 92-71-7 20 mg/l 1.8 kg bis-MSB 13280-61-0

Table 3.5: Composition of muon veto, buffer, gamma catcher and neutrino target and the individual amounts used for the far detector. Values for gamma catcher and target are taken from [62] subsequently written to hard disk. The trigger-system groups the different PMTs to check the plausibility of the trigger signal. The ID-PMTs are split in two groups of 195 each, while the IV-PMTs are split in multiple groups of 8 PMTs each. The trigger condition is fulfilled, when the energy-sum of a single PMT-group is above 5 MeV in the muon veto or any other energy deposition above 700 keV in the ID. The trigger efficiency, shown in figure 11.1, increases from +0 50 % at 400 keV to 100−0.1% between 700 and 800 keV [65].

The main data acquisition depends on the capabilities of the ν-FADC, which samples at 500 MHz and holds a buﬀer of up to 4 µs per trigger and channel. The read out system is supposed to work dead-time free for an expected rate of 300 Hz, dominated by cosmogenic muons and correlated backgrounds at the near detector-site. In order to reduce the amount of recorded data to <10 Gb/day (on average), an online data reduction scheme has been conceived using the VME-crate computers and/or the event builder computers to ﬂag and characterize each energy deposition. Figure 3.11 provides an overview of the readout system of IV and ID (excluding OV).

3.5.9 Detector Calibration System

In order to calibrate the detector and to test and understand the detector response, a broad calibration program was conducted. It comprises the use of multiple calibration techniques using diﬀerent radioactive sources, as well as light calibration sources. The systems used hereby are brieﬂy presented in the following. Guide Tube [66]: is a small stainless steel tube, that allows to guide a wired and encap- sulated calibration source through the inner PTFE-layer. The tube is a vertically installed

33 The Double Chooz Experiment

Inner Veto IV HV cable 26m cable 18m μ-FADC PMT RG303 Splitter RG303 78 PMT‘s (8") 500 Mhz CAEN-V1721 Hammamatsu RI 406 VME (from IMB)

IV: energy + pattern HV-supply Front End Trigger VME Crate Electronics 16 FADC Cards CAEN-AI535P ID: energy

Hammamatsu R7081 MOD-ASSY ν-FADC Inner Detector ID HV 125 Mhz 22m cable 18m cable Custom 390 PMT‘s (10") PMT RG303 Splitter RG303 VME

Figure 3.11: Data acquisition system for muon veto and inner detector. Information from [64].

circle line. Installed in the GC, the tube is bent in such way that the source runs down the target-chimney, along the top-lid and side-wall of the target, before it passes horizontally through GC and runs back along the GC-wall. The path of the tube is indicated in red in figure A.1. Light Injection System [67]: The buffer vessel is equipped with multiple optical fibers mounted to the walls. Multiple LEDs with different wavelengths allow to inject light (focused or diffuse) into the detector. Z-Axis System [68]: uses the straight target chimney and the glove box mounted on top to deploy different calibration sources. The system is installed in the glove box (GB) and allows to descent a calibration source of choice on a wire. The wire holds either a light diffusing ball or a standard calibration source. Buffer Tube [69]: a small tube that runs vertically from the top of the buffer to the bottom and allows to insert and deploy radioactive calibration sources. Articulated Arm [70]: a massive telescopic arm that enters through the target chimney. Once the telescopic arm is extended far enough into the main body of the target, the articulated joint allows a 90 degree elevation of the lower part of the arm. This allows to deploy a calibration source not only along the z-axis, but also along the off-axis. The additional rotation of the arm enables then to move the source along a circular path within the target. The calibration method normally used during data taking is the injection of light, as it avoids to introduce a hardware source into the detector. Light injection can be used safely and also quickly between different data taking runs. The LI-system is mainly used to calibrate timing and gain of the different PMTs and provides the possibility to monitor the detector response for variations. The other calibration systems insert calibration sources, which involves always a significant risk for the detector: apart from the risk of loosing a source, a tool submersion is always connected to the risk of a dangerous change of the liquid level (especially for the big articulated arm in the small chimney) or a contamination of the detector. Considering all these concerns, a comprehensive calibration program comprising the use of multiple gamma sources (137Cs (662 keV), 68Ge (106 keV), 60Co (1173 keV, 1332 keV), neutron sources (252Cf, tagged and untagged), as well as the use of a light-ball, has been conducted. These systems, as well as natural radioactivity (as it is unavoidable in the detector), allows to calibrate the energy scale of the detector and helps to understand and test the detector response.

34 The Double Chooz Experiment

3.6 Background

Double Chooz distinguishes between two diﬀerent kinds of backgrounds, accidental and correlated. While correlated background is the result of a single process that produces an IBD-like signature, accidental background is the result of two individual processes that pass (accidentally) the neutrino-selection-cuts. For a low background experiment like Double Chooz a comprehensive knowledge of these backgrounds is crucial, and their contribution has to be kept as low as possible or, if necessary, identiﬁed and subtracted.

3.6.1 Accidental Background

Accidental background describes all neutrino-like signatures, which are produced by two independent processes which accidentally meet the neutrino-selection-cuts, meaning the (muon uncorrelated) occurrence of a positron-like signal (0.7 - 12.2 MeV) and a neutron-like signal (6- 12 MeV) between 2 and 100 µs after the prompt signal. The largest part of all prompt-like signals in the detector is coming from β- and γ-emitters. The delayed signal is mainly provided by cap- −1 tured neutrons (≈18 h [22]) and, to a little part, also by rare high-energetic prompt events. Prominent and long living sources, contained in all materials, are uran, thorium or potassium, as well as their decay daughters (among them Bi and Po in coincidence, as well as Tl or Rn). Table 3.12 summarizes the most prominent radioisotopes and their endpoints normally found in liquid scintillators. Although in small amounts, these elements are found in all materials. A certain radioactive contamination of a big detector like Double Chooz, which uses a significant amount of metals for the detector and, in addition, roughly 220 m3 of mineral oils, is inevitable. These single events and those coming from additional contamination due to contaminated materials or careless liquid handling are the main source for the accidental trigger rate in the experiment. Careful material selection and proper Prominent Radioisotopes in Scintillators liquid handling are therefore a fundamental prerequisite for the success of Isotope Decay Emax T1~2 a low counting experiment like Double 14C β− 0.15 MeV 5730 a Chooz. An extensive material screen- − 40 β 1.31 MeV 9 ing, using germanium-spectroscopy and K + 1.3⋅10 a β 1.50 MeV NAA-measurements, has been conducted 222Rn α 5.59 MeV 3.8 d to pre-select all materials used in and for 210Pb β− 0.06 MeV 22.3 a 210Bi β− 1.16 MeV 5 d the detector [72]. As the detector liq- 210Po α 5.40 MeV 138.4 d uids can not be tested in advance, great 208Tl β− 5.0 MeV 3.1 m care has to be taken during the produc- β− 2.25 MeV tion of the liquids in order to avoid any 212Bi 60.6 m α 6.20 MeV contamination during mixing, transport 214 Pb β− 1.02 MeV 26.8 m or handling. These efforts allowed Dou- 214 − Bi β 3.28 MeV 19.9 m ble Chooz to limit the individual rates 212Po α 7.833 MeV 164.3 µs for prompt-like events to 8.2 Hz (65 Hz in Figure 3.12: Primary decay modes, line or end point CHOOZ [30]), and for delayed-like events to 5 10−3 Hz (0.24 Hz in CHOOZ [30]). energies (Emax) and half-life of different radioiso- ⋅ topes (T1~2). Values from [71]. Combining these two values allowed to limit the accidental background rate for the far detector to 0.261 ± 0.002 per day, which is a factor of three below the anticipated limit of one accidental event per day [55].

35 The Double Chooz Experiment

3.6.2 Correlated Background

Correlated background is the result of a single process, which produces two correlated events that meet the neutrino selection cuts. Prominent example for such a background are fast neutrons, which are produced by spallation processes of cosmogenic muons in the rock surrounding the detector. Those fast neutrons have enough energy to reach the ID, where the neutron is decelerated via proton recoils what produces a higher energetic prompt-like signal compared to those of a normal IBD-event. Subsequent to the deceleration, the thermalized neutron is captured on gadolinium, what produces a delayed coincidence signal around 8 MeV [34]. The contribution of this background can be studied by comparing the occurrence of a muon passing the nearby rock with the occurrence of a neutrino-like signal in the inner detector, which has a very high energetic prompt event (Eprompt ≈ 13 - 30 MeV). Using this technique and assuming the same number of events also in the lower energy region (Eprompt ≈ 0.7 - 12.2 MeV), the contribution of fast neutrons could be determined to 0.89 ± 0.10 correlated events per day [22]. Another source of correlated background are βn-emitters. These isotopes are produced by high energetic (showering) muons, which produce a hadron shower and lead to a large energy deposition in the detector. The following spallation processes in the scintillator (normally on 12C) produce neutron rich isotopes, as 9Li or 8He. Both isotopes are instable and reduce their neutron excess via a β−-decay, followed by the emission of a neutron. The β−-decay leads to a prompt-like signal up to 7.4 MeV for 8He and 11.2 MeV for 9Li, respectively. The emitted neutron is thermalized and ﬁnally captured on gadolinium, what produces a delayed coincidence signal around 8 MeV. Due to the life time of the insta- 9 8 Cosmogenic Induced Radioisotopes ble isotopes ( Li: τ1~2=178 ms, He: τ1~2=119 ms), these background events Isotope Decay Emax T1~2 are correlated in time to the occurrence 8 − He β n ≤7.4 MeV 119 ms of large energy deposition in the detector. 9 − Li β n 11.2 MeV 178 ms Thus, the background events produced 7 + Be β ,EC 0.48 MeV 53.3d by βn-emitters can be determined by 11Be β− 11.5 MeV 13.8s searching for neutrino-like events, which 10C β− 2.9+0.72 MeV 19.3s 11C β+,EC 1.98 MeV 20.4m follow a large energy deposition in the detector (> 600 MeV) and occur in cor- Figure 3.13: Decay modes, beta end-points and half- relation with the lifetime of 9Li or 8He. life of diﬀerent radio isotopes found in liquid scintil- A related analysis studying the contribu- lators. Values from [71]. 9 +0.62 tion of Li, yielded 2.05−0.52 background events per day [22]. Spallation leads not only to the production of 9Li and 8He, but also other cosmogenic induced isotopes as 7Be, 10Be, 10C and 11C. These elements are pure β-emitters and do not produce delayed events, because of which their contribution can be neglected in Double Chooz.

3.6.3 Artiﬁcial Background

Apart from natural background sources, Double Chooz suffers from instrumental light in the inner detector. Some of the PMTs in the inner detector show electrical discharges at their transparent electrical bases. These discharges are accompanied by the emission of light, which is seen by the other PMTs. Unfortunately, these events are too intense and too frequent (60 Hz) to be ignored in the data analysis because of which these events are handled as background. As the detector cannot be opened to replace the PMTs, this artificial background is unavoidable and its effect has to be reduced to a minimum. Currently, two means of background reduction are applied: the first one excludes the most intense flashing PMTs from the data taking (HV off), and the second one removes the remaining events of the taken data set by using two data

36 The Double Chooz Experiment selection cuts (light noise cuts) in order to identify and veto these events. Unlike the wanted IBD-events, which are exclusively produced in the target, light noise events are produced directly at the PMTs and thus near the buffer wall. While a real IBD-event (near the center ) produces a homogeneous signal in the detector, an event near the wall produces an inhomogeneous signal (regarding energy deposition as well as onset-timing of the PMTs). This difference can be used to identify light noise events. The first selection possibility, also referred to as max Q/total Q- cut, takes the ratio between the maximum observed charge by one PMT (max Q) and the total charge seen by all PMTs (total Q). This ratio is close to one, if one PMT observed most of the charge (light noise event), and close to zero, when all PMTs observe more or less the same charge. The second selection cut considers the light propagation time from the source to the PMTs. A central event will show only small variation between the trigger times of all PMTs, a decentral event, however, will result in a wider spread of trigger times, which can be used for identification. This second selection cut, also referred to as RMS-cut, takes the squared means of all trigger times and rejects events with a wider spread than 40 ns, what requests a certain homogeneity of the observed events.

3.7 Neutrino Selection

3.7.1 Pre-Selection Cuts for the Neutrino Search

The neutrino search starts with a pre-selection of the acquired data. The pre-selection applies three pre-selection cuts, which are supposed to remove all events originating from light-noise, muons or muon correlated events. This is done by rejecting all events which fall into a 1000-µs- window upon each muon-trigger or which fall into a 0.5 s-window upon each muon with an energy above > 600 MeV to account for production of βn-emitters (1-Muon Veto cut, 1a-cosmogentic- isotope-cut). In addition, the outer veto cut rejects all prompt events, which follow within 224 ns upon an outer veto trigger (1b-Outer veto cut). Artificial background, respectively light noise, is rejected by a Qmax/Qtotal-cut (2), as well as a RMS-cut (3), as they have been described in the previous section. For the pre-selection, a Qmax/Qtotal of ≤ 0.09 for the prompt event and ≤ 0.055 for the delayed event has been chosen. Additionally, both events had to meet a RMS (τs) of ≤ 40 ns. The pre-selected data, also referred to as “single-spectrum”, for prompt and delayed energy window are presented in figure 11.6. The cuts 1,2,3 were used for the first analysis of Double Chooz presented in [34]. For the later data analysis as presented in the second publication all of the here presented cuts were applied now including also 1a, 1b, which allowed to reduce the correlated background events caused by cosmogenic isotopes, however, at the expense of a increased veto time from 4.4% at the first and 9.2% [22] at the second publication [22].

3.7.2 Neutrino Selection Cuts

Out of these pre-selected data, Double Chooz applies four additional selection cuts to search for neutrino candidates. The neutrino selection is based on the search of a delayed coincidence, a prompt-signal with an energy between 0.7 - 12.2 MeV (4: prompt energy-cut) and a delayed signal between 6 - 12 MeV (5: delayed energy cut). A neutrino candidate is ﬂagged when the second event occurs between 2 µs5 and 100 µs after a prompt event (6: coincidence time) and no other prompt-like signal was found 100 µs before and 400 µs after the primary prompt event (7: multiplicity cut). Some background events (correlated as well as uncorrelated) survive the neutrino selection cuts (4-7), their contributions have to be determined and subtracted as described in the previous section.

5The time 0 - 2 µs is excluded to eliminate after-pulses and correlated events.

37 38 Part II

Development and Production of two Detector Liquids

39 Joint Venture

The Technische UniversitätMünchen (TUM) and the Max Planck Institut fürKernphysik (MPIK) in Heidelberg were responsible for the production of the four detector liquids used in the Double Chooz experiment. This task included the development and installation of the necessary infrastructure, as well as its subsequent usage during the production of the detector liquids. TUM and MPIK shared this responsibility and cooperated closely in planning and production in order to realize this comprehensive working package in a common effort. In this cooperation, TUM was responsible for production and storage of the muon veto- and the buffer-liquid, while MPIK overtook the responsibility for gamma catcher- and target-liquid. Furthermore, TUM and MPIK cooperated closely during the instrumentation of the underground laboratory and shared the responsibility for the filling and handling of the DC-far detector. When helpful for a better understanding of the global system the contributions from MPIK will be mentioned and are explicitly marked in related drawings. The following two parts are dedicated to summarize the contributions of TUM, which were realized by the author in course of the here presented thesis. Part II concentrates on the production of two detector liquids and presents the instrumentation of the liquid storage area (LSA) in chapter 4, the requirements and selection process of the used components in chapter 5 and in chapter 6 finally the on-site production of the muon veto scintillator and the buffer liquid. Part III is then dedicated to the filling and handling of the Double Chooz far detector and presents in chapter 7 all necessary systems to fill, handle and monitor all detector liquids in the underground laboratory. Chapter 8 then presents the usage of these systems and describes the preparation and filling of the DC far detector.

40 Chapter 4

Hardware Installations for Detector Liquid Production

TUM has been responsible for the production and storage of 90 m3 of muon veto scintillator and 110 m3 of buffer liquid. The large amount of liquid, as well as a missing infrastructure, prevented a production at TUM, because of which it has been necessary to install a large scale liquid- and gas-handling-system at the experimental site in Chooz. All installations had to meet the strict cleanliness requirements for the detector liquids, as well as the safety regulations applied within nuclear power plants. The following chapter will start with a presentation of all installations in the liquid storage area (LSA), necessary for the production of muon veto and buffer liquid. This comprises a detailed presentation of the liquid handling systems in section 4.2, the gas-handling systems in section 4.3 and the monitoring systems in section 4.2.3. In addition, this chapter presents in section 4.4 also the trunk line system (TLS), which is used to transfer the detector liquids between the surface installations and the underground laboratory. Table 4.1 summarizes the different surface installations in and around the LSA and separates the work of the author from the work of Dr. C. Buck and his group from MPIK. Realized Surface Installations for the Detector Liquid Production

Liquid Storage Area TUM MPIK PS (MU,BF) PS (GC,NT) Liquid Handling ST (MU,BF) TT (GC,NT) ECS ECS & TLM LPV LPV LPN LPN Gas Handling HPN LN2 Plant N2 Filter N -blanket N -blanket Monitoring 2 2 Liquid Level

Trunk Line System TUM MPIK MU, BF, GC, NT, N Street Part 2 Separation-Valve Boxes MU, BF, N GC, NT Tunnel Part 2 Emergency-N2 Supply

Table 4.1: Summarized surface installations which were realized by TUM and MPIK and subsequently used for the reception, storage, mixing and transfer of the detector liquids.

41 Hardware Installations for Detector Liquid Production

4.1 Liquid Storage Area (LSA)

The setup of the far detector is located at the Chooz-A reactor site (see ﬁgure 3.3). The experiment uses a 17 m deep underground lab that is connected to the surface through a drivable tunnel of 160 m length. Directly across the tunnel entrance is a simple surface building (LSA), which has already been used as storage facility for the CHOOZ experiment [30](1995-1996). The LSA is supposed to receive, store and process all detector liquids prior to the ﬁlling process of 2 the far detector. The building itself is 9 m high and has a footprint of 14×12 m . The ribbed

Figure 4.1: (top left): outside view of the LSA: The LSA has three large sliding gates (red), which allowed to enter the big storage tanks horizontally into the building. The black gate marks the tunnel entrance to the far laboratory; (top right): inside view of the LSA, including the first two storage tanks installed in October of 2009; (bottom): top view of the LSA-building, indicating the positions and affiliation of the storage tanks and handling systems installed in the LSA. The scheme correlates all liquids (and related systems) with a separate color to improve clarity of the drawing. The color code is: MU = yellow, BF = orange, GC = purple, NT = red. All systems of MPIK are marked with the MPIK-logo. roof and the plain walls provide no thermal insulation, because of which the environmental conditions in the building are subdued to seasonal changes. The front-side offers three unsealed

42 Hardware Installations for Detector Liquid Production

2 3×3 m sliding doors, which lead to very poor cleanliness and environmental conditions in the hall. Although these conditions complicate the production of the detector liquids, chapter 9 will show that these difficulties could be compensated and did not effect the final quality of the detector liquids. The interior of the LSA is dominated by a big liquid-tight safety-pit that offers 3 3 a total volume of 117 m (9×13×1 m (d/w/h) ), in which the entire liquid has to be handled and stored. At the inside, a first floor balcony extends from the front wall to the inside, providing space for the ventilation system (LPV), however restricting the available surface for storage tanks. Figure 4.1 shows two pictures and an overview drawing of the LSA. The scheme at the bottom shows a top view of the different installations in the LSA, providing an overview of the individual liquid-handling and gas-handling systems for muon veto (MU), buffer (BF), as well as gamma catcher (GC) and target (NT). Each of these systems is composed of multiple storage tanks (ST), or in case of the MPIK systems, transport tanks (TT), as well as an individual pumping station (PS), which allows to transfer liquids in, out or between the tanks. In order to provide a better overview of the different liquid handling systems, all liquids and their related systems are color coded. This color code is consistently used throughout this thesis and presents the muon veto in yellow, the buffer in orange, the gamma catcher in purple and the target in red. In the following, the liquid handling systems of muon veto and buffer will be presented in more detail. This includes a presentation of the pumping stations in section 4.2.1, the storage tanks in section 4.2.2, as well as the monitoring system in section 4.2.3. The systems from MPIK will not be described in detail, but mentioned when it is helpful for a better understanding of the whole system. A detailed description of the latter can be found in [62].

4.2 Liquid Handling System

The liquid handling systems in the LSA are supposed to realize all liquid handling operations necessary for the production of the scintillator and the handling of the final liquids. This includes, for muon veto and buffer, the reception of all liquid components from standard delivery trucks, the scintillator mixing and the provision of the underground lab with the final scintillator. In order to achieve the anticipated cleanliness, each system was hermetically sealed in order to protect the detector liquids from the conditions in the LSA and the harmful influence of normal air. In order to do so, each system is kept under a permanent low pressure nitrogen atmosphere. Due to the properties of the different detector liquids and in order to avoid any cross-contamination, the LSA uses four separate liquid handling systems. A detailed drawing of the two liquid handing systems1 used for buffer and muon veto is shown in figure 4.2 and presents the two pumping stations and its respective connection to the storage tanks and the trunk line module (TLM).

1Piping & Instrumentation Diagram (P&ID)

43 Hardware Installations for Detector Liquid Production

Figure 4.2: Piping and instrumentation diagram of the liquid handling systems in the LSA: The pumping stations of muon veto (right, yellow) and buffer (left, orange) and their connections to the three storage tanks (1-3 for the MU and 4-6 for the BF) are presented in the yellow boxes. Both pumping stations receive liquids from the delivery trucks, handle the liquids and are able to send the final scintillator to the trunk line module, which regulates the liquid flow to the underground laboratory. The systems from MPIK are indicated, however not in detail.

44 Hardware Installations for Detector Liquid Production

The systems for MU and BF are made of stainless steel, each comprising a pumping station (PS) 3 3 and three interconnected storage tanks (ST) (3×33 m for MU and 3×40 m for BF respectively; see figure 4.11). Apart from the liquid circuit, each pumping station includes two gas supply manifolds (see figure 4.15). The combinations of gas- and liquid-handling valves in one module allows to operate the entire system (of MU and BF) from one position. In order to receive feedback from the system and to monitor the progress of different handling steps, all storage tanks are equipped with sensors. These sensors monitor the different liquid- and gas-pressure levels as well as the temperature in all tanks and trigger alarm if critical values are reached. All sensors are connected to a central PC, situated between the two pumping stations, which allows to monitor the different operations. A picture of these installations is presented in figure 4.3, which shows the two pumping stations, connected tubing, and the white tent in between, which hosts the monitoring system and protects the electrical set-up from dust, humidity or the fire sprinkling systems installed in the LSA. The safety regulations on a nuclear power plant furthermore requested the installation of an emergency closure system (ECS), which is independent from electricity and allows to isolate all tanks in case of fire or any other emergency (see figure 4.14). In the following, the individual parts of the liquid handling system (pumping station, storage tanks and monitoring system) will be presented in more detail.

Figure 4.3: Picture of the liquid handling system in the LSA. Showing parts of the muon veto pumping station (MU-PS) on the left, the monitoring tent with LM-PC in the middle and the BF-PS on the right. In the background, the MU-storage tank 1, as well as the buﬀer tanks 4 and 5 can be seen. Mounted to the BF-PS, the emergency closure system (ECS) and the necessary air compressor are additionally indicated.

45 Hardware Installations for Detector Liquid Production

4.2.1 Pumping Stations

The pumping stations are the centerpiece of each liquid handling system, as they include all active parts and allow to regulate all liquid- and gas-handling operations. Figure 4.4 represents a zoom of figure 4.2, depicting in detail the liquid routing within one pumping module. Each pumping station is 2 m high and mounted within a transportable frame with a footprint of 2 (1.4×1 m ). Each module was assembled by the author at TUM and subsequently transported and installed in Chooz. Each frame comprises three active parts: a nitrogen-driven membrane pump, a particle filter and a mass flow meter. These parts shall be briefly introduced in the following paragraphs.

Pumping Station Tank top connections Tank bottom connections BU V 08 BU V 11 VCR Buffer & Muon Veto VCR

BU V 07 BU V 09 BU V 12 BU V 10 1 Inch, SS-Tube

1 Inch, PFA-Tube BU V 06 BU V 13 Membrane Valve

Active Parts System Connection Point II Sample BU V 17 Port

BU V 14 Flow meter Filter BU V 05 BU V 04 0,5µm

BU V 16 BU V 03 System PTFE Connection Swagelok Pump BU V 15 Point I

BU V 02 BU V 01

Figure 4.4: Pumping and instrumentation diagram, indicating the layout of the pumping stations for muon veto and buffer. Using the system-connection-point-I (SCPI) as inlet allows to pump the liquid through a particle filter (poresize: 0.5 µm) and a mass flow meter, before the liquid is distributed into the storage tanks or directed to the SCP II, which connects the PS with the trunk line module (TLM). Furthermore, the individual valve identification numbers used for the description of individual flow patterns through the system are indicated. A picture of this system is presented in figure 4.9.

With the Mega 960, Trebor oﬀers a high capacity membrane pump in which all wetted parts are fully made of PTFE/PFA. The pump is pneumatically driven and requires a working pressure between 2 and 5.5 bar [73]. The pump is supposed to provide a capacity of 5700 l/h, the observed values, however, were within the range of 1500 - 2200 l/h. The reason for this discrepancy was the combination of internal impedance of the liquid handling system and the limited driving force of max. 5.5 bar. For the use in DC, the pump was mounted on a metal plate at the lowest Figure 4.5: membrane pump [73] point in the pumping station. The pneumatic pump was actuated with the high pressure nitrogen supply system (HPN) and regulated by a needle- valve in the 1/2-inch supply line.

46 Hardware Installations for Detector Liquid Production

With the KFE 6 20S, MTS offers a high capacity filter device with a 15 l- stainless steel housing, that provides space for six cartridges with a length of 20-inch. The nominal flow capacity is 4000 l/h. The housing can tolerate a maximum pressure of 10 bar and offers an additional thread for ventilation at the top-lid, as well as drainage at the lower end [74]. For the use in DC, each filter was equipped with six nylon-filter cartridges with a nominal poresize of 0.5 µm. During the delivery of the liquids, these cartridges were checked after each truck and replaced when necessary. A picture of these cartridges (new and after pumping 26 m3 of LAB) can be found in Figure 4.6: particle figure C.1 in the appendix. In order to have removable and tight connec- filter [74] tion points, the filter was implemented using 1-inch flanges and additional clamps, which reduced the weight on the flange connection. With the Promass 83a, Endress & Hauser provides a high precision Coriolis mass flow meter, which measures mass flow with an accuracy of 0.25 % of the measured value. The density is measured with an accuracy of ○ ± 0.02 kg/l and the temperature with ± 0.5 C. All wetted parts are made of either stainless steel or PTFE. The flow meter offers various counters and a comprehensive software menu, which can be read-out or set via a small touch screen [75]. In order to be removable from the system, the flow meter uses an 1-inch flange connection. In DC, the flow meter was essential for the handling and mixing of the detector liquids as its values allowed to control the volume/mass, as well as the density of the uploaded liquids. These information were used for the mixing process and allowed already Figure 4.7: flow me- during the delivery of the components to distribute the correct amounts ter [75] to the indvidual storage tanks. During circulation of the final liquids, the flow meter allowed to monitor the circulation process and the density of the detector liquids during the production process. With the SELFA M20S, Rotarex provides an ultra-high-purity diaphragm valve with a diameter of one inch. The valve body is made of electro-polished stainless steel, the o-rings as well as the inner diaphragm are made of PTFE. The valve offers two purge connections on both sides of the valve, as well as a position indicator below the handle. The valve is −9 helium tight up to a rate of ∼10 mbar/s, at a pressure of 15 bar [76]. In order to avoid mechanical connections and to ensure the radon tightness of the pumping station, all tubing and valve connections (except the active parts) are welded with a high-purity welding technique, referred to as Figure 4.8: membrane valve [76] orbital welding. In order to identify individual valves, each valve has an individual identification number composed of two letters, identifying the respective system, a ”‘V”’ for valve and two digits for the valve number (BF V 01), as already indicated in figure 4.2. These active parts are interconnected by an 1-inch high-purity stainless steel tubingm as depicted in figure 4.9, the connection logic is presented in figure 4.4 and allows to realize all necessary liquid handling tasks, as the unloading of the delivery trucks, the circulation or the transfer of the liquids. Although the pump is only one-directional, a prudent tubing-layout allows to use the system in both directions. Multiple bypasses in the tubing provide furthermore the possibility to exclude active parts (pump or the filter) from a flow path. A summary of the different flow patterns, which can be realized with each pumping station, is presented in table 4.2. In order to achieve the necessary cleanliness, the entire system is exclusively made of stainless steel or high quality plastics as PTFE, PFA or FEP. This ensures not only the material com-

47 Hardware Installations for Detector Liquid Production

Pumping Station Flow Paths Table

Task Option Detail Liquid ﬂow path with Pump SEP I, 1, 2, P, 3, 16, FM, 6, (7, or 8, or 9) Truck with Filter SEP I, 1, 15, 4, F, 5, FM, 6, (7,8,9) Emptying with P+F SEP I, 1, 2, P, 3, 4, F, 5, FM, 6, (7,8,9) Storage Tank without P+F SEP I, 1, 15, 16, FM, 6, (7,8,9) Filling with F SEP II, 14, 1, 15, 4, F, 5, FM, 6, (7,8,9) Detector with P SEP II, 14, 1, 2, P, 3, 16, FM, 6, (7,8,9) Emptying with P+F SEP II, 14, 1, 2, P, 3, 4, F, 5, FM, (7,8,9) without P+F SEP II, 14, 1, 15, 16, FM, 6, (7,8,9) from bottom with P (10,11,12), 13, 2, P, 3, 16, FM, 6, (7,8,9) Circulation to top with P+F (10,11,12), 13, 2, P, 4, F, 5, FM, 6, (7,8,9) with P (10,11,12), 13, 2, P, 3, 16, FM, 17, 14, SCP I Truck with F (10,11,12), 13, 15, 4, F, 5, FM, 17, 14, SCP I Loading with P+F (10,11,12), 13, 2, P, 3, 4, F, 5, FM, 17,14, SCP I Storage Tanks without P+F (10,11,12), 13, 1, SCP I Emptying with P (10,11,12), 13, 2, P, 3, 16, FM, 17, SCP II Detector with F (10,11,12), 13, 15, 4, F, 5, FM, 17, SCP II Filling with P+F (10,11,12), 13, 2, P, 3, 4, F, 5, FM, 17, SCP II without P+F (10,11,12), 13, 1, 5, SCP II Sampling from Tank (10,11,12), 13, 1, 14, Sample Port

Table 4.2: Summary of the different flow patterns, which can be realized by the pumping stations in the LSA. Indicated are the different tasks, the possible option and the detail with which this option can be realized. The set of numbers describes the liquid flow path through the pumping station. The given numbers describe the flow path by naming the valve identification number, as to find in figure 4.2, as well as the different active parts which are presented as P=pump, F=filter, FM=flow meter, PS=pumping station and SCP I=system connection point. For a more detailed explanation regarding the flow path description, refer to section B.1.1 in the appendix. patibility of the system, but also a good cleanability due to the tube’s smooth inner surface. In order to avoid radioactive contamination, coming either from radon emanation, metal surfaces or welding with thorium, the entire pumping station is welded using orbital welding. This easy and clean technique provides smooth and clean welding joints and has already been used by the BOREXINO collaboration, which successfully installed an ultra-high-purity liquid- and gas handling system for the BOREXINO experiment [77]. Orbital welding uses an inert gas (mostly argon), which is flushed around the welding joints prior, during and after the welding process, which avoids the oxidation of carbon. In addition, this technique uses an automated welding head without thorium, which produces a very even welding joint on both sides of the tube. Apart from the shown liquid valves, each pumping station includes two gas-manifolds, the low pressure manifold used to provide a nitrogen blanket above the liquids, and the high pressure manifold used for the membrane pump and turbulent bubbling of the liquids. The combination of liquid- and gas-control valves within the pumping station allows the user to control all liquid- and gas-operations from one point. Figure 4.9 shows a picture of the buffer pumping station, indicating the different active parts as well as the just mentioned high- and low-pressure manifolds.

48 Hardware Installations for Detector Liquid Production

Figure 4.9: Picture of the buffer pumping station module indicating the different active parts: 1) membrane pump, 2) particle filter, 3) flow meter, 4) HPN sub-manifold, 5) LPN sub-manifold, 6) inlet- connections, 7) outlet-connections.

49 Hardware Installations for Detector Liquid Production

4.2.2 Storage Tanks for Buﬀer and Muon Veto

The liquid handling system in the LSA comprises six large storage tanks, three for the muon veto scintillator with a total volume of 99 m3, and three storage tanks for the buffer liquid with a total volume of 120 m3. All tanks are made of stainless steel, have cylindrical shape and stand upright on four adjustable feet. Each tank offers a bottom- and top-flange, as well as a manhole at the top, which allows to enter the tank for instrumentation and cleaning. A technical drawing of the storage tanks of MU and BF, as well as their most important technical details, are summarized and compared in figure 4.11. Produced by a German company2, the six tanks were transported to France, where they were installed in the LSA with the help of a local company3. Horizontally introduced into the LSA, each tank was erected by hoisting them on their feed (without touching the ground, protecting the oil-tight coating of the pit). Figure 4.10 presents the some pictures from the production, transport and finally the installation of the tanks. The available area within the safety pit of the LSA was only 7×7 meters for the six tanks, which

Figure 4.10: Storage Tanks for muon veto and buffer liquids. (upper left): bottom side of the storage tanks after the production in Germany, Regensburg. (upper right): Arrival of the transport tanks in France. (bottom row): Installation of the first two storage tanks in the LSA. required the company to handle and position each tank with an accuracy of a few centimeters. The restricted space led to the finally chosen positions, as depicted in figure 4.1, leaving only centimeters between some of the tanks. In order to meet the needs for cleanliness and material compatibility, the producing company exclusively used stainless steel and TIG-welding4 to assemble the tanks, which were then pickled and passivated on the inside. After the installation in the LSA, all tanks were instrumented and cleaned by the author. The cleaning was done manually, using a scrubber and industrial detergent, as well as a high-pressure cleaner and ultra-pure water for rinsing. This cleaning procedure was realized twice before the tanks were finally closed and flushed with nitrogen. Apart from the here presented tanks, the LSA additionally hosts two

2Co. Gresser, Auweg 34, 93055 Regensburg, Germany 3Co. Dumonceau, 08600 Chooz, France 4Tungsten Arc Welding (GTAW), also referred to as TIG welding, is a high quality welding technique, produces clean and smooth welding joints and avoids the use of welding rods (source for radioactive contamination with the thorium-chain).

50 Hardware Installations for Detector Liquid Production custom made transport tanks aﬃliated to MPIK: a 5 m3-PTFE-transport tank for the target scintillator and a 24 m3-iso-container with PTFE-inliner for the gamma catcher.

Storage Tank Instrumentation

Each of the six storage tanks is equipped with two flanges, one at the bottom and one at the top. These flanges host all instruments and feed-throughs necessary for gas handling, liquid handling and monitoring. The top flange (and the nearby manhole) allowed the installation of all internal instruments, like a high-pressure nitrogen tube used for turbulent bubbling of the liquids, a long filling tube going to the bottom of each tank, as well as two sensors to monitor liquid- and gas-pressure-levels within the tanks. Apart from that, the top-flange hosts all connections for the nitrogen blanket (LPN) and the ventilation (LPV). The bottom flange is equipped with only two outlet connections: the first, standard used and secured by a pneumatic valve, and the second, which is only used as back-up solution in case of emergency. The following two paragraphs briefly summarize the connection provided by the top and the bottom flange. Top Flange The top-flange has an usable diameter of 200 mm, which is closed by a custom-made and PTFE- sealed stainless steel flange (Ø=340×24 mm), which hosts: 1 liquid connection

1 liquid feed through: for the long ﬁlling tube (1 inch, Swagelok) 4 gas connections

1 HPN-feed through: for the N2-purging (1/4 inch, Swagelok)

1 LPN-connection: for the N2-blanket (3/4 inch, Swagelok)

1 LPV-connection: for the N2-ventilation (1 inch, ball valve) 2 monitoring connections

1 thread connection: for the pressure sensor (1/2 inch, G1/2-thread)

1 cable feed through: for the liquid level sensor (5 cm thread with 1/4 inch, Swagelok). Bottom Flange The bottom-ﬂange has an usable diameter of 150 mm, which is closed by a custom-made and PTFE-sealed stainless steel ﬂange (Ø=285×24 mm), which hosts: 2 liquid connections

1 pneumatic valve: connected to the PS (1 inch, ball valve (ECS5.), welded)

1 manual valve: back-up connection (1 inch, ball valve VCR, caped). More information about these connections can be found in section B.1.2 in the appendix. Apart from these two ﬂanges, each tank is additionally equipped with an externally mounted ladder, a manhole for cleaning and tank instrumentation, a rupture disc that bursts above 1.3 bar internal pressure, as well as safety- and lifting-hooks. A technical drawing of the storage tanks and a detailed list of the instrumentation and further technical details can be found in ﬁgure 4.11.

5Emergency closure system, pneumatic valve directly welded to the bottom ﬂange

51 Hardware Installations for Detector Liquid Production

Technical Details for MU & BF Storage Tanks

Dimension Buﬀer Muon Veto Height 7890 mm 7560 mm Diameter 2800 mm 2600 mm Wall thickness 3 mm 3 mm Bottom/Top thickness 4 mm 4 mm Weight empty 2.150 kg 1.910 kg Weight full 32.160 kg 26.532 kg Nom. Volume 40 m3 33 m3 Max. Volume 43 m3 35 m3

Pressure Buﬀer Muon Veto Certiﬁed Pressure 1300 mbar 1300 mbar Rupture disk Pressure 1100 bar 1100 bar Maximum pressure 900 mbar 900 mbar Blanket Pressure 0-100 mbar 0-100 mbar

Material Buﬀer Muon Veto Walls SS 1.403 SS 1.403 Bottom/Top SS 1.403 SS 1.403 Joints PTFE PTFE

Instrumentation Buﬀer Muon Veto 1-Manhole 600 mm 600 mm 2-Top Flange 200 mm 200 mm 3-Rupture disk 80 mm 80 mm 4-Lifting hooks 2 2 5-Bottom Flange 150 mm 150 mm 6-Feet (adjustable) 4 4 7-Type plate 1 1 8-Safety hooks 1 1 9-Ladder+Back protection yes yes

Figure 4.11: Technical drawing and technical details of the storage tanks of muon veto and buffer: top- and side-view of the tanks, as well as the details A and B. Detail A shows the design of the tank-feet, while detail B shows the upper side of the tanks, indicating the number and position of the tank instrumentation. The table summarizes and compares the most important technical details of the different tanks. Bigger full-scale version of the drawing can be found in the appendix, see figure B.1.

52 Hardware Installations for Detector Liquid Production

4.2.3 Monitoring- and Safety-Systems

Monitoring System

For security reasons and to monitor the liquid handling operations, each tank is equipped with two sensors: a gas-pressure sensor6 and a hydrostatic-pressure sensor7, which offers also a temperature measurement. Both sensors are mounted at the top-flange and can be replaced easily in case of problems. The gas pressure sensor is directly screwed into the top-flange and compares the internal with the atmospheric pressure. The hydrostatic pressure sensor uses a relative measurement technique, which compares the pressure at the bottom of the tank with the pressure in an internal capillary hidden in the sensor cable. This capillary was extracted from the cable and opened to the in- Pressure Monitoring Sensors ner nitrogen atmosphere, which provides a clean measurement System Unit Gas pressure Liquid Level of the liquid level, independent Company – Boie STS from the actual blanket pres- Trade name – LDK 121 ATM/N/T sure. Both sensors are sup- Pressure Range mbar 0 - 1000 0 - 800 plied with 24 VDC and provide an Temp. Range ○C -25 - 80 5 - 25 analog output signal between 0- Material Cable – PA PTFE Material Head – SS SS 10 VDC, which is collected by a Accuracy % 0.2 FS 0.1 FS standard data-acquisition-system Input VDC 11-32 15-30 (DAQ) from Nation Instruments Output VDC 0-10 0-10 (NI) [80]. The main properties of ATEX – yes yes both sensors are summarized in Length mm 110 137 table 4.2.3. A lab-view program Diameter mm 40 24 records and displays the acquired data on the correlated monitor- Figure 4.12: Technical details about the hydrostatic- and gas- pressure-sensor installed in the storage tanks [78, 79]. ing PC. The program displays the acquired data and alerts the handling personnel if any measurement reaches critical values. On top of this monitoring system, the safety regulations of the power plant required the installation of an additional safety feature, which is able to isolate the LSA in case of emergency, independently from electrical supply. Consequently, all storage tanks and the N2-supply line of the LSA are equipped with pneumatically driven valves. The next paragraph shall be used for a brief presentation of the emergency closure system.

Emergency Closure System (ECS)

For safety reasons, the LSA is equipped with an emergency closure system, which isolates the LSA in case of emergency. In order to do so, each storage tank, as well as the N2-supply line, is equipped with a pneumatically driven ball valve. The valves (type normally-closed) can be remotely controlled via a “control box”, which is mounted to the buffer pumping station (see figure 4.13). Produced at TUM, this custom made box allows to distribute compressed air (4-6 bar) to the pneumatically actuated valves, which allows to open or close individual valves by pressurizing or venting them. The pressure is supplied by a standard air compressor with a storage volume of 5 l, which is placed on top of the BF-PS. The connection logic between valves, control box and compressor is indicated in figure 4.14. It has to be mentioned, that in case of compressor malfunction or a long term electricity cut, the pressure in the ECS could fall

6Sensor: LPK 121 [78] Boie GmbH & Co. KG, Rudolf-Diesel-Str. 5a, 82205 Gilching, Germany 7Sensor: ATM/N/T [79], STS Sensoren Transmitter Systeme GmbH, Poststr. 7, 71063 Sindelﬁngen, Germany

53 Hardware Installations for Detector Liquid Production below the necessary 4 bar, which could trigger the (normally closed) valve to shut. This would automatically isolate all six storage tanks. Each valve is additionally equipped with a position indicator. The position of each valve can therefore be monitored either by looking directly at the indicator (which is possible but not comfortable), or by looking at the colored LEDs (red=closed, yellow=defective, green=open) installed in the control box. Figure 4.13 shows a set of pictures, indicating the diﬀerent parts of the ECS. The air-compressor is shown in ﬁgure 4.3.

Figure 4.13: Emergency closure system in the LSA: (left): bottom ﬂange of a storage tank, indicating the pneumatic valve and the position indicator mounted on top; (right): control box for the nitrogen supply, the pneumatic valve and the position indicator mounted in the N2-supply line; (bottom): pneumatic control box for the six storage tanks, indicating the muon veto tanks as open (green LEDs) and the buﬀer tanks as isolated (red LEDs). The main switch at the center allows to close all tanks at the same time.

54 Hardware Installations for Detector Liquid Production Pneumatic control valve for tank 2 Tank 1 Tank BU V 17 Nitrogen supply main valve control box MU Tank 2 Tank 33 m³ BU V 18 Nitrogen pneumatic main valve XNV03(P) Tank 3 Tank BU V 19 signal collection box signal collection box Compressor 4 - 6 Bar Pneumatic valve (normally closed) with position indication 3-2 valve Liquid level pressure sensor 0-800mBar Gas pressure sensor 0-1Bar

Vent Vent Pneumatic valve is vented closed moving open Valve Closed Valve Tank 4 Tank BU V 17 From: pneumatic valve 3-2 Valve BF Valve Valve position indicaton Tank 5 Tank 40 m³ BU V 18 Vent Vent Pneumatic Valve Control Box Pneumatic Valve From: Compressor Pneumatic valve is pressureized Valve Open Valve LED‘s To: To: pneumatic valve Tank 6 Tank BU V 19 signal collection box signal collection box signal collection box Gas pressure level read out Liquid temp. Monitoring and Storage PC

pressure read out Liquid level Monitoring System Liquid Storage Area (LSA) Liquid Storage Monitoring & Instrumentation Scheme for MU BF

Figure 4.14: Overview of the monitoring- and ECS-system installed in the LSA: All tanks are monitored with a gas- and a liquid-level sensor. The sensor information are collected and transferred to a data acquisition system, which records and displays liquid-level, liquid-temperature and blanket-pressure. The ECS-system allows to isolate the LSA independently from electricity, using pneumatically actuated valves and a control-box. Supplied by an air-compressor, the normally closed valves are pushed open and can be regulated by three 2-way-valves in the control box, which either pressurizes or vents the valve. The connection logic of these valves is indicated at the bottom.

55 Hardware Installations for Detector Liquid Production

4.3 Gas Handling System

When liquid scintillators are exposed to normal air, and therefore to oxygen, radon or dust particles, they suffer quickly from contamination or degradation. Therefore, Double Chooz handles and stores all detector liquids under a permanent dry and clean nitrogen atmosphere. This, however, requires a comprehensive gas handling system, which supplies not only the surface installations in the LSA, but also the underground installations in the neutrino laboratory. This is realized by two individual gas handling systems, one in the LSA and one in the underground laboratory. Both systems are supplied by a common liquid nitrogen plant (LN2), which is situated in front of the LSA. The following section concentrates on the gas handling system in the LSA, as well as the trunk line system (TLS), which is used to transfer the nitrogen (and the liquids) from the LSA to the underground lab. The gas handling system in the LSA is composed of different sub-systems, providing different pressure levels for liquid handling, bubbling or blanketing. Table 4.3 provides an overview of the different sub-systems and indicates the different pressure ranges as well as the used color code. An overview of the gas handling system and its different sub-systems is provided in figure 4.15, indicating the connection logic between LPN, HPN, LPV and the storage tanks.

Gas handling system in the LSA

Abbr. Name Press. Range Color Code LN2 Liquid Nitrogen Supply 8.5 bar red TLS-N2 Nitrogen Trunk Line System 8.5 bar red HPN High Pressure Nitrogen 0–5.0 bar dark blue LPN Low Pressure Nitrogen Underground 0–100 mbar light blue LPV Low Pressure Ventilation 0–30 mbar green

Table 4.3: Overview of the diﬀerent sub-systems of the gas handling system in the LSA, their pressure ranges and the color code used in the following drawings.

Supplied by the LN2-plant, the gaseous nitrogen for the experiment is passing a gas filter station, which includes a particle filter that removes all residual particles above 4 nm. Beyond that point, the nitrogen flow is split up in two lines, one going to the underground lab (using TLS), the other into the LSA. Inside the LSA, the nitrogen is supplying two separate distribution systems: the LPN-system, which supplies low-pressure nitrogen (for blanketing), and the HPN-system, which supplies high-pressure nitrogen to the pumping station. The HPN is not only used to run the pneumatically driven membrane pumps, but also to support the mixing process by bubbling the liquids turbulently. In addition to the LPN and HPN-connections, each storage tank has a LPV- connection. The LPV-system collects and purifies the outbound gas and provides furthermore an adjustable impedance to the nitrogen flow. This impedance leads to a back-pressure in the tanks, which can be used to set a low pressure nitrogen blanket on the storage tanks. The following subsections shall be used to introduce the gas handling system in more detail and to provide a brief overview of the different systems.

56 Hardware Installations for Detector Liquid Production XNV03(P) XN Purge Valve FIPI01 XN V02 FI V01B FI V01A MU LPN V01 MU LPN V02 MU LPN V03 MU LPV V04 MU LPN V05 LPN Manifold A B 40 m³ Tank 1 Tank XN V01 FI V02A FI V02B 2 LN - plant 3000 l Gas filter station BF FIPI02 Tank 2 Tank LSA outer wall LSA Muon Veto Pumping Station (MU PS) Muon Veto

HPN & LPN Sub-Manifold

HPN V06 V06 HPN Spare MU HPN V01 MU HPN V02 MU HPN V03 MU HPN V04 V05 HPN Tank 3 Tank Safety valve HPN Manifold

Flow Meter 0-12 L/min HPN V04 V04 HPN Muon veto FM

fer FM V03 HPN Buf

FM & GC HPN V02 V02 HPN NT

HPN PR HPN V01 LPN-Distributor 0-1bar 1/4 Inch, SS-Tube 3/4 Inch, SS-Tube 3/4 Inch, SS-Tube TL N2 V01 TL LPN V01

Pressure Gage + 2 Manometer LPN V02 V02 LPN LPN Manifold

NT& GC HPN Neutrino Lab. Line Trunk 8,5 bar 40 m³

Tank 4 Tank LPN V03 V03 LPN

LPN V04 V04 LPN Buffer LPN V05 V05 LPN Muon veto BF Flow meter Pneumatic Valve Membrane Valve Tank 5 Tank

LPN V06 V06 LPN Spare

Flow meter Buffer Pumping Station (BF PS) Buffer HPN & LPN Sub-Manifold HPN-Distributor 4-8 bar Tank 6 Tank HPN Manifold Flow Meter 0-12 L/min FM Myon veto Buffer Y-catcher Target FM FM (MPIK) Gas Module (MPIK) Trunk Line Module Trunk h adjustable impedance

oil bubbler with adjustable feet LPV-System (first floor balcony) LPV-System Gas handling scheme for MU & BF Liquid Storage Area (LSA) Liquid Storage Active charcoal filter

Figure 4.15: Overview of the gas handling system in the LSA: The LSA has three independent gas supply systems, the TLS in red, which supplies the underground laboratory, the HPN-system in dark blue, used for purging and pumping, as well as the LPN-system in light blue, which is used for blanketing. The supplied nitrogen is collected and puriﬁed by the LPV-system, which is presented in green. The LPV- system allows furthermore to adjust the blanket pressure in the storage tanks by using an adjustable impedance (oil-bubbler). 5 m³ Target Target Pump station (MPIK) 57 25 m³ Pump station (MPIK) Gamma Catcher Hardware Installations for Detector Liquid Production

4.3.1 Liquid Nitrogen Plant and Gas Filter Station

The Liquid Nitrogen Plant is a stand-alone solution from Air Liquide8, installed in front of the LSA, as indicated in figures 4.15 and 4.1. It provides a 3000 l-liquid-N2-reservoir with a four- stage evaporizer, which supplies 99.999 % pure nitrogen gas (Nitrogen 5.0) with a pressure of 8.5 bar and a temperature of about 13○C to the gas filter station. This setup allows uninterrupted gas supply also during a re-filling process. It includes two main isolation valves (XN V01 and V02 indicated in figure 4.15) and two overpressure-safety-valves, directly attached to the N2- tank, which opens at an internal pressure of 13 bar. The usage of liquid-N2 has a couple of advantages. Firstly, it allows to store a large amount of nitrogen on a small volume, and secondly, the evaporating systems provide a higher nitrogen quality compared to gas bottles. A liquid nitrogen plant with 3000 l generates an usable nitrogen volume of 2073 m3 (comparing to 156 m3 for a bottle pack) before it has to be refilled and has therefore the necessary dimension and capacity to flush the storage tanks or the detector prior to the filling (which is about 3 3 8×220 m for the storage tanks and 8×250 m for the detector).

Figure 4.16: (left): gas ﬁlter station and emergency closure system in the LSA; (right): liquid nitrogen plant in front of the LSA.

The Gas Filter Station is an additional installation of TUM to increase the cleanliness with respect to particles bigger than 4 nm, which may have been accumulated in the N2-system during installation or transport. The filter panel offers two parallelly mounted stainless steel filter housings, which can be used alternately, guaranteeing an uninterrupted nitrogen supply. Each filter housing hosts a single PE-cartridge with a nominal pore size of 4 nm. Two manometers, one mounted ahead of the filter, and one after, allow to recognize a clogged or malfunctioning filter by a rising pressure difference. For the exchange of a filter cartridge or a repair, the gas flow can be stopped or redirected (way A or B in fig. 4.15, see also fig. 4.16), which allows an uninterrupted and safe manipulation of the system. The filtered N2-flow is equally split in two supply lines. One is directed into the LSA, where it supplies the distribution manifolds of

8Air Liquide, Hans-G¨unther-Sohl-Str. 5, 40235 D¨usseldorf,Germany

58 Hardware Installations for Detector Liquid Production

HPN and LPN, the other is directed to the underground laboratory, where it supplies the gas handling system of the detector.

4.3.2 High Pressure Nitrogen

The HPN-distributor is the main HPN-supply in the LSA and provides a nominal pressure of 5 bar to all following systems. The HPN-manifold includes a main isolation valve, an adjustable pressure reducer (0-10 bar), as well as 5 distribution valves (3/4 inch). One connection point is used to supply the gas module of MPIK, which individually provides nitrogen to GC and NT. Two connection points are used to supply the pumping stations of muon veto and buﬀer and the HPN-sub-manifolds integrated therein. The remaining two connections are used as spare connections, of which one hosts an overpressure safety valve that opens above 5.5 bar. A picture of the HPN-distributor is presented in ﬁgure 4.17.

The HPN-sub-manifolds in the muon veto and buffer pumping stations are used to distribute high-pressure nitrogen to the pneumatically driven membrane pumps and to the individual storage tanks, where the HPN is used for turbulent bubbling of the liquids during the mixing process. The sub-manifold comprises four connection points, each equipped with an isolation valve. Three of these connection points, namely those which go to the storage tanks, are additionally equipped with mechanical flow meters. The flow meters allow to regulate the gas 9 flow through the N2-purging-flower , installed at the bottom of each storage tank. The fourth connection point is equipped with a needle-valve (1/2 inch), used to supply and regulate the membrane pump, which drives the liquid handling. The mechanical details of all manifolds, the HPN-distribution and the two sub-manifolds are summarized in table B.3 in the appendix.

Figure 4.17: N2-distribution system in the LSA: The LPN-distributor is presented on the left, the HPN- distributor on the right. The indications present the supplied system: MU-PS = muon veto pumping station, BF-PS = buﬀer pumping station, N2-TLS = Nitrogen trunk line.

9A perforated 1/4-inch PFA-tube mounted at the bottom of each storage tank. The perforated part of the tube is ﬂower-shaped and provides four leafs, which cover the bottom of each tank. Using HPN, the liquids are purged turbulently.

59 Hardware Installations for Detector Liquid Production

4.3.3 Low Pressure Nitrogen

The LPN-distributor is the main LPN-supply in the LSA and provides a nominal pressure between 0 and 100 mbar to all following systems. The LPN-manifold includes a main isolation valve, an adjustable pressure reducer (0-1 bar), as well as four distribution valves (3/4 inch). One connection point is used to supply the gas module of MPIK, individually providing nitrogen to GC and NT. Two connections are used to supply the pumping stations of muon veto and buﬀer and the LPN-sub-manifolds integrated therein. The remaining connection is used as spare and hosts an overpressure safety valve that opens above 1.0 bar.

The LPN-sub-manifold allows to provide a common low pressure blanket to the storage tanks. The sub-manifold includes ﬁve connection points (3/4-inch), all equipped with regulation valves. Three of them are used to supply a common low pressure nitrogen blanket to the three related storage tanks. The fourth connection point is directly connected to the exhaust system and allows to vent the system pressure. The last connection is a spare connection and yet unused. The mechanical details of the LPN-distribution, as well as the two sub-manifolds, are summarized in table B.3 in the appendix.

4.3.4 Low Pressure Ventilation

The Low Pressure Ventilation System is supposed to collect and purify all nitrogen that has been used in the LSA. Apart from that, the LPV-system allows to regulate the blanket pressure in the storage tanks. Due to spatial problems in the LSA, the entire system is installed on the 1st-floor balcony inside the LSA. The LPV-system collects the outbound gas from all storage tanks and merges it in two main exhaust lines: one line for the scintillating liquids (MU, GC, NT) and one line for the non-scintillating buffer (see fig. 4.16). Each of these main exhaust lines has a diameter of 63 mm, is 10 m long and offers five (3/4 inch) connection points: three for the storage tanks, one for the pumping station going to the LPN-sub-manifold, and one as spare connection. An oil bubbler at the end of each main line provides an impedance to the nitrogen flow and prevents the back-flow of gas to the storage tanks. Both oil-bubblers

Figure 4.18: LPV-System: (left): gas collection container with the active charcoal ﬁlter on top. Each exhaust line ends in an individual oil bubbler, which provides an adjustable impedance, what allows to set the blanket pressure. (center): gas collection box and the two main exhaust lines connected to the storage tanks (The part of the main exhaust line indicating ﬁve connection points and a manometer to monitor the gas blanket pressure). (right): storage tanks and the necessary tubing for liquid- and gas-handling.

60 Hardware Installations for Detector Liquid Production are contained in a gas-tight collection-container and can be adjusted in height, what allows to regulate the impedance and therefore the blanket pressure in the storage tanks. Both oil bubblers use transparent mineral oil (Ondina-909). After the bubbler, the mixed gas-phase is collected in the container and has to pass an active charcoal ﬁlter, which removes any vaporized aromatics, before it is ﬁnally released to the environment. Figure 4.18 presents three pictures of the LPV-system installed in the LSA, indicating the main exhaust lines, the gas-collection container and the storage tanks including the necessary tubing.

4.4 Trunk Line System (TLS)

The trunk line system connects the surface installations with the underground laboratory. It is a joint eﬀort of MPIK and TUM, which made it necessary to divide it into three parts. TLS-Street part, TLS-Tunnel part and TLS-DFOS part.

The TLS is composed of five individual 3/4-inch trunk-lines: A stainless steel line for the N2- supply and four liquid lines for muon veto, buffer, gamma catcher and target. Although the trunk line system is technically not very difficult, its cleanliness is of great importance for the experiment. The contact of the scintillator with the 160 m of tubing is not negligible. The TLS is therefore a potential source for re-contamination of the detector liquids stored in the LSA. Improper installation, as well as inadequate welding or incompatible materials, could have led to a degradation or contamination of the detector liquids and delimit the sensitivity of the experiment. In order to avoid such a scenario, all tubes were installed and subsequently purged with nitrogen. In case of gamma catcher and target the tubes were additionally cleaned by flushing with an industrial detergent and a weak acid-solution. Finally, all tubes were flushed with a batch of final detector liquids, before they were used for the liquid transfer to the filing system in the underground laboratory. The next paragraphs will provide a brief overview of the different trunk line parts, while table 4.4 summarizes the most important technical details. The street part : The first part spans 15 m from the trunk line module (TLM, see figure 4.1) in the LSA to the tunnel entrance of the laboratory. The TLM is part of the emergency closure system installed by MPIK and allows to isolate the LSA in case of emergency. The TLM includes the five trunk lines and secures each line with a manually- and a pneumatically-driven valve. After the TLM, the tubes exit the LSA through the wall. Mounted on the outside of LSA, a waterproof PE-box contains additional isolation valves (manual), which allow to isolate the LSA from the outside (see fig. 4.19). The tubes are leaving the isolation valve box vertically to reach below the street level, where the tubes run across the street. At the other side of the road, the tubes run vertically into the tunnel entrance, where they are connected to a second isolation-valve box, which marks the end of the street part. Except for the nitrogen line, which is stainless steel, all tubes are made of flexible PFA. The PFA tubes are furthermore double contained, as required by reactor safety regulations, within a bigger, stiff and transparent PVC- tube in order to secure the PFA-tube from mechanical forces. In order to secure the tubes from static discharge (non-conducting liquid passes through non-conducting PFA-tube), the PVC- tube is additionally mantled by a metal mesh. A detailed technical description of the used valves (in the TLM as well as in the valve boxes) can be found in table B.5 in the appendix.

TLS-Tunnel Part : The tunnel part spans a length of 145 m and a vertical height of 17 m from the entrance of the tunnel to the entrance of the underground lab. As already in the street part,

61 Hardware Installations for Detector Liquid Production

Figure 4.19: Picture of the street part of the trunk line system (TLS); (left): isolation valve boxes mounted outside of the LSA, indicating the manual isolation valves for MU, BF and N2 in the right box and the valves for GC and NT in the left box. The four liquid tubes are made of 3/4-inch-PFA- tubes, which are additionally protected by a stiﬀ 60 mm PVC-tube, the N2 is stainless steel and without PVC-mantle. (center): trunk lines below the street-level, going horizontally from the LSA to the tunnel entrance. (right): tunnel entrance, trunk lines enter the 145 m long connection tunnel to the neutrino laboratory. All trunk lines are double contained and protected by a concrete channel. Not on the picture: the concrete channel is additionally lined with plastic foil and furthermore covered with concrete plates on top of the channel. Pictures from [81] the GC and NT lines are made of PFA, mantled with PVC-tube and additionally wrapped with a metal mesh to avoid electro-static discharges. MU and BF liquids, however, are compatible with stainless steel, because of which these trunk lines do neither need mechanical protection (PVC-tube) nor a metal-mesh to prevent discharges. All tubes are installed within a concrete channel that is used as spill tray and protection against external mechanical forces. The channel is lined with plastic foil and covered with concrete plates. In this trunk-line-segment, there are no mechanical connectors or valves installed. The four liquid lines run straight into the laboratory, where they, still within the concrete channel, connect to the DFOS-part. Just in front of the underground lab, the N2-line connects to the main gas-supply valve and a second particle ﬁlter (poresize: 4 nm) of the underground laboratory. This main valve is also the connection point for the emergency nitrogen supply for the underground lab, which is installed just outside of the underground lab.

Tunnel Part—Emergency N2 Supply : The emergency N2-supply is supposed to deliver the detector with nitrogen, even in the case that the main supply is interrupted or can not be used due to contamination. For this reason, it was necessary to install an independent N2-supply right in front of the lab entrance. This was realized with a 50 l N2-gas-bottle (5.0 Nitrogen, 200 bar) connected to a pressure reducer, which is able to supply pressures between 0-12 bar. The pressure reducer connects to the main isolation valve and supplies the underground lab with nitrogen. The connection valve is in front of the particle filter, which allows to filter the bottled nitrogen before it enters the gas handling system in the underground lab. With a nitrogen consumption of 0.7 m3/h (nominal consumption during data taking), this emergency supply could cover a time-span of 14 hours before the detector would have to be exposed to laboratory air. The DFOS part spans the last few meters (about 8 m) from the laboratory entrance to the detector fluid operating system (DFOS). This part uses flexible PFA-tubes including PVC-

62 Hardware Installations for Detector Liquid Production containment and metal-mesh for all four liquid lines. This segment has to cross the laboratory ﬂoor, because of which it is additionally protected by a metal-cover to withstand mechanical forces induced by people. This segment is a non-permanent installation and was removed after ﬁlling, in order to allow heavy machinery to enter the lab without harming the TLS.

Trunk Line System

TLS-Lines Detail Street part Tunnel part DFOS part Material PFA SS PFA Encased PVC+CC+M CC PVC+MP Muon Veto Length 10 m 145 m 7 m Installed by MPIK TUM TUM Material PFA SS PFA Encased PVC+CC+MM CC PVC+MP Buﬀer Length 10 m 145 m 8 m Installed by MPIK TUM TUM Material PFA SS PFA Encased PVC+CC+M CC PVC+MP Gamma Catcher Length 10 m 145 m 9 m Installed by MPIK MPIK TUM Material PFA PFA PFA Encased PVC+CC+M CC PVC+MP Target Length 10 m 145 m 10 m Installed by MPIK MPIK TUM Material SS SS SS Encased CC CC CC Nitrogen Length 10 m 145 m 7 m Installed by MPIK TUM TUM

Table 4.4: The ﬁve trunk lines used to connect LSA and underground laboratory. Each line is composed of a street-, tunnel- and DFOS-part. The table summarizes the technical details of all TLS-parts: Material (PFA=Perﬂuoroalkoxylalkane, SS=stainless steel), encased (PVC=60 mm×5 mm transparent PVC-Tube, CC= Concrete Channel, MM=Metal Mesh, MP= Metal Cover Plate).

63 Chapter 5

Material Selection for the Detector Liquid Production

5.1 Organic Liquid Scintillators and Requirements for Double Chooz

As particles pass through matter, they deposit energy. A possibility to measure this energy deposition is provided by certain organic, as well as inorganic materials, which have the characteristic to re-emit a proportional part of this energy as scintillation light. Consequently, this scintillation light can be used to determine the amount of energy, which has been deposited by the particle [82]. However, an inherent problem of all scintillation materials is the re-absorption of the initially emitted scintillation light on the neighboring molecule of the same species, because of which all scintillating materials are opaque for its own scintillation light. A possibility to suppress this re-absorption is provided by the stokes shift, which allows to shift the wavelength of the initial scintillation light above the absorption bands of the emitting material. In organic liquid scintillators, this wavelength shift requires a certain chemical compound, also referred to as wavelength shifter, which absorbs the initial fluorescence light (Eabs.=hν1) and re-emits the absorbed energy at a significant higher wavelength (Eem.=hν2), which is defined as stokes shift.

5.1.1 Scintillating Mechanism and Stokes Shift

Organic scintillation bases on the use of a solvent that includes benzene, which has a chemical ring-structure composed of six carbon atoms. The individual carbon atoms are bound by strong σ- and π-bonds, resulting from overlapping s- and p-orbitals [83]. The overlap of the p-orbitals leads to a delocalization of the p-electrons and therefore a multi-electron-system. The excitation and subsequently following relaxation of these electrons leads to the emission of scintillation light, normally in the UV region, which is composed of fluorescence and phosphorescence light, of which the latter is emitted on longer time scales. This can be understood by considering the electron configurations of benzene, its energy levels and the different transitions between them. The delocalized electrons can be described as multi-electron-system with a total spin quantum number (S). The coupling between spin and orbit leads to a multiplicity term (defined by 2S+1) and an energetic splitting of the electronic states. Important for the multiplicity is the relative orientation of the electron-spin in the ground state, compared with the orientation in the excited state. For an even number of electrons, (2S+1) produces two distinguishable states:

64 Material Selection for the Detector Liquid Production

1. spins (↑↑) S=1 (2S+1)=3 triplet state and 2. spins (↓↑) S=0 (2S+1)=1 singlet state.

In benzene, this leads to a singlet ground state (S0), as well as two sorts of excited states: the higher singlet states, described by S1,S2...Sn, and the higher triplet states, described by T1,T2...Tn [84, 85], which are in general lower in energy than the corresponding singlet levels (compare with figure 5.1). These individual energy levels are additionally influenced by molecular vibrations. Describing these vibrations as harmonic oscillation-modes (1,2,3...n), they lead to an additional splitting of the individual states (Sn,Tn), producing various sub-levels written as Sn1,Sn2...Snn or Tn1,Tn2...Tnn. The energy gap between vibrational modes is significantly smaller as the gap between the different electronic states (S0,S1...Sn).

Jablonski-diagram

Ionization energy

Singlet Triplet

S21

T21 S1n - S 0= ΔE = Δλ T2 Stokes Shift non radiative

S13 internal non-radiative S12 Inter-system conversion S11 vibrational modes S crossing 1 τ~10-12 s

spin-flip T13

T12

T11 T1 -9 τ ~10 s -6 spin-flip τ ~10 s Inter-system crossing 1 1 2 2 Absorption E = h ν Fluorescence E = h ν Phosphorescence S03 S02 S01 S 0 ground state

Figure 5.1: The Jablonski-diagram provides a simplified illustration of the different energy levels of the p-electrons in benzene as to find in organic liquid scintillators [86]. Depending on the spin of the excited electron relative to the one in the ground state (parallel or anti-parallel), the electronic states split into singlet and triplet states. Shown are the electronic states for singlet and triplet states (full lines), as well as the vibrational sub-levels (dashed lines) induced by molecular vibrations. While the vibrational sub-levels relax quickly and most importantly non-radiative, the relaxation of singlet-states is mainly radiative and leads to the emission of fluorescence light. In contrast to the singlet states, the relaxation of triplet-states requires a spin-flip and is therefore less probable, which leads to a delayed emission of scintillation-light, referred to as phosphorescence. In addition, the diagram indicates the stokes shift, which is used to increase the transparency of a scintillating solvent to its own scintillation light. The used fluors absorb scintillation light (E1 → S13) and disexcite non-radiatively into the vibrational ground state (S10), from which the radiative decay leads to the emission of fluorescence light with a bigger wavelength defined by E2.

65 Material Selection for the Detector Liquid Production

Starting with an excitation from the ground state S0, the absorption of electro-magnetic energy (E1=hν1) leads exclusively to the excitation of higher singlet states, but not to the population of triplet states, as this would require a spin flip and is forbidden by the rules of selection [84, 85]. Hence, triplet states can not be populated by em-radiation, but have to be populated by other processes like inter-system-crossing, which describes the excitation of T1 by decay of the higher S1 state (S1 →T1) or the recombination of electrons and an ionized molecule, which leads for 75% to the population of triplet states [84]. The disexcitation of these higher states has two different possibilities: non-radiative transitions, which dissipate the absorbed energy mechanically via collisions with other molecules [86], as well as radiative transitions, which use luminescence to disexcite the higher energy levels. Both of these scenarios are presented in figure 5.1, showing the Jablonski-diagram1, which indicates a simplified scheme of the different energy levels and possible transitions. Indicated are: Radiative transitions between:

– states of the same multiplicity (S1 → S0,S2 → S1,T2 → T1). These transitions do not require a spin flip because of which these transition happen on time scales of few nano-seconds [88, 85]. The energy gap between S1 and S0 is normally between 2-4.5 eV [84, 85], which results in a fluorescence light emission in the UV region around 270 nm. Due to the fast and non radiative relaxation of vibrational modes, the fluorescence emission originate only from the vibrational ground state (Sn0), as indicated in figure 5.1 by the blue emission arrow.

– states of diﬀerent multiplicity (T1 → S0) These transitions require a spin ﬂip and are therefore highly forbidden by the rules of selection. These processes are less probable, because of which the direct emission of phosphorescence light has a time scale of microseconds or longer. More probable, however, is the indirect and delayed relaxation (T1 ↝ S1 → S0), which necessitates the repopulation of the higher S1 state. This can be done by the absorption of thermal energy or by the interaction of two molecules. Once the higher S1-state is repopulated by these processes, radiative transitions can disexcite the molecule into the ground state. The delay accounts for the time, which has been necessary to repopulate the higher S1-state. Furthermore the interaction between two molecules with the same level of excitation allows to relax two triplet states into the ground state via: ∗ T1+T1 →S +S0 → S0+S0 + photon [84, 85]. Non-radiative transitions between:

– states of the same multiplicity (S1 ↝ S0,S2 ↝ S1,T2 ↝ T1) The non radiative relaxation of excited levels has to be divided into internal conversion, describing the relaxation between electronic states (S2 ↝ S1), and vibrational relaxation, describing the decay of vibrational modes of individual electronic states −12 (S11,S12,S13 ↝ S0). Especially the latter happens very fast (τ ∼ 10 s)[84, 85] and therefore long before radiative emission can occur. This has two consequences: firstly, radiative transitions happen only from the vibrational ground states S10,S20...Sn0, and secondly, the quick vibrational relaxation reduces the available energy for a radiative emission. In consequence, the emission spectrum (E2 = hν2) is shifted to higher wavelength. The difference between absorbed and emitted energy ∆E = h(ν2-ν1) is known as stokes shift with ∆E ≈ 0.1 eV. 1Aleksander Jablonski, Ukraine, 1898-1980. According to A. Jabolnski, it is possible to approximate the spectroscopic properties of many organic molecules by a simplified energy level diagram, considering only the ground state and the deepest excited states S1 and T1. In 1935, he developed the first energy-level-diagram to explain the energy transfer in molecules, today known as Jablonski-diagram [87].

66 Material Selection for the Detector Liquid Production

– states of different multiplicity (S1 ↝ T1,T1 ↝ S0) These transitions require a spin flip and are the result of a direct interaction between electrons. These transitions are referred to as inter-system-crossing and are shown as wavy lines in figure 5.1.

Non Radiative Energy Transfer

The non radiative energy transfer is dominated by two different processes: The Dexter-process [89, 84], also referred to as collisional energy transfer, depends on spatial overlap of interacting molecular orbitals and describes the exchange of electrons between the −r interacting orbitals. The energy transfer rate (k) is strongly depending on the distance (∼ e ), because of which this transfer is mainly responsible for interactions below 50 A.˚ The direct interaction and the exchange of electrons provide the possibility for a spin-flip, because of which this process is the dominant energy transfer for the above mentioned inter system crossing. The Förster-process [90, 84] describes a dipol-dipol interaction between the transition dipoles of two interacting molecules [91]. This energy transfer depends on the orientation of the interacting 6 dipoles as well as on their distance (k∼1/r ). Typically, this process describes interactions up to 100 A˚ and is used to describe the non-radiative energy transfer between charged particles and the scintillating-solvent, as well as the energy transfer between solvent and wavelengths shifting molecules.

Loss of Scintillation Light

In order to use scintillation for the detection, it is important to maximize the emission of fluorescence light. This, however, can be reduced by different effects, as for instance: 1. a higher rate of internal conversion, which is correlated to the temperature of the solvent, 2. a contamination of the solvent with a (quenching) molecule-species or 3. the formation of molecule-complexes. A prominent example for a quenching molecule, which additionally can lead to a degradation, is oxygen. When liquid scintillators are stored or handled under normal atmosphere, they saturate with oxygen. Depending on the duration of the exposure, this leads to two different quenching effects: 1. a reversible one, which is caused by the presence of quenching-molecules. Oxygen, for example, absorbs energy from the solvent and dissipates it in non-radiative relaxation channels. This effect can be reversed by removing the oxygen form the solvent, which is normally done by purging the solvent with nitrogen. 2. an irreversible one (caused by a longer exposure), which leads to a chemical reaction of the solvent with oxygen. The oxidation of organic molecules can lead to the formation of molecule-complexes (dye), which alter the absorption- and emission-bands from UV into the visible spectra. This reduces the fluorescence emissions in the UV-region and leads to a visible and yellowish discoloring of the solvent. As this chemical-quenching can not be reversed, it reduces the optical properties of the solvent permanently. This is an inherent problem of all liquid scintillators because of which these materials have to be handled and stored in an oxygen-free environment.

67 Material Selection for the Detector Liquid Production

5.1.2 Requirements for Double Chooz

Double Chooz uses three different liquid scintillators to measure the energy depositions in the detector vessels by observing the produced scintillation light, because of which the experiment depends on the stability and cleanliness of the detector liquids. Produced in the target, the scintillation light has to pass through the different liquid layers of the detector before the photons can be detected by the photo multipliers. All processes which change either the production of fluorescence light (quenching) or its subsequent detection at the PMTs (optical degradation of the liquids) are critical for the experiment and must be prevented. In order to be usable for Double Chooz, all liquids have to provide certain minimum requirements, regarding: Optical purity: In order to minimize the loss of scintillation light, the final detector liquids have to be highly transparent. Attenuating effects, as absorption and scattering, have to be avoided, because of which all detector liquids have to demonstrate an attenuation length above 5 m at a wavelength of 430 nm. In addition, all liquids have to be free from impurities. This is especially true for suspended particles which are similar in size compared to the emitted scintillation wavelength, as those particles would lead to additional Mie-scattering effects [92]. Density: Double Chooz uses four different detector liquids separated only by thin and fragile walls. Any density difference between these liquids leads to buoyancy forces and therefore to stress on the different vessels. In order to avoid dangerous stress levels, the density of the different liquids should not vary more than one percent. Based on this, all liquids should be adjusted to 0.804 g/cm3 at 15○C [55] with a maximal variation of 0.008 g/cm3. Radio purity: Double Chooz is a low background experiment, which aims to limit the accidental background rate below 1 event per day [55]. The accidental background is driven by radio-chemical contamination, which is either induced by the detector materials (PMTs, metals, acrylics etc.) or by the detector liquids. Based on detector simulations, the activity of the detector materials (mainly caused by the PMTs) accounts for 0.4 accidental-background-events per day. Using additional 0.4 events per day as upper limit caused by the detector liquids, the maximal contamination with 40K, 238U and 232Th for each detector liquid can be calculated. For the inner detector vessels, these limits are in the range of 10−13 g/g for 238U and 232Th and of 10−10 g/g for 40K. The individual limits for the different liquids are summarized in table 5.1. Light yield: In order to recognize also small energy depositions, all liquid scintillators have to provide a certain minimum light yield. The muon veto, as optically separated detector module, is independent from the other detector liquids and should provide a minimum light yield of 5000 photons per MeV [55]. The light yield of the inner detector liquids has to be equal in order to provide homogeneous response and should provide a light emission of minimum 6000 photons per MeV [55]. The buffer liquid, used as transparent and inactive shielding, however must not scintillate at all, because any scintillation in the buffer would lead to a significant limitation of the anticipated detector performance. Chemical stability: In order to provide a stable detector response, all detector liquids have to provide constant properties, especially regarding light yield and transparency. Any degradation of one or the other detector liquid would immediately limit the performance of the entire detector or, in the worst case, could lead to the premature end of the experiment as experienced by the parent experiment CHOOZ [93] or the Paloverde-experiment [94]. The stability of the detector liquids is therefore a major concern for Double Chooz. In order to ensure the chemical stability, each liquid is handled and stored under a permanent nitrogen atmosphere, protected from UV- light and only in contact with compatible materials as stainless steel or fluorinated plastics.

68 Material Selection for the Detector Liquid Production

Minimum Requirements on the Detector Liquids

Property Unit MU BF GC NT Radio purity 238 232 −10 −10 −13 −13 U, Th g/g < 10 < 10 < 10 < 10 40 −7 −7 −10 −10 K g/g < 10 < 10 < 10 < 10 Transparency m > 6 > 6 > 5 > 5 Light yield Ph/MeV > 5000 0 > 6000 > 6000 3 Density g/cm 0.804±0.008 0.804±0.008 0.804±0.008 0.804±0.008

Table 5.1: Summary of the diﬀerent requirements on the detector liquids, indicating the limits on density, light yield, transparency, radio purity and required long term stability. The limits are extracted from [55].

In order to meet all of the above mentioned requirements, comprehensive laboratory measurements were conducted by different institutes of the Double Chooz collaboration. TUM supported this process by radio purity measurements (germanium spectroscopy and NAA [72]), as well as transparency measurements (absorption, attenuation length [95]), light yield- and density- measurements [95]. These measurements were used prior to the production process in order to select suitable candidates for the detector liquids. The following section is dedicated to this material selection process and presents in section 5.2 the selection process for the components of the muon veto scintillator and in section 5.3 measurements to find the ingredients for the buffer liquid. The finally selected components for both liquids are summarized in section 5.4. Information about the mixing process of gamma catcher- and the Gd-doped target-scintillator are summarized in [62].

5.2 Component Selection for the Muon Veto Scintillator

The muon veto scintillator is a composition of LAB, n-paraffine and two chemical additives. The LAB is used as scintillating solvent, the n-paraffine is used as dilution in order to tune the density and the two additives are used to facilitate the scintillation process (PPO and bis/MSB). The combination of both shifts the initial fluorescence light in two steps from ∼280 nm to about 450 nm. In the following, the different ingredients and their selection process is presented.

5.2.1 Scintillating Solvent

Linear Alkylbenzene

Chemically, Linear Alkylbenzene (LAB) is composed of a benzene-ring and a saturated and linear hydrocarbon-chain of varying length [96, 97]. LAB is insoluble in water, very transparent, almost odorless and has a density of 0.863 g/cm3 and a comfortably high flash point of 140○C. It is ecologically harmless and non-hazardous, what simplifies all sorts of safety- and legal-issues regarding transport, storage, handling and material compatibility [96, 97]. As standard product in the detergent industry, LAB is available by default and provided by various companies for a comparable low prize of a few Euros per liter. The chemical structure of LAB is presented in figure 5.2, along with structure of PXE (Phenyl-Xylyl-Ethane) [98], which is used as scintillating solvent for the inner detector liquids gamma catcher and target. For the use in Double Chooz,

69 Material Selection for the Detector Liquid Production

Figure 5.2: Chemical structure of two organic solvents, often used in organic liquid scintillators: (left): linear alkylbenzene (LAB), (right): phenyl-xylyl-ethane PXE.

LAB-samples from different companies (Petresa/Cepsa2, Wibarco3, and Helm4) were compared, density and optical properties were studied. The absorbance (A) of all samples was measured with a standard UV/vis-spectrometer5, which compares the intensity of two equal light beams, one going through the sample, the other is used as reference. The absorbance is then given by A(x) = log10(I(0)/I(x)) where I(0) is the intensity of the reference beam and I(x) the measured intensity after the light beam traveled through the sample with the length x. A second measurement was made of the empty sample cell in order to account for the intensity losses due to reflections on the sample cell. Subsequently, the absorbance was corrected for reflections and used to calculate the attenuation length (Λ), using x 1 Λ = ⋅ log10(e) A(λ)k = A(λ) − min(A(λ)) A(λ) = Asample(λ) − Acell(λ) . A(λ)k 2 An elaborate description of this measurement method can be found in [95]. The measured

Figure 5.3: Attenuation length measurement of diﬀerent LAB samples which have been in consideration for the use in Double Chooz. The provided samples from Petresa/Cepsa and Wibarco showed higher absorbance and an attenuation length far below 6 m at 430 nm, which would have been the minimum requirement for the use in Double Chooz. The two samples from Helm on the other hand demonstrated with 7.14 m (sample from Belgium) and 9.73 m (sample from Spain) a superior transparency at 430 nm. Both samples from Helm were produced in Egypt and later distributed over Belgium and Spain. Graph from [95]. absorbance (A) as well as the determined attenuation length (Λ) of the diﬀerent samples are

2CEPSA, Oude Graanmarkt 63, B-1000 Brussels, Belgium 3WIBARCO GmbH, Hauptstrasse 21, 49479 Ibbenb¨uren,Germany 4HELM AG, Nordkanalstrasse 28, D-20097 Hamburg, Germany 5Co. Perking&Elmer, UV/vis spectrometer, Lambda 850, 10 cm long sample cell [99]

70 Material Selection for the Detector Liquid Production presented in figure 5.3. Within the interesting region between 400 nm and 450 nm, the samples from Petresa/Cepsa and Wibarco show a significantly higher absorption, leading to an attenuation length of only 2.64 ±0.24 m, and 3.00 ±0.27 m at 430 nm respectively, which is both far below the minimal required attenuation length of 6 m. The samples of Helm showed significantly better values. Both samples were produced in Egypt and were subsequently transported to facilities in Spain and Belgium. The sample received from Spain showed the best value with 9.73 ±0.88 m, followed by the sample from Belgium with 7.14 ±0.64 m at 430 nm. The difference between these two samples is significant and clearly favors the distribution facility in Spain. The attenuation length at 420 nm, 430 nm and 440 nm of all measured samples is summarized in table 5.2, together with the results of the density measurements, which were done with a digital density-meter6. As result of these measurements, LAB from Helm, distributed over Spain, has been chosen for the muon veto scintillator. Attenuation Length and Density of Different LAB Samples

Company Trade Name Density Attenuation Length g/cm3 m@420 nm m@430 nm m@440 nm Helm Belgium LAB 0.860±0.001 6.18±0.56 7.14±0.64 7.98±0.72 Helm Spain LAB 0.860±0.001 8.36±0.75 9.73±0.88 11.25±1.01 Cepsa LAB P-550Q 0.859±0.001 2.19±0.20 2.64±0.24 3.08±028 Wibarco Wibracan 0.867±0.001 2.15±0.19 3.00±0.27 4.00±0.36

Table 5.2: Attenuation length of the LAB samples from Helm, Cepsa and Wibarco, measured at 420 nm, 430 nm and 440 nm. The highest transparency was measured in the sample from Helm, distributed over facilities in Spain, which is indicated in bold letters. The densities of the diﬀerent samples are additionally presented. Values from [95].

5.2.2 Non-scintillating Dilution

Alkane, n-paraﬃne

Alkanes describe the group of saturated hydrocarbons with the general formula CnH2n+2. Alka- nes exist in different variations, the hydrocarbon-chain can be linear (n-alkanes), branched (iso- alkanes) or circular (cycloalkanes). Physical properties like density, boiling, melting, flash point, etc. depend on the size of the molecule and thus on the number of carbons-atoms. For the use in Double Chooz, two different n-alkanes have been in consideration: n-paraffine and the higher refined tetradecane. N-alkanes are suitable dilutions for a liquid scintillator, as they are highly transparent and dissolve very well in organic solvents. This and the low density (ρ = 0.6-0.8 g/cm3) allow to use them as dilution in order to tune the density of the scintillator. Figure 5.4 provides an illustration of the chemical structure of linear and branched alkanes (left), tetradecane and (right) n-paraffine.

CH3 CH3

Tetradecane N-paraffine C H 2 +2 n =14 C H n =18-32 n n n 2n+2 Figure 5.4: Chemical structure of: (left): tetradecane and (right): n-paraﬃne, two n-alkanes, which have been in consideration for the use in Double Chooz.

6Co. Anton Paar, DMA38, [100] providing an accuracy of 0.001 g/cm3

71 Material Selection for the Detector Liquid Production

As n-alkanes are a standard product for industry, they are highly available and provided by different companies. For the use in Double Chooz, n-paraffine samples from Wibarco, Helm and CBR7 as well as a tetradecane sample from Petresa/Cepsa were compared. Absorbance (A), attenuation length (Λ) and the density of all samples were investigated as described above. The results of these comparisons are summarized in figure 5.5 and table 5.3. With the exception of tetradecane, all samples demonstrated an excellent transparency, showing an attenuation length between 12 and 20 m. Finally chosen for the use in Double Chooz was the Cobersol C70 from CBR. As all n-parraffines easily met the minimum requirements of 6 m, CBR was chosen due to the high flexibility of the distribution center, which allowed to test and choose the n-paraffine-batch with the highest quality out of different storage tanks.

Figure 5.5: Attenuation length measurement of different alkane-samples, which have been considered for the use in Double Chooz. The provided samples from Petresa/Cepsa showed the highest absorbance between 400 nm and 450 nm and an attenuation length of only 4.57 m@430 nm. All other samples showed significantly higher transparencies well above 12 m@430 nm. Although the n-paraffine from Wibarco showed the highest transparency, Cobersol C70 was finally selected due to the higher flexibility of the CBR logistic-center. Graph from [95].

Attenuation Length and Density of Diﬀerent Alkane Samples

Company Trade Name Density Attenuation Length g/cm3 m@420 nm m@430 nm m@440 nm Cepsa Tetradecane 0.767±0.001 3.23±0.29 4.57±0.41 6.95±0.63 Wibarco n-paraﬃne 0.749±0.001 19.34±1.74 20.68±1.86 22.15±1.99 Helm n-paraﬃne 0.749±0.001 12.40±1.12 12.46±1.12 12.32±1.11 CBR Cobersol C70 0.749±0.001 15.50±1.40 17.17±1.55 19.13±1.72

Table 5.3: Density and attenuation length at 420 nm, 430 nm and 440 nm of diﬀerent alkane-samples, which have been considered for the use in Double Chooz. The ﬁnally chosen component is printed in bold letters. Values from [95].

7CölnerBenzin Raffinerie K. Kroseberg GmbH & Co. KG, Eupener Straße 128-144, D-50933 Köln,Germany

72 Material Selection for the Detector Liquid Production

5.2.3 Wavelength Shifter

Double Chooz uses two different fluors, a primary one, which absorbs the initial scintillation light from the solvents, and a secondary one, which absorbs the emission of the primary fluor. This allows to shift the emission of the solvent in two steps from about 280 nm to about 450 nm, where the solvent is transparent to the scintillation light and the PMTs are most sensitive. An illustration of this process is presented in figure 5.6, which indicates the absorption- and emission-bands of LAB, PPO and bis/MSB.

Secondary bis/MSB Fluor Emission: 380 - 450 nm Absorption: 320 - 370 nm

270 280 290 300 310 320 330 340 350 360 370 380 400 410 420 430 440 450 460 470 480 Primary PPO Fluor Stokes Shift Emission: 350 - 400 nm Absorption: 280 - 325 nm

270 280 290 300 310 320 330 340 350 360 370 380 400 410 420 430 440 450 460 470 480 LAB Emission: 283 nm Absorption: 260 nm 250 260 270 280 290 300 310 320 330 340 350 360 370 380 400 410 420 430 440 450 460 470 480 nm

Figure 5.6: Overview scheme, indicating the absorption- and emission-bands of LAB, PPO and Bis/MSB: The absorption-bands are indicated in blue, emission-bands are indicated in red. The absorption- and emission-bands of the fluors show a stokes shift, which allows to shift the initial fluorescence light in two steps from 280 nm up to 450 nm. Measured spectra of the absorption- and emission-bands of PXE, PPO and bis/MSB can be found in the appendix (see figure 5.6).

Primary Fluor, PPO

2.5-diphenyloxazol, also referred to as PPO, is a chemical stable white pow- der, which disolves well in organic solvents. The polar molecule8 absorbs in the region between 250-350 nm and re-emits the scintillation light between 320-420 nm [101]. At this higher wavelength, the solvent is transparent for the scintillation light, which increases the light yield of the solvent. Lab- oratory measurements indicated a significant increase of the light yield for already small concentrations of a few gram per liter. Figure 5.7 presents these measurements and indicates the light yield of a LAB-based scintillator for a varying concentration of PPO. Based on these measurements, the PPO concentration in the muon veto was fixed to 2 g/l, as this provided the best balance between light yield and costs. For the inner detector liquids, the concentration was chosen to be 5 g/l in the gamma catcher and 7 g/l in the target. Based on these concentrations, Double Chooz required 180 kg for the muon veto, 45 kg for the gamma catcher and 7 kg for the neutrino target. Industrially man- ufactured PPO is normally contaminated with impurities. Based on the mentioned amounts, also small radio-chemical impurities could have negative influence on the detector liquids. For

8The polarity of PPO has to be considered during scintillators-puriﬁcation in order to avoid an accidental removal of PPO together with other polar impurities.

73 Material Selection for the Detector Liquid Production the use in Double Chooz, PPO from Sigma Aldrich9 and Perkin & Elmer10 was considered and screened for radio chemical-impurities, using NAA at TUM [72] and AAS at MIPK [62]. Based on these measurements, Perkin & Elmer produced an individual batch of PPO with increased radio-chemical cleanliness (”neutrino grade PPO”). The found concentrations of 40K in the diﬀerent samples showed only low concentrations, which are summarized in table 5.4. Based on these measurements, the PPO from Perkin & Elmer was selected and used for the production of muon veto, gamma catcher and target. Concentration of 40K in PPO

Company Trade Name 40K concentration −11 Sigma Aldrich PPO (standard) (2.52±1.87)⋅10 g/g −11 Perkin & Elmer PPO (standard) (15.4±0.49)⋅10 g/g st −11 Perkin & Elmer PPO (neutrino grade) 1 (2.58±1.38)⋅10 g/g nd −11 Perkin & Elmer PPO (neutrino grade) 2 (1.36±1.32)⋅10 g/g

Table 5.4: Concentration of 40K in diﬀerent PPO samples from Perkin & Elmer and Sigma Aldrich, measured with NAA-measurement at TUM. Values from [72].

Figure 5.7: Light yield of an LAB-based scintillator, using 37.5% LAB, 62.5% tetradecane, 20 mg/l bis/MSB and a varying concentration of PPO between 0 and 4 g/l. Based on these measurements, the best balance between maximal light yield and required PPO is provided by a concentration of 2 g/l PPO and 20 mg/l bis/MSB, because of which this concentration has been chosen for the muon veto scintillator. Plot taken from [95].

Secondary Fluor, bis/MSB

1,4-bis(2-methylstyryl)-benzene, also referred to as bis/MSB, is composed of three benzene-rings, which absorb between 300-380 nm and re-emit between 380-500 nm [102]. Bis/MSB is nonpolar, chemical stable and solutes sufficiently well in organic solvents. This second wavelength shifter is used to shift the PPO-emissions to even higher wavelength, where the used pho- tomultipliers provide the best detection efficiency. Already small concentrations of a few mg per liter provide a sufficient stokes shift. Laboratory measurements indicated the best performance for 20 mg/l, which has finally be chosen for all

9SIGMA-ALDRICH CHEMIE GmbH, Eschenstr. 5, Am Wald, 82024 Taufkirchen, Germany 10Perkin & Elmer, Ferdinand-Porsche-Ring 17, 63110 Rodgau, Germany

74 Material Selection for the Detector Liquid Production the detector liquids. Based on these concentrations, Double Chooz required 1.8 kg for the muon veto, 0.4 kg for the gamma catcher and 0.2 kg for the target, which were purchased together with the PPO from Perkin & Elmer. Due to the small concentration of bis/MSB in the detector liquids, separate measurements of the radio purity were not necessary. Figure 5.8 indicates the measured emission- and absorption-bands of bis/MSB and PPO, indicating their individual stokes shift together with the emission-band of PXE.

Figure 5.8: Absorption- and emission-bands of PXE, PPO and bis/MSB, measured by [103].

5.3 Component Selection for the Buﬀer Liquid

The buffer tank is the optical separation between the muon veto and the inner detector. The buffer tank is made of stainless steel and equipped with 390 photomultiplier tubes. The volume between these tubes and the inner acrylic vessel of the gamma catcher is filled with 100 m3 of a non-scintillating buffer oil, which is supposed to shield the inner detector liquids from internal radioactivity, mostly coming from the PMTs and the used detector materials. Any fluorescence light from the inner volumes has to pass the buffer before it can be detected by the PMTs. Hence, the chemical and optical properties of the buffer are very important. In order to tune the density 3 of the buffer liquid to the finally required 0.804 ± 0.008 g/cm , the buffer liquid is composed of two different mineral oils: a highly transparent medical white oil from Shell with the trade-name Ondina, and the lighter n-paraffine, which has been introduced in the last section.

5.3.1 Non-scintillating Mineral Oils

The company Shell produces various highly refined mineral oils, also referred to as medical white oils. By standard, these oils are used in cosmetic or medical products and thus are highly available and comparably cheap. Two of these oils, with the trade names Ondina-909 and Ondina-917, have been considered for the use in Double Chooz. They are composed of branched (iso-alkane) as well as unbranched (n-alkane) saturated hydrocarbons with the general formula CnH2n+2 , with n=15-40. Both offer the same optical properties and low reactivity, differ however in their number of carbon atoms, which leads to different chemical properties regarding

75 Material Selection for the Detector Liquid Production density and viscosity. The other component of the buffer-oil is n-paraffine, a higher refined mineral oil, which is composed of linear (saturated) hydrocarbons only. Figure 5.9 provides an illustration of the chemical structure of Ondina and n-paraffine and table 5.6 summarizes their main properties. For the use in Double Chooz, absorbance (A), attenuation length (Λ) and

Figure 5.9: Chemical structure of branched and unbranched mineral oils: (left):n-paraffine, a higher refined mineral oil, composed of only unbranched saturated hydrocarbons, also referred to as n-alkane. (right): Ondina-917, composed of branched (iso-alkane) and unbranched (n-alkane) saturated hydrocarbons. density of different Ondina-samples (Ondina-909, samples from 2006 and 2008, and Ondina- 917, samples from February and April 2010) from Shell have been compared by Meyer [95]. The results of this comparison are shown in figure 5.10, which indicates the absorbance and attenuation length between 400 nm and 450 nm. Although Ondina-909 demonstrated better optical properties, Ondina-917 was finally chosen for the use in Double Chooz. The reason for this was the questionable availability of Ondina-909 in the future, as Shell announced to remove Ondina-909 from their port folio. In order to guarantee the same buffer oil composition for near and far detector, the choice was made for Ondina-917, whose availability is ensured. The results of attenuation length- and density-measurements are summarized in table 5.5.

Figure 5.10: Absorbance (A) and attenuation length comparison between Ondina-909 and -917 (Ondina- 909 samples were taken in 2006 and 2008, Ondina-917 samples were taken in February and April 2010), which have been considered for the use in the buﬀer-liquid. Although the Ondina-917 samples, both produced in 2010, should provide the same optical properties, only one sample (April 2010) showed a very high absorbance between 400 nm and 450 nm, and only an attenuation length of 1.52 m@430 nm, which was most probably caused by neglectant sample preparation or handling. All other samples, however, demonstrated with an attenuation length between 7, 9 and 10 m at 430 nm much better optical properties above the required 6 m at 430 nm. Finally chosen for the the use in Double Chooz was Ondina-917, which is, in contrast to Ondina 909, available in future. The graph is taken from [95].

76 Material Selection for the Detector Liquid Production

Attenuation Length and Density of Diﬀerent Ondina Samples

Company Trade Name Density Attenuation Length Ondina g/cm3 m@420 nm m@430 nm m@440 nm Shell 909 (2006) 0.825±0.001 9.33±0.84 10.45±0.94 11.40±1.03 Shell 909 (2008) 0.825±0.001 8.76±0.79 9.62±0.87 10.29±0.93 Shell 917 (02/2010) 0.854±0.001 7.13±0.64 7.70±0.70 8.52±0.77 Shell 917 (04/2010) 0.854±0.001 1.33±0.28 1.52±0.30 1.60±0.32

Table 5.5: Density and attenuation length at 420 nm, 430 nm and 440 nm of different Ondina-samples, which have been considered for the use in Double Chooz. The Ondina-909 samples were taken in 2006 and 2008, the Ondina-917 samples were taken in February and April 2010. For the use in the buffer liquid, Ondina-917 was finally chosen, here presented in bold letters. Although Ondina-909 demonstrated better optical properties, it was not selected, because this product will not be available for the near detector, as Shell terminated the production of this mineral oil type. In consequence, Ondina-917 was chosen for the use in the buffer, what is indicated in bold letters. Values taken from [95].

5.4 Selected Components for Muon Veto Scintillator and Buﬀer Liquid

Table 5.6 summarizes the most important properties of the finally selected liquid components, which have been chosen for the muon veto scintillator and the buffer liquid. Selected Components for the Muon Veto Scintillator and the Buffer Liquid

Muon Veto Unit Solvent Dilution Wavelength Shifter Trade Name LAB Cobersol C70 PPO bis/MSB Atten.length m@430nm 9.73±0.88 17.17±1.55 – – Viscosity mm2/s 5.8-11.6@20○C 1.9@20○C – – Flash Point ○C 140 70 – – ○ 3 Density g/cm@15 C 0.860±0.001 0.749±0.001 0.3 0.45 Absorption nm 260 – 250-350 300-380 Emission nm 283 – 320-420 380-500 CAS no. 68890-99-3 64771-72-8 92-71-7 13280-61-0 Supplier Helm CBR Perkin Elmer Perkin Elmer

Buﬀer Unit Mineral Oil Dilution Trade Name Ondina-917 Cobersol C70 Atten. length m@430nm 7.70±0.70 17.17±1.55 2 ○ ○ Viscosity mm ~s 18@40 C 1.9@20 C Flash Point ○C 200 70 3 ○ Density g/cm @15 C 0.854±0.001 0.749±0.001 CAS no. 8042-47-5 64771-72-8 Supplier Shell CBR

Table 5.6: Main properties of all components of the muon veto scintillator, indicating LAB as scintillating solvent, n-paraffine as dilution and PPO as well as bis/MSB as wavelength shifter. [97, 96, 104, 105, 101, 102]. In addition, the main properties of the selected buffer liquid components n-paraffine and Ondina-917 are summarized. [106, 107, 104, 95].

77 Chapter 6

Detector Liquid Production

6.1 Composition of Muon Veto and Buﬀer

After identifying the different components of the muon veto scintillator and the buffer liquid, which are presented in table 6.1, the individual compositions of both could be determined [108]. The aim of this was to produce 90 m3 of muon veto scintillator and 110 m3 of buffer liquid, 3 ○ both with a density of 0.804 ± 0.008g/cm at 15 C. The fluor concentration in the muon veto was chosen to be 2 g/l of PPO and 20 mg/l of bis/MSB, which is the result of measurements by Meyer [95], who investigated the light yield of different fluor concentrations (see figure 5.7). Table 6.1 summarizes the finally chosen composition for muon veto and buffer and lists the individual amounts of the different ingredients.

Muon veto Ingredient Composition Amount Density@15○C 3 3 N-paraﬃne 49.8 %vol. 44.7 m 33700 kg 0.749 g/cm LAB 50.2 % 45.3 m3 36000 kg 0.860 g/cm3 90 m3 MU-LS vol. PPO 2 g/l – 180 kg 0.300 g/cm3 bis/MSB 20 mg/l – 1.8 kg 0.450 g/cm3

Buﬀer Liquid Ingredient Composition Amount Density@15○C 3 3 3 Mineral Oil 54 %vol. 60 m 52000 kg 0.854 g/cm 110 m BF-Oil 3 3 N-paraﬃne 46 %vol. 50 m 39300 kg 0.749 g/cm

Table 6.1: Composition of the muon veto scintillator and the buﬀer liquid, as well as the necessary amounts of the diﬀerent ingredients [108] used for the far detector. A full summary including the gamma catcher and neutrino target can be found in appendix C.1.

6.2 Preparation of the LSA

After the instrumentation of the LSA, both liquid handling systems had to be prepared for the reception of the different liquids. This included the cleaning of all storage tanks and the thorough flushing of the pumping station as well as the connected tubing. The inner surfaces of the storage tanks were cleaned with industrial detergent and iso-propanol, as well as a high pressure cleaner with 5 m3 of ultra pure water. The cleaning was done in two steps: During the first step, the inner walls of the storage tanks were manually cleaned, using a scrubber

78 Detector Liquid Production and a water-detergent-mixture. The second step was done by using a scrubber with a water- propanol-mixture. After each cleaning step, the inner walls were flushed with ultra pure water, using the high pressure cleaner. In order to clean also the interior of the pumping station and all related tubing, the accumulated water-mixture (of about 1 m3 per tank) was circulated and subsequently disposed. After this cleaning process, all tanks were dried with a flow of warm air in order to remove the residual water-propanol-mixture. After drying, the entire liquid handling system was isolated and flushed with dry nitrogen in order to remove oxygen and still remaining water-propanol-mixture from the system. By using the gas-flow-meters in the pumping station, the nitrogen purging was maintained until 1800 m3 of nitrogen was pushed through the system, corresponding to eight times the storage volume. After the initial inertization of the liquid handling system, the small nitrogen flow was maintained in order to produce a low pressure nitrogen blanket in the storage tanks.

6.3 Parallel Production of the Muon Veto Scintillator and Buﬀer Liquid

The liquid handling system in the LSA was designed to receive and mix large amounts of liquids, but not to dissolve and blend large amounts of crystalline compounds, as PPO and bis/MSB. Such a functionality would have risen the costs for the liquid handling system in the LSA tremendously. In order to avoid these costs and due to the possibility to cooperate with Wacker-Chemie, the chemicals PPO and bis/MSB were dissolved in a small batch of LAB at Wacker-Chemie in Munich. This highly concentrated solution, also referred to as Master Solution1, contained all the chemicals in liquefied form. In a second step, the MS was transported to Chooz, where the liquid handling system was used to dilute the MS until the desired density and concentration of the scintillator was reached. Subsequently, the mixture was blended, fine tuned and tested in order to verify the final density, transparency, as well as the light yield.

6.3.1 Master Solution

The master solution for the far detector was made in cooperation with Wacker-Chemie in Munich, which operates a chemical research and provides the necessary infrastructure to process and mix liquids and chemicals under controlled, clean and adjustable conditions. This industrial setup, also referred to as “Technikum”, includes a 1000 l stainless steel mixing tank, which provides a stirring- and purging-unit along with the possibility to heat the medium inside. The production of the master solution started with the delivery of 5000 l of LAB, 180 kg of PPO and 1.8 kg of bis/MSB to the “Technikum” in Munich. The process tank was cleaned and evacuated before a batch of 960 l LAB was transferred to the tank. In order to remove oxygen from the LAB and to facilitate the dissolving of the crystalline PPO and bis/MSB, the LAB was heated to +50○C and continuously purged with nitrogen. After weighing, both chemicals were added to the tank and the mixture was blended under constant stirring and purging. In order to guarantee a homogeneous mixture, purging and stirring was maintained for three additional hours. Subsequently, the master solution was filtered using a 3 µm particle filter, in order to remove all residual particles, which did not dissolve during the mixing process. After the production, each batch was transferred to a nitrogen-filled 1000 l transport tank (IBC- international bulk container), which was used to deliver the LAB. The four remaining batches

1A master solution is a highly concentrated solution, which includes the entire amount of chemical additives of an anticipated scintillator batch. The production of a master solution eases the production process, as only a small amount of liquid has to be handled to dissolve the chemical additives. In addition, the production of a master solution simpliﬁes the requirements on the on-site mixing facility, as only liquids have to be handled.

79 Detector Liquid Production were produced correspondingly, what allowed to prepare 4800 l of master solution with a ﬂuor concentration of 40 g/l PPO and 0.4 g/l bis/MSB.

6.3.2 Mixing Process

The on-site mixing process of the muon veto started with the delivery of five IBCs filled with master solution (MS). For the unloading of the MS, the containers were placed in front of the storage area and connected to the pumping station, using an 1-inch PTFE-tube, mantled with metal mesh. Using the pumping station, the master solution was equally distributed, loading each of the three muon veto storage tanks with 1600 l of MS. In order to avoid a possible oxygen contamination during the uploading process, the IBC containers and the storage tanks were constantly purged with nitrogen. All other liquids for the muon veto (i.e. 40.5 m3 of LAB as well as the 44.7 m3 of n-paraffine) and the buffer liquid (i.e. 60 m3 of Ondina-917 as well as the 50 m3 of n-paraffine) were supplied directly by the different companies and delivered by standard delivery trucks. In order facilitate the mixing process and the restricted space in front of the liquid storage area, the delivery trucks were coordinated. At first, the liquids with higher density (LAB, Ondina) were delivered, followed by the lighter n-paraffine. The arriving trucks were parked in front of the LSA, supplied with nitrogen and connected to the corresponding pumping station. Before the unloading of an individual truck started, the transport compartment of each truck was pressurized (up to 1 bar) with nitrogen. This allowed to check the tightness of all connections and provided a higher liquid flow to the membrane pump in the pumping station. The delivered components were equally uploaded (max. 2 m3/h) into the respective storage tanks, strictly separating the scintillating LAB from the other liquids. Figure 6.1 provides a set of pictures, showing the liquid delivery of LAB/n-paraffine/mineral oil with standard delivery trucks and delivery and unloading of the master solution using five IBC-containers. For the purpose of avoiding a possible pollution of the storage tanks with sediments (which might have accumulated in the delivery trucks), the first 50 l of each new truck were disposed. In order to blend the new arriving liquid with the already stored mixture, the uploading process was accompanied by turbulent nitrogen purging. The equal uploading of MS, LAB, n-paraffine and Ondina-917 into the three storage tanks provided similar, but not identical batches in the three storage tanks. In order to blend these three batches to one final batch, the pumping station was used to circulate the scintillator between the three tanks. This circulation was done for the muon veto and for the buffer liquid.

After mixing and thorough blending, the density of the muon veto was fine-tuned by adding either the heavier LAB (0.860 g/cm3) or Ondina-917 (0.854 g/cm3) and the lighter n-paraffine (0.749 g/cm3). For this process, 1 m3 of LAB, 1 m3 of n-paraffine, as well as 1 m3 of Ondina-917 were available, which were set aside in separate IBC-container during the uploading process. This fine tuning process finally led to 90 m3 MU-scintillator with a fluor concentration of 2 g/l 3 3 PPO and 20 mg/l bis/MSB, with a density of ρ=0.804 ± 0.001 g/cm , as well as 110 m buffer 3 liquid with a density of ρ=0.805 ± 0.001 g/cm . In order to monitor the quality of the detector liquids, different liquid samples were taken, one after the mixing process from the different storage tanks in the LSA, and a second one from the intermediate tanks of DFOS in the underground laboratory, after the liquid passed the 160 m trunk line. The results of these measurements showed a successful and clean production of the different detector liquids and a clean transfer to the underground laboratory, what will be presented in chapter 9.

80 Detector Liquid Production

Figure 6.1: Liquid delivery to the LSA: (bottom): ﬁve 1000 l-container (IBC-international bulk container) used for the delivery of 4.8 m3 of master solution (MS). Using the MU-pumping station with a 1-inch PTFE-tube (mantled with metal mesh), the MS was equally distributed between the three muon veto storage tanks. (top left): inside view of the LSA, indicating the 3-inch-standard-hose used to empty the delivery truck. In order to facilitate the unloading process, each delivery truck was pressurized with nitrogen. (top right): one of eight standard delivery trucks used for the delivery of 45.3 m3 LAB, 95.7 m3 n-paraﬃne and 60 m3 Ondina-917.

81 82 Part III

Filling and Handling of the Double Chooz Far Detector

83 Chapter 7

Hardware Installations for the Filling and Handling of the DC far Detector

Apart from the production of the detector liquids, TUM was responsible for the ﬁlling of the DC-far detector and the later handling during data taking. Within this frame, the underground laboratory had to be equipped with various systems: a liquid handling system to handle the detector liquids, a gas handling system to supply the underground laboratory with nitrogen, and a detector monitoring system to supervise the ﬁlling process. As the detector has a complex structure and is, above all, composed of fragile vessels, these systems had to be customized to the needs of the detector and its liquids, what prevented the use of standardized solutions. Due to the various tasks, each system is composed of multiple sub-systems, which are summarized in table 7.1. Installed Hardware in the Underground Laboratory

Systems TUM MPIK DFOS Weighing Tank Liquid Handling XTOS Connections & Tubing HPN-U FPN-U Gas Handling LPN-U LPV-U Liquid Level Measurement Detector Monitoring Gas Pressure Monitoring

Table 7.1: Hardware systems installed by TUM and MPIK in order to ﬁll and handle the Double Chooz far detector. Each of the three main systems is composed of various customized sub-systems, which have been developed, produced and installed by the author in course of the here presented thesis in close collaboration with MPIK.

The development1, production2 and installation of these systems was part of the here presented thesis and the responsibility of the author. The following chapter will be used to introduce the necessary hardware to ﬁll and handle the DC far detector. The liquid handling system will be presented in section 7.1, the gas handling system in section 7.2 and the detector monitoring system will be presented in section 7.3.

1The development of the here presented systems was realized in close collaboration with Dr. Christian Buck and his group from the MPIK in Heidelberg. 2The production of the here presented systems was realized in TUM workshops as well as with the help of various diﬀerent companies, which were instructed and supervised by the author.

84 Hardware Installations for the Filling and Handling of the DC far Detector

7.1 Liquid Handling System

The underground laboratory is equipped with two diﬀerent liquid handling systems. Firstly, the Detector Fluid Operating System (DFOS), which supports all tasks correlated to the ﬁlling, handling or emptying of the DC far detector, and secondly the Expansion Tank Operating System (XTOS), which secures the detector during data taking by increasing the tolerance to thermal variations. Figure 7.1 presents the entire liquid handling chain between the LSA and the DC-far detector, indicating the four individual modules of DFOS and the three expansion tanks of XTOS as well as their connections to the DC-far detector.

Figure 7.1: Global overview of the entire liquid handling chain in the DC-far detector; (top): the liquid storage area (LSA) and the connection of the trunk line system (TLS) to the underground lab; (center): the four individual modules of DFOS for MU, BF, GC and NT, which are similar but not identical; (bottom, right): XTOS and the three expansion tanks used for BF, GC and NT; (bottom, center): stylized picture of the detector and its connections to DFOS, indicating a long and short ﬁlling tube for each vessel and a single connection for XTOS. The used color code equates the previously used one: yellow for the muon veto, orange for the buﬀer, purple for the gamma catcher and red for the target.

7.1.1 Detector Fluid Operating System (DFOS)

The detector ﬂuid operating system is a multi-purpose tool and supposed to realize all tasks correlated to the ﬁlling, handling or emptying of the DC-far detector, beginning with the arrival

85 Hardware Installations for the Filling and Handling of the DC far Detector of liquids in the neutrino laboratory and ending with sending liquid back to the LSA after the experiment. In between, the DFOS realizes all liquid transfers to or from the detector. The DFOS has a modular set up and is composed of four independent liquid handling modules, each providing a similar instrumentation. In order to facilitate the operation of DFOS, all modules are supervised by a programmable logic controller (PLC), which monitors multiple sensor values (pressure, temperature, liquid ﬂow, etc.) and allows to operate pneumatic valves and pumps. This admits to monitor the performance of the system and to automate standard handling processes. The correlated control unit (touch screen) is directly mounted to the muon veto module. Figure 7.2 shows a picture of DFOS, indicating the four modules (1-4), the control unit (5) and some parts of the gas handling system (6-8), which will be explained in section 7.2. The following sections will be used to introduce the instruments and piping-layout used in the diﬀerent modules.

Figure 7.2: Detector Fluid Operating System (DFOS), installed in the underground laboratory of the far detector: 1) target module, 2) gamma catcher module, 3) buﬀer module, 4) muon veto module, 5) PLC-control unit (touch screen) as well as installations correlated to the gas handling system, 6) over-/under-pressure safety box, 7) LPN-Box, 8) LPN-U manifold.

86 Hardware Installations for the Filling and Handling of the DC far Detector

Instrumentation of DFOS Modules

In order to provide all necessary function- and handling-options, each module is equipped with a different instrumentation, which is summarized in table 7.2 and will be introduced shortly in the following. For further information about the different instruments can be found in the appendix see section D.1.1. Instrumentation of the Different DFOS-Modules

Instrument MU BF GC NT Volume 185 l 300 l 100 l 85 l 1. Intermediate Tank Material SS SS SS PVDF Temp.Range +2○/+25○ +2○/+25○ +2○/+25○ – 2. Heat Exchanger Material SS SS SS – Pore Size 0.2 µm 0.2 µm 0.2 µm 0.5 µm 3. Particle Filter Material PE-HD PE-HD PE-HD PVDF Type Pyrus 20 Pyrus 20 Pyrus 20 Maxim 110 4. Membrane Pump Material PTFE PTFE PTFE PTFE Type Promass 83a Promass 83a Promass 83a Siem.M 2100 5. Flow Meter Material SS SS SS SS Volume – 4.8 l 3.6 l 0.2 l 6. Fine Filling Tank Material – PVDF PVDF PVDF Diameter 0.5 inch 0.5 inch 0.5 inch 0.5 inch 7. Tubing Material SS SS SS PFA

Table 7.2: Instrumentation of the diﬀerent DFOS-modules in direct comparison, indicating almost equal systems for MU, BF, and GC made of stainless steel and a special instrumentation for the target module made of ﬂuorinated plastics. These instruments will be shortly presented below. For further information about these instruments see section D.1.1.

Each of the DFOS modules adapts to the individual needs coming either from the vessels or from the liquids. A major difference between the modules originates in the material compatibility of the detector liquids. The target scintillator is, in contrast to the other liquids, incompatible with metals, because of which the target module has to be completely made of fluorinated plastics as PFA, PTFE, FEP or PVDF. Figure 7.3 shows a picture of the target and buffer-module indicating the different instruments presented in table 7.2.

Intermediate Tank (IMT) The intermediate tanks are the centerpiece of each module and used to decouple the LSA from the detector. The size of each IMT (see table 7.2) is chosen in a way that an accidental draining of a full IMT would not endanger the respective detector vessel. The IMTs for MU, BF and GC are made of stainless steel and were designed to tolerate a pressure range between -1 and +3 bar. The vacuum provides the option to remove liquid from the detector without using a liquid pump, which will be necessary for the emptying of the 7 m high detector due to the limited suction lift of the used membrane pumps. The upper limit protects the tank from the hydrostatic pressure between LSA and DFOS (max. 2.5 bar) and gives the option to push liquid back into the LSA. The target IMT is made of PVDF and therefore less pressure-resistant. Each IMT is equipped with diﬀerent monitoring sensors displaying temperature, gas pressure and liquid level. The liquid level is normally visually monitored using a side glass. This side glass is additionally equipped with three capacity sensors, which indicates special levels as full, mid-full and empty.

Heat Exchanger (HE) Thermal control about the detector liquids was a fundamental design request for the DFOS. Large temperatures between the detector liquids could lead to density

87 Hardware Installations for the Filling and Handling of the DC far Detector

Figure 7.3: Picture of two DFOS modules installed in the underground lab of the far detector: design differences between MU, BF and GC modules which are made of stainless steel (left) and the target module which is made of fluorinated plastics (right). The numbers indicate the positions of the different instruments: 1) individually sized intermediate tank, 2) heat exchanger, 3) flow meter, 4) particle filter, 5) pneumatically driven membrane pump, 6) individually sized fine filling tank. differences and therefore to buoyancy forces between the vessels. In order to avoid such stress on the vessels, all liquids should have roughly the same temperature. The thermal control system consists of a heating-cooling-unit situated in front of the lab, which is able to send hot or cold water through a stainless-steel heat-exchanger (HE) mounted on top of the IMTs of MU, BF, GC but not the target. Unlike the other liquids the target scintillator is stored in the underground lab and needs therefore no active thermalization.

Particle Filter (F) The liquids were already filtered during its production process in the LSA but had to pass 160 m of trunk line as well as the DFOS-module, which could be a possible source for a re-contamination. In order to avoid this risk all liquids are filtered a second time in the underground lab before entering the individual IMTs. The used filter cartridges have a nominal pore size of 2 µm for MU, BF and GC and 5 µm for the target.

Membrane Pump (P) The Pyrus 20 [109] is a pneumatically driven membrane pump. It is

88 Hardware Installations for the Filling and Handling of the DC far Detector fully made of PFA and offers a theoretical capacity of 20 l/min but demonstrated not more than 10 l/min when implemented in DFOS. The pump is the anticipated driving mechanism to all liquid transfers as IMT-filling, circulation, detector filling or emptying processes. The pump is connected to the PLC what allows to start the pump automatically or manually by the user. As the direction of membrane pumps can not be reversed, the liquid tubing had to be set up in such way to allow also bi-directional pumping.

Flow Meter (FM) The promass 83a is a coriolis flow meter, which measures the mass flow with an accuracy of 0.25 % of the measured value (0.1 % in case of the target flow meter), the ○ density with an accuracy of ± 0.02 kg/l and the temperature with ± 0.5 C. Together with the PLC, which is opening pneumatic valves and starting the pump, the flow meter allows to dose a preset amount of liquid either into or out of the IMT.

Fine Filling Tanks (FFT) The fine filling tanks are a smaller version of the intermediate tanks and are used to fill the chimney. The size of each FFT (see table 7.2) is chosen in a way that an accidental draining of a full FFT would not lead to a dangerous liquid level increase in the respective chimney. All tanks are made of PVDF and withstand also vacuum, which allows to suck liquid out of the IMT (to fill the FFT) or out of the detector to actively control the liquid level in the chimney.

Programmable Logic Controller (PLC) The programmable logic controller is supposed to support the handling-personnel. The PLC acquires sensor data (temperature, pressure, flow rate, etc.) and a custom-made software displays these values on a touch screen. The touch screen is the main interface and allows to monitor and control the four modules as well as the gas handling system, to set the alarm and critical limits, to monitor filling and to indicate the status of pumps and valve-positions (see figure D.8). If one of the monitored values reaches the alarm limit, the PLC notifies the user by acoustic and visual alarms. If values further exceed and reach critical values, the PLC stops any operation, isolates the detector and secures the system. The PLC supports the IMT-filling mode and assists by automating the three included filling steps (IMT-filling, thermalization, and IMT-emptying), which will be explained in section 7.1.2. In order to avoid an accidental filling in the automated mode, each automated process requires also the setting of manual valves. Apart from these “semi”-automated steps, the PLC offers the possibility to operate every pneumatic valve and pump also manually. This allows to use also non-standard flow-paths if necessary.

More details about the instrumentation of the diﬀerent modules are collected in two separate sections in the appendix. For information on MU, BF and GC see section D.1.1 and for information on the target module, see section D.1.4.

Piping of DFOS Modules

The tube routing and the instruments in each module provide various possibilities to transfer liquid. The four main transfers are: 3 1. LSA ↔ IMT: allows to receive liquid from the LSA, to store and process detector liquids in the underground lab as well as to send4 the liquids in all directions, also back to the storage tanks in the LSA when the detector has to be emptied.

3via pump or by using gravitational pull 4via pump or by pressurizing the IMTs

89 Hardware Installations for the Filling and Handling of the DC far Detector

2. IMT → IMT: allows to circulate liquid within DFOS what provides the possibility to thermalize or re-ﬁlter the liquids before they are ﬁlled into the detector.

3. IMT ↔ Detector: allows to send liquid from the IMTs into the detector. The two filling lines enable to transmit the liquid either to the bottom (long filling tube) or to the top (short filling tube) of the detector. Furthermore DFOS provides three different filling modes5, in order to fill the detector safely and homogeneously. Due to the flexible tube routing, these filling modes can be reversed, what allows to regulate the liquid levels and to empty the detector.

4. Detector → Filter → Detector: in combination with the two detector connections of DFOS (long and short-filling tube), it allows to circulate detector liquids from top to bottom out of the detector and vice versa. This enables to re-filter the detector liquids or to relocate warmer liquids. In order to provide these different functions, the tube-routing within each module has to be flexible and is therefore comprehensive. As all modules have to provide the same functions, the tube routing is roughly the same in all modules with slight exceptions for muon veto and target module. The muon veto module provides no fine filling tank and the target module has no heat exchanger. Furthermore, the target module connects to the 10 m3-weighing tank, which also changes the tube routing. Figure 7.4 presents exemplarily the piping- and instrumentation- diagram (P&ID) of the buffer module, which allows to see the tube routing and the implementation of the different instruments. The piping diagram can be divided in two parts: the main line, which includes the different instruments and the side lines, which are bypasses and allow to reverse processes or to exclude instruments from a flow path. The main line starts at the system connection point (SCP) and aligns the instruments in the following order: membrane pump6 (P), particle filter (F), flow meter6 (FM), heat exchanger6 (HE) and intermediate tank6 (IMT). In figure 7.4 the main line is indicated in red, the bypasses are presented in black and the different instruments are indicated with letters. The main-line and side-lines include various valves, what allows to set the individual flow path through the module. The mixture of pneumatic-6 and manual-valves avoids an accidental filling, as all liquid handling processes require the setting of manual valves. Those flow paths that are required for the different liquid handling tasks are summarized in table 7.3. For an explanation how to read the flow patterns in table 7.3, see section D.1.2. Apart from main- and side-lines, which connect to the intermediate tank, each module provides two connections to the detector. A long filling tube, which runs straight to the bottom of the detector, and a short filling tube, which enters 20 cm below the final liquid level. Shortly before the filling lines enter the detector, a valve station provides the last isolation valves ahead of the detector. This valve station enables to cross-connect both filling lines, what allows to clean the tubes prior to filling. A more detailed description about the valve station, filling tubes and laboratory connections can be found in the appendix, see section D.1.6.

5IMT-filling mode, continuous mode and fine filling mode, see section 7.1.2 6can be monitored and/or regulated by the PLC

90 Hardware Installations for the Filling and Handling of the DC far Detector FFT short filling tube IMT

D E T E C T O R T E C T D E HE HE long filling tube FM F P SCP

Figure 7.4: P&ID-Scheme of BF-DFOS. The tube routing can be divided into a main line shown in red, which includes the different instruments, and various bypasses shown in black, which offer a variety of alternative flow paths. The P&IDs of the other modules can be found in the appendix: presenting MU-DFOS in figure D.5, the GC-DFOS in figure D.6 and NT-DFOS in figure D.7.

91 Hardware Installations for the Filling and Handling of the DC far Detector

DFOS Modules Flow Pattern Table

IMT Filling Mode

Task Option Detail Liquid Flow Path with F SEP, 0, 2, 3, 6, 7, F, 8, FM, 14, HE, IMT by Gravity without F SEP, 0, 2, 3, 11,FM, 14, HE, IMT IMT Filling with F SEP, 0, 2, 4, P, 5, 7, F, 8, FM, 14, HE, IMT by Pump without F SEP, 0, 2, 4, P, 5, 9, FM, 14, HE, IMT with F+FM IMT, 15,19,13,1,2,4,P,5,7,F,8,FM,14,HE,IMT Way A Circulation no F+FM IMT, 15,19,13,1,2,4,P,5,6,11,14,HE,IMT from/to IMT with F IMT,15,18,17,3,4,P,5,7,F,8,FM,14,HE,IMT Way B without F IMT,15,18,17,3,4,P,5,9,FM,14,HE,IMT with F IMT,15,18,17,6,7,F,8,FM,12,13,21,23,58,Det Gravity without F IMT,15,19,21,23,58,Det Detector Filling with F IMT,15,18,17,3,4,P,5,7,F,8,FM,12,13,21,23,58,Det.L by Pump without F IMT,15,18,17,3,4,P,5,9,FM,12,13,21,23,58,Det.L

Continuous Filling Mode Filling

Task Option Detail Liquid Flow Path with F SEP,0,2,3,6,7,F,8,FM,12,13,21,23,58,Det.L Gravity without F SEP, 0,2,3,6,9,FM,12,13,21,23,58,Det.L Detector Filling without F/FM SEP,0,1,13,21,23,58,Det.L with F SEP,0,2,4,P,5,7,F,8,FM,12,13,21,23,58,Det.L Gravity without F SEP,0,2,4,P,5,9,FM,12,13,21,23,58,Det.L

Fine Filling Mode Filling

Task Option Detail Liquid Flow Path FFT Filling Gravity/HPN IMT,15,19,56,FFT Chimney Filling Gravity/HPN FFT,56,21,23,58,Det.L

Circulation and Sampling

Task Option Detail Liquid Flow Path Top to with F Det.S,59,24,17,3,4,P,5,7,F,8,FM,12,13,21,23,58,Det.L Circulation bottom without F Det.S,59,24,17,3,4,P,5,9,FM,12,13,21,23,58,Det.L from/to Detector Bottom to with F Det.L,58,23,21,13,1,2,4,P,5,7,F,8,FM,11,17,22,24,59,Det.S Top without F Det.L,58,23,21,13,1,2,4,P,5,9,FM,11,17,22,24,59,Det.S at IMT IMT,15,54 IMT Sampling Sample I IMT,15,18,22,60 by HPN or Draining Sample II IMT,15,19,21,57 from detector IMT,15,19,21,23,62

Detector Emptying

Task Option Detail Liquid Flow Path long ﬁll.tube Det.L,58,23, 21,19,18,IMT by Vacuum short ﬁll.tube Det.S,59,24,22,16,IMT IMT Filling with F Det.L,58,23,21,13,1,2,4,P,5,7,F,8,FM,14,HE,IMT by Pump without F Det.L,58,23,21,13,1,2,4,P,5,9,FM,14,HE,IMT with F IMT,15,18,17,6,7,F,8,FM,12,1,0,SEP by HPN without F IMT,15,19,13,1,0,SEP IMT Emptying with F IMT,15,18,17,3,4,P,5,7,F,8,FM,12,1,0,SEP by Pump without F IMT,15,18,17,3,4,P,5,9,FM,12,1,0,SEP

Table 7.3: The table presents standard operations and separates between different options (with/without filter or pump). The flow pattern is described in the last column and presented by a set of numbers. The given numbers in the liquid flow path are the valve identification numbers (VX.03=3), as to find in the P&ID-schemes for each module. For further information about the flow path description see section D.1.2 in the appendix. Abbreviations: P=pump, F=filter, FM=flow meter, SCP=system connection point, Det.L=detector long filling tube, Det.S=detector short filling tube, IMT=intermediate tank, HE=heat exchanger.

92 Hardware Installations for the Filling and Handling of the DC far Detector

7.1.2 DFOS Main Operation Modes

Detector Filling

An equal and homogeneous increase of all liquid levels necessitates the adjustment of the liquid flow over a wide dynamic range, from large flows in the main body of the detector and very small flows in the different chimneys. Each DFOS-module therefore provides three different filling modes with different dynamic ranges and sensitivities. From high to low flows, these modes are Continuous Filling IMT-Filling Fine Filling. While Continuous- and IMT-filling mode are only used in the main-bodies of the detector, fine filling is exclusively used to regulate the liquid levels in the different chimneys. These three modes and their working principle are shortly summarized in the following paragraphs and can be found in more detail in chapter E in the appendix. 1. Continuous Filling Mode (CM): Continuous filling is the quickest way to fill the detector. The flow path excludes the IMT and provides a continuous flow directly into the different detector vessels. In this mode, DFOS is predominantly passive as it only filters the liquid (0.5 µm) and measures the flow rate. The only active roll is the regulation of the different liquid flows in order to realize a homogeneous increase in all four vessels. The liquid flows can be regulated by a manual membrane valve (Vx.23) and are monitored by the flow meter. This mode provides a flow rate of 8 - 9 l/min (only driven by gravity) and allows to increase the liquid level in the detector by about 3.3 cm per hour. Although it is the most efficient way of filling, it is only used during non-critical filling phases where the surface area of the detector vessels is not changing. A disadvantage of this mode is the inability to thermalize the arriving liquid before it enters the detector. Figure E.3 in the appendix presents a technical drawing of DFOS with different liquid paths superimposed. 2. IMT-Filling Mode (IMT): IMT filling is the anticipated standard filling mode. It uses the IMTs to decouple the liquid flow from the LSA to the detector. The liquid enters the IMTs over the main-line. Once the IMT is full, the liquid can be probed, thermalized, re-filtered, or event sent back to the LSA if necessary. When the liquids in the IMT are thermalized, the IMT is emptied into the detector (by pump, gravity or pressure). The volume of each IMT has been individually chosen to increase the liquid level by not more than 2 cm in the corresponding volume. Using this mode, the detector is filled in a batch-mode with an IMT as filling increment. The simultaneous filling of all detector volumes requires therefore the constant iteration of IMT-filling, thermalization and IMT- emptying parallel in all modules. These three steps are automated by the PLC. Including the thermalization-step, this filling mode allows to fill about 2 cm/h. This mode has been used only during the beginning of the filling and at critical points, where CM-filling would have been too dangerous. An illustration of the different liquid flow patterns of IMT-filling, thermalization and detector filling can be found in figure E.2 in the appendix. 3. Fine Filling Mode (FF): This mode is exclusively used in the chimney, where already the addition of a small volume increases the liquid level significantly and thus the pressure on the whole vessel. This mode uses the IMTs as liquid storage that supplies the smaller fine-filling-tanks. These small PVDF-tanks can be pressurized or evacuated, what allows to insert or extract liquid from the chimney. Figure E.4 presents the flow pattern used during the fine filling mode.

93 Hardware Installations for the Filling and Handling of the DC far Detector

Detector Circulation

Apart from the filling, DFOS provides the possibility to recirculate the different detector liquids. Each module is therefore equipped with two filling lines. A long filling tube that runs straight to the bottom of the detector and a short filling tube that enters at a different point and ends already 20 cm below the final liquid level. These lines allow to circulate the detector liquids from bottom-to-top or vice versa. This circulation can in- or exclude certain instruments like the particle filter or heat exchanger. Furthermore the system allows to in- or exclude the IMT and to circulate liquid directly out of the detector.

Detector Emptying

After data taking, DFOS is supposed to remove liquids from the detector. All modules are therefore able to reverse the three filling scenarios. Normally, this is done by pumping using a different flow path. Due to the depth of the detector and the limited suction lift of the used membrane pump, it is also possible to empty the detector without pump. In this scenario, the IMT-mode is reversed using under-pressure to fill the IMT and over pressure to empty it. The pressure resistance of each7 IMT has been chosen from vacuum to +3 bar, which allows to retrieve liquid from the detector and push this liquid back into the LSA.

7.1.3 Expansion Tank Operating System (XTOS)

The design of the detector vessels, a small and long chimney attached to a big main body, is optimized for physics, however, at the expense of technical aspects. Once the liquid is inside the chimney, such a design is very vulnerable to thermal expansion, as the volume in the vessels can only expand into the chimney. The small cross-section of the chimney, however, leads to a significant increase of the liquid level in the respective chimney and therefore to a significant increase of the hydrostatic pressure in this vessels. In the case of Double Chooz, thermal expansion could lead to large liquid level differences between the different vessels and cause therefore significant pressure differences which can quickly lead to a fracturing of the acrylic vessels. The thermal expansion ∆Vt and the related liquid level increase ∆L in the chimneys can be calculated via ∆V ∆V V γ ∆T and ∆L t t = 0 ⋅ = A where V0 is the total expanding volume, γ the thermal expansion coefficient, ∆T the thermal variation and A the respective chimney surface. Using the expansion coefficient of mineral oil, −4 −1 γoil = 7.6⋅10 K [110], and a thermal variation of 1 K, the expansion leads to a volume increase in the different detector vessels as summarized in table 7.4. The target vessel for instance has a volume of 10 m3, offers, however, only a cross-section of 176 cm2 in the chimney. The expansion, correlated to a thermal change of 1 K, would push 7.8 l into the target chimney and induce a level increase of 45 cm where only 3 cm are considered to be safe. Due to this extreme sensitivity of the target to volumetric changes is the submersion of calibration tools highly critic and can pose a threat to the detector. Table 7.4 summarizes the situation for all detector vessels and presents the total volume (V0) stored in the detector vessels, the expansion volumes (∆Vt) for a thermal increase of 1 K, the available surface area (A) in the chimneys, the expected level change for thermal variation of 1 K, as well as the thermal variation (∆t) necessary to increase the liquid level up to the critical level of 3 cm. As can be seen in the mentioned table, the liquid levels in the chimneys react very sensitive to thermal variations. The target, for example, reaches the

7This is only true for MU, BF and GC, the target IMT is with 0-400 mbar less pressure resistant.

94 Hardware Installations for the Filling and Handling of the DC far Detector

Figure 7.5: Drawing of the expansion tank operating system (XTOS; not true to scale): In case of the MU, there is no chimney, meaning the chimney surface of the MU is the same than the detector surface. (top): Detector top view comparing the available cross-section in the detector with the cross section available in the XTOS system; (bottom): Detector side view indicating the connection between the detector-chimneys and the XTOS-tanks, realized by 3/4-inch tubes with a steady slope.

3 cm-mark already for a thermal variation of 0.07 K. Considering this sensitivity and the normal thermal variation in the detector induced by seasonal changes (∆t ∼ 0.9 K, see ﬁgure 10.5), it is clear that a technical intervention is unavoidable and mandatory to ensure the safety of the detector. The expansion tank operating system (XTOS) increases the tolerance of thermal or mechanical8 induced volume changes in the detector. Figure 7.5 presents a top- and side-view of the detector and XTOS, indicating the available surface in the detector chimneys and in the XTOS-tanks at the ﬁnal liquid level.

XTOS is composed of three separate tanks, each directly connected to the correlated chimney at the height of the ﬁnal liquid level. The connection is realized by three 3/4-inch tubes, which steadily ascend from the chimney to the bottom of each XTOS-tank. This slope allows gravity to adjust to a rising as well as a falling of liquid levels autonomously. All tanks are 15 cm high, have a cuboid form and provide an ambient environment for the detector liquids using only

8volume changes due to the submersion of tools

95 Hardware Installations for the Filling and Handling of the DC far Detector

Thermal Expansion in the Chimney with and without XTOS

Unit MU BF GC NT 3 Detector Volumes V0 m 90 100 22.5 10.3 Expansion ∆VT l 68 76 17 7.6 XTOS Volumes Vx l – 550 162 135 Chimney Surface without XTOS A l/cm (330) 1.6 0.9 0.17 Chimney Surface with XTOS A+ l/cm – 33.6 10.8 9.0 Calculated Increase without XTOS ∆h/○K cm 0.2 47.5 19 45 Calculated Increase with XTOS ∆h/○K cm 0.2 2.0 1.6 0.8 ○ Thermal Variation without XTOS ∆t/3 cm K ±15 ±0.06 ±0.15 ±0.07 ○ Thermal Variation with XTOS ∆t/3 cm K ±15 ±1.5 ±1.9 ±3.5

Table 7.4: The table summarizes the situation for all detector vessels and presents the total volumes (V0) stored in the detector vessels, the expansion volumes (∆VT ) for a thermal increase of 1 K and a thermal −4 1 expansion coefficient of γoil = 7.6 ⋅ 10 K , the total volumes available in the tanks of XTOS (Vx), the available surface area in the chimneys with (A) and without (A+) XTOS, the expected level change in the detector for thermal variation of 1○K with and without XTOS, as well as the thermal variation (∆t) necessary to increase the liquid level by 3 cm with and without XTOS (in case of the MU, there is no chimney, meaning the chimney surface of the MU mentioned in the table is the same than the detector surface). The here presented values correspond to a homogenous thermal variation of the entire detector liquid compatible materials9 and a permanent nitrogen blanket. In addition, all tanks are equipped with liquid-level- and gas-pressure-sensors. The expansion tanks are installed in a pit next to the detector and are mounted in a way that the final liquid level reaches to the mid-level of the XTOS-tanks. Using the extra surface of the XTOS-tanks the tolerance to volume changes increases, as can be seen in table 7.4. Apart from the in general reduced increase per degree, the different sizes of the individual XTOS-tanks lead to a more evenly increase (or decrease) within homogeneous thermal variations (compared to equal sized expansion tanks). That reduces the occurrence of liquid- level differences due to thermal variations and therefore additionally increases the tolerance to thermal variations. Table 7.4 summarizes the improved situation for all detector vessels and presents the total volume (V0) stored in the detector vessels, the expansion volumes (∆Vt) for a thermal increase of 1 K, the available surface area (Ac) in the chimneys, the available surface area (Ax) in XTOS, the expected level change for thermal variation of 1 K, as well as the thermal variation (∆t) necessary to increase the liquid level by 3 cm.

Due to the fixed capacity (±7 cm) of the tanks, the function of XTOS is limited. The buffer- XTOS-tank for instance has a capacity of ±275 l and allows to compensate a variation of ±3.6 K before it would overflow or run empty. The other tanks can tolerate variations up to +4.7 ○K in GC and +8.5 K in NT. Thus, XTOS increases the acceptable thermal variation in the detector from 0.07 K up to 1.5 ○K and even higher if one considers that a single volume will never heat or cool without affecting the other liquids. Figure 7.6 provides some pictures of the expansion tanks during the installation phase, further details about the XTOS tanks can be found in section D.1.7. An important consequence of the rather flat tanks is that XTOS ensures the safety of the detector only ±7 cm around the final liquid level. The critical chimney-filling phase, however, starts long before XTOS increases the tolerance of the detector. Thus, the entire chimney filling phase is particularly dangerous as the liquid can only expand into the chimney.

9stainless steel in case of BF and GC and PVDF in case of the target tank

96 Hardware Installations for the Filling and Handling of the DC far Detector

Figure 7.6: Pictures of XTOS installation: (top left): top view of the open XTOS-pit with the XTOS- tanks during installation; (top right): front view of the GC-XTOS-tank and the mounted detector connection as well as the side-glass used to monitor the liquid level manually; (bottom left): detector connections of XTOS; (bottom right): top-ﬂange of all XTOS tanks indicating the diﬀerent connections for LPN, LPV and sensor connections.

97 Hardware Installations for the Filling and Handling of the DC far Detector

7.2 Gas Handling System

In order to handle the detector liquids under a permanent nitrogen atmosphere, the underground laboratory is equipped with a comprehensive gas handling system, presented in ﬁgure 7.7. It can be divided into three main parts:

1. the Nitrogen Supply Systems, which provide N2 with diﬀerent pressure levels for the various consumers,

2. the Consumers, which utilize N2 with the diﬀerent pressure levels for liquid handling, ﬂushing and blanketing, 3. the Ventilation Systems, which collect and purify the nitrogen before it is extracted from the underground lab.

Figure 7.7: Overview of the entire gas handling chain of the far-detector-laboratory; (top): presents in red the N2 trunk line between the LSA and the underground lab; (yellow-part): summarizes the diﬀerent N2-supply systems in the underground lab (HPN-U (dark blue), FPN-U (mid-blue) and LPN-U (light blue)); (purple-part): shows the diﬀerent consumers (WT, PLC, DFOS, GB, Detector, XTOS) with the established color code for the detector liquids; (green-part): presents the ventilation system of the underground lab, which was used to collect and purify nitrogen from the detector (light green) and from the auxiliary systems (dark green).

98 Hardware Installations for the Filling and Handling of the DC far Detector

7.2.1 Nitrogen Supply System

The nitrogen supply system consists of three different sub-systems each providing a different pressure range: 1. the High Pressure Nitrogen Underground (HPN-U), which provides a control pressure of 4 bar to the liquid handling system, that allows to run pumps or to actuate pneumatic valves. 2. the Flushing Pressure Nitrogen Underground (FPN-U), which provides a nominal pressure of 150 mbar to the DFOS-IMTs in order to flush the detector vessels prior to the filling process. 3. the Low Pressure Nitrogen Underground (LPN-U), which provides a nominal pressure of 3 mbar to the detector and all other parts of the liquid handling system in order to maintain a permanent nitrogen blanket above the detector liquids. In the following these systems will be presented in more detail. Table 7.5 summarizes the different systems and their pressure levels as well as the color code used for the individual systems. An overview of the different supply systems and their technical realization is presented in figure 7.9, where the correlated piping and instrumentation diagram is also shown. Nitrogen Supply Systems in the DC-Far Lab

Abbr. Systems Pressure Range Color Code

TLS-N2 N2-Trunk Line System 0–8500 mbar red HPN-U High Pressure Nitrogen Underground 0–5500 mbar dark blue FPN-U Flushing Pressure Nitrogen Underground 0-200 mbar mid blue LPN-U Low Pressure Nitrogen Underground 3-9 mbar light blue

Table 7.5: Summary of N2-supply systems in the underground laboratory; Indicated are the names and pressure ranges of the individual systems as well as the used color code.

Figure 7.8: Picture of the nitrogen supply system in the underground laboratory; (left): Two gas handling stations, left one holding the HPN- and FPN-manifolds, right one the LPN-manifold; (right): Detail of the HPN and FPN-manifold, indicating single components as the pressure reducer (PR), the main isolation valves (MV), the flow meters (FM), the pressure indicators (M, PI) as well as the different connection points of the manifold (CP). A detailed presentation of the individual components of HPN-, FPN- and LPN-manifold can be found in the piping- and instrumentation-diagram in figure 7.9.

99 Hardware Installations for the Filling and Handling of the DC far Detector

Figure 7.9: Piping- and instrumentation-diagram (P&ID) of the nitrogen distribution system in the underground laboratory: the nitrogen supply by the trunk line system (red) and the three diﬀerent supply manifolds of HPN-U (black), FPN-U (blue) and LPN-U (light blue). Open valves are indicated in green, closed valves indicated in red. The presented valve positions indicate the nitrogen supply situation in the detector during ﬁlling. Plot from [111].

100 Hardware Installations for the Filling and Handling of the DC far Detector

High Pressure Nitrogen - Underground (HPN-U)

The HPN-manifold is supplied by the trunk line system and reduces the arriving 8 bar to nominal 4 bar. The HPN-pressure is mainly used as control pressure for the PLC, which uses the high pressure to actuate pneumatic valves and pumps within DFOS. Apart from that, the HPN- manifold supplies the FPN- and LPN-manifolds with nitrogen as those pressure-reducers would not tolerate an inlet pressure of 8 bar. In order to secure the HPN-manifold and all following systems, the HPN-manifold has two independent safety installations. The first one is a manual pressure-relieve-valve (tripping above 5.5 bar) and the second one an electronic pressure sensor that communicates with the DFOS-PLC. If the HPN-pressure exceeds the nominal value, the PLC notifies the user by visual- and audio-alarm on the touch panel (PLC-control unit). If the pressure is beyond that exceeding a critical limit, the PLC isolates the detector (automatically within 300 ms) by closing the pneumatic main valve (MV, V017) of the LPN-system, which supplies the detector. Another mean of protection is the emergency nitrogen supply unit (introduced in section 4.4), which provides the HPN-manifold with nitrogen in case that the normal supply from the liquid nitrogen plant is interrupted. This 50 l N2-bottle allows to supply the far-lab, and thus the detector blanket, independently for 14 hours (for a nominal N2-flux of 0.7 m3/h during data taking).

The HPN-manifold is composed of a main valve (MV), a pressure reducer (PR), pressure indicators and manometers (PI, M) as well as various valve regulated connection points (CP), which are used to distribute the nitrogen to the diﬀerent sub-systems. A detailed overview of the HPN-manifold, its instrumentation, connection points and supplied systems is presented in table 7.6. Additionally, the HPN-manifold is depicted in dark blue in ﬁgure 7.9, which shows a piping- and instrumentation-diagram of the entire nitrogen supply system. High Pressure Nitrogen Manifold

No. Abbr. Instrumentation ID Valve 1. MV Main Isolation Valve V000 yes 2. PG Pressure Gauge 8 → 4bar EP1.401 no 3. PI Pressure Indicator EP1.402 no 4. CP 5 Connection Points V00X yes

No. Abbr. Connection Points ID Valve 1. SV Over Pressure Safety Valve V003 no 2. GB Glove Box V004 yes 3. SP Spare Ball Valve V002 yes 4. PLC Programmable Logic Controller V006 yes 5. SP Spare Membrane Valve V005 yes

No. Abbr. Supplied Systems 1. FPN-U Flushing Pressure Manifold 2. LPN-U Low Pressure Manifold 3. GB Glove Box 4. PLC Programmable Logic Controller

Table 7.6: Technical details of the HPN-U manifold, indicating the instrumentation of the manifold, its connection points as well as the supplied sub-systems. In addition, the table summarizes the abbreviations and valve identification numbers (ID) for all parts of the manifold as they can be found in figure 7.9, which shows a P&ID of the supply systems and in figure 7.8, which shows some pictures of the supply systems during the installation phase.

101 Hardware Installations for the Filling and Handling of the DC far Detector

Flushing Pressure Nitrogen - Underground, FPN-U

The FPN-manifold is supplied with the HPN and reduces the arriving 4 bar to nominal 150 mbar. Before any detector liquid can be transfered to the underground lab, all systems have to be dry and free of oxygen. This flushing process is realized with the FPN-system. It provides a sufficient nitrogen flow (0-7 m3/h) to flush the detector vessels in due time (about 3 weeks). In order to do so, the FPN-manifold is equipped with a main valve (MV), a pressure reducer (PR), a pressure indicator (PI), a manometer (M) as well as various valve regulated connection points (CP), which are used to send a nitrogen flow through the weighing tank and to DFOS. In order to secure the FPN-System and to avoid over-pressure in the detector, the manifold has an overpressure-safety- valve (SV), which ventilates pressures above 200 mbar into the laboratory. The FPN-system is not directly connected to the detector but to the different intermediate tanks of DFOS and uses the long filling line to send nitrogen directly to the bottom of each detector vessel. Then the nitrogen is removed by default over the nitrogen exhaust lines at the top of the detector (LPV-U system). This detour (using DFOS) has two advantages: firstly, it increases the flushing efficiency and secondly, it flushes the entire liquid handling system (DFOS+detector+LPV-U) prior to the filling process. A detailed overview of the FPN-manifold, its instrumentation, connection points and supplied systems is presented in table 7.7. The FPN-manifold is additionally depicted in mid-blue in figure 7.9, as well as in figure 7.8, which shows a picture of the FPN-manifold during the installation. The flushing of the big and fragile acrylic vessels is a very critical process. It requires high flow rates to minimize the flushing time and thus higher pressures, however differential pressures between the vessels must be avoided. The high gas flow produces an overpressure in different volumes. Differential pressures occur, when the differently sized vessels adjust on different time scales to the new pressure situation. During this re-adjustment, the vessels are subdued to differential pressures. Thus, flushing has to be prudently regulated and thoroughly monitored to avoid dangerous pressure differences, what turned out to have worked very well. FPN-U Manifold

No. Abbr. Instrumentation Connection Valve 1. MV Main Isolation Valve V010 yes 2. FM Main Flow Meter EF202 no 2. PR Pressure reducer 4 bar → 150 mbar V011 no 4. M Manometer EP1.403 no 5. CP 6 Connection Points V00X yes

No. Abbr. Connection Points Connection Flow Meter 1. SV Over pressure safety valve V030 no 2. MU-IMT MU-Intermediate Tank V031 yes 3. BF-IMT BF-Intermediate Tank V032 yes 4. GC-IMT GC-Intermediate Tank V033 yes 5. NT-IMT NT-Intermediate Tank V034 yes 6. WT Weighing Tank V035 no

No. Abbr. Supplied Systems 1. DFOS DFOS-Intermediate Tanks 2. WT Weighing Tank

Table 7.7: Technical details of the FPN-U manifold, indicating the instrumentation of the manifold its connection points as well as the supplied sub-systems. In addition, the table summarizes the abbreviations and valve identification numbers (ID) for all parts of the manifold as they can be found in figure 7.9, which show a P&ID and figure 7.8, which shows some pictures of the supply systems during the installation phase.

102 Hardware Installations for the Filling and Handling of the DC far Detector

Low Pressure Nitrogen - Underground, LPN-U

The LPN-manifold is supplied by the HPN-system and reduces the arriving 4 bar to nominal 3 mbar. The LPN-pressure is used to provide a low pressure nitrogen blanket for the detector and all other parts of the liquid handling system. In order to do so, the LPN-manifold supplies a homogeneous and one-directional nitrogen flow through the detector-system (LPN → Consumers → LPV). The LPV-system allocates an adjustable resistance to the gas flow, leading to an adjustable back-pressure and therefore to a LPN-blanket in the detector. The LPN-U is the only N2-supply-system that is directly connected to the detector. This has two implications: firstly, the N2-supply of the detector depends on the reliability of the LPN-system and secondly, all vessels are completely subdued to the capability of the LPN-system to provide a homogeneous and stable low-pressure-blanket. Consequently, the detector depends on a reliable function of this system and is quickly endangered by any instability of malfunctions of the LPN-manifold. For instance, the loss of nitrogen supply could lead to under pressure in the hermetically closed detector. Low Pressure Nitrogen Manifold

No. Abbr. Instrumentation ID Valve 1. MV Main Isolation Valve V013 yes 2. FM Main Flow Meter EF201 no 3. PR Pressure Reducer 4 bar → 3-9 mbar V014 no 4. M Manometer (0-16 mbar) EP404 no 5. PI Pressure indicator EP1.404 no 6. SV Over Pressure Safety Valve V0016 yes 7. PV LPN Main Valve (pneu.) V017 yes 8. CP 12 Connection Points V00X yes

No. Abbr. Connection Points ID Valve 1. SV Oil-bubbler in LPN-Box V016 yes 2. MU-IMT MU-Intermediate Tank V019 yes 3. BF-IMT BF-Intermediate Tank V020 yes 4. GC-IMT GC-Intermediate Tank V021 yes 5. NT-IMT NT-Intermediate Tank V022 yes 6. GB Glove Box V023 yes 7. WT Weighing Tank V024 yes 8. SP Spare Valve V025 yes 9. XTOS Expansion Tank Operating System V026 yes 10. DET Detector LPN-Distributor V027 yes 11. SV Over- and Under-Pressure Safety Box V028 yes 12. DET Detector LPN-Distributor V029 yes

No. Abbr. Supplied System 1. DET Detector 2. DFOS Detector Fluid Operating System 3. XTOS Expansion Tank Operating System 4. GB Glove Box 5. WT Weighing Tank 6. OPSB Over pressure Safety Boxes

Table 7.8: Technical details of the LPN-U manifold, indicating the instrumentation of the manifold, its connection points as well as the supplied sub-systems. In addition, the table summarizes the abbreviations and valve identification numbers (ID) for all parts of the manifold as they can be found in figure 7.9, which shows a P&ID, and figure 7.8, which shows some pictures of the supply systems during the installation phase.

In order to protect the detector, the LPN-manifold is equipped with several means of protection: the ﬁrst one is an oil-bubbler that works as pressure-relieve-valve. The bubbler is mounted in

103 Hardware Installations for the Filling and Handling of the DC far Detector the LPN-Box (see figure 7.2 and figure 7.11) and allows to set an upper limit for the pressure in the LPN-manifold. In case of a sudden pressure increase in the manifold, this oil-bubbler would absorb the pressure shock and ventilate the gas away from the detector. The second means of protection comes from an electronic pressure indicator (PI), which monitors the pressure in the LPN-system. As soon as the pressure exceeds 5 mbar, the PI triggers the PLC to isolate the detector from the LPN-system by shutting the pneumatic LPN-supply-valve (V017). If this isolation has to be maintained over a longer time period, the detector is endangered again by changes in the atmospheric pressure. For those cases, the detector is protected by a third means of protection, which allows the detector to breathe lab-air before the vessels are harmed of under- or over-pressure. This protection is realized with an individual box (over- & under-pressure-safety-box), which contains four oil-bubblers. The bubblers are connected in a way that two of them allow to vent detector-pressures above 7 mbar, while the other two allow to suck lab-air into the detector before the detector reaches a negative pressure of -2.5 mbar. This last safety system ensures the mechanical integrity of the detector and accepts a contamination with oxygen. In those cases, the detector could be recovered by purging the detector liquids with nitrogen using the long filling lines. This box is indicated in figure 7.2 and illustrated in more detail in figure D.17. The LPN-manifold is composed of a main valve (MV), a flow meter (FM), a pressure reducer (PR), pressure indicators and manometers (PI, M) as well as a pneumatic main valve and various valve regulated connection points (CP), which supply the detector and all other systems with a homogeneous low-pressure-nitrogen-blanket. A detailed overview of the manifold, its instrumentation, connection points and supplied systems is presented in table 7.8. Additionally, the LPN-manifold is depicted in light blue in figure 7.9, which shows a piping- and instrumentation- diagram of the entire nitrogen supply system.

7.2.2 Consumers

As already shown in ﬁgure 7.7, the supply systems provide diﬀerent nitrogen consumers in the underground laboratory. These consumers are summarized in table 7.9 and will be shortly introduced in the following section. Nitrogen Consumer in the DC-Far Lab

No. Abbr. System Supplied with Supplied for 1. PLC Programmable Logic Controller HPN pneu. pumps & valves 2. GB Glove Box HPN, LPN blanketing, flushing 3. DFOS Detector Fluid Operating System FPN, LPN blanketing, flushing 4. XTOS Expansion Tank Operating System LPN blanketing 5. DET Detector LPN blanketing 6. WT Weighing Tank FPN, LPN blanketing, flushing

Table 7.9: Summary of nitrogen consumers in the underground, indicating the name and abbreviation of the supplied systems as well as the provided function.

1. PLC: The programmable logic controller is supplied by a separate 1/2-inch stainless-steel (ss) tube, which ends in a separate gas distribution system. The supplied HPN is used as control pressure to actuate the pneumatically driven valves and pumps in DFOS. 2. GB: The glove box is supplied with two separate lines: a HPN-line (1/2-inch, ss) and a bigger LPN-line (3/4-inch, ss). Both lines connect an internal gas distribution system, which is part of the glove box. The LPN-line provides the nitrogen blanket, while the

104 Hardware Installations for the Filling and Handling of the DC far Detector

HPN-pressure is reduced and used for flushing. The GB has in addition a ventilation line (LPV-line, 1-inch, PFA), which allows to extract the nitrogen from the GB and send it into the ventilation system (LPV). 3. DFOS: Each intermediate tank within DFOS is equipped with two supply lines, a LPN- line (1/2-inch) and a FPN-line (1/2-inch), and, in addition, a bigger ventilation line (LPV- line, 3/4-inch, ss). Each of these lines can be isolated at the top of each tank (see figure D.14), what allows to apply various pressure situations in the different IMTs. This includes a steady or a flowing LPN-blanket as well as over-pressure oder under pressure situations, which allow to fill or empty the tanks without pumping. 4. XTOS: XTOS is supplied by a single LPN-line (3/4-inch, ss), which runs from the gas handling system to the XTOS-pit. In the pit, the supply line splits up into three equal lines (3/4-inch, PFA), each connecting to an individual XTOS-tank. In addition, each tank is equipped with an individual exhaust line (1-inch, PFA), which connects to the exhaust line of the correlated detector vessel. This merged nitrogen flow is then led to the LPV-system, where an adjustable impedance leads to a common back pressure. This merging ensures that the XTOS-tanks and the correlated detector-vessels have exactly the same blanket pressure. Any differential pressures between the detector and XTOS would lead to a displacement of detector liquids, inducing liquid level differences between the detector vessels. 5. Detector: The detector is supplied by two separate LPN-lines. Both LPN-lines (3/4- inch, ss; 3/4-inch, PFA) run from the LPN-manifold to the detector, where they merge at a small distribution piece compsed of a small cylinder with two inlets (3/4-inch), a manometer (0-15 mbar) and four outlets (1/2-inch). This common volume distributes the equalized pressure evenly to the different detector vessels. A picture of the LPN-distributor is presented in figure D.15. Each detector vessel is supplied by a single LPN-line (1/2-inch, ss). This individual supply guarantees a separation of the different gas phases. The nitrogen is then pushed through the detector-vessels and expelled by a bigger LPV-line (3/4-inch, ss and PFA for the target). The nitrogen flow through each detector-vessel is supposed to be one-directional because the out-bound nitrogen should not flow back into the detector. This is ensured by the LPV-system, that provides the back-flow protections to each nitrogen flow individually, as well as a common impedance (oil-bubbler), which also prevents a back flow of gas into the detector. This oil bubbler also allocates a common impedance to the out-flowing nitrogen and therefore produces equal back pressures in all four detector vessels. That allows to have equal, but still individual nitrogen blankets in the detector. A picture of the detector connections can be found in the appendix, see figure D.12. Illustrating pictures of the LPV-system can be found in the next section, see figures 7.12 and 7.13. 6. Weighing Tank: The weighing tank is equipped with two supply lines, a LPN-line (1/2-inch, PFA) and a FPN-line (1/2-inch, PFA). The weighing tank stores the target- scintillator in the neutrino lab prior and during the detector filling. It was used to measure the target-mass in order to determine the number of protons in the target. Before the target scintillator was transfered to the underground lab, the weighing tank was thoroughly flushed and subsequently supplied with a permanent LPN-blanket. In order to be used for the near detector, the weighing tank was isolated and removed from the lab after the filling process was finished.

105 Hardware Installations for the Filling and Handling of the DC far Detector

7.2.3 Ventilation System

As can be seen in ﬁgure 7.10, the LPV-system in the underground laboratory is divided into two separate ventilation systems: the ventilation system of the detector (Detector-LPV, light green) and the ventilation system of the auxiliary systems (DFOS-LPV, dark green). Both systems have the same purpose: collecting and purifying the used nitrogen before it is handed to the air-condition-system and extracted from the underground lab. The detector-LPV-system furthermore provides the possibility to regulate the blanket pressure in the detector-system and to measure the O2-content in the outbound nitrogen.

Figure 7.10: Overview of the entire gas handling chain of the far-detector-laboratory; (yellow-part): diﬀerent N2-supply systems in the underground lab (HPN-U (dark blue), FPN-U (mid-blue) and LPN- U (light blue)); (purple-part): diﬀerent consumers (WT, PLC, DFOS, GB, Detector, XTOS) with the established color code for the detector liquids; (green-part): ventilation system of the underground lab used to collect and purify nitrogen from the detector (light green) and from the auxiliary systems (dark green).

The separation in two ventilation systems guarantees that possible pressure shocks (which are possible in the DFOS-LPV due to the ventilation of pressurized intermediate tanks or weighing tank) do not transmit into the detector, and is therefore an inherent safety feature of this ventilation system. Table 7.10 outlines the individual parts of both ventilation systems of DFOS

106 Hardware Installations for the Filling and Handling of the DC far Detector and the detector. Figure 7.10 shows an overview of the gas handling system and indicates the individual parts of the DFOS-LPV in dark green and of the detector-LPV in light green. The following section will introduce these two ventilation systems in more detail: Parts of the DFOS- and Detector-LPV-System

Abbr. DFOS-LPV LPN-Box Low Pressure Nitrogen Box Filter-Box Charcoal Filter Box

Abbr. Detector-LPV O2 O2-Panel LPV-Box Low Pressure Ventilation Box Filter-Box Charcoal Filter Box

Table 7.10: Individual parts of the ventilation system of DFOS and the detector installed in the underground lab.

Ventilation System of the Liquid Handling System (DFOS-LPV) The ventilation system of the liquid handling system has the purpose to collect and purify nitrogen that has been used in DFOS or in the weighing tank. In order to do so, the individual exhaust lines (LPV-lines) of each intermediate tank as well as the weighing tank are lead to the LPN-box. This gas-tight box has a single compartment and collects the outbound nitrogen. In this box the outbound gases mix and therefore must not flow back to the detector. This is avoided by individual back-flow protections, which are mounted inside the LPN-box covering the incoming LPV-lines. The LPN-box provides only one exit which is connected to an active charcoal filter, which purifies the outbound nitrogen from vaporized aromatics. After the filter- ing, the nitrogen flow is lead to the air-extraction-point of the air-condition-system installed in the underground lab. This system finally removes the nitrogen from the underground lab end relives the nitrogen to the environment. The LPN-box furthermore contains an height adjustable oil bubbler, which allows to apply an artificial impedance onto the outbound gas what allows to regulate the back pressure between 0-6 mbar. Connected to the LPN-manifold, this bubbler allows to set an upper limit on the LPN- pressure and to use the bubbler as overpressure-protection for the LPN-manifold. In addition, this oil-filled glass provides an efficient protection against pressure shocks in the LPN-system and therefore in the detector. A pressure shock would empty the oil-reservoir and ventilate the gas into the LPN-box (away from the detector). A technical drawing as well as some illustrating pictures of the LPN-box are summarized in figure 7.11.

107 Hardware Installations for the Filling and Handling of the DC far Detector

from LPN coal filter

from DFOS-IMT

height adjustable oil bubbler

Figure 7.11: LPN-Box: (top left): shows an overview picture of the gas handling system including the LPN-box, which is closely mounted to the LPN-manifold in the underground lab. (top center): shows a detail picture of the height adjustable oil-bubbler in the LPN-box and its connection to the LPN-system. By looking at the oil displacement in the oil bubbler (and the cm-scale) it is possible to measure the LPN- pressure without electronic devices; (right): shows a technical drawing of the LPN-box and indicates the position of the oil-bubbler in the upper part and the four back flow protections at the bottom; (bottom left):shows a picture of the four individual back flow protections in the LPN-box. The inlet tubes are loosely caped with thin PTFE-plates, which open easily and avoid back flow into the DFOS-IMT’s

108 Hardware Installations for the Filling and Handling of the DC far Detector

Ventilation System of the Detector (Detector-LPV) The ventilation system of the detector is supposed to collect and purify the outbound nitrogen that has been in contact with the detector liquids. Apart from this, the LPV-system produces an adjustable, stable and common low pressure blanket in the detector. In addition, the LPV- system monitors the quality of the nitrogen blanket by measuring the O2-content in the outbound nitrogen. Due to the different tasks, the detector-LPV-system has a different setup, which is summarized in table 7.10 and shown in figure 7.12. The LPV-system of the detector collects the outbound nitrogen of the detector and all systems, which are in direct contact (therefore XTOS and the glove box). The individual exhaust lines of the detector vessels (1-inch, ss and PFA for NT) and the exhaust lines of the XTOS tanks (1-inch, PFA) are merged in order to ensure a common pressure in both systems. The collected nitrogen is then lead through the O2-panel (sensitive between 1000 ppm down to 40 ppm) in order to measure the O2-content in the outbound gas. A technical drawing of the O2-panel can be found in figure D.16.

Figure 7.12: Picture of the LPV-system in the underground laboratory, indicating the individual exhaust lines, the O2-panel, the LPV-box with an adjustable oil bubbler and four back-ﬂow protections and the main exhaust line, which is connected to an active charcoal ﬁlter. The panel on the right presents the XRS-system and the Loris tube both used for the level measurement system, which will be described in more detail in section 7.3.1.

After the O2-panel the nitrogen ﬂow is lead to the LPV-box. This gas-tight box has two compartments: a lower compartment, which avoids back-ﬂow and collects the nitrogen, and an

109 Hardware Installations for the Filling and Handling of the DC far Detector upper compartment, which provides an adjustable impedance for the gas coming from the lower compartment. In order to avoid back-flow, the lower compartment is equipped with four plastic covers which cover the four nitrogen inlets. The lower compartment has only one exit, which leads the total nitrogen flow directly into an oil-bubbler in the upper compartment. This oil- bubbler is adjustable in height, what allows to regulate the impedance of the bubbler between 0-5 mbar. The impedance in the gas flow leads to a common back pressure in all detector vessels and at the same time avoids differential pressures between the different detector vessels. After the oil bubbler the gas is lead through big exhaust lines (3-inch, PVC) to an active charcoal filter, which removes a possible organic contamination of the nitrogen. Finally, the gas is transfered to the air extraction point of the air-conditioning-system in the underground laboratory, which ventilates the nitrogen to the surface. Figure 7.12 provides an overview of the detector- LPV-system, indicating the LPV-lines of the detector, the LPV-lines of XTOS, the O2-panel, the LPV-box and the main exhaust lines leading to the active charcoal filters. A detailed illustration of the LPV-box can be found in figure 7.11 and figure 7.13 in the appendix. Using the bubbler in the LPN-box (LPN-inlet-pressure, Pi) and the bubbler in the LPV-box (LPV-outlet-impedance, Po) allows to operate the gas handling system in two different modes: The first mode (Pi Po) provides a flowing blanket through the detector. Furthermore the transparent oil bubblers allow to measure and regulate the inlet as well as the outlet pressure without the usage of electronic devices just by looking at the oil-displacement in the transparent bubblers (compare with figure 7.11).

110 Hardware Installations for the Filling and Handling of the DC far Detector

Figure 7.13: LPV-box: (top left): shows a detail picture of the height adjustable oil-bubbler in the LPV- box and its connection to the lower compartment. By looking at the oil displacement in the oil bubbler (and the cm-scale) it is possible to measure the LPN-pressure without electronic devices; (right): shows a technical drawing of the LPV-box and indicates the position of the oil-bubbler in the upper compartment and the four back-flow protections at the lower compartment. (bottom left):shows a picture of the four individual back flow protections in the LPV-box. The inlet tubes are loosely caped with thin PTFE- plates, which open easily and avoid back flow into the detector.

111 Hardware Installations for the Filling and Handling of the DC far Detector

7.3 Detector Monitoring System (DMS)

Due to the fragility and difficult geometry of the detector vessels, the Double Chooz detector can easily be harmed. The rigid but fragile acrylic vessels tolerate only little stress and suffer already damage upon small differential pressures (>3 mbar) or liquid level differences (>3 cm). These small and yet critical values can easily be exceeded, especially during the flushing or the parallel filling of the detector vessels. Therefore, all operations on the detector have to be constantly monitored. In order to handle the detector safely and to monitor the different operations, the detector is equipped with a dedicated monitoring system. It is composed of two separate systems: firstly, a liquid level monitoring system, which measures the absolute and differential liquid-levels in all detector vessels, and secondly, a gas pressure monitoring system, which measures the absolute and differential pressure-levels in all detector vessels. The design goal for the DMS was the redundant monitoring of the all liquid- and gas-pressure-levels with a minimum accuracy of 1 cm and 1 mbar, respectively. This was realized with the help of different individual level measurement systems, which are summarized in table 7.11 and will be introduced in the following two sections. Detector Monitoring System

Liquid Level Monitoring Monitored value Monitored vessels Measuring System MU BF GC NT Hydrostatic Pressure Sensors (HPS) absolute x x x Laser Level Measurement (LLM) absolute x x x Cross Reference System (XRS) diﬀerential x x x x XTOS-Level Measurement (XTOS-LM) absolute x x x Critical Point Sensors (CPS) critical levels x x Tamago System absolute x

Pressure Monitoring Monitored value Monitored vessels Measuring System MU BF GC NT Gas Pressure Monitoring (GPM) absolute/ x x x x diﬀerential XTOS-LM absolute – x x x

Table 7.11: The table summarizes the diﬀerent systems to monitor the liquid- and gas-pressure levels in the Double Chooz far-detector.

7.3.1 Liquid Level Monitoring Systems

Following the design request to measure every liquid level with two independent systems, the liquid levels of MU, BF, and GC were measured with hydrostatic pressure sensors (HPS), which scale the liquid column, and the laser level measurement system (LLM), which measures the distance from the top-lid to the liquid surface. Due to the material incompatibility of the target scintillator with metals and the additional need to remove all level measurement systems from the target after filling, HPS and LLM could not be used in the target-vessel. In consequence of these restrictions, the target vessels had to be equipped with an alternative system called Tamago (Japanese for egg), which uses a suspended PTFE-weight (egg-shaped) to measure the absolute liquid level. Figure 7.14 shows an illustration of the detector and indicates the geometrical situation within the detector as well as the implementation of the level measurement systems in the different vessels. Apart from these absolute level measurement systems, the cross reference system (XRS) provides the possibility to monitor the liquid level differences between MU, BF, GC and NT

112 Hardware Installations for the Filling and Handling of the DC far Detector

Laser-System (LS)

Cross-Reference System (XRS) Tamago

Hydrostatic Pressure Sensors (HPS)

Loris tube Long Filling Tubes

MU Laser guide tube

GC CPS III+IV

NT CPS I+II

PMT

Figure 7.14: Illustration and position indication of the level measurement systems used in the DC far detector: side view of the detector and the geometrical situation within; (colored lines): different level measurement systems as well as the filling lines in the different detector vessels. just by applying a common under pressure to the four liquid levels. This system allows to cross- check the absolute measurements of HPS, LLM and the Tamago and provides furthermore the possibility to measure the liquid level differences without electronic devices. In addition to these continuously measuring systems, the critical point sensors (CPS) allow to survey the reach of critical-filling-points in the detector as, for instance, the onset of the two acrylic chimneys. Each acrylic chimney is equipped with two sensors, one shortly before the chimney in order to provide a slow-down signal for the filling team. The second sensor is just within the chimney and marks the end of the vessel-slope and the start of the chimney filling. These independent information about the filling level is an additional safety feature and can furthermore be used to re-calibrate the absolute level measurement systems. After the filling process, most of the monitoring systems, namely the XRS, the Laser- and the Tamago-system, have to be de-installed in order to facilitate10 an undisturbed data taking. As the detector is subdued to thermal variations, the liquid levels in the chimneys can vary

10XRS, LLM and Tamago introduce light into the detector, either actively (laser) or passively by producing unavoidable light leaks. Due to the sensitivity of the PMT’s in the inner detector, light leaks could endanger the electronics and must therefore be avoided. Consequently these LM-systems have to be de-installed before calibration and data taking start.

113 Hardware Installations for the Filling and Handling of the DC far Detector dangerously (even with XTOS as it has a limited capacity). Therefore, it is necessary to monitor the liquid levels also during data taking. The expansion tanks of the XTOS provide a possibility to do so of XTOS. Separate from the detector, the XTOS-level measurement can monitor the liquid levels of BF, GC and target. Due to the big surface of the muon veto, it is not necessary to follow the MU-liquid level, anyhow the levels of the muon veto and additionally those from BF and GC are monitored within the detector by the HPS. In the following, the individual level measurement systems, their working principle and their accuracy shall be presented in more detail.

Laser-System

The Laser-system measures the absolute liquid levels in MU, BF and GC. This system uses three industrial lasers [112] to measure the distance between the top-lid and a custom made PTFE-float that is guided within a vertical stainless steel tube (ø=60 mm). The float rises with the liquid level and provides a target for the laser beam. The laser emits a frequency-modulated laser beam (650 nm, red) and analyses the diffuse reflected light. A micro-processor analyses the phase shift between the emitted beam and the diffuse reflected light, what allows to determine the distance between laser and the float11. All lasers are mounted on the muon veto top-lid using custom made flanges. Each flange is equipped with a low-reflective glass and separates the gas blanket from the lab atmosphere. As the flanges are distributed over the muon veto, a possible tilt of the muon veto top-lid has to be considered as that would distort the level measurement. The position of the individual flanges was measured. It was found that the muon veto top-lid is tilted and that the laser flanges are vertical displaced, as shown by the values in table 7.12. Correcting for the vertical displacement of the individual flanges, the laser system provided an excellent resolution and measured the absolute liquid level with an accuracy of 2 mm [113, 114].

Laser MU BF GC NT Type – M10 M10 M10 – Range m 0.1-100 0.1-100 0.1-100 – Accuracy mm 1 1 1 – Flange no. 18 42 26 – Vertical displacement mm 0 3 7 –

Table 7.12: Technical details of the Laser level measurement system, presented are the used laser type, their range and accuracy as well as the installed position (ﬂange no.) and the vertical displacement of the lasers as result of a tilted muon veto top-lid.

The measurements in the muon veto and in the buﬀer could be realized in the related volume, however, the measurement of the gamma catcher level could not be measured in the GC-vessel as the geometry of the vessels prevented the installation of a laser guide tube. Therefore, the liquid level of the gamma catcher had to be translated into a separate and isolated tube12 (GC-laser-guide tube) within the muon-veto-vessel. This transfer required a separate system, called Loris-tube (see ﬁgure 7.19), which assigns the liquid level of the gamma catcher into the GC-laser-guide tube, where the translated GC-liquid level can be measured in a straight line. This separate system is composed of two arms (3/4-inch-PFA-tube), which connect the bottom of the GC-vessel with the bottom of two tubes in the muon veto. These two interconnected metal pipes (ø=60 mm) were vertically installed, but isolated from the muon veto. One of these connected tubes holds one arm of the Loris-tube, while the other pipe is used as GC-laser guide

11For further information regarding the used laser, see [112]. 12A 60 mm stainless steel tube, which is isolated from the muon veto liquid.

114 Hardware Installations for the Filling and Handling of the DC far Detector tube, which hosts a PTFE-float. The upper part of the Loris-tube provides a vacuum-pump, which allows to suck up the liquids into the arms. Both arms can be connected in such way that an up-side-down siphon is established and can be maintained. As a consequence of this siphon, any liquid level differences between the pipes and the GC-vessels would automatically have been equalized, what allowed to translate the liquid level. This translation-solution was not possible for the target, due to which the target had to be equipped with an alternative system, called Tamago. Figure 7.15 provides and illustration of the laser system and shows some pictures of the used laser, the laser guide tube in the muon veto as well as the used PTFE-float during the installation of this system. In the lower part of figure 7.15, the laser level measurement system is shown in an overview, indicating the different installation details.

Figure 7.15: (left): overview of the working principle of the laser level measurement system; (center): pictures of the MU-laser mounted at the top-lid and the PTFE-ﬂoat at the bottom of the muon veto; (right): picture of the laser guide tube in the muon veto.

115 Hardware Installations for the Filling and Handling of the DC far Detector

Hydrostatic Pressure Sensors (HPS)

The HPS system provides an independent measurement of the absolute liquid levels of MU, BF and GC. The sensors are situated at the lowest possible points in the related vessels (compare figure 7.14) and measure the hydrostatic pressure as well as the temperature at the sensor- head. The three industrial immersion sensors13, which are fully made of stainless steel are supplied by a PTFE-mantled cable. This sensor type measures differentially and compares the pressure on the stainless-steel-membrane with the pressure in a capillary, which is hidden in the sensor cable. For the HPS- system, the different capillaries were extruded from the cables (shortly before the cable exits the detector) and exposed to the nitrogen blanket. This allows to directly measure the liquid level independently of the nitrogen blanket in each vessel. In order to measure the absolute and differential levels, it is necessary to know the z-levels of each installation point to cross- calibrate the individual measurements. These levels are gathered Figure 7.16: HPS and cable feed through at the BF-flange during the the first moments of filling, when the different liquid level sensors are established. The sensors enter the detector through sliding seals and can be lifted or removed if necessary (except in the GC, where it is glued to the vessel). Although the specification of the sensor predicted an accuracy of 1 cm, a good calibration and well chosen pressure range of the sensor allowed to measure the absolute liquid level with an accuracy of 2 mm ± 1 mm [114]. Table 7.13 summarizes the different pressure ranges and other technical details of the individual sensors. Hydrostatic Pressure Sensors

System Unit MU BF GC NT Pressure Range mbar 0 - 600 0 - 550 0 - 450 – Temperature Range ○C 5 - 25 5 - 25 5 - 25 – Accuracy mm 2 2 2 – Flange No. – 2 10 Chimney – Length mm 137 137 137 – Ø mm 24 24 24 – Material Cable – PTFE PTFE PTFE – Material Head – SS SS SS – Material Membrane – SS SS SS –

Table 7.13: Technical details of the hydrostatic pressure sensors (HPS) installed in MU, BF and GC, indicating the chosen pressure range, the temperature range, the number of the ﬂange in which the sensor is installed, the dimensions of the sensor head as well as the used materials.

Tamago

Tamago is the nickname for an industrial level measurement system from Endress & Hauser, actually called Proservo NMS5 [115]. The system uses an egg-shaped PTFE-weight suspended on a string to measure the absolute liquid level in the target. The system scales the tension in the PTFE-coated stainless steel string (ø=0.2 mm), which is strained by the mentioned weight14. After measuring this tension, the system is able to lift or descend the weight on sub-mm-level. Once the liquid level in the target rises and submerges the PTFE-weight, the buoyancy force reduces the tension in the string and the system lifts the weight until the tension is re-established.

13Sensor: ATM/N/T from Co. STS, further information can be found in [79]. 14PTFE weight: height=55 mm, Ø=50 mm, weight=286 g.

116 Hardware Installations for the Filling and Handling of the DC far Detector

Figure 7.17: Overview of the Tamago level measurement system; (left): Picture of the Proservo NMS5 installed at a custom made chimney extension, whith the PTFE-weight visible. After ﬁlling, Tamago and the chimney extension are replaced by the glove box; (right): illustration of the target vessel indicating installation details and the working principle of the Tamago system.

The lifting is realized by a highly sensitive step-motor in the upper-part of the Proservo. In the standard conﬁguration, this system requires a minimum liquid level of 30 mm. This could be improved by good calibration and elaborate testing: as the Tamago-system measures the weight of the egg, it has been possible to recognize the weight-reduction even before the weight is lifted by the step-motor. This allowed to improve the starting level from standardly 30 mm down to 6 mm. In general, this system provides an excellent resolution and measured the absolute liquid level with an accuracy of 2 mm [113, 114]. Figure 7.17 provides an overview of the Tamago system as well as a picture of the Proserve NMS5 shortly before the ﬁlling started. Apart from the Tamago, the target is equipped with a parallel suspended XRS-tube. Both systems are installed through the chimney-extension and can be de-installed15 after using without corrupting the nitrogen blanket or the cleanliness of the target, what was one of the conditions for the NT-LM-system.

15The NT-LM-systems are de-installed by retracting the XRS-tube and Tamago back into the chimney- extension. Once this is done, the ball valve, mounted below the chimney extension, is closed and allows to dismount the chimney extension in one piece.

117 Hardware Installations for the Filling and Handling of the DC far Detector

Figure 7.18: (left): picture of the critical point sensors installed in the target-chimney, indicating the chimney and the 1/2-inch ﬁlling line; (center): installation details and dimensions of the sensor-tips; (right): overview of the CPS-installation in gamma catcher and target and its connection to the LM-PC.

Critical Point Sensors (CPS)

The critical point sensors monitor the conical transition (slope) between the main body and the chimneys of the acrylic vessels. Each slope is equipped with two contact sensors16, which are composed of two optical fibers housed in a PFA-tube (ø=6 mm), both leading to an optical cone (see figure 7.18). An amplifier sends light through one of the fibers into the optical cone, the light is totally reflected and sent back via the second fiber to the amplifier. When the cone is submerged, the refractive index around the cone changes (from air to liquid) and the light is not totally reflected anymore. The intensity loss is recognized by the amplifier and interpreted as contact. Since the level increase is slowly and light loss gradual, the amplifier recognizes not only the contact but also the gradual submersion of the 7 mm high cone. Each slope is equipped with two sensors: one sensor-tip is mounted 3 cm below the chimney and therefore still within the slope. This sensor provides a slow-down-signal for the filling team and indicates that only a few more liters are needed to reach the chimney. This point is used to change from the faster filling modes17 to the fine filling mode used in the chimney. The second sensor-tip is mounted already 1 cm within the chimney and indicates the filling team that the slope is full and the chimney reached. From this point on until the XTOS system is reached, the detector filling is highly critical as already a thermal variation of 0.07 K of the scintillator in the main body would lead to a fatal liquid level difference in the chimney (compare with section 7.1.3). Using this system, an independent safety feature is provided and a re-calibration of the absolute level measurement systems allowed. So the CPS-1 and -2 in the target enables to cross- check the Tamago-system, which anticipated a sensor contact 2504 mm above the target bottom. The CPS-1 was triggered at 2504.04 mm and thus showed the reliability of the Tamago-System and the quality of the target vessel construction.

16Sensor: FU-93z from Keyence, further information can be found in [116]. 17IMT- or continuous-ﬁlling mode

118 Hardware Installations for the Filling and Handling of the DC far Detector

Cross Reference System (XRS)

The cross reference system allows to monitor the diﬀerential liquid level between MU, BF, GC and NT. The XRS is composed of the XRS-panel mounted above the detector and four individual PFA-tubes, which run from the XRS-panel straight down to the bottom of each vessel. Each tube is charged with an additional weight at the end, which straightens the tube and ensures that the opening is at the lowest point. Table 7.14 summarizes the installation positions as well as some mostly technical details of the four XRS-tubes. Cross Reference System

XRS Unit MU BF GC NT Flange no. 2 10 34 Chimney Ext. Tube diameter inch 0.5 0.5 0.5 0.5 Accuracy mm 2 2 2 2 Tube Material – PFA PFA PFA PFA Weight Material – SS SS SS PTFE

Table 7.14: Technical details of the cross reference system installed in the diﬀerent detector vessels. Indicating the ﬂange number where the PFA-tubes enter the detector, the tube diameter, the tube material and the material used for the weight.

The XRS measurement bases on the fact that a common under pressure, applied to multiple tubes (which are differently submerged) pulls up the different liquid levels but leaves their differentials unchanged. Using this effect, it enables to lift the different liquid levels in the detector, above the detector and into the XRS-panel, where the liquid level differences can easily be measured with the help of a mounted scale. The XRS panel is presented in figure 7.19 and indicates the different instruments (a small vacuum pump, a vacuum resistant 1 l-stainless steel tank, a vertical mm-scale, a distribution chamber with four valves connecting to the four PFA-tubes). The XRS provides an independent measurement of the liquid level differences. This correlation between the different liquid levels can then be used to cross-check the other level measurement systems. Using this system enables to measure the differential liquid levels with an accuracy of ±2 mm. Apart from the system mentioned above the XRS-panel also hosts the Loris-tube, which is part of the Laser-system (compare figure 7.14).

119 Hardware Installations for the Filling and Handling of the DC far Detector

Figure 7.19: (upper, left): Overview of the cross reference system, indicating the XRS-panel and its instruments: (1) vacuum pump, (2) 1 l-stainless steel-volume, (3) distribution chamber, (4) mm-scaled area where all (5) XRS-tubes can be compared; (upper right): two details of the overview picture, which indicate the final liquid levels in the detector right after the filling process was finished; (bottom): scheme of the XRS-system and the XRS-panel including the installations for the Loris-tube (upside-down siphon), used for the translation of the GC-liquid level into the GC-laser guide tube.

120 Hardware Installations for the Filling and Handling of the DC far Detector

XTOS-Level Measurement (XTOS-LM)

Figure 7.20: Overview of the XTOS-level measurement system; (upper left): connection scheme of the XTOS-LM-system; (upper right): vertical cut through the buﬀer XTOS-tank indicating the installation details used to measure the liquid- and gas pressure-levels in XTOS; (lower left): picture of the XTOS top-ﬂange indicating gas handling (LPN-supply, LPV-line) and level measurement (pressure-sensor, liquid level sensor) connections; (lower right): picture of the GC-XTOS-tank, indicating the side glass (including mm-scale) and the connection to the detector.

In order to measure the liquid levels and the gas pressure levels in the expansion tanks, the XTOS-LM-system uses highly sensitive differential gas pressure sensors18, which are mounted outside of the tank. Each tank is equipped with two sensors. Mounted at the top-flange, one sensor monitors the blanket pressure while the other one uses an indirect measurement method to monitor the liquid level. Each sensor is a combination of an amplifier (AP-V40) and a small sensor-head (AP-47), which has two connection ports. The sensor uses a piezo-element to compare the pressures between the ports and transmits the measured difference to the amplifier. While the sensor heads are mounted on the tanks, the amplifiers are collected in a custom made box (XTOS-sensor-box) and provide analog signals to the LM-PC. The liquid levels in the XTOS tanks can be measured in two ways: manually, by using a side-glass, or electronically, by using the sensor, as indicated in figure 7.20. For the latter, the first port of the sensor connects to a straw (PFA-tube, ø=6 mm), which enters the tank vertically and runs straight down to the bottom. The second port connects to the gas-blanket of the same tank using a normal plastic tube (PE-tube, ø=6 mm). Once the tank is filled with liquid, the gas volume in the straw is

18Ampliﬁer (AP-V40) + Sensor (AP-47) from Keyence company. Further information can be found in ﬁgure D.19 and in [117].

121 Hardware Installations for the Filling and Handling of the DC far Detector trapped and further compressed with an increasing liquid level. This compression (inside the straw) is measurable and allows to deduce the liquid level. Hence, the liquid level measurement is independent from the LPN-blanket and its variations. The blanket pressure in the tanks is monitored by an additional sensor of the same type. For this measurement, one port of the sensor connects to the gas-blanket while the other port of the sensor is open to the atmosphere in the lab. This setup allowed to measure the liquid level with an accuracy of ±1 mm and the gas pressure levels with an accuracy of ±0.05 mbar. Figure 7.20 provides an overview of the XTOS-LM-system indicating the connection logic of the sensors and ampliﬁers with the level measurement-PC (upper left), a vertical cut through one XTOS tank indicating the installation of the pressure sensors and the side glass (upper right), and two pictures of the XTOS tanks during the installation of XTOS.

Level Measurement Computer and Data Acquisition

Figure 7.21: Overview of the level measurement PC and its connections to the individual level measurements systems; (top): connection logic between LM-PC and individual sensors indicating the used protocol; (bottom left): picture of the level measurement computer (LM-PC); (bottom center): picture of the PXI-chassis and the individual PXI-read-out cards; (bottom, right): Picture of one terminal board (PXI-SCB-68), which is used as hardware-interface between sensor boxes and the analog read-out card.

The level measurement computer (LM-PC) is a standard data acquisition system from National Instruments [118]. The PXI-standard of NI allows to assemble an individual DAQ-system by choosing the necessary read-out cards and to combine them in one chassis. The LM-PC, for

122 Hardware Installations for the Filling and Handling of the DC far Detector instance, is composed of a standard PXI-chassis (PXI-1042), which offers space for 8 different PXI-cards. Two of these slots are equipped with a controller (PXI-8106), which offers a windows platform and the standard PC connection ports. The controller uses the software package Lab- view [119] and a custom made level-measurement-program19 to visualize and record the acquired data. Apart from that, the chassis holds two analog read-out cards (PXI-6225, 1-slot) and a serial read-out card (PXI-8433/4, 1-slot). Most of the sensors (HPS, CPS and the gas pressure sensors) provide analog signals, the other sensors, namely the lasers and the Tamago-system, use a serial protocol. In order to provide a clear arrangement, all analog signals were collected in custom made sensor-boxes [113, 114] and sent collected to the read-out cards. This allowed to or- ganize the LM-system and to reduce the amount of cables and the number of power-supply units.

Apart from the analog and serial read-out cards, the PC provides standard connections as USB and LAN. These connections are also used to read out information from other systems as indicated in ﬁgure 7.21. For instance, the LM-PC is connected to the programmable logic controller (PLC) of DFOS. By using a LAN-connection, the LM-PC is able to read-out the diﬀerent pressure levels in the gas handling system of the detector. Furthermore this connection allows to monitor the condition20 of the LPN main valve (V017) of the detector.

The connections of the controller are used to read out an additional pressure sensor, which monitors the atmospheric pressure in the laboratory. Furthermore the LM-PC connects to the Internet as well as two web cams, which monitor the side glasses of the XTOS-tanks and the gas-flow-meter in the LPN-supply of the detector. These last connections allow to monitor the most important detector values (liquid level and current gas flow) also remotely and independent from the running monitoring program. The LM-PC reads-out 32 different sensors, the acquired data are recorded and visualized at the LM-PC. In addition, the entire data volume is transfered to a MySQL-database, which provides an online access of the stored data.

19Details related to the level measurement program as well as a screen shot of the program are presented in [114]. 20open/malfunction/close

123 Hardware Installations for the Filling and Handling of the DC far Detector

7.3.2 Gas Pressure Monitoring System (GPM)

The gas pressure monitoring system is divided into two systems: one monitors only the various absolute and differential pressure-levels in the detector, and the second one monitors the blanket pressure in the XTOS-tanks. Furthermore, the gas handling system in the underground laboratory is equipped with various pressure indicators, manometers and oil bubblers, which allow to monitor the LPN-pressure as well as the exhaust-impedance of the ventilation system without electronic devices. Table 7.15 provides an overview of the different systems, observed pressure levels and monitored vessels. The following section will be used to introduce the different monitoring systems in more detail. Gas Pressure Monitoring System

Monitoring System Monitored value Monitored vessels

Detector MU BF GC NT Detector Monitoring absolute/ x x x x diﬀerential pressure

XTOS MU BF GC NT XTOS Monitoring (XTOS-LM) absolute pressure – x x x XTOS Monitoring (XTOS-LM) diﬀerential pressure – – – –

Gas handling inlet-pressure outlet-impedance LPN-oil bubbler absolute pressure x LPN-manometer absolute pressure x LPN-distributor absolute pressure x LPV-oil bubbler absolute pressure x

Table 7.15: The table summarizes the gas pressure monitoring systems for the XTOS and the detector and shows which volumes are monitored.

Detector Monitoring

In order to monitor the absolute and differential pressure levels in the four different detector vessels, the DC-far detector is equipped with eight highly sensitive differential pressure sensors. Each sensor is composed of a small and separate sensor-head21 (AP-47) and a control- ling amplifier (AP-V40). The sensor-head has two connection ports and uses a piezo-element to compare the pressures between the two ports. All sensor-heads and amplifiers are collected in a custom made box (detector sensor box), which is supplied and connected to the level measurement-PC. The sensor-box is situated at the center of the muon veto top-lid and therefore below the shielding. From this central position, the different sensor-heads are connected to the different vessels. The connection between sensors and detector vessels is re- Figure 7.22: Amplifier and sensor head. Picture from alized with equally long plastic tubes (PE-LD, [117]. ø=6 mm). The absolute overpressure (LPN- blanket pressure) in MU, BF, GC and NT is monitored by four separate sensors, each connecting

21diﬀerential pressure sensor with two connection ports from Keyence (AP-47). Further information can be found in ﬁgure D.19 and in [117].

124 Hardware Installations for the Filling and Handling of the DC far Detector to an individual vessel. One port of the sensor-head connects to the gas-blanket of the measured vessels, while the other connection is open to the atmosphere in the lab. The differential pressures between the vessels are measured by the other four sensors. For this measurement, each sensor-head connects to two vessels, measuring the pressure between MU-BF, BF-GC, GC-NT and NT-MU. The connection logic is presented in figure 7.23 and made in a way, that each vessel is monitored by three different sensors. This generates a redundant measurement and allows to identify faulty pressure sensors, as a true pressure variation would be seen in all three sensors. Figure 7.23 provides an overview of the gas pressure monitoring system of the detector, indicating the connection logic of the sensors and amplifiers with the level measurement-PC. Using this system allowed to monitor the gas pressure levels with an excellent accuracy of 0.05 mbar and therefore well within the specifications of 1 mbar, as indicated at the beginning of this chapter.

Figure 7.23: (left): scheme of the connection logic used for the gas pressure monitoring of the far detector; (right): the detector-sensor-box, indicating the eight differential gas pressure sensors (AP-47 [117]) and related amplifiers, used for the absolute and the differential gas-pressure-monitoring in the detector. Apart from that, the sensor box connects to the critical point sensors in the detector and additionally holds the related amplifiers (marked as CPS-amplifiers).

125 Hardware Installations for the Filling and Handling of the DC far Detector

XTOS

The gas pressure levels in XTOS are measured with a separate sensor box, which is part of the XTOS-LM-system. Due to the solid construction of the individual XTOS-tanks, it is not necessary to monitor the diﬀerential pressures between the tanks. The XTOS-LM-system therefore measures only the absolute over-pressures by comparing the pressure in the tanks with the atmospheric pressure in the underground laboratory. A detailed presentation of the XTOS-level measurement system can be found in section 7.3.1 and shall not be recapitulated at this point. Figure 7.24 provides an overview of the XTOS-LM-system and the connection logic used to measure the liquid- as well as the gas-pressure-levels within XTOS.

Figure 7.24: (left): scheme of the connection logic used for the gas pressure monitoring of the expansion tank operating system (XTOS); (right): Picture of a XTOS top ﬂange, indicating the two diﬀerential gas pressure sensors (AP-47 [117]) used for liquid-level- and gas-pressure level-monitoring in the XTOS-tanks.

126 Chapter 8

Detector Filling

Figure 8.1: Simplified picture of the Double Chooz Far Detector: The Double Chooz detector has a cylindrical shape and is composed of four concentrically arranged vessels. The outer two vessels, muon veto and buffer, are made of 12 mm steel and 3 mm stainless steel and are therefore more resistant to stress than the gamma catcher or target-vessel, which are optimized for physics and made of only 12 mm and 8 mm thin acrylic walls. The vessels are filled with 90 m3 muon veto scintillator, 110 m3 buffer oil, 22.5 m3 gamma catcher scintillator and 10.3 m3 target scintillator.

127 Detector Filling

After the production of the detector liquids and the realization of all liquid-, gas- and monitoring- systems in the underground laboratory, the Double Chooz far detector could be filled. This required not only absolutely reliable hardware but also a prudently planned filling process, which anticipated imminent dangers and avoided unnecessary stress on the vessels. The development of this filling procedure, the preparation of the detector and finally, the realization of the filling process had been the responsibility of the author and Dr. C. Buck from MPIK in Heidelberg and were realized together with a dedicated filling team between October and December of 2010. The following chapter is dedicated to the filling of the Double Chooz far detector and will start with a presentation of the necessary preparations, followed by a detailed description of the individual filling steps.

8.1 Preparations for Filling

8.1.1 Filling Team

The filling of the detector required the parallel use of all systems, which have already been introduced in this thesis. This included the surface installations in the LSA (chapter 4), which provided the detector liquids to the underground lab, as well as all liquid-, gas-handling, and monitoring systems in the underground lab (chapter 7), which were used to fill the detector. The parallel use of all these systems, scattered over different locations, clearly necessitated several people. In order to handle the different systems and to meet the safety requirements of the nuclear power plant, a filling-team was composed of minimum 5 people: two shifters, who monitored all surface systems and supplied the underground laboratory with different liquids, two trained experts, who handled the underground systems and were responsible for the correct realization of the filling procedure, and a filling coordinator, who was responsible for the safety of the detector, the filling process and the coordination of the filling team.

8.1.2 Detector Flushing

After the assembly of the detector, the lab-atmosphere (ambient air with suspended particles and significant humidity) had to be removed from the different detector vessels. While air and dust particles can lead to radioactive contamination and/or a degradation of the scintillator properties, water harms1 the target scintillator in concentrations of more than 100 ppm. A useful means to avoid such negative effects is therefore a thorough flushing process with dry and clean nitrogen. The standard system for supplying the detector with nitrogen was the low pressure nitrogen system (LPN-U). The low pressure provided only a small nitrogen flow, which was, additionally, supplied to the top of the detector. Both of these features were a disadvantage for flushing. Because of this, the gas handling system in the underground lab provided an individual system exclusively used for the flushing of the detector, the FPN-U-system (see section 7.2). This offered elevated pressure-levels and allowed to send a controlled nitrogen flow directly to the bottom of the detector. In order to do that, FPN-U was not connected to the detector, but to the intermediate tanks of the filling system. Using this system, the nitrogen ran from the gas handling system to the intermediate tanks and then over the filling lines to the bottom of the detector. The nitrogen was vented at the top over the standard ventilation system (LPV- U).

1The polarity of water is able to disintegrate the soluted gadolinium-complex in the target scintillator, what causes the gadolinium to precipitate from the solution. Furthermore, the target vessel is made of acrylic, which is a hygroscopic material and therefore saturates with the humidity of the surrounding air.

128 Detector Filling

The flushing of fragile structures, what the acrylic detector vessels in the Double Chooz are, is highly critical. The big volumes and the need to keep the flushing time short, required large nitrogen flows through the different detector vessels. High flow rates, however, have to be handled carefully, because any sudden change induces2 differential pressures between the detector 3 vessels. In addition, the big detector volume (∼240 m ) was hermetically closed and subjected to atmospheric pressure changes, what again could have lead to high gas flows in or out of the detector. In order to ensure the safety of the detector and to monitor the flushing progress, the detector was monitored with pressure and oxygen-sensors. While the pressure sensors allowed to monitor stress on the vessels, the oxygen-sensors (installed within the individual exhaust lines at the O2-panel) provided information about the oxygen content in the outbound nitrogen flow. Aiming for an oxygen concentration of less than 100 ppm, the sensors indicated between 1000 and 40 ppm. Once the oxygen levels were constantly below 40 ppm, the FPN-U-system could be stopped and the LPN-U-system could overtake the nitrogen-supply of the detector. By using this procedure, the detector was flushed for about three weeks, with a nitrogen flow between 1 and 4 m3/h, adapted to the size of the individual detector vessels.

8.1.3 DFOS Cleaning

Before the individual liquid handling modules in DFOS could start to fill their corresponding detector volume, it had to be ensured that all systems were clean and not source of a possible re-contamination of the liquids. Each system therefore was thoroughly rinsed at two different occasions. Once with ultra pure water in the manufacturing hall of the constructing company, and a second time in the underground lab after DFOS was installed and connected to the detector. The second rinsing was done with the final detector liquids. Supplied by the LSA, each IMT was filled and the liquid was circulated: in a first step through DFOS, and in a second step also through the two filling lines3, which connected DFOS with the detector. After rinsing, the liquids used for cleaning were removed and replaced by new liquids from the LSA. These new liquids were the first to enter the detector and were therefore also used to monitor the quality of the final detector liquids. The results of these measurements are summarized in chapter 9.

8.2 Detector Filling

The parallel filling of the four nested vessels was dangerous, especially for the fragile acrylic vessels. In order to determine the stability of the acrylic vessels, the vessel-constructing-group 4 (CEA) conducted a related FEM-analysis , which indicated that liquid level differences of ≤ 3 cm, 3 density differences of ≤ 0.01 g/cm or gas pressure differences of ≤ 3 mbar could be tolerated by the vessels. During the filling process, when the vessels were held under a permanent nitrogen atmosphere and were additionally filled with different liquids, all vessels were exposed to a combination of gas-pressure-differences, liquid-level-differences and density-differences. The total stress on the vessels therefore was the sum of these influences. Consequently, the filling process had to be carefully executed and well monitored in order to limit the total-stress on the detector vessels below the mentioned values. Considering this, it is clear that a safe detector filling required equal densities (within the percent level), a reliable and stable working gas handling

2The reason for this are the different detector volumes, which adjust to sudden pressure changes on different time scales. During this adjustment, the detector is subjected to pressure differences. 3The circulation sent liquid through the long filling line to the valve-station shortly ahead of the detector. At the valve station, the liquid used the bypass valve and was sent back to DFOS via the short filling tube. 4The finite element analysis (FEM) was conducted by CEA. They considered the geometry of the vessels and the strength of acrylic materials. Although they neglected the gluing-joints of the vessels, which are mechanical weak points, this study provided stress limits which could be used as a guideline for a safe detector handling.

129 Detector Filling system as well as a liquid handling system which allowed to fill and handle the detector liquids with the necessary precision. A critical filling step is always the submersion of an empty inner-vessel5 or the transition between main-body and chimney, also referred to as the vessel-slope (compare figure 8.2). In the target- slope, for example, the surface reduces from 4.15 m2 in the main-body down to only 176 cm2 in the chimney. The necessary volume to increase the liquid level in the target by 1 cm thus changes from 41.5 l to only 176 ml. This large dynamic range could not be covered by a single filling- mode, because of which each DFOS-module provided three different filling modes, as described in section 7.1.2. Each of them provided different flow ranges between 550 l/h in continuous-mode down to a few ml/min in the fine filling mode. Detector Filling Phases

Phase Aim Filling Mode Illustration

Nr. Description MU BF GC NT Figure 1 Establish BF-LM IMT 2 Establish MU-LM IMT 8.3 3 Incr. LL to BF-bottom CM 4 Adjust LL IMT 8.4 5 Establish GC-LM IMT 6 Incr. LL to GC-bottom CM CM 7 Adjust LL CM IMT 8.5 8 Establish NT-LM IMT 9 Incr. LL to NT-bottom CM CM CM/IMT 10 Adjust LL CM CM IMT 8.6 11 Incr. LL to Center CM CM CM CM 12 Change LL-Order CM CM CM CM 8.7 13 Incr. LL to NT-Slope CM CM CM CM 8.8 14 Incr. LL to NT-chimney CM CM CM IMT/FF 15 Incr. LL to GC-slope CM CM CM FF 8.9 16 Incr. LL to GC-chimney CM CM IMT/FF FF 17 Incr. LL to BF-slope CM CM FF FF 8.10 18 Incr. LL to BF-chimney CM CM/IMT FF FF 19 Incr. LL to XTOS CM FF FF FF 8.11 20 Fill XTOS CM FF FF FF 21 Increase to Final LL CM FF FF FF 8.12 22 Adjust Final LL FF FF FF

Table 8.1: The filling process was composed of 22 individual filling phases: the table summarizes the number, the aim, the applied filling mode used in the different detector vessels and the figure, in which each individual filling step is illustrated. The individual filling phases will be explained in detail in section 8.2.1. Abbreviations: CM = continuous mode, IMT = intermediate tank mode, FF = fine filling mode, LL = liquid level.

In order to charge the detector safely, filling followed a defined sequence of filling phases, which anticipated critical steps and avoided unnecessary stress on the detector. In the case of Double Chooz, the filling process was composed of 22 individual phases, which are summarized in table 8.1 and indicated in figure 8.2. The filling of each vessel started with a phase called pre-filling. This phase allowed to establish the level measurement system of the vessel (finding 0-level) and increased in addition the weight

5The submersion of an inner vessel would lead to immense buoyancy forces, what thereupon would lead to a fracturing of the inner vessel.

130 Detector Filling of the vessel. For example, the first filling phase in DC was the pre-filling of the buffer. The 0-level in the buffer provided a reference mark, to which the upcoming MU-liquid level could be adjusted. In addition, the extra weight in the buffer helped to reduce the buoyancy forces, which emerged during the adjustment of the MU-liquid level to the 0-level of the buffer (Phase 4). After this adjustment, the gamma catcher vessel was pre-filled (Phase 5) and the liquid levels in MU and BF were homogeneously increased until they matched the 0-level of the GC- vessel (Phase 6). The implementation of the target vessel was done accordingly (Phases 8-11). This procedure was maintained until the center of the detector was reached. At the center the remaining level differences were re-arranged in anticipation of the upcoming chimney filling phase (Phase 12). The chimney filling phase was the most critical part of the filling, because already small volume- or thermal changes could endanger the detector. Important for the safety of the detector therefore was the identification of the vessel-slopes and the controlled transition into the chimney. Once the transition was done, the liquid level in the chimney was adjusted stepwise (with FFM) to the continuously increasing liquid level of other vessels, which were filled by the standard filling modes IMT or CM. In these filling modes, the detector was filled until the gamma catcher slope was reached (Phase 15). From this point on, the next chimneys were implemented accordingly (Phases 16-19). This procedure was maintained until the XTOS-tanks and the final liquid levels were reached (Phases 20-22).

The ﬁlling of the Double Chooz far detector started on 12.10.2010, 10:30 am (MEZ) and lasted, including all interruptions, two month until 13.12.2010, 1.46 am (MEZ). A detailed description of the individual ﬁlling phases will be presented in the next section.

131 Detector Filling

8.2.1 Filling Sequence

The filling of the DC-far detector followed a well defined filling procedure, the development and execution of this sequence had been the responsibility of the author and Dr. C. Buck (MPIK) and was realized in course of the here presented thesis. As indicated in table 8.1, the filling process was composed of 22 individual filling phases. In the following section, each of the filling phases will be described and illustrated in chronological order. Figure 8.2 provides an illustration of the detector and the position of the different level measurement systems, which were established and used during filling. Furthermore figure 8.2 provides an overview of the different filling phases and where these phases lay in the detector.

Laser-System (LS)

Tamago Cross-Reference System (XRS)

Hydrostatic Pressure Sensors (HPS)

Loris tube Filling Long Filling Tubes phases

21 - 22 19 - 20 MU Laser guide tube 17 - 18 BF 14 - 16

GC CPS III+IV 13 -14 NT 11 - 12 CPS I+II

7 - 10 5 - 7

Buffer Slope PMT 3 - 4

1 - 2 PMT PMT

Figure 8.2: Illustration of the Double Chooz far detector indicating the critical filling points and the correlated filling phases. Furthermore, the different level measurement systems and their positions in the detector are shown.

132 Detector Filling

Phase 1-3 Level: 0-535 mm

Muon Veto (MU) 1 1a MU + BF Laser Buffer (BF) 390 (8") BF BF-PMT‘s MU BF Long Long XRS Filling Filling

Side Center Vessel Tube Tube Slope BF HPS

2 3

MU XRS

78 (10") MU-PMT‘s MU HPS

Figure 8.3: Phase 1: pre-filling of the buffer vessel and establishment of the BF-HPS; Phase 1a: filling of the BF-slope and establishment of the BF-laser; Phase 2: pre-filling of the muon veto and establishment of the MU-HPS; Phase 3: liquid level increase of the MU-level until buffer-bottom is reached.

Phase 1: Establish BF-LM The detector filling started with the pre-filling of the buffer vessel in IMT-mode. Pre-filling, as described before, means to enter just enough liquid to establish the level measurement system for the buffer (BF-LM), which is composed of a hydrostatic pressure sensor (BF-HPS), a laser (BF-laser) and the cross reference system (XRS). HPS and XRS are installed in the BF-side-tube and will be submerged instantly when the side-tube is filled with liquid. By starting to fill the side tube, the HPS-level rose quickly and stopped once the liquid entered the main body of the buffer-vessel. The excess-liquid ran down the BF-slope and started to fill the center-tube. With liquid in the center tube, the laser-float started to rise until the liquid started to fill the BF-main-body itself. At this point, the laser-float stopped rising, marking the beginning of the vessels and the 0-level for the BF-LM (with an amiability of 1 cm). The BF-XRS-system could be established once there was enough liquid inside the side-tube to fill the XRS-tube (15 m×1/4-inch, PFA-tube). Important for the safety of the detector was, that the XRS-tube was fully submerged, otherwise the vacuum pump (-800 mbar) would have sucked directly on the carefully maintained gas phase, what finally could have lead to pressure differences between the vessels.

Phase 2: Establish MU-LM The second step was the establishment of the level measurement system in the muon veto. Analog to the buﬀer, the muon veto was equipped with a MU-laser, a HPS-sensor (MU-HPS) and a XRS-tube. Using the IMT-mode6, the liquid level in the muon veto

6A detailed description can be found in section 7.1.2.

133 Detector Filling was slowly increased. This first application of the IMT-mode was used to test the performance of this filling mode. Below the buffer vessel, the 100 l of a full MU-IMT increased the MU-level by 5 mm. The IMT-mode demonstrated an IMT-filling-time of 20 min, a thermalization-time of 20 min/K (100 l for 2K) as well as an IMT-emptying-time of 40 min, when the IMT was pressurized with 550 mbar. Consequently, the IMT-mode allowed to fill the lower part of the muon veto (A=36 m2) with a rate around 3 mm/h with and 5 mm/h without thermalization. Using the IMT-mode, the Muon veto was filled for the first 20 mm, what allowed to establish the MU-LM-systems. The laser-float was lifted when the liquid level reached 12 mm. The HPS-sensor started to measure with an offset of 5 mm. The XRS could be established once the liquid level was above 15 mm. A cross-calibration between the different levels could be done once the entire bottom of the muon veto was fully covered and both systems showed an equal increase.

Phase 3: Increase LL to BF-bottom Once the LM-systems of muon veto and buffer were established, the MU-level could be increased to the buffer bottom. As this next and uncritical phase required a level increase of 515 mm, it could be used to test the performance of the faster continuous filling mode (CM). This mode allowed to insert unthermalized liquid directly into the detector. Only driven by gravity, this mode showed a filling rate of about 8 l/min and, supported by the DFOS-pump, this mode demonstrated a flow rate around 9.3 l/min. Consequently, the continuous-mode allowed to fill the lower part of the muon veto with a filling speed of about 15 mm/h, and 19 mm/h respectively, using the DFOS-pump. By using the continuous mode, the MU-level was increased until the MU-level matched the pre- filling level of the buffer. Due to the reduced surface in the muon veto, the MU-level accelerated once the MU-level reached the buffer-slope. This increase in the filling-rate could clearly be recognized and provided an independent indicator for the reach of the BF-bottom. A plot related to the accelerated levels can be found in the appendix, see figure E.1. With the reduced surface in the muon veto, the performance of the IMT- and the continuous-mode quadrupled and allowed to fill the muon veto in this region with a filling speed of about 20 mm/h in IMT-mode and 98 mm in CM.

Phase 4-5 Level: 535-585 mm

4 XRS 5 Loris 5a + tube HPS

BF>MU +10 mm

Figure 8.4: Phase 4: liquid level adjustment for the MU-level to the 0-level of the buﬀer; Phase 5: establishment of the GC-HPS and the loris tube; Phase 5, step a: establishment of the GC-laser.

Phase 4: Adjust LL of MU/BF Once the MU-liquid reached the lower end of the buffer- slope, filling was slowed down and the filling mode in the muon veto was changed from CM to IMT. Slowly, the MU-level was increased until muon veto and buffer showed the same level. In order to ensure an outward pressure on the buffer-walls, the BF-level was further increased

134 Detector Filling until it exceeded the MU-level by +10 mm. By maintaining this difference, the buffer-slope and the muon veto were homogeneously filled until the BF-HPS-sensor in the BF-side-tube showed a level increase, what marked the upper end of the BF-slope. Once this point was reached, BF-HPS and BF-laser increased equally and both LM-systems could be cross-calibrated to the 0-level of the buffer vessel.

Phase 5: Establish GC-LM Before the liquid level of MU & BF could be increased to the GC-bottom, the GC-vessel had to be pre-filled. Using the volume of one GC-IMT (100 l), the GC-vessel was pre-filled, ensuring the submersion of the HPS-sensors, the XRS-tube and one arm of the Loris-tube, which were commonly installed at the deepest point of the GC-vessel. Since the geometry of the GC-vessel prevented the installation of a direct laser measurement, the GC-liquid level had to be translated. This translation was done by the Loris-tube (for further description, see section 7.3.1), which would automatically have equalized any liquid level differences between the pipes and the GC-vessels, what allowed to translate the liquid level. The upper-parts of the Loris-tube are presented in figure 7.19, while the lower-parts are indicated in figure 8.4. Once both ends of the Loris-tube were sufficiently submerged (pre-filling of GC-vessel with 100 l and the GC-guide-tube with 8 l), the up-side-down-siphon was established. In consequence, the liquid levels in the fully filled guide-tubes were translated into the GC-vessels until the difference was equalized. The laser in the tubes indicated a falling level and stopped when both levels were equal. This self-adjusted-level marked the pre-filling level in the gamma catcher. Knowing in addition the geometry of the vessel and the inserted volume, allowed to calculate the pre-filling level, and subsequently to define the 0-level in the gamma catcher. Using this sequence, the GC-vessel was pre-filled in IMT mode and the Loris-tube was established. According to the geometric calculation, the pre-filling level correlated to 108 l was 35 mm. After the levels were equalized, the GC-HPS showed showed 33 mm. Using this level, the 0-level of the GC could be defined and both LM-systems could be cross-calibrated and additionally cross-checked by the XRS-system.

Phase 6-8 level: 585-1635 mm

6 7 8

+10 mm

BF>GC=MU

Figure 8.5: Phase 6: homogeneous increase of MU- & BF-level to the bottom-slope of the GC-vessel; Phase 7: liquid level adjustment of the BF-level to the 0-level of the gamma catcher; Phase 8: pre-ﬁlling of the NT-vessel including the establishment of the NT-LM (Tamago).

Phase 6: Increase LL to GC-bottom After pre-filling of the gamma catcher, the liquid levels of MU & BF could be homogeneously increased, maintaining the previously adjusted 10 mm- liquid-level difference. With monitoring the level difference, the filling of the next 1000 mm until the GC-slope was not critical and could be realized in continuous mode. In order to increase

135 Detector Filling both levels homogeneously, the flow rates of MU and BF were balanced. After some testing, homogeneous increase could be realized for a flow rate of 3.3 l/min in the muon veto and 8.8 l/min in the buffer, respectively, what allowed to fill the detector with a speed of 24 mm/h. During this phase, the XRS has been constantly used to monitor the level difference and to cross-check the LM-system. Once the BF-level reached the GC-slope, the BF-level increased faster (due to the reduced surface). This increase could be clearly recognized and provided an independent indicator for the reach of the GC-bottom (see, figure E.1).

Phase 7: Adjust LL of MU/BF/GC With the submersion of the GC-slope, the BF-LM indicated an accelerated level increase. In addition, the HPS-sensor in the gamma catcher showed a temperature drop, caused by the contact of the colder BF-liquid with the warmer GC-bottom. Both of these indications could be seen and clearly indicated the submersion of the GC-slope. Subsequently, the levels of MU and BF were homogeneously increased until the BF-level exceeded the GC-level by 10 mm. The higher BF-level lead to an inward pressure on the GC-vessel, which 7 was better for the gluing-joints of the vessel. After the adjustment (BF>MU=GC) of all three levels, all levels were slowly and homogeneously increased until the GC-slope was fully ﬁlled (additional 22 mm).

Phase 8: Establish NT-LM Before the liquid levels of MU, BF and GC could be increased to the target-slope, NT-LM had to be established. Due to the incompatibility with metals, the target scintillator prevented the installation of HPS- and the laser-LM-system within the target vessel. The target therefore used the Tamago-system, which was installed directly in the chimney and through the chimney-extension (see figure 7.17). Before starting the target filling, the entire target scintillator was stored in a weighing tank in the underground lab. This tank was used to measure the mass of the scintillator, once before the filling (the whole mass of the target scintillator) and once after the filling (the rest of the target scintillator, which didn’t fit into the target vessel), what allowed to determine the number of protons filled into the target. For the establishment of the target LM-system, the target-slope (V=53 l, h=38 mm) was pre- filled in IMT-mode with 10 l of scintillator. This increased the liquid level by 7 mm at the center and allowed to establish the Tamago-, and the parallelly installed XRS-system. In order to cross-calibrate the Tamago measurement with the laser measurements in the other vessels, the off-set between the laser-position (muon veto flange) and the Tamago-position (chimney extension-flange) was determined with the help of a leveling instrument (Nivellier). The off-set was subtracted and allowed to correlate the four different liquid levels. This cross-calibration could additionally be checked by the XRS-system.

Phase 9-11 level: 1635-3421 mm

Phase 9: Increase LL of MU, BF, GC to NT-bottom With an established LM- and XRS- system in the target, the liquid levels of MU, BF, and GC could be increased quickly for the next 520 mm, until the GC-liquid reached the bottom of the neutrino target. All liquid levels were increased in continuous mode. The individual flow-rates were again adjusted to: MU: 5 l/min, BF: 8 l/min, and GC: 2.6 l/min. This allowed to maintain the anticipated liquid level differences and to fill the detector with a rate of 32 mm/h. In order to ensure the safety of the detector, the

7The vessels were glued in a way, that an outward pressure pulls the gluing-joints apart, while an inward pressure pushes the joints together. Consequently, an inward pressure is less stress-full for the gluing-joints of the vessels.

136 Detector Filling

center line 9 10 11

-10 mm

GC>NT 10 Figure 8.6: Phase 9: homogeneous increase of MU-, BF- and GC-level to the bottom-slope of the NT- vessel; Phase 10: liquid level adjustment of the MU-, BF- and GC-level to the pre-ﬁlling-level of the target; Phase 11: homogeneous liquid level increase of MU, BF, GC & NT up to the center of the detector.

ﬁlling-speed of the outer levels was reduced shortly before the bottom of the target was reached. Using IMT-mode in the gamma catcher, all levels were slowly increased until the NT-slope was recognized by the LM-systems, the XRS-system and additionally by an accelerated increase of the GC-level, due to the submersion of the NT-vessel (see ﬁgure E.1).

Phase 10: Adjust LL to MU/BF/GC/NT Closely monitoring the LM-system and the XRS, the three upcoming liquid levels of MU, BF and GC were further increased until the GC-level matched the pre-ﬁlling level in the target. By ﬁlling the GC in IMT-mode and MU and BF in CM, all levels were homogeneously increased until the GC-level exceeded the target-level by 10 mm. While maintaining this order, the general liquid level in all vessels was slowly increased until the bottom-slope of the target was full.

Phase 11: Increase LL to Center After filling the slope of target, the non-changing geometry for the next 1229 mm allowed to fill all vessels in continuous mode. In order to ensure a homogeneous level increase, the different flow rates were harmonized, what led to filling rates of 5 l/min in the MU, 8 l/min in the BF, 2.6 l/min in the GC and 2.5 l/min in the target. Using this filling mode, the general liquid level could be increased with a rate of 32 mm/h. This filling phase was stopped once the BF-level reached the center-level of the detector.

137 Detector Filling

Phase 12 level: 3421-3441 mm 12a 12b 12c

Center Line +10 +20mm +10 0mm 0mm 0mm 0mm -20mm -10 -10mm -10mm -10mm

Figure 8.7: Phase 12, step a: liquid level order maintained in the lower part of the detector, which lead to an inward pressure on the acrylic vessels and an outward pressure on the muon veto vessel; Phase 12, step b: re-arrangement of the liquid levels, which changed the pressure situation in the detector; Phase 12, step c: new liquid level order maintained in the upper part of the detector, which lead to an outward pressure in all vessels and avoided a ﬂooding of the diﬀerent top-lids.

Phase 12: Change LL order Anticipating the chimney filling, and thus the flooding of the various top-lids, it was necessary to re-arrange the liquid levels. Continuing filling with the established level-order would have meant to flood the top-lids of NT and GC before the vessels were filled. To avoid this kind of stress, it has been decided to change the level-order from the maintained one to a new step-function, falling in 10 mm-steps from target to muon veto. The levels were re-arranged by homogeneously increasing the GC and NT in continuous mode, until the GC-level exceeded the BF-level by 10 mm. The GC-filling was stopped and the NT-level further increased until the NT-level exceeded the GC-level by 10 mm. Figure 8.7 indicates the previous level order (a), the re-arrangement (b) and the new level order (c). This re-arrangement changed the net-forces on the acrylic vessels from inward to outward. This was unfortunate, however better than the alternative to flood the top-lids before the vessels were fully filled. After setting the new level order, the detector filling was interrupted and the liquid-level-differences in the detector were monitored. In the case of any leakage, the liquid-level-differences would have equilibrated. After monitoring the levels for several days, no indication of leakage could be detected, because of which filling was finally resumed.

Phase 13-14 level: 3441-4688 mm

Phase 13: Increase LL to NT-Top-Lid Being sure that all vessels were tight, all liquid levels were homogeneously increased in continuous mode by using the already established flow rates of MU=5 l/min, BF=8 l/min, GC=2.6 l/min and NT=2.5 l/min. This allowed to maintain the level-order and to raise the general level for the next 1211 mm with a speed of 33 mm/h. Reach- ing the target top-lid in continuous-mode was highly critical as the surface in the target reduced significantly and could have lead to an overshooting of the NT-level. Missing this critical point during filling would have been highly dangerous and would have quickly lead to a fatal level difference. In order to avoid this, the target was equipped with two critical point sensors (see figure 7.18). The first one was mounted 30 mm below the start of the chimney and indicated that the slope was reached and already filled by 8 mm. This signal was the indication to slow down the filling from CM to IMT in the target and to prepare the NT-DFOS for the upcoming chimney filling. Geometric calculations of the target vessels anticipated the first contact (between sensor and liquid) exactly 2504 mm above the NT-0-level. The Tamago-LM-system

138 Detector Filling 13a 13b

center line center line

st 2 nd CPS 1st CPS 2 nd CPS 1 CPS

13c contact 14 contact contact

Chimney Start Chimney Start +10mm -30mm Top-Lid (slope)

A NT-Main Body = 41.50 l/cm Main body end

A NT-Chimney = 0.17 l/cm

Figure 8.8: Phase 13, step a: homogeneous increase of the MU-, BF-, GC-, and NT-level in continuous- mode. The detector was filled until the first critical point sensor (CPS) was reached. Phase 13, step b: liquid levels when the first CPS was reached. At this point, the filling was stopped for about two weeks in order to let the detector liquids equilibrate thermally, before the liquid level was increased into the chimney. Phase 13, step c: detailed view of the target chimney, indicating the transition between the main body and the target-chimney as well as the two critical point sensors, which were used as independent indicators for the reach of the target slope and the target chimney. Phase 14: filling of the target-slope and the reach of the second CPS. At this point, the filling mode in the target was changed from IMT-mode to fine filling mode and used henceforth to fill the target-chimney. indicated exactly 2504.4 mm, when the first sensor began to show contact. The good agreement revealed beautifully the accuracy of the LM-systems and the quality of the vessel construction. In addition, these fixed sensors provided the possibility to re-calibrate the other LM-systems, which, however, was not necessary. During chimney-filling-phase, thermal variations in the detector would have been highly dangerous and had to be avoided. The constant use of the CM-filling-mode, however, introduced large amounts of cooler liquids (MU, BF, GC) from the unheated LSA into the detector. The target liquid, on the other hand, was stored in the underground lab and therefore already thermalized. Consequently, the detector was not in thermal equilibrium, because of which the chimney filling phase was not started right away, but postponed for about two weeks, what allowed the liquids to equilibrate thermally.

Phase 14: Increase LL to NT-chimney Once the detector was thermally stable, ﬁlling could be resumed. Using IMT-mode for the target and CM for the other volumes, all levels were homogeneously increased until the second critical point sensor indicated contact. This second CPS was mounted already 10 mm within the chimney and revealed therefore the deﬁnitive start

139 Detector Filling of the chimney-filling-phase. In parallel, the NT-top-lid was flooded with GC-liquid. This could independently be observed by a decelerating filling-level in the gamma catcher, although the filling-speed was maintained. With the chimney filling, the most critical phase of the entire filling process began, as the detector liquid could only expand into the very small chimney. Only the 2055 mm higher situated expansion tank operating system (XTOS) could ease this situation, because of which all following filling phases belonged to this critical detector filling phase. In order to minimize this critical filling time, the filling team extended the filling efforts from from 10 h on 5 days to 24 h on 7 days.

Phase 15-16 level: 4688-5263 mm

rd 4 th CPS 3 CPS 15a contact 15b

569mm A Main Body = 49.50 l/cm

A Chimney = 0.88 l/cm

4 th CPS 3 rd CPS 1057mm 16 contact contact

557mm

Figure 8.9: Phase 15, step a: detailed view of the GC-slope and the homogeneous increase of MU, BF, GC in CM and the target in FF until the third critical point sensor indicated the reach of the GC-slope. Phase 15, step b: overview of the current liquid level in the detector; Phase 16 : detailed view of the GC-slope, the filling of the GC-slope and the reach of the fourth CPS. At this point, the filling mode in gamma catcher was changed from IMT-mode to fine filling mode and used henceforth to fill the GC-chimney.

Phase 15: Increase LL to GC-Top-Lid Using the fine filling mode (FF) in the target and continuous mode in the outer vessels, the NT-level could be increased for the next 557 mm until the GC-top-lid was reached. The fine filling mode used a smaller version of the IMT-tank, called filling tank (FFT). The FFT had a volume of 250 ml and allowed to handle target liquid on the 10 ml-scale. In the target-chimney, a volume of 176 ml was necessary to increase the liquid level by 1 cm. Expecting the liquid-level to rise by 1 cm, the first FF-tank was filled with 180 ml and emptied into the detector. Unexpectedly, the NT-level did not rise even after the addition of a second FF- tank. The reason for this was the unavoidable flexibility of the acrylic-vessel and its expansion. Due to the expansion, the liquid level difference disappeared and left a stressed target vessel. Consequently, the measured level difference was not a reliable stress-indicator anymore, what meant that the NT-LM system could not have been used to navigate the filling process in the chimney-area any more. The only possibility to keep track of the stress on the target vessels, was to closely monitor the amount of liquid which was inserted (in the chimney leading to an expected liquid level) and to

140 Detector Filling compare this with the value the NT-LM showed. When these two values didn’t agree with one another, it was obvious that there was stress on the target vessels. The only way to clear this stress was to increase the outer liquid levels. For example: The target chimney was filled with 180 ml (means 1 cm), although there was no visible level difference, the vessels were subdued to an outward-stress, which could only be compensated by increasing the outer liquid levels by 1 cm. Remembering that the chimney has already been filled with two FF-tanks (by which the level did not rise), the outer liquid levels in MU, BF and GC were slowly increased by 2 cm. Although the target was not filled, the level in the target rose accordingly due to the compression and relaxation of the target-vessel. The success of this filling method and therefore the safety of the detector furthermore depended on a close monitoring of the outer liquid levels and a good logging of how much liquid should be in the chimney. Using this fine-filling mode, filling was resumed and the detector levels could be increased for another 557 mm until the GC-top-lid was reached. Like the target vessel, the GC-vessel was equipped with two equally installed critical point sensors. They indicated the liquid level 30 mm below (3rd-CPS) and 10 mm above (4rd-CPS) the start of the GC-chimney. All liquid levels were homogeneously increased (MU, BF in CM, GC in IMT and NT in FF) until the third CPS indicated contact. A geometrical calculation anticipated the first contact with the GC-liquid exactly 3654 mm above the 0-level of the GC-vessel. The GC-LM-system indicated 3653 mm, when the liquid was detected by the third CPS-sensor. The good agreement between expectation and measurement revealed the accuracy of the GC-LM-system and the quality of the vessel construction. As already in the target, the CPS-system provided the possibility to re-calibrate the LM-systems in the GC or also over the XRS-system in the other vessels.

Phase 16: Increase LL to GC-chimney The general level in all volumes was further increased until the fourth CPS-sensor indicated the start of the gamma-catcher-chimney (MU, BF in CM, GC in IMT and NT in FF). The fourth sensor has been expected at a GC-level of 3694 mm and was found at 3692 mm. During this phase, the BF-liquid flooded the top-lid of the GC-vessel, which could be clearly identified by the decelerating level increase, although the filling speed in the BF was maintained. With the signal of the fourth CPS, the GC-IMT was re-filled and the GC-filling mode was changed from IMT to fine filling. Starting to fill the second chimney induced an additional stage of difficulties to the filling process as the flexibility of the GC-vessel added up to the level-dynamics in the other vessels. Filling the GC-chimney now led to the expansion of the GC-vessels and, at the same time, to the compression of the target. Both effects had to be considered, as already small compressions of the vessels lead to a significant displacement of liquid in the chimney. In order to keep track of the different stress-levels in the target, and now also in the gamma catcher, each filling step had to be observed precisely and the reaction of each vessel had to be understood. This required a detailed knowledge of the detector-vessels, the different pressure situations in each vessel, the thermal development of the liquids as well as a detailed knowledge about all detector handling systems. This comprehensive and detailed knowledge had been provided by the filling coordinators and allowed to understand the individual filling steps and the reaction of the detector.

141 Detector Filling

Phase 17-18 level: 5263-6336 mm

17a 17b Detector Chimney Muon Veto Top-Lid XTOS-connection-tubes 569mm steady slope of 1.7°

A BF-main Body = 146 l/cm

A = 1.74 l/cm 18 1057mm BF-Cimney

557mm

Figure 8.10: Phase 17, step a: detailed view of the BF-slope and the homogeneous level increase of MU, BF, GC in CM and the target in FF. The filling was stopped when the level increase in the buffer-vessel accelerated due to the surface reduction in the BF-slope. Phase 17, step b: upper part of the detector, surface reduction from main body into the chimney and the connection tubes to the three expansion tanks; The latter marked the end of the critical chimney-filling-phases. Phase 18 : filling of the buffer slope and the slow transition into the buffer chimney; At this point, the filling mode in the buffer was changed from IMT-mode to fine filling mode.

Phase 17: Increase LL to BF-Slope With gamma catcher and target in fine filling mode and MU and BF in continuous mode, the general liquid-level in the detector was slowly increased until the GC-top-lid was fully submerged. Beyond this point, the geometry of the vessels was not changing for the next 1057 mm, because of which the general filling speed could be increased to the maximum. Using continuous mode in MU and BF and fine filling mode in the chimneys of NT and GC, allowed to fill the detector with a rate of 38 mm/h. Unlike the acrylic vessel, the buffer-slope was not equipped with CPS-sensors. Nevertheless, the BF-slope was a critical point and had to be monitored, because of which a different indicator had to be found. As the acrylic-vessels, the buffer-vessel has a slope, however with a height of 73 mm and a volume of 574 l. With a level difference of 1 cm to the muon veto and a slope height of 73 mm, the MU-liquid was flooding the BF-top-lid before the BF-slope could be fully filled. In consequence, the decreasing filling-speed in the muon veto and the characteristic deceleration of the level-increase in the MU-level allowed to independently recognize the reach of the BF- slope. Once the LM-system indicated the necessary height (supported by the flooding of the top-lid), the filling-speed was reduced and the BF-filling-mode was switched from continuous- to IMT-mode. Filling the buffer in IMT-mode, the BF-level was further increased until the liquid level in the buffer indicated an accelerated increase as response to the shrinking surface in the buffer-slope (see figure E.1).

Phase 18: Increase LL to BF-chimney With increasing the BF-level and the shrinking surface, the inserted amount of liquid was reduced until the buffer-level reacted to small amounts of liquid. This was maintained until the BF-level reacted to the liquid inserted by one fine filling tank (4.8 l). At this point, the BF-IMT was filled once more and the DFOS-module was prepared to work in fine-filling-mode. Starting to fill the third chimney again increased the difficulties to understand the behavior of the detector and to predict the stress on the different vessels. The

142 Detector Filling

ﬁlling of one vessel now had measurable impact on three other vessels, which were deformed and accordingly displaced liquid in all detector vessels.

Phase 19-20 level: 6336-6522 mm

Final Liquid Level XTOS-Tubes

Bottle Neck GC-Bellow

NT GC BF MU 20 Expansion tank operating system (XTOS) Final Liquid Level 2cm below MU-TopLid

XTOS -Tubes

Figure 8.11: Phase 19: 3D-CAD drawing of the upper detector, indicating the different chimneys in detail. The homogeneous level increase to the XTOS-tanks and the bottle-neck in the BF-Chimney is shown. The liquid levels are indicated in their related colors. The buffer chimney can be seen in brown, the GC-chimney in purple and the muon veto is presented in gray, the bottle neck is additionally marked by the black circle. Furthermore, the different XTOS-connection tubes are shown. They have a steady upward slope of 1.7○ and connect the individual chimneys with the correlated XTOS-tanks; Phase 20: overview of the connection between detector and XTOS, indicating the XTOS-filling phase, realized in fine filling mode.

Phase 19: Increase LL to XTOS Filling the three chimneys in fine filling mode and the muon veto in continuous mode allowed to increase the general liquid level towards the expansion tanks, which marked the end of this highly critical chimney-filling-phase. However, the BF-chimney has a bottle neck, which had to be passed before the XTOS-tanks could be reached. Figure 8.11 indicates a three dimensional CAD-drawing of the upper chimney-area [59]. The liquids of buffer and gamma catcher are separated by the GC-chimney and therefore by the flexible GC- bellows. This bellows protected the acrylic vessels from mechanical stress, however, extended

143 Detector Filling significantly into the BF-chimney and therefore reduced the available surface area significantly. This sudden area change in buffer and gamma catcher was highly critical and likely to provide stress on the vessels. With exception of the LM-system, no independent indication for the start of this bellows-region existed. The levels were slowly increased until they were supposed to meet the bellows, here the fine filling volume was reduced as well as the maintained liquid-level- differences, from 10 mm to 5 mm per step. Monitoring closely the new level differences, the levels showed only slight variations during the filling of the bellows. Subsequently chimney filling was resumed as before (level differences were kept at 5 mm) and the levels were slowly increased until the liquids started to fill the XTOS tubes and finally the XTOS-tanks.

Phase 20: Start filling XTOS The expansion tank operating system has two main tasks: Firstly, the artificial increase of the chimney surfaces, what allows the detector to tolerate bigger thermal variations, and secondly, the monitoring of the different liquid detector levels after the standard LM-systems were de-installed in order to facilitate data taking. Each XTOS-tank is therefore equipped with a LM-system, which does not disturb the detector performance (see figure 7.20). The Tamago-system as well as the XRS-system use transparent parts (see figures 7.17, 7.19), which undermine the light tightness of the detector and therefore permit to power-on the PMT’s. Although the XTOS-tanks have large volumes, the detector filling and therefore the filling of the XTOS-tanks was realized in fine filling mode. For these last steps, the continuous filling of the muon veto was temporarily stopped and, when necessary, resumed to catch up the general level.

Phase 21-22 level: 6522-6790 mm

Phase 21: Increase to Final Liquid level With the arrival of the XTOS-tanks, the most critical chimney-filling-phase was overcome. Maintaining the fine filling mode in BF, GC and target, the different volumes were filled until the muon veto reached the final liquid level of 6780 mm, which was 2 cm below the muon veto top-lid. Once the final liquid level was reached, the remaining liquid in the target-IMT was pumped back to the weighing tank (WT), where the remaining target-mass was measured. The comparison between the initially and the remaining target-mass allowed to determine the number of protons in the target.

Phase 22: The final step of filling was done on December the 13th of 2010 at 1.46 am, and comprised solely the reduction of the liquid level differences from 5 mm to 1-2 mm and the adjustment of the nitrogen blanket from 0.3 mbar to 1.7 mbar. Subsequently, the filling process was interrupted and the XRS-system was used to measure the finally adjusted liquid levels. A picture of this measurement is presented in figure 8.12. Afterwards, the detector was kept in filling mode and closely monitored for additional five days. During this time, nothing unexpected happened and the level measurement systems used during filling could be stopped and de- installed in order to facilitate the installation of the still missing detector parts.

144 Detector Filling

NT Final Liquid Level GC 2cm below MU-TopLid BF MU

NT GC BF

Final Liquid Level

XRS BFMU GC NT mm

Figure 8.12: Phase 21 : common level increase to the final liquid levels and their adjustment to the final differences; Phase 22 : the XRS-measurement indicating the liquid levels of MU, BF, GC and NT shortly after the final liquid levels were reached, on 13.12.2010 at 2.00 am.

145 Detector Filling

Figure 8.13 shows an overview of the liquid level increase during the entire filling process, which started on 12.10.2010 and lasted, including all interruptions, two month until 13.12.2010. The filling curve shows several plateaus, in which the filling process was slowed down, i.e. for the submersion of a new detector vessel, or stopped completely, i.e. for overnight filling stops or for the thermalization of the detector liquids. Beyond that, filling was interrupted twice for a longer time period due to technical8 and administrative9 reasons, which caused a total delay of about 14 days. Before the liquid level reached the target chimney, the detector was only filled during the working days and thermalized overnight. As the detector is very vulnerable to thermal variations during the chimney filling phase, the filling process was interrupted shortly before the detector liquids reached the first chimney. At this point, filling was interrupted for about two weeks in order to thermalize all detector liquids. After this thermalization phase, filling could be resumed. In order to shorten the time of this critical phase, the detector was filled non-stop (24 h-filling) until completed. Figure 8.13 indicates the liquid levels of MU, BF, GC and NT and their development during the filling. After the successful filling, the detector was prepared

Figure 8.13: Overview of the liquid level increase in the DC far detector between October and December of 2010: The presented liquid levels are the mean values of two independent measurements. For better illustration, the different liquid levels are color coded and slightly shifted (MU = yellow, BF = orange, GC = purple, NT = red). On the right side, a scaled picture of the detector is presented and allows to correlate the liquid level development with inner structure of the detector as well as the different filling phases, which are marked on the right. The presented data-set shows three gaps, which are the result of sudden interruptions of the monitoring system i.e. sudden power loss. Due to those interruptions, the (at this time) currently written data-file suffered damage and could not be recovered. Consequently, these gaps in the monitoring data will appear in all other plots, which cover the same time period. for data taking. This included the de-installation of the level measurement systems, the de- installation of the weighing tank from the underground laboratory and the light tightening of the detector. After these preparations, the still missing detector parts could be installed. This comprised the installation of the upper steel shielding, the installation of the outer muon veto, the installation of the glove box on top of the target chimney and finally the last part of the outer muon veto, which covers the area above the glove box. Once the detector was light tight, commissioning started and lasted until April, 13th, which marks the start of data taking.

8A missing flange of the target chimney prevented the start of the target filling, what caused a significant interruption of about 10 days around 09.11.2010. 9missing paper work regarding the safety of the filling process prevented the start the gamma catcher filling, what caused an interruption of about 4 days around 30.10.2010.

146 Part IV

Performance and Results

147 Chapter 9

Quality of the Produced Detector Liquids

The Double Chooz detector is filled with four different liquids. The production of two of these liquids, namely the muon veto scintillator and the buffer oil, has been the responsibility of the author and was realized with the system and processes presented in part II. In order to be usable for Double Chooz, all liquids have to demonstrate a set of minimum requirements. In general, all liquids have to be equal in density, compatible with the detector materials, highly transparent and free from large radioactive contamination, as these would limit the performance of the detector. Different requirements, however, exist for the light yield of the detector liquids. While the muon veto is supposed to provide a minimum light yield of 5000 photons per deposited MeV, the buffer liquid must not scintillate at all. The individual limits of these requirements are summarized in table 9.1 and presented in section 5.1.2 in more detail. Requirements for the Detector Liquids

Requirement Detector Liquid Unit MU BF GC NT 3 Density g/cm 0.804 ± 0.008 0.804 ± 0.008 0.804 ± 0.008 0.804 ± 0.008 Light Yield p/MeV > 5000 0 > 6000 > 6000 Transparency m@430 nm > 6 > 6 > 6 > 6 Radio Purity 238 −10 −10 −13 −13 U g/g < 10 < 10 < 10 < 10 232 −10 −10 −13 −13 Th g/g < 10 < 10 < 10 < 10 40 −7 −7 −10 −10 K g/g < 10 < 10 < 10 < 10

Table 9.1: Summary of the diﬀerent requirements for the detector liquids, indicating the limits on density, light yield, transparency and radio purity.

In order to monitor the quality of the produced detector liquids and the cleanliness of the production process, various samples were taken and analyzed. The analysis comprised the comprehensive laboratory measurements at TUM, investigating the optical transparency using an UV/vis-spectrometer1, the density using a density-meter2, and the light yield using an individ- ual3 setup at TUM. Details of these measurement methods and the used instruments can be found in [95]. In addition, germanium spectroscopy and neutron activation analysis (NAA) have been used to investigate the radio purity of the used PPO and the ﬁnally mixed muon veto scintillator.

1Co. Perking&Elmer, UV/vis-spectrometer, Lambda 850, 10 cm long sample cell, [99] 2Co. Anton Paar, DMA38, [100] 3Experimental setup at TUM for the determination of the absolute light yield, indicated in ﬁgure C.2.

148 Quality of the Produced Detector Liquids

A detailed presentation of the used instruments as well as the two measurement methods is presented in [72]. Prior to the on-site production process, various samples from different supplying companies were analyzed and compared, what allowed to identify and select the ingredients of the different detector liquids. Having identified the several ingredients and considering the different requirements for muon veto and buffer, their individual composition could be determined. The material selection process has been presented in section 5.2 and finally led to the composition summarized in table 9.2. Detector Liquid Ingredient Composition Amount Density@15○C 3 3 N-paraffine 49.8 %vol. 44.7 m 33700 kg 0.749 g/cm 3 3 Muon veto LAB 50.2 %vol. 45.3 m 36000 kg 0.860 g/cm 90 m3 MU-LS PPO 2 g/l – 180 kg 0.300 g/cm3 bis/MSB 20 mg/l – 1.8 kg 0.450 g/cm3

3 3 Buﬀer liquid Mineral Oil 54 %vol. 60 m 52000 kg 0.854 g/cm 3 3 3 110 m BF-Oil N-paraﬃne 46 %vol. 51 m 39300 kg 0.749 g/cm

Table 9.2: Composition of the muon veto scintillator and the buﬀer liquid. The necessary amounts of the diﬀerent ingredients in order to produce the liquids for one detector are also given. A full summary, including the gamma catcher and neutrino target, can be found in table C.1 in the appendix.

After the proper installation of the infrastructure in the LSA and a thorough cleaning process of both liquid handling systems, the entire infrastructure has been used to receive, mix and handle the different components and to produce 90 m3 of muon veto scintillator and 110 m3 of buffer liquid. In order to monitor the quality of the production process, four samples of each of the both detector liquids were taken and analyzed. Three samples were taken from each of the three storage tanks in the LSA, and a fourth sample from the intermediate tank in the underground laboratory after the liquid had passed the 160 m long trunk line system and was processed by DFOS. The analysis of these samples allowed not only to monitor the quality of the detector liquids, but also to determine the cleanliness and radio purity of the installed systems as well as the quality of the applied mixing process. The following two sections will summarize the experimental results found by analyzing the different samples of the muon veto and buffer liquid.

9.1 Muon Veto Scintillator

9.1.1 Transparency, Light Yield and Density

Using the instruments and measurement methods mentioned above, transparency, light yield and density of the scintillator have been measured. The measurements of these four samples yielded a 3 ○ density of about ρ=0.804 ± 0.001 g/cm at 15 C, a light yield of about 9000 ± 1000 photons/MeV and an attenuation length above 8 m at 430 nm. Figure 9.1 presents the absorption (A) and attenuation length (Λ) of the three muon veto samples taken from the three diﬀerent storage tanks as function of wavelength for the interesting region between 410 nm and 445 nm. Figure 9.2 indicates the same measurement for the intermediate tank sample, which has been done twice. The results of these measurements are summarized in table 9.3.

149 Quality of the Produced Detector Liquids

Figure 9.1: Absorption (A) and attenuation length (Λ) measurements of the three muon veto samples taken from the three diﬀerent storage tanks. A and Λ are presented as function of wavelength between 410 nm and 445 nm. Within the interesting region around 430 nm, the measurements indicated no variation between the tanks and an attenuation length above 8 m. All samples easily met the minimum requirement of 6 m @ 430 nm, which is indicated by the dashed line. The equal and high transparency between the diﬀerent tanks implies, that the entire liquid handling system in the LSA (pumping station and storage tanks) was free from pollution and met the cleanliness requirements of the experiment. Plot taken from [95].

Figure 9.2: Absorption (A) and attenuation length (Λ) measurements of the muon veto sample taken from the intermediate tank in the underground laboratory. The measurement has been done twice. Absorption and attenuation are presented as function of wavelength between 415 nm and 460 nm. For the interesting region around 430 nm, both measurements showed a mean value of 7.95 ± 0.73 m meeting the minimum requirement of 6 m, which is indicated by the dashed line. The comparison between the IMT-samples and the LSA-samples implies, that the trunk line system (TLS) and the ﬁlling system in the underground lab (DFOS) were free of signiﬁcant pollution and met the cleanliness requirements of Double Chooz.

150 Quality of the Produced Detector Liquids

Attenuation length, Density and Light yield of the Muon Veto Scintillator

Sample Sample Attenuation Length in m Density Light Yield no. from @ 420 nm @ 430 nm @ 440 nm g/cm3 Ph/MeV 1 Tank 1 5.10 ± 0.46 8.16 ± 0.73 9.94 ± 0.89 0.805 ± 0.001 – 2 Tank 2 5.20 ± 0.47 8.39 ± 0.76 10.49 ± 0.94 0.805 ± 0.001 – 3 Tank 3 5.33 ± 0.48 8.46 ± 0.76 10.30 ± 0.93 0.804 ± 0.001 – 4 IMT 5.30 ± 0.47 7.95 ± 0.73 9.33 ± 0.88 0.804 ± 0.001 9000 ± 1000 Requirement for MU-LS > 6 0.804 ± 0.008 > 5000

Table 9.3: Attenuation length, density and light yield of the muon veto samples (1-4). The ﬁnal sample (4) has been taken from the DFOS-IMTs in the underground laboratory. The measurement of the last sample has been done twice, presented are the mean values. Values from the storage tanks were taken from [95, 108].

9.1.2 Radio Purity

In order to test the muon veto scintillator for large radioactive contamination, a sample of muon veto scintillator was screened at TUM, using germanium spectroscopy [72]. Although −13 the used setup was not able to reach the required sensitivity of 1⋅10 , the measurement still allowed to identify larger radio chemical contamination’s and to determine upper limits for their concentration. Using this germanium detector with a scintillator sample of 80 g (maximal sample size), the measurement showed no contamination above the normal background of the setup. Considering these background limits, it was possible to determine upper limits for the activity and a mass concentration of 40K, 238U and 232Th in the muon veto scintillator. According to these limits, the muon veto scintillator does not contain larger amounts of radio chemical isotopes. The deﬁned upper limits are summarized in table 9.4. Radio Purity Analysis of the Muon Veto Sample using Germanium Spectroscopy

Muon Veto Sample Units 40K 238U 232Th Activity Bq/kg < 0.192 ± 0.01 < 0.567 ± 0.332 < 0.0301 ± 0.0159 −10 Concentration 10 g/g < 6.42 ± 0.34 < 457 ± 268 < 74.2 ± 39.2 −10 Requirement 10 g/g n.s. <1.0 <1.0

Table 9.4: Upper limits of the activities of 40K, 238U and 232Th contamination in an 80 g muon veto scintillator sample taken from the LSA. The germanium spectroscopy showed no contamination above the sensitivity limits of the detector. Taking these background values allowed to obtain upper limits of 40K, 238U and 232Th. According to these values, no contamination above the given values was introduced by the liquid handling system or the mixing process. Presented values are from [72].

9.2 Buﬀer Liquid

9.2.1 Transparency, Light Yield and Density

Using the same methods as for the muon veto scintillator, all four buffer samples (three from the storage tanks in the LSA and one from the IMT in the underground lab) were investigated, regarding their density, light yield and transparency. The samples showed a density of 3 ○ ρ=0.805 ± 0.001 g/cm at 15 C, no measurable light yield and, with an attenuation length of 14.75 ± 1.30 m, an excellent transparency. Figure 9.3 presents the absorption (A) and attenuation length (Λ) of the three buffer samples taken from the three different storage tanks as

151 Quality of the Produced Detector Liquids function of wavelength for the interesting region between 400 nm and 450 nm. Only one sample in the LSA showed with 11.98 ± 1.08 m a significant variation compared to the 15 m @ 430 nm, which were measured in the other LSA-samples. Since the buffer was constantly circulated and thus homogenized between the different storage tanks, it seems unlikely that one tank holds a less pure liquid, more likely is a negligent sample preparation. In any case, the buffer liquid met the minimum required attenuation length of 6 m.

Figure 9.3: Absorption (A) and attenuation length (Λ) measurements of the three buffer samples taken from the three different storage tanks. A and Λ are presented as function of wavelength between 400 nm and 450 nm. The measurements constantly indicated excellent optical values, only one sample (Storage Tank 1) showed with 11.98 ± 1.08 m a significant reduction, most probably caused by negligent sample preparation. Nevertheless, all samples easily met the minimum requirement of 6 m @ 430 nm. The extremely high transparency of the buffer liquid implies that the entire liquid handling system of the buffer (pumping station and storage tanks) was free from pollution and easily met the cleanliness requirements of the experiment. Plot taken from [95].

Figure 9.4 presents the same measurement for the intermediate tank sample and indicates an attenuation length of 14.71 ± 1.30 @ 430 nm and therefore no significant reduction of the optical quality after the liquid had passed the 160 m long trunk line systems and parts of the DFOS. The measurements have been done twice and showed no significant variation. Density, light yield and attenuation length of the different samples are collected in table 9.5. Attenuation Length, Density and Light Yield of the Buffer Liquid

Sample Sample Attenuation Length in m Density LY no. from @ 420 nm @ 430 nm @ 440 nm g/cm3 Ph/MeV 1 Tank 4 11.12 ± 1.00 11.98 ± 1.08 13.27 ± 1.19 0.805 ± 0.001 – 2 Tank 5 13.93 ± 1.25 15.07 ± 1.36 16.64 ± 1.50 0.805 ± 0.001 – 3 Tank 6 13.95 ± 1.26 15.46 ± 1.39 17.00 ± 1.53 0.805 ± 0.001 – 4 IMT 13.39 ± 1.18 14.57 ± 1.30 16.05 ± 1.47 0.804 ± 0.001 0 Requirement for BF-liquid > 6 0.804 ± 0.008 0

Table 9.5: Attenuation length at 420 nm, 430 nm and 440 nm, density and light yield of the four buffer samples. Samples (1-3) were taken from the different storage tanks. The final sample (4) has been taken from the DFOS-IMTs in the underground laboratory. The measurement of the last sample has been done twice, presented are the mean values. Values from the storage tanks are taken from [95, 108].

152 Quality of the Produced Detector Liquids

Figure 9.4: Absorption (A) and attenuation length (Λ) measurements of the buffer sample taken from the intermediate tank in the underground laboratory. A and Λ are presented as function of wavelength in the region between 400 nm and 450 nm. The measurement was done twice and showed no significant variation. In the interesting region around 430 nm, both measurements showed a mean attenuation length of 14.57 ± 1.30 @ 430 nm far above the required 6 m. The mean value of both measurements is presented in table 9.5. The comparison between the IMT-sample and the LSA-samples implies, that the liquid handling system of the buffer, which has been used to transfer the liquid from the LSA to the underground lab (trunk line system and DFOS), was free from significant pollution and easily met the cleanliness requirements of Double Chooz.

9.2.2 Radio Purity

Unlike the muon veto scintillator, the radio purity of the buffer liquid was not measured in the laboratory. Although not measured explicitly, it can be assumed that the gamma activity of the buffer is less than the upper limits found in the muon veto, because of the following reasons. Firstly, the buffer does not include chemical additives (as PPO or bis/MSB, which are normally a source for contamination), and secondly, the buffer is composed of 46 % n-paraffine, which was already screened as part of the muon veto. Furthermore, the buffer liquid was handled and processed with the same care as already the muon veto scintillator. Based on these facts, it can be assumed, that the buffer liquid is equally clean and also well below the sensitivity threshold of the germanium spectrometer at TUM. Apart from that will allow the analysis of detector data and here especially the accidental background rate in the detector to draw a conclusion about the integrated radio purity and therefore also the radio purity of the individual components.

9.3 Performance of the Liquid- and Gas Handling Systems in the LSA

After the intensive use of all systems during the production of the detector liquids, the performance of the liquid handling as well as the gas handling system in the LSA could be tested. The liquid and the gas handling system provided all functions which were necessary for the reception, mixing and storage of the different liquid components. All these functions could be realized successfully and allowed to produce the 90 m3 of muon veto scintillator and 111 m3 of buffer liquid. The quality of the final detector liquids as well as their cleanliness allowed to state that both the gas and the liquid handling system in the LSA met the cleanliness requirements for Double Chooz. Based on this, it can be concluded that the production, installation and usage of all these systems was successfully realized and with the necessary care.

153 Quality of the Produced Detector Liquids

The only limitation of the liquid handling system is related to the flow rates provided by the pumping stations. Although designed to provide a flow rate of 5000 l/h, the liquid handling system demonstrated a maximal flow rate of only 2200 l/h during uploading, and even less (1600 l/h) during the circulation of the detector liquids. The reason for this limitation was the used membrane pump4 in combination with the used tube size. Although the pump was supposed to provide a capacity of 5000 l/h [73], the pump was overburdened by the impedance of the 1-inch-tubing, which led to 70 % decrease of capacity. Due to the large discrepancy between promised and observed flow rate, consequently the usage of an alternative pump with higher driving force as well as a continuous liquid flow (which was a general drawback of all membrane pumps) would be advisable. A rotary pump could be a suiting alternative, as this pump type is available in the required materials (fluorinated plastics), offers a continous liquid flow and additionally, significantly larger driving forces. Using the maximal driving force of 10 bar (maximal allowed pressure in the liquid handling system in the LSA), such a pump could increase the flow rate significantly. This would not only allow to safe time during the production process, but also during the transport of the liquids to the near-detector, for which the detector liquids have to be transfered to nine ISO-containers (each 26 m3). An increased flow capacity would also reduce the holding time of the delivery trucks during uploading and therefore reduce some of the delivery and rental costs. These costs, however, have to be balanced with the costs for new pumps, which are also significant.

4Co. Trebor, Mega 960, biggest pneumatically driven (max. 5.5 bar) PTFE-membrane pump on the market, which oﬀers 1-inch in- and outlet connections, [73].

154 Quality of the Produced Detector Liquids

Properties of the Detector Liquids

Muon Veto Scintillator Property Unit Required value Measured value 3 Density g/cm 0.804 ± 0.008 0.804 ± 0.001 [95] Transparency m@430 nm > 6 7.93 ± 0.73 [95] Light Yield Ph/MeV > 5000 9000 ± 1000 [95] Radio Purity 238 −10 −10 U g/g <10 < (457 ± 268)⋅10 [72] 232 −10 −10 Th g/g <10 < (74.2 ± 39.2)⋅10 [72] 40 −7 −10 K g/g < 10 < (6.42 ± 0.34)⋅10 [72]

Buﬀer Liquid Property Unit Required value Measured value 3 Density g/cm 0.804 ± 0.008 0.805 ± 0.001 [95] Transparency m@430 nm > 6 14.57 ± 1.30 [95] Light Yield Ph/MeV 0 0 [95] Radio Purity 238 −10 U g/g < 10 n.s. 232 −10 Th g/g < 10 n.s. 40 −7 K g/g < 10 n.s.

Gamma Catcher Scintillator Property Unit Required value Measured value 3 Density g/cm 0.804 ± 0.008 0.8035 ± 0.001[62] Transparency m@430 nm > 6 13.5 ± 1.0 [62] Light Yield Ph/MeV >6000 7560±1000 [120] Radio Purity 238 −13 −14 U g/g < 10 (0.87 ± 0.05)⋅10 [72] 232 −13 −14 Th g/g < 10 (1.34 ± 0.08)⋅10 [72] 40 −10 −9 K g/g < 10 < 2⋅10 [62]

Target Scintillator Property Unit Required value Measured value 3 Density g/cm 0.804 ± 0.008 0.8041 ± 0.001[62] Transparency m@430 nm > 6 7.8 ± 0.5 [62] Light Yield Ph/MeV >6000 7830±1000[120] Radio Purity 238 −13 −14 U g/g < 10 (0.33 ± 0.03)⋅10 [72] 232 −13 −14 Th g/g < 10 (14.8 ± 1.0)⋅10 [72] 40 −10 −9 K g/g < 10 < 2⋅10 [62]

Table 9.6: Comparison between the minimum requirements on MU, BF, GC and NT with the actually measured values for density, transparency, light yield and radio purity. n.s.= not speciﬁed

155 Chapter 10

Accuracy and Performance of Detector Filling and Handling

As described in part III of this thesis, the author was responsible for the development and realization of all detector systems in the underground laboratory, which were necessary to fill and handle the Double Chooz far detector. This required the development of a comprehensive liquid- and gas handling concept and the realization of all necessary systems, which have been presented in chapter 7. This mainly included the development and realization of: A liquid handling system (DFOS, see section 7.1), that allowed to fill and handle each detector liquid individually and with the necessary precision as well as a system, which increased the tolerance of the detector to thermal variations and therefore allowed to handle the detector safely during data taking (XTOS, see section 7.1.3). A gas handling system (HPN, FPN, LPN, see section 7.2), that provisioned the underground lab and cared for a homogeneous and permanent nitrogen blanket in the four different detector vessels during all stages of detector life. A detector monitoring system (DMS, see section 7.3), which measured all absolute and differential liquid-levels as well as gas-pressure-levels in the detector. Subsequently to the proper installation in the underground laboratory, these systems were used to flush, fill and handle the Double Chooz far detector. Using the gas handling system to provide a common and permanent nitrogen blanket, the liquid handling system was used to fill the Double Chooz far detector according to the sequence presented in chapter 8. In order to supervise the filling process and to monitor actual stress on the detector vessels, the DMS measured all absolute and differential liquid-levels as well as gas-pressure-levels in the detector. The following chapter will be used to present the performance of these systems and the accuracy, with which the Double Chooz far detector could be filled and handled. Section 10.1 will concentrate on the accuracy of the filling process and present the development of all liquid- and gas pressure levels in the detector and demonstrate, that the detector was not harmed during the filling process. Section 10.2, on the other hand, will concentrate on the performance of the XTOS and the gas handling system, which were used to maintain the detector safety also during data taking.

10.1 Detector Filling Process

The gas handling system in the underground laboratory is supposed to supply the four diﬀerent detector vessels with a homogeneous and stable low pressure nitrogen blanket. The blanket

156 Accuracy and Performance of Detector Filling and Handling is produced by two interacting systems, the LPN-U-system, which exclusively provisions the hermetically closed detector with nitrogen, and the LPV-U system, which avoids back flow and provides an adjustable impedance (between 0 and 3 mbar). These systems allow to provide each vessel with a separated nitrogen blanket during and after filling. Figure 10.1 presents the absolute over pressures in MU, BF, GC and NT and their development during filling. The individual gas blankets are presented in the same color code as the related liquid levels.

Figure 10.1: LPN-blanket pressure measured in MU, BF, GC and NT. The presented values were monitored by the gas pressure monitoring system of the detector (GPM) and indicate a stable pressure blanket below 0.5 mbar during day-time ﬁlling (before 7.12) and a pressure of about 1.4 mbar in the later chimney ﬁlling phase. The stable and equal values indicate a homogeneous nitrogen supply, a successful gas handling concept and a properly working gas handling system.

During the filling of the main body, the blanket pressures were intentionally kept below 1 mbar and later increased to 1.4 mbar during the chimney filling phase. The levels show several spikes, a prominent one (around 16.11.2010), which is the result of a test of the gas handling system during which the blanket pressure was commonly increased, to about 1 mbar and several small ones, which are the result of pressure variations during the filling process. As can be seen, all detector vessels were permanently provided with a homogeneous low pressure nitrogen blanket.

10.1.1 Performance of the Filling Systems

The basic requirement for all operations during filling was the safety of the detector. Any difference, between the liquid levels, the gas pressure levels or the density, would have led to mechanical stress. In order to ensure the safety of the detector, the total stress on the vessels 3 had to be kept below the critical limits (∆p = 3 mbar, ∆h = 3 cm, ∆ρ = 0.008 g/cm ). Important for the safety of the detector therefore was a homogeneous increase of the different liquid levels, a stable and homogeneous nitrogen blanket as well as equal dense detector liquids. The following section will be used to demonstrate the accuracy of the filling process and summarize the different stress factors, which have been observed during filling.

Observed Liquid Level Diﬀerences during Filling

Figure 10.2 presents the liquid level differences between MU, BF, GC and NT. The presented values are the subtracted mean values of the absolute liquid levels, which were presented in figure 8.13. As can be seen, all liquid level differences remained within the allowed region of

157 Accuracy and Performance of Detector Filling and Handling

± 30 mm. The stress on the muon veto vessel is indicated by the level difference BF-MU and shows a maximum of 23 ± 2 mm at one point of the filling. The stress on the acrylic vessels was smaller and strained gamma catcher vessels at one point with maximal 15 ± 2 mm and the target vessels with 14 ± 2 mm level difference.

Figure 10.2: Liquid level differences between MU, BF, GC and NT observed during detector filling: The presented values are the subtracted mean values of the absolute liquid levels in the detector, which were presented in figure 8.13. The difference between BF-MU is presented in black, the difference between GC-BF in red and the difference between NT-GC is presented in green. None of these level differences exceeded the critical limit of ±30 mm, which is indicated by the dashed lines. The observed values allow to state, that the detector filling was realized homogeneously and well within the anticipated limits. This indicates not only a successful liquid handling concept with a reliable filling system, but also the accuracy with which the detector was filled.

These measurements were in agreement with the visual inspection made with the XRS-system, that was conducted constantly and during each day of filling. Taking these measurements as basis, it can be stated that the filling process did not induce dangerous liquid levels, neither during main-body-filling and the submersion of vessels nor during the difficult chimney filling phase. Based on these measurements, it can be revealed that the filling process was carefully realized and well within the anticipated limits. Considering the dimensions of the detector and its difficult geometry, these small liquid level differences were a great success and expression of a carefully realized filling process.

Observed Gas Pressure Diﬀerences during Filling

During the filling process, each detector vessel was permanently supplied with a low pressure nitrogen blanket. The differences between these individual blankets are presented in figure 10.3.

The observed differences were extremely small, because of which the differences are presented on two different scales, once in comparison with the critical limit and once on smaller scale to present the observed variation. As can be seen, the pressure induced stress levels were extremely small and remained constantly well below 0.5 mbar and are therefore far away from the critical limit of ± 3 mbar. Considering the observed values, the buffer vessel was subdued to a maximal

158 Accuracy and Performance of Detector Filling and Handling

Figure 10.3: Gas pressure differences between MU, BF, GC and NT observed during detector filling: The presented values were monitored by the gas pressure monitoring system of the detector (GPM). Presented are the differences between BF-MU (black), GC-BF (red), NT-GC (green), as well as MU-NT (blue). (top): indicates the observed gas pressure differences in comparison with the maximal allowed pressure difference of ± 3 mbar (dashed lines); (bottom): presents the gas pressure differences on a smaller scale, indicating that no pressure difference exceeded 0.5 ± 0.05 mbar. These low differential pressures impressively show the success of the developed gas handling concept for the DC far detector, which not only compensates variations of the gas handling system but also atmospheric variations in the underground laboratory (see figure 10.8).

159 Accuracy and Performance of Detector Filling and Handling stress of 0.35 ± 0.05 mbar, the gamma catcher vessel to maximal 0.43 ± 0.05 mbar and the target vesselto maximal 0.31 ± 0.05 mbar. Based on these measurements, it can be stated that the detector was successfully supplied with four individual and yet very equal low pressure nitrogen blankets. The stability of the blanket and the low stress levels furthermore allow to conclude that the gas handling concept of the far detector was successful.

Thermally Induced Density Diﬀerences During Filling

Providing equal liquid levels in the detector, density variations between the detector liquids can lead to buoyancy forces and therefore also to mechanical stress on the detector vessels. Large density variations between the liquids could already be avoided during the production 3 during which all densities were matched to 0.804 ± 0.001 g/cm and therefore already a factor of eight better than the critical variation of 1 %. In order to keep track of also small density variations induced by thermal variations, the temperature in the detector liquids was monitored. Figure 10.4 shows the thermal development in MU, BF and GC measured by the hydrostatic pressure sensors during filling. The sensor-heads were installed near the filling lines, because of which the sensors measured the temperature of the arriving liquid during filling, and the temperature of in the detector only during filling-stops. As can be seen, the repeated usage of the continuous filling mode led to a repeated decrease of the averaged temperature in the detector. The largest thermal variation was measured during the chimney filling phase, where the continuous filling mode decreased the average temperature by about two degrees. Considering −4 −1 the thermal expansion coefficient of mineral oil (γmin.oil= 7.6⋅10 K ), the density variation per degree can be calculated by: γ∆T ∆ρ(∆T ) = ρ , 1 − γ∆T where ρ is the density, ∆T the averaged thermal variation of the liquid and γmin.oil the thermal expansion coefficient. Based on this calculation, the density in the liquids varied only by 1.5 ⋅ −3 3 3 10 g/cm , the critical limit of one percent (∆ρ = ± 0.008 g/cm ) would be reached if one of the detector liquids would show a thermal variation of ± 13 K. As all detector liquids decreased homogeneously and only by two degrees, the detector was not subdued to buoyancy forces.

Summary

During the filling process, neither the individual stress factors nor their sum exceeded critical values. The largest mechanical stress during the filling process was induced by liquid level differences: the buffer vessel (stainless steel) was subdued to a maximal level difference of 23 mm, the acrylic vessels, meaning the vessels of gamma catcher and target, were subdued to a maximal level difference of 15 mm and 14 mm. The mechanical stress induced by pressure differences was significantly lower and reached a maximum of 0.35 mbar on the buffer vessel, 0.43 mbar on the gamma catcher vessel and 0.31 mbar on the target vessel. The mechanical stress induced by density differences was only of minor order as the detector liquids were already matched in density (to the per-mil-level) during the production process. Thermal variations in the detector could be observed, however, these variations were homogeneous in all vessels and, in addition, relatively small compared to the critical limits. Consequently, density differences between the detector liquids did not lead to any significant mechanical stress during the filling process. As the different detector vessels have no real indicator for stress, the integrity of each vessel can therefore only be tested with two things. Firstly by monitoring the liquid level differences,

160 Accuracy and Performance of Detector Filling and Handling

Figure 10.4: Temperature variations in MU, BF and GC during filling, measured by the hydrostatic pressure sensors in the detector. The sensor heads were placed near the filling lines, consequently the sensors measured the temperature of the arriving liquids. Due to the use of the continuous filling mode (no thermalization), the arriving liquid was colder than the average temperature in the detector, what is indicated by the various temperature drops. The small spikes (before 30.11.) indicate the day-time filling and the over-night thermalization of the detector. The large spike (after 7.12.) on the right presents the chimney filling phase, in which the HPS-sensors could not thermalize overnight. The actual temperature in the detector liquids can be measured during the filling stops (30.10., 9.11., 30.11.) and demonstrate that the detector has a mean temperature about 14.3 ○C. Also prominent is the pre-filling phase of the gamma catcher, which can be seen at 2.11., in which the temperature drops from 21 ○C in the gas-phase down to 15.5 ○C when the sensors got in contact with the detector liquids.

161 Accuracy and Performance of Detector Filling and Handling which were maintained during the filling process (any leakage would have equalized such level differences, this, however, could not be observed) and secondly by analyzing the detector data. Any leakage in one of the vessels would have led to a mixing of the concerned liquids and therefore to a change of their individual properties: For instance, a leakage in the muon veto or in the gamma catcher would have led to scintillation in the buffer, any leakage in the target, on the other hand, would have led to gadolinium-events in the gamma catcher. As will be shown later (see figures 11.8 and 11.11) such a dramatic change of properties could not be observed however. On the contrary, the event reconstruction indicated clearly separated detector liquids, thus proving the integrity of the different detector vessels and the successful and safe filling of the detector.

10.2 Detector Handling

In order to limit the potential risks for the detector, the liquid levels, the gas pressure levels and the temperatures of the detector liquids are constantly monitored. The gas pressures are monitored with the same system already used for the filling. The liquid levels of BF, GC and NT are measured directly at the expansion tanks (MU has no XTOS-Tank), while the liquid levels and temperatures of MU, BF and GC (no HPS in NT) are measured by the hydrostatic pressure sensors (HPS). The following section will present the development of the liquid- and gas-pressure-levels in the detector observed in the first 18 month after the detector has been filled. The analysis of these data will allow to demonstrate the performance of XTOS and the reliability of the gas handling system.

10.2.1 Performance of XTOS

Figure 10.5 presents the temperature development of MU, BF and GC between December of 2010 and June of 2012. The development shows a heating phase after the start of data taking, which is the result of waste heat introduced by turning on electronics in the underground lab. A quick heating phase of the detector liquids was induced by turning on the PMTs (12.02.). The following dip in the development is caused by an active cooling of the detector top-lid, initialized as response to the increasing temperature. This active cooling of the top-lid was stopped with the installation of the outer veto. This led to a re-heating of the detector before the winter season in France finally dominated the thermal development and caused a constant cooling of the detector. Integrated over 1.5 years, the maximal observed thermal variation in the detector was measured to be 0.9 K. Figure 10.6 indicates the development of the liquid levels of MU, BF, GC and NT during the 18 month of data taking. The levels of BF, GC and NT were obtained by visual inspection of a side-glass mounted on the XTOS-tanks. The muon veto level was measured by the hydrostatic pressure sensor. Cross-calibrated with the information of the XRS-system, both measurements are presented in one plot. As the muon veto liquid is not limited to a chimney but can use the entire surface of the detector (36 m2) for thermal expansion, the level of the muon veto is only slightly affected by a thermal change of 0.9 K. The other liquid levels, however, follow the thermal development of the detector liquids and in- or decrease accordingly. The buffer liquid (as biggest volume with chimney) is affected most and shows a maximal variation of 17 ± 4 mm for a thermal variation of 0.9 K. The smaller gamma catcher liquid varies up to 15 ± 4 mm and the target liquid, as innermost vessel, reacts slowly and only to long term variations. During the first 18 month of data taking, the target scintillator showed a variation of about 10 ± 4 mm. Figure 10.7 presents additionally the development of different liquid level differences in the detector caused by thermal variation. The

162 Accuracy and Performance of Detector Filling and Handling

Figure 10.5: Temperature development measured by the hydrostatic pressure sensors (HPS) in MU, BF and GC after the detector has been ﬁlled. During the commissioning phase of the detector, the PMTs were switched on and caused a quick heating phase (12.02.), which was antagonized by an active cooling of the detector top-lid. The cooling had to be stopped due to the installation of the outer muon veto (OV), what allowed the detector to follow the normal seasonal variation of the temperature. The maximal observed variation (between 12.12. and 12.06.) was 0.9 K and represents the natural thermal variation in the detector.

Figure 10.6: Liquid level variations of BF, GC and NT during the first 1.5 years of data taking. Values were obtained by visual inspection of the XTOS tanks in case of BF, GC and NT, and hydrostatic measurements in case of the muon veto. The plot uses the established color code for the different liquids. The vessels with chimney are more sensitive to thermal variation because of which the levels of BF, GC and NT are more affected by thermal variation.

163 Accuracy and Performance of Detector Filling and Handling differential values were obtained by simple subtraction of the absolute values using the sequence indicated in the legend of figure 10.7. As can be seen, the maximum liquid level differences remained well below the given limit of ± 30 mm.

Figure 10.7: Liquid level differences between MU, BF, GC and NT observed in XTOS during data taking: The presented values were obtained by off-line analysis subtracting the absolute liquid levels in the expansion tanks, presented in figure 10.6. Illustrated are the differences between BF-MU (black), GC-BF (red), NT-GC (green), as well as NT-MU (blue). Despite a thermal variation of 0.9 K, all the level differences remained well within the maximal allowed difference of ±30 mm (dashed lines), indicating that XTOS works as anticipated and successfully increased the tolerance of the detector to thermal variations.

Without XTOS, the liquid levels in the chimneys would increase extremely and lead to a fracturing of the different vessels. Consequently, the safety of the detector depends on the proper function of XTOS. Table 10.1 indicates the capability of XTOS and presents the calculated level increase in MU, BF, GC and NT for a thermal increase of 0.9 K, once with XTOS and once without. These values are finally compared to the maximal observed level changes measured during the 18 month of data taking. Based on these measurements, it can be concluded that XTOS performs as anticipated and increases the tolerance of the DC-detector to thermal variations from ± 0.06 K up to ± 1.5 K. The successful realization of this system increases the safety of the detector significantly and allows to operate the detector without the need of constant adjustment of the liquid levels. Expected Liquid Level Increase for a Homogeneous Thermal Variation of 0.9K

Level Increase/0.9K MU BF GC NT Without XTOS 0.2 cm 42.7 cm 17.1 cm 40.5 cm With XTOS 0.2 cm 2.0 cm 1.6 cm 0.8 cm Observed 0.3 ± 0.4 cm 1.7 ± 0.4 cm 1.5 ± 0.4 cm 1.0 ± 0.4 cm

Table 10.1: Expected liquid level increase in the detector with and without XTOS for a thermal variation of 0.9 K. These values are compared to the actually observed level variations over 1.5 years of data taking.

164 Accuracy and Performance of Detector Filling and Handling

10.2.2 Performance of the Gas Handling System

As already during filling, the gas handling system is supposed to supply the four different detector vessels with a homogeneous and stable low pressure nitrogen blanket. Being subjected to significant atmospheric pressure variations, the nitrogen blanket of the hermetically closed detector depends fully on the capability to compensate for external influences, i.e. atmospheric pressure changes. Thus, the main task of the gas handling system was to compensate for arising pressure differences as those from atmospheric pressure changes or those coming from detector manipulations (insertion and submersion of calibration tools). Figure 10.8 presents the atmospheric pressure variations observed in the underground laboratory during the first 18 month of data taking, recorded between 15.12.2010 and 15.6.2012. As can be seen, the atmospheric pressure normally varies between 10 and 20 mbar during

Figure 10.8: Atmospheric pressure variation during data taking, monitored in the underground lab of the DC-far detector. The plot shows standardly a pressure variation around 10-20 mbar, which has to be adjusted by the gas handling system of the detector. During a strong storm around 15.12.2011, the gas handling system had to adjust a variation of 65 mbar. The three initial peaks, showing an atmospheric pressure of 1050 mbar, are not physical and were induced by electronics. one day and at peak times (heavy thunder storm in Chooz on 15.12.2011) even 40 mbar within only a few hours. Figure 10.9 presents the absolute pressure levels of MU, BF, GC and NT in comparison during the same time period. As evident from figure 10.9, the gas handling system compensates these external influences and provides a homogeneous and stable nitrogen blanket with a nominal overpressure of 1.6 mbar. The visible spikes are induced by detector manipulations (calibration and system tests) during which the absolute pressure levels in the detector were intentionally decreased or increased. Apart from that, the gas handling concept of the detector is supposed to avoid differential pressures between the detector vessels. As it results from figure 10.10, which presents the differential pressures observed during data taking, the stress on the different detector vessels was normally below 0.1 mbar and deviated rarely up to 0.4 mbar during the adjustment of the blanket pressure. When the blanket pressure changes, the differently sized blanket-volumes adjust on different timescales, what temporarily leads to unequal pressures. The varying pressures during the first month of data taking are the result of intentionally increased pressure levels in BF and GC, which allowed to study the operation of the gas handling system and the tightness of the detector vessels. Considering these measurements, it can be stated that the gas handling system provided all detector vessels permanently and stable with individual low pressure nitrogen

165 Accuracy and Performance of Detector Filling and Handling

Figure 10.9: Nitrogen blanket pressure during data taking in the four detector vessels, measured by the detector monitoring system. The plot shows stable levels and a blanket pressure of 1.6 mbar in all detector volumes, indicating a stable operation of the gas handling system of the far detector. blankets. The small diﬀerential pressures furthermore indicate that the gas handling concept of the DC far detector is successful, stable and perfectly able to compensate for external inﬂuences, always keeping the detector within the allowed limits.

166 Accuracy and Performance of Detector Filling and Handling

Figure 10.10: Gas pressure differences between MU, BF, GC and NT observed during data taking: The presented values were monitored by the gas pressure monitoring system of the detector (GPM). Presented are the observed differences between BF-MU (black), GC-BF (red), NT-GC (green), as well as MU-NT (blue); (top): observed gas pressure differences in comparison with the maximally allowed pressure difference of ± 3 mbar (dashed lines); (bottom): gas pressure differences on a smaller scale, indicating that no pressure difference exceeded 0.5 mbar. These low differential pressures impressively indicate the success of the developed gas handling concept for the DC far detector, which not only compensates variations of the gas handling system but also atmospheric variations in the underground laboratory as indicated in figure 10.8. The spikes result from intended manipulations of the LPN-blanket in the detector (see fig. 10.9), which led to non-critical differential pressures between the detector vessels.

167 Chapter 11

Detector Performance & Results from 2012

After the filling and commissioning of the DC-far detector, data taking started on April, 13th th of 2011. The Double Chooz collaboration published its first result for Θ13 on March, 12 of 2012 after analyzing 101 days of data [34] and updated this result with a second publication [22] on August, 30th of 2012. For the second publication, a total data set of 228 days was analyzed. Based on these results, the following chapter will shortly review the performance of the DC far detector and the quality of the used detector liquids. Figure 11.1 presents the data taking efficiency of the Double Chooz far detector indicating a constantly working detector with a data taking efficiency of about 87%. The data acquisition system was triggered upon the identification of a muon-event in the IV or any other energy deposition in the ID. The trigger +0 efficiency increases from 50 % at 400 keV to 100−0.1% at 700 keV.

Figure 11.1: Data taking efficiency of the first 398 days. Using a total detector live time of 398 days, the averaged data taking efficiency was 87 % and lead to 346.2 days of data. Physics data were acquired with 79.8 % efficiency, corresponding to 317 days data, out of which a total of 227.9 days were analyzed and used for the second publication of the DC-collaboration. Plot taken from [65].

168 Detector Performance & Results from 2012

11.1 Cosmogenic Muons in the Inner Muon Veto

Performance & Data of the Inner Muon Veto

The interaction of primary cosmic rays with earth’s atmosphere leads to spallation processes and the production of instable mesons (mostly K±, π±) in the lower atmosphere. Their subsequent ± ± ± decay K , π → µ + νµ(ν¯µ) produces high-energetic muons, leading to a constant shower of ion- izing particles arriving at the surface. This high-energetic shower of hadrons, as well as muons, is a major problem for all particle detectors at the surface, because of which most experiments, and especially those with low counting rates, choose to go deep underground in order to shield themselves from such background. As Double Chooz is built underground, but shielded only with little overburden (enough to shield the hadronic components), the influence of muons is significant and produces an unpreventable and dominating background for the experiment. A proper identification of muons, as well as muon-correlated events, is therefore crucial for Double Chooz and was realized by surrounding the ID with 90 m3 of liquid scintillator, used as inner veto (IV), and covering the whole detector with an additional plane of plastic scintillator, called outer veto (OV). As the outer veto was installed during data taking, it could be used for 68.9% of the here presented data. As atmospheric muons have very high energies, they easily pene- trate matter, produce long tracks through the detector and deposit large amounts of energy in the scintillator. Using the Bethe-Bloch-formula with the Landau-Vavilov-correction [82, 121], a muon deposits 1.8ρ MeV/cm (where ρ is the density of the penetrated material in units of g/cm3), finally leading to an energy deposition of 1.47 MeV/cm [122] in the muon veto scintillator. Measuring the energy depositions in IV and ID, as well as the trigger times of the individual PMTs, allows to identify muons and to reconstruct their tracks through the detector. A muon

Figure 11.2: (left): Time distribution between two muons indicating a muon rate of 39 Hz in the IV. (right): Energy spectrum measured by the IV. The spectrum displays a distinct energy distribution depending on the length of the ionization path of the muon, which allows a rough classification of the muon path as it is indicated in the little graphic. The excess to lower energies is caused by muon events recognized in the ID as well as pre-scaled trigger events, which monitor events below the data-taking- threshold for muons. Pictures taken from [121, 123]. is identified by a total energy deposition of ≥ 5 MeV in the IV by events which deposit more than 30 MeV in the ID. The OV identifies a muon upon multiple correlated hits in neighboring scintillator stripes of which each has to be above 0.1 MeV. If one of these conditions was met the data acquisition systems was triggered and recorded an muon event. In order to study also events below this threshold, a pre-scaled trigger recorded also every thousand event with lesser energy. Both kind of events are summarized in figure 11.2, which presents (∆t) the time between

169 Detector Performance & Results from 2012 two muons events, which indicates a muon rate of 39 Hz and the observed energy spectrum in MeV-scale. Taking an energy deposition of 1.47 MeV/cm and the dimensions of the IV, the spectral features in the energy spectrum correlate to the different muon tracks through the detector. These tracks (1, 2, 3) are indicated in the little detector graphic and roughly correspond to the marked sections in the energy spectrum. The first part of the spectrum complies with a smaller energy deposition and therefore a shorter muon-track, resulting of a single penetration of the IV-layer (path 1, of about 70 cm), as for instance produced by muons, which have only a short path through the muon veto or are stopped in the inner detector. The second part corresponds to an energy deposition between 130 and 300 MeV, describing muons which pene- trate the ID and pass the IV twice (path 2 between 90 and 200 cm). The last section describes higher energetic events, which are either the result of a very long muon-track (path 3, vertical path through the IV) or stopped muons, which produce a particle shower within the detector. Due to the fixed correlation between energy deposition (dE) and track-length (dx), muons can be used as probes to test the energy response and stability of the inner muon veto. In order to measure dE and dx independently, the track length of the muon (in IV) was obtained with a different method. For this measurement only muons, which passed the inner detector were used. Using the timing information of the different ID-PMTs, the muon tracks through the ID were reconstructed and used to extrapolate the muon paths (dx) through the inner veto. As dE/dx Figure 11.3: Variation of the energy deposition per is defined by the scintillator implies a vari- unit of length (dE/dx) of the inner muon veto. The ation of dE/dx an loss of detected scintilla- measurement comprises 300 days and demonstrates an absolutely stable muon veto performance, which tion light. This could be caused by unstable is an indicator for the chemical stability of the muon electronics (gain variations), decreasing light veto scintillator and its compatibility with the used yield (quenching) or decreasing transparency detector materials. For this measurement, the track (degradation). Figure 11.3 presents the vari- length in the muon veto was measured indepen- ation of dE/dx observed over a time period dently, based on geometrical considerations using the of 300 days [124]. The measurement indicates muon tracks which were observed in the ID. Plot a variation of less than 1% over the full time from [124]. period, which demonstrates a stable performance of the inner muon veto (IV) in general and the stability of the used scintillator in par- ticular. Hence, this measurement allows to conclude that the produced muon veto scintillator performs well, is chemically stable in light yield and not subdued to optical degradation.

11.2 Cosmogenic Muons in the Inner Detector

Performance & Data of the Inner Detector

A muon in the inner detector is identified by a total energy deposition of more than 30 MeV. All events which meet this condition are presented in figure 11.4. Fitting the exponential decrease allowed to determine a muon rate of 11 Hz in the ID. Additionally, the related energy spectrum is presented, which indicates muon energies between 30 and 860 MeV. The plot shows different spectral features, what corresponds to different muon tracks (1, 2, 3), indicated in the little graphic of the detector. The first part of the spectrum corresponds to muons, which pass only a segment of the active volumes (path 1). The second part in the spectrum corresponds to muons,

170 Detector Performance & Results from 2012

Figure 11.4: Muon rate and energy spectrum in the ID; (left): Time distribution between two muons in the ID, indicating a total muon rate of 11 Hz. (right): Energy spectrum measured in the ID. The spectrum displays a distinct energy distribution depending on the ionization path of the muon. The increased count rate below 100 MeV is caused by muon decay and the subsequent detection of Michel-electrons with ≈50 MeV. Plots taken from [121, 123]. which fully cross the detector (path 2). The last section describes higher energetic events (path 3), which are produced by spallation processes, hadron showers or stopped muons. Due to the shallow depth of the underground laboratory, muons are a major problem for the experiment, not only due to their large energy deposition, which saturates electronics and covers IBD-events, but due to their capability to produce high energetic neutrons in or nearby the detector. Although these neutrons are background and have to be distinguished from the neutrino signal (for the later neutrino search), these neutrons are a copious calibration source for the detector. The capture of these neutrons on either hydrogen, carbon or gadolinium produces a mono- energetic signal in the scintillator, which can be used as reference to monitor the energy response of the ID and therefore the performance and stability of the inner detector liquids. Figure 11.5 shows the energy spectrum of the inner detector, measured between 150 and 1000 µs after a muon event occurred. The left plot indicates the muon-correlated neutron captures on H (2.2 MeV), C (4.4 MeV) and Gd (8 MeV). The right plot shows the energy response of the detector for the n-capture on gadolinium over a time period of 300 days, which shows no sign of degradation and only a slight variation in the range of ±1%. This indicates an absolutely stable energy response of the inner detector, which implies not only stable electronics, but also the quality and stability of all inner detector liquids: target, gamma catcher and buﬀer.

Muon Correlated Background

Muon induced spallation processes in or nearby the detector form the largest source for correlated background. A prominent example for this background are β-n-emitters, mainly produced by high energetic muons with energies above 600 MeV, which generate a hadron shower. The following spallation processes in the scintillator (normally on 12C) trigger the production of neutron-rich isotopes, as 9Li or 8He. Both isotopes are unstable and reduce their neutron-excess via a β−-decay, followed by the emission of a neutron. The β−-decay leads to a prompt-like signal up to 7.4 MeV for 8He and 11.2 MeV for 9Li, respectively. The emitted neutron is thermalized and ﬁnally captured on gadolinium, what produces a delayed coincidence signal. Due to the life time of the unstable isotopes (9Li: τ = 257 ms, 8He: τ = 172 ms [22]), these background events

171 Detector Performance & Results from 2012

Figure 11.5: (left): Muon correlated energy spectrum indicating neutron captures on hydrogen (2.2 MeV), carbon (4.4 MeV) and gadolinium (8 MeV). (right): Detector stability, measured over a time period of 300 days, using muon-induced neutron captures on gadolinium. The stable energy response of the inner detector indicates a stable detector performance and therefore also the quality and stability of the inner detector liquids. Plots taken from [125, 22]. are correlated in time to the occurrence of a large energy deposition in the detector. Thus, the background events produced by β-n-emitters can be determined by searching for neutrino-like events, which follow a large energy deposition in the detector (>600 MeV) and occur in correlation with the lifetime of 9Li or 8He. A related analysis, including also lower energies, estimates a +0.62 total contribution of 2.05−0.52 events per day. The background contributions from muons, which are recognized by the inner- or outer-muon-veto, can efficiently be reduced by rejecting all events observed in a 1000 µs-window after each muon, to avoid muon-induced neutron captures, and a 0.5 s-window for muons with energies above 600 MeV, to reduced the contribution of long lifetime of cosmogenic isotopes. (a total removal is not possible as the muon rate and the required veto-times would cause unacceptable large dead-time for the experiment.) More problematic for Double Chooz, however, are those muons, which are not detected by the inner- or outer-veto. Those background events can not be rejected, because of which their contribution has to be studied and subsequently subtracted from the measured neutrino rate. A prominent example for such background are fast neutrons. Produced by spallation-processes in the surrounding rock, they have enough energy to reach the ID. Here, the neutron is decelerated via proton recoils, what generates a prompt-like signal, which covers a larger energy range than the prompt signal of the IBD. Subsequent to the deceleration, the thermalized neutron is captured on gadolinium, what produces a delayed coincidence signal. The contribution of this background can be studied by looking at neutrino-like events, which have a high-energetic prompt event (Eprompt ≈ 13-30 MeV). Using this technique and assuming a flat energy distribution, the contribution of fast neutrons could be determined in the lower energy region of the IBD (Eprompt ≈ 0.7-6 MeV) to 0.83 ± 0.38 correlated events per day. Another source are stopped muons, which enter the detector over the uncovered area above the chimney (before the completion of the OV). These muons produce only a short path in the upper region of the detector and subsequently decay (2.2 µs) into Michel1-electrons, which provides a delayed signal. A related analysis yielded 0.30 ± 0.14 events per day. Combining these different contributions, a total muon-correlated background of 2.9 ± 1.1 events per day has to be considered for the neutrino sample.

1Louis Michel, French Mathematical Physicist (1923-1999)

172 Detector Performance & Results from 2012

Muon uncorrelated events in the ID

The first step in the search for neutrino events is the removal of all muon and muon-correlated events from the inner detector data. In order to do so, four pre-selection cuts (described in section 3.7) are applied onto the inner detector data. The remaining data-set, also referred to as the muon-uncorrelated-data or single-spectrum, is the basis for the following neutrino search. This spectrum, however, is yet dominated by uncorrelated events coming from radioactive contamination. Any contamination within the detector (or the scintillator) would lead to an increasing number of single events and, at the same time, to a higher accidental background rate. Con- sequently, the radio-purity of the detector and the detector liquids is vital for the experiment. Figure 11.6 shows the energy spectrum of the pre-selected ID-data. The two spectra represent the prompt (left, 0.7 - 12.2 MeV) and delayed energy window (right, 6-12 MeV), which will later be used for the coincidence search. The single spectrum is largely dominated by radioactivity, clearly indicating two prominent peaks below 3 MeV: the first one at about 1.5 MeV, resulting from γ-emissions of 40K, and a second at about 2.6 MeV, resulting from 208Tl. In addition, the spectrum shows a first indication for neutron captures on carbon at 4.4 MeV and gadolinium at 8 MeV, of which the latter is also shown in the delayed energy window. While 40K is inherently included in the used detector materials (PMT-glass, metals), the ob-

Figure 11.6: (left): Single spectrum in the prompt energy window between 0.7 and 12.2 MeV, indicating the γ-emissions of 40K and 208Tl, as well as neutron captures on carbon and gadolinium; (right): Single spectrum in the delayed energy window between 6 and 12 MeV, indicates the neutron capture on gadolinium and high energetic background events, which are correlated to proton recoils, 12B and gamma emission from neutron capture on 56Fe. Plots taken from [126]. served 208Tl indicates the existence of thorium in the detector, as it is part of its decay-chain. The accumulated activity in the detector leads to uncorrelated single events mostly in the lower energy region. The probability to accidentally identify two uncorrelated events as a neutrino signal increases with the single rates in the respective energy windows and the length of the coincidence window. Figure 11.7 indicates the diﬀerent single rates for the prompt and the delayed energy window. The prompt energy window showed a single rate of 8.20000 ± 0.0006 Hz (65 Hz in the CHOOZ experiment), while the delayed energy window showed only a single event-rate of 0.00491 ± 0.00002 Hz (0.24 Hz in the CHOOZ experiment). This background can best be determined by using an oﬀ-time-window-method [127], which uses a valid prompt event and searches for a valid delayed event in 198 consecutive time-windows each is 98 µs long and shifted by 500 µs, starting 1 s after the initial prompt event. Repeating this measurement thirty times, the accidental background for the DC far detector was found

173 Detector Performance & Results from 2012

Figure 11.7: (left): prompt energy window showing an single event rate of 8.20000 ± 0.0006 Hz; (right): delayed energy window showing a single event rate of 0.00491 ± 0.00002 Hz. Using the off-time window method the accidental background rate could be determined to be 0.261 ± 0.002, which is a factor of three below the anticipated value of one event per day. Both spectra indicate an increasing rate around day 135, which was caused by an increasing artificial background coming from a finally excluded PMT. After excluding the PMT, the previously measured single rates were regained. Plot taken from [126].

−1 to be 0.261 ± 0.002 d . Comparing this value with the anticipated limits of one event per day [55] impressively shows the experimental success to limit the radioactive contamination within the detector. The low accidental background rate is therefore not only a proof of successful and thorough material screening and clean detector assembly, but also of the radio purity of the detector liquids and the cleanliness of the entire liquid-handling-chain, which has been used to mix, handle and ﬁll the detector.A possibility to quantify the contamination of the detector liquids with uranium and thorium oﬀers the distinct signature of the Bi-Po-coincidence, which describes the sequential decay of bismuth and polonium. The decay-chain of 232Thorium includes the isotope 212Bi, which decays into 208Tl (35.8%) and 212Po (64.1%), of which the latter 208 quickly decays into Pb (τ1~2 = 0.3 µs) [71].

τ =60.6 min, β− = 2.25 MeV τ =0.29 µs, α = 8.95 MeV 212 1~2 212 1~2 208 Bi Ð→ P o Ð→ P b This decay-chain leads to a consecutive signal, also referred to as Bi-Po-coincidence, which can be used to determine the amount of 232Th. A similar decay is part of the uranium-decay-chain, given by: τ =19.9 min, β− = 3.27 MeV τ =164.3 µs, α = 7.83 MeV 214 1~2 214 1~2 210 Bi Ð→ P o Ð→ P b . Due to the short lifetime of polonium, both coincidences can be tagged and distinguished, what allows to determine their individual contribution to the single rates. Based on this, it is possible to calculate the concentration of uranium and thorium suspended in GC and NT. Small traces are expected in all materials, because of which Double Chooz deﬁned an upper −14 limit of 20 ⋅ 10 g/g for both isotopes. A corresponding analysis by M. Hofmann [72] shows values well below these limits, the individual concentration of U/Th are summarized in units of 10−14g/g in table 11.1. The low concentration of uranium and thorium proves the cleanliness of Uranium and Thorium Concentration in Target and Gamma Catcher Isotope Unit Limit NT+GC NT GC Uranium −14 g 20 1.10 ± 0.05 0.33 ± 0.03 0.87 ± 0.05 10 g Thorium 20 5.82 ± 0.35 14.8 ± 1.0 1.34 ± 0.08

Table 11.1: 238U and 232Th concentration in NT- and GC-scintillator, determined via the coincidence −14 signals of BiPo-212 and BiPo-214. The measured values are below the acceptable value of 20 ⋅ 10 g/g, indicating the radio purity of the inner detector liquids. Values from [72]. the scintillator production process, as well as the cleanliness of the used liquid-handling-chain.

174 Detector Performance & Results from 2012

This implies permanent cleanliness (on the level of 10−14g/g for U/Th) during the production- process at MPIK, the liquid transport to Chooz, the liquid handling in the LSA, the liquid transfer to the underground lab (TLS) and finally also during the filling process (DFOS). As the cleanliness efforts in Chooz were equal for all the detector liquids, it can be assumed that the liquid handling of muon veto scintillator and buffer liquid were of similar quality. In order to identify individual sources of contamination, which could be induced by installations or detector materials, it is possible to reconstruct the position of the single events and therefore the possible position of the contaminating source. The accuracy of this reconstruction depends on the energy deposition and is about 20 cm for lower energies and improves to 10 cm for energies above 5 MeV. Figure 11.8 presents a 2D-reconstruction of single events in the prompt (top row) and delayed energy window (bottom row). The reconstruction of single events in the prompt energy window shows a higher event rate in the gamma catcher, as well as point like sources at the center of the detector. The gamma-catcher-ring is the first scintillating barrier for the incoming gammas, thus, a homogeneously elevated event rate can be expected. The hot spot in the center, however, is caused by two point-like sources within the detector. The correlated side view of the detector indicates two sources, one near the target-chimney the other near gamma catcher bottom. The upper one is partially caused by incoming radiation (γ, µ, etc. which enter through the uncovered target chimney) and partially caused by a small metal box, which is installed just outside the target chimney as part of the calibration system guide-tube. The lower source is caused by the hydrostatic pressure sensor (GC-HPS, see figure 7.3.1), which had to be installed at this position as part of the detector monitoring system. In order to limit the contamination in the detector, the HPS-sensor was screened at TUM and MPIK. The depicted events corresponds mainly to 40K and other less active isotopes found in the sensor. A full radio assay of this sensor and the mentioned guide-tube-box, is summarized in the appendix see figure D.18. Analyzing the accidental background events accumulated around the HPS-sensor, allowed to determine the impact of the HPS sensor on the accidental background. Based on this selection, the HPS-sensor is responsible for 0.0023 events per day or ∼1 % of the total accidental background rate [128]. The bottom row of figure 11.8 shows reconstructed events in the delayed energy window. Due to the higher energy deposition, the accuracy of reconstruction is higher and indicates a clear separation between GC and BF vessel as well as the absence of events in the buffer. While a higher event rate can be expected from the n-captures on gadolinium, the equal high event rate in the gamma catcher is unexpected. Excluding correlated events this homogenous distributed events are caused by proton-recoil, 12B and an additional contribution coming from n-capture on 56Fe, which emits two high energetic gammas with 6 and 7.6 MeV. Based on the in general very low accidental background rate and the absence of unexpected radioactive sources, this reconstruction shows a very an effective cleanliness concept, a clean detector assembly and clean detector liquids.

Summary

In summary, it can be stated, that all parts of the detector and the detector liquids exceeded the cleanliness requirements of Double Chooz. In addition, the radio purity of the detector liquids demonstrates the cleanliness of the entire liquid-handling-chain, which was used for the production and handling of the liquids. Apart from that, the stable performance of the detector implies that all liquids are chemically stable and compatible with the used detector materials. The clear separation between the diﬀerent detector vessels furthermore allows to state that all inner detector vessels are intact and free from leakage. Consequently, it can be reasoned that the detector was not harmed before, during or after the detector ﬁlling process.

175 Detector Performance & Results from 2012

Figure 11.8: Single event reconstruction in the inner detector vessels: The top row shows the reconstruction of single events in the prompt energy window, the bottom row shows the same reconstruction for the delayed energy window. The top views show the X-Y-plane of the detector in mm. The side view shows the detector-height z in mm (y-axis) and the squared radius ρ2 in mm2 (x-axis); (top left): The top view shows a homogeneously increased event rate in the GC (resulting from external γ’s and a hot spot at the center of the target (coming from two point sources). Within the errors of the reconstruction algorithm, the buffer-liquid shows no scintillation, which implies the integrity of GC and the MU-vessel; (top right): The side view shows two hot spots within the detector, the upper one coming from the combined effect of incoming radiation through the uncovered chimney and a metallic box, which is part of the guide tube. The lower spot is caused by a stain less steel pressure sensors (HPS), which contributes about 1 % of the total accidental background rate; (bottom left): The top view indicates single events between 6 and 12 MeV, which are homogeneously distributed in NT and GC. In addition, the top view shows two areas with elevated event rates, one in the center, which is caused by stopped muons and a second, which is caused by mis-reconstructed artificial background events; (bottom right): The reconstruction shows a vertical line-like feature directly below the central chimney of the detector. This feature is caused by unidentified muons, which enter over the chimney and decay into a michel-electron. In addition, the GC shows a higher event rate. Unexpected in this energy window this the homogenous event rate in the GC (and in the NT) is caused by 7.6 MeV γ’s, which are the result of n-captures on 56Fe, which is part of the BF-vessels,the IV-vessel or the steel shielding. Plot from [128].

176 Detector Performance & Results from 2012

11.3 First Neutrino Data

In order to select the neutrino events out of the acquired data set, diﬀerent selection cuts were applied. The pre-selection cuts (1-3, see section 3.7), which mainly exclude light noise, muons and muon-correlated events, as presented above. The remaining data sample, also referred to as the single spectrum, is then used for the neutrino search. In order to separate the neutrino interactions from the dominating and uncorrelated single events, four additional (neutrino-) selection cuts were applied, explicitly, these cuts extract all events which provide the following coincidence.

4. prompt energy cut: 0.7 > Eprompt > 12.2 MeV,

5. delayed energy cut: 6.0 > Edelayed > 12 MeV, 6. coincident time: 2 µs > ∆T > 100 µs and 7. multiplicity cut: no additional prompt event 100 µs before and 400 µs after the initial prompt event. Applying these cuts on the pre-selected single spectrum, a total number of 8249 neutrino- candidates could be extracted from the data sample. This number is composed of the wanted IBD-events and background events, which passed the neutrino selection cuts. In order to vali- date the origin of these candidates, the selected coincidences can be screened for the individual properties, regarding time-, space- and energy-distribution. In the following, the validation of these selected candidates shall be presented. Figure 11.9 shows the energy distribution of the selected events, separated into the prompt and the delayed energy window. The yellow surface represents Monte-Carlo-simulations and the expectation for prompt and delayed event. In case of the prompt event, the non-oscillation hypothesis is presented. The actual measured energy distribution, including error-bars, is presented in black. The energy distribution of both events shows the expected energy distribution, a variable signal in the prompt energy window and a almost the mono-energetic signal from the two gadolinium-peaks in the delayed energy window, no sign of unexpected energy depositions can be found.

Figure 11.9: Prompt and delayed signal after applying the neutrino selection cuts (4-7) to the single spectrum. Measured data are presented in black, expectations for no-oscillation coming from Monte- Carlo-simulations are presented in yellow. (left): prompt energy window (1.5 - 7 MeV), showing the measured reactor spectrum, peaking between 2 and 3 MeV. (right): delayed energy window (5 - 12 MeV), showing a clearly visible Gd-peak with an average energy of about 8.0 MeV. The visible energy shift is the result of a not fully tuned Monte-Carlo-simulation. Plots taken from [129].

177 Detector Performance & Results from 2012

The absorption of the anti-neutrino on a free proton in the scintillator leads to the production of a positron and a neutron. The time and space correlation between prompt and delayed signal of a true IBD-event is deﬁned by the capture time of the free neutron. In case of gadolinium, the averaged lifetime of the neutron is τ ∼30 µs, which limits the possible distance between both signals. Figure 11.10 presents the time and space correlation of the selected neutrino candidates and indicates a very good agreement between the Monte-Carlo-simulations and the actual measurement. The time correlation shows only one prominent capture time, which indicates the purity of the data sample and the successful exclusion of all neutron captures on other nuclei. The spatial correlation between prompt and delayed event meets the expectation for a mean free path-length between 20-30 cm for neutrons in the target scintillator. The weak interaction

Figure 11.10: Time and space correlation between prompt and delayed event, which can be used for tagging. For the neutrino search in Double Chooz, only the time correlation is used. (left): time difference in µs between prompt and delayed event, showing only one prominent capture time. (right): spatial correlation in cm between prompt and delayed event, which indicates a mean distance of about 25 cm between prompt and delayed event. Plots taken from [129]. of neutrinos with matter should lead to a homogeneous distribution of IBD-events in the target vessels. Any clustering of either prompt or delayed events is unexpected and would imply an IBD-mimicking contamination. In order to avoid such unexpected background, the selected coincidences are reconstructed to their position in the detector. Figure 11.11 presents a top- and side-view of the inner detector, showing the inner volumes of target, gamma catcher and buffer. The plots indicate the reconstructed vertices for the prompt (top row) and the delayed events (bottom row). The color code indicates the number of events presented per pixel. As can be seen, both event-types are homogeneously distributed over the target region and cleanly separated from the gamma-catcher-vessel. The absence of delayed events in the gamma catcher and in the buffer indicate the containment of gadolinium in the target and therefore the integrity of the target- and the gamma-catcher-vessel. Based on this validation, the selected neutrino candidates are free from a large contamination with unexpected events and are, to the larger part, the result of inverse beta decays in the target area. As the neutrino rate depends on the reactor power, the daily neutrino rate should vary with the power levels of the two reactor cores. Here, the Double Chooz experiment has an inherent advantage to all other reactor disappearance experiments currently operating, as Double Chooz measures the flux of only two power cores, which are subdued to alternating and regular maintenance stops. In addition, only a small number of power cores provides the possibility to find a time window in which both reactors are off-line. These rare but extremely valuable phases will allow unique background studies, during

178 Detector Performance & Results from 2012

Figure 11.11: 2D-vertex reconstruction of prompt-and delayed events in the ID. (top left): top view of the ID, indicating reconstructed prompt events. (top right): side view of the detector, indicating the same prompt events in a projection of the squared radius. (bottom left): top view of the ID, indicating reconstructed delayed events. (bottom right): side view of the detector, indicating the same delayed events in a projection of the squared radius. As can be seen, both event types are homogeneously distributed over the target, no sign of clustering can be seen. Within the limits of the reconstruction, all events are contained within the target, implying separated detector liquids and consequently also tight acrylic-vessels. Plots from [130].

179 Detector Performance & Results from 2012 which the contribution of correlated and accidental background events to the observed neutrino rate can be directly measured. During the preparation of this thesis, the two power cores in Chooz operated in all possible operation modes, what allowed to acquire 139.27 days of data with both reactors on, 88.66 days of data with one reactor oﬀ (Pth < 20%) and even 7.53 days during which both reactors were oﬄine. Figure 11.12 presents the observed neutrino rate per day over a time period of one year. The 8249 neutrino candidates were measured over a live time of 227.93 days, corresponding to an averaged neutrino candidate rate of 36.2 ± 0.4 events per day. The shaded boxes indicate time periods, where either one of two reactors was running with less than 20 % power. As can be seen, the shut-down of one reactor reduces the neutrino rate by half, going from about 42 down to about 22 neutrinos per day. The parallel shut-down of both reactors, indicated in the dark shaded box, demonstrates the very low level of background rate of 2.9 ± 1.1 events per day, which is in excellent agreement with the background estimation presented in the last section. Using these estimations, the entire data set of 227.93 days includes 7751.9 neutrino events and 497.1 background events.

Figure 11.12: Observed neutrino rate per day over a time period of one year, indicating the different operation modes of the nearby power cores. The analysis of 227.93 days off neutrino data showed 8249 events, what corresponds to an averaged ν-candidate rate of 36.2 ± 0.4 events per day. The different power- levels of the two reactor cores allowed to observe 139.27 days of data with both reactors on, 88.66 days of data with one reactor off (Pth < 20%) and 7.53 days during which both reactors were offline. Plot from [130].

11.4 First Result for Θ13

During the first phase of the Double Chooz experiment, with only the far detector taking data, the expected neutrino rate has to be determined either by reactor simulations or by adapting an already measured reactor-neutrino flux. With the near detector yet missing, Double Chooz used the measured neutrino flux from the Bugey4-experiment. With the help of newly made reactor simulations, DC adapted this measurement to the experimental situation in DC and was therefore able to obtain the expected neutrino rate for no-oscillation at the far detector. Based on this calculation, the non-oscillation hypothesis predicted a total number of 8440 neutrinos at the position of the far detector. The analysis of 227.93 days, however, yielded a background- subtracted number of 7751.9 neutrinos observed at the far detector and therefore 8.15 % less than expected, providing a ratio of Robs~exp = 0.918. Even though this ratio already indicates the disappearance of electron-anti-neutrinos on short base lines, only the spectral distortion of the prompt energy spectrum allows to unmistakably identify the energy-depending suppression of electron-anti-neutrinos caused by neutrino oscillations. A combined rate and shape-analysis

180 Detector Performance & Results from 2012 is performed by dividing the measured positron spectrum into 19 energy bins (between 0.7 - 12.2 MeV). The ratio Robs~exp is then individually determined for each energy bin, what allows to measure Robs~exp at different energies. A corresponding analysis has been performed by the collaboration and used a standard χ2-estimator, which considered uncertainties in reactor flux, detector response, efficiencies, signal and background statistics. A combined rate- and shape- 2 −3 2 analysis over the full data sample of 227.93 days, using ∆m13 = 2.32 × 10 eV from Minos [31], 2 allowed to find a best-fit value for sin (2Θ13) of 2 sin (2Θ13) = 0.109 ± 0.030(stat.) ± 0.025(syst.), excluding the non-oscillation hypothesis with 99.8% CL (2.9 σ). Figure 11.13 presents the measured positron-spectrum (black) and superimposes the expected 2 spectra for the non-oscillation hypothesis (dashed blue) and the best-fit (red), using ∆m13 = −3 2 2.32×10 eV . In addition, the summed background is indicated in green, combining accidental and correlated background events to 2.9 ± 1.1 events per day. The inlet presents a zoom, in −1 which the individual backgrounds are shown in detail and split into accidental (0.261 ± 0.03 d ) −1 and correlated events, which are composed of fast neutrons (0.83 ± 0.38 d ), stopped muons −1 9 +0.62 −1 (0.30 ± 0.14 d ) and Li-contributions (2.05−0.52 d ).

Figure 11.13: Energy-spectrum for the prompt events (black) and combined background-spectrum (green) for the ﬁrst 227 days of data taken by Double Chooz. The expected energy spectrum for the non-oscillation hypothesis is superimposed in blue, and the best-ﬁt value in red, found by a rate- and shape-analysis; (Inset): stacked histogram of correlated and accidental background events; (Bottom attachment): visualization of the oscillation signal, showing the ratio and the subtraction of observed and expected neutrino rate. Plot from [22].

181 Detector Performance & Results from 2012

Results on Θ13 from Other Experiments

Besides Double Chooz also accelerator-based appearance-experiments, as T2K and Minos [33] aimed for a measurement of Θ13 and published their first results in 2011. The T2K-experiment ○ used a conventional off-axis νµ-beam (2.5 ) produced at J-PARC to search for a νe-appearance at Super-Kamiokande experiment, which is 295 km downstream. T2K reported in [32] the ap- 2 pearance from electron neutrinos consistent with 0.03 < sin (2Θ13) < 0.28 for normal hierarchy 2 and with 0.04 < sin (2Θ13) < 0.34 for inverted hierarchy, both for δCP =0 and both with 90% C.L. Apart from T2K also the MINOS experiment searched for a νe-appearance in a νmu-beam. MINOS is situated along the NuMI neutrino beamline [131] and uses two detectors: a near one 1.04 km downstream of the NuMI target (0.98-kton located at Fermilab) and a far detector with 5.4-ktons located 735 km downstream in the Soudan Underground Laboratory. The two detectors have nearly identical designs, each consisting of alternating layers of steel (2.54 cm) and plastic 20 scintillator (1 cm). After the exposure of 8.2⋅10 protons on the NuMI target, the MINOS- 2 2 collaboration reported an improved limit on Θ13 corresponding to 2 sin (Θ23) sin (2Θ13) < 0.12 2 2 for normal-hierarchy and 2 sin (Θ23) sin (2Θ13) < 0.20 for inverted-hierarchy, both with a con- fidence level of 90% and assuming δ=0. Only recently, the reactor-based disappearance-experiments Daya Bay [23] and RENO [24] confirmed the observation of flavor-oscillations on short base lines and published their first result on Θ13 in 2012. The Daya Bay experiment in China usedν ¯e’s emitted from 6 nearby reactor blocks each with a thermal power of 2.9 GW and employs 6 identical detectors (each with 20 t of target mass) at different distances to search for their disappearance. After an exposure of 139 days and an analysis which based on the disappearance rate only, Daya Bay reported in [132] a best- 2 fit value of sin (2Θ13)= 0.089 ± 0.010(stat.) ± 0.005(syst.), excluding non-oscillation with 7.7 σ. Similar to Dayabay, the RENO-experiment in south Korea employs 6 reactor blocks each with a thermal power of 2.8 GW but only 2 detectors, a near and a far one, each with a target mass of 16 t. Based on a data taking period of 229 days and again an analysis based on rate only the RENO-collaboration reported in [25] a neutrino disappearance cor- 2 responding to a best-fit value of sin (2Θ13)= 0.113 ± 0.013 (stat.) ± 0.019(syst.), excluding non-oscillation with 4.9 σ. Figure 11.14 summarizes the currently measured values for 2 sin (2Θ13) with their 1 σ-error-bars. The successful measurement of Θ13, as well as its rather large value, open the door for several new experiments, which will allow to address CP-violation as well as the neutrino mass hierarchy. The recent measurements of Double Chooz (and all other experiments), are the basis for this future search. Although Double Chooz has a smaller target and offers lesser 2 Figure 11.14: Recent results on sin (2Θ13) measured statistics than Daya Bay and RENO, Double by accelerator- and reactor-based experiments. The Chooz has a very high potential to measure presented results are from Double Chooz [34, 22], Θ13 with the lowest systematical errors. Dou- Daya Bay [26], RENO [25], T2K [32] and Minos [31]. ble Chooz not only was able to build the de- Presented error bars correspond to 1 σ, with the ex- tector with the lowest accidental background ception of T2K, which is indicated with the 90% CL. For T2K and Minos, the CP-violating-phase δ has rate (compared to DB and RENO), it more- been set to δ = 0. Plot taken from [22].

182 Detector Performance & Results from 2012 over compares the neutrino signal from only two instead of six power cores. This alone provides two advantages: firstly, with only two power cores the probability to find them both off line is high, and secondly the disappearance signal from two power cores is significantly easier to disentangle than the one provided by six power cores. Since the start of data taking, Double Chooz was already able to acquire 7.53 days of data during which both reactors were off line. This allowed to measure a the accidental and the correlated backgrounds of the DC-far detector with unprecedented accuracy, what further reduced the systematic error of the experiment. Within the next two years, Double Chooz will begin the second phase of the experiment and start measuring with near and far detector in parallel. With both detectors online, Double Chooz will be able to increase its sensitivity and further reduce systematic as well as statistical errors. The combined analysis of near and far detector data will finally allow to measure Θ13 with highest precision.

Update on the Double Chooz Analysis

During the preparation of this thesis, Double Chooz was able to continue its analysis efforts and published three additional papers. In [133] the collaboration reports about a first test for Lorentz-violation by searching for an annual (sidereal) variation in the neutrino signal. Using the same data set as in [22] (227.93 days of detector live time), no variation could be found, which allowed for the first time to set limits on fourteen different Lorentz-violation coefficients. In [134] the collaboration presents an correlated and accidental background measurement of the DC-far detector based on the analysis of 7.53 days during which both reactors were off line. Using the same selection process as in [22], the measurement provided a combined background rate of 1.0±0.4 events per day, which is less than predicted by the background model used in [22], however, correct within the uncertainties. Most recently, the Double Chooz collaboration could profit from the very high cleanliness of the far detector setup and could exploit the very low accidental background rate. In [1] DC presents the first results on Θ13 obtained by the analysis of neutron capture on hydrogen (Eγ−delayed,H ≈ 2.2 MeV) instead of Gd (Eγ−delayed,Gd ≈ 8 MeV) which were analyzed and presented in [22]. While the higher γ-emissions of Gd allowed to separate the IBD-signal efficiently from the background events, the ν-search with hydrogen is more challenging as the delayed event can not be distinguished from natural background (compare with figure 3.7). Using this hydrogen-analysis, Double Chooz was able to increase the total target volume from 10.3 m3 in the target to 32.8 m3 in target and gamma catcher. 29 This allowed to increase the effective number of target protons from (6.747±0.02) ⋅ 10 [22] 30 in the target by (1.582±0.016)⋅10 protons provided by the gamma catcher. Analyzing the same the data-set as in [22], however, with adapted selection cuts regarding the coincidence windows and for the first time also the spatial relation between prompt and delayed events, allowed to identify 36284 neutrino candidates (ν-events and background). This corresponds to −1 daily candidate rate of 77.69 ± 0.81 d , which is due to the different selection cuts in the same −1 dimension as the accidental background rate of 73.45 ± 0.16 d . A combined rate and shape analysis of the mostly in the GC-found events, equal to the one presented in [22], yielded a 2 preliminary best-fit value of sin (2Θ13)= 0.097 ± 0.034 [1] in very good agreement with previous values for Θ13 reported in [22]. Taking only this hydrogen analysis as basis Double Chooz is able to exclude the non-oscillation hypothesis with 2.0 σ. An overview plot presenting a comparison between the observed and simulated energy spectra of the prompt events of the hydrogen analysis are summarized in figure 11.15. A combined analysis using gadolinium- and hydrogen-events is currently under preparation and will be presented soon.

183 Detector Performance & Results from 2012

Figure 11.15: Hydrogen analysis: Energy-spectrum for the prompt events (black) and combined background-spectrum (green) based on a analysis of n-captures on hydrogen for a total detector live time of 227 days. The expected energy spectrum for the non-oscillation hypothesis is superimposed in blue, and the best-ﬁt value in red, found by a rate- and shape-analysis; (Inset): stacked histogram presents the same spectrum in a logarithmic plot, indicating the diﬀerent contributions of correlated and accidental background events; (Bottom attachment): visualization of the oscillation signal, showing the ratio and the subtraction of observed and expected neutrino rate. Plot from [1].

184 Summary & Outlook

The Double Chooz experiment is, besides to Daya Bay [26] and RENO [25], one of three currently data taking reactor disappearance experiments, which recently succeeded to discover a non-zero value for the last neutrino mixing angle Θ13. This result improves the current knowledge of the standard model of particle physics and will have a considerable impact on the current field of neutrino, as well as astro particle physics. A non zero value for Θ13 provides the opportunity to investigate a possible CP-violation in the leptonic sector, which would be measurable as additional phase (δ) in the leptonic mixing matrix. Providing the respective δ-phase has a measurable influence (δ ≠ 0, ±π), future accelerator experiments will have the chance to observe the influence of a CP-violation by comparing the oscillation probabilities of neutrinos and anti- neutrinos P (να → νβ) ≠ P (ν¯α → ν¯β) [27, 3].

Regarding the search for Θ13, Double Chooz will use two identical liquid scintillator detectors, installed at two different baselines (400 m & 1050 m) from the two power cores, which provide a total thermal power of 8.5 GW [55]. While the near detector measures the unoscillated neutrino flux, the far detector observes an oscillated flux, as it is located near to the first Θ13-oscillation maximum for 2 MeVν ¯’s. A comparison between emitted and observed neutrino rate, as well as its spectral distribution, then allows to observe a disappearance effect and thus to determine Θ13. Double Chooz is realized in two phases: the currently running phase I, which uses only the far detector, and phase II (starting in 2014), in which the near detector joins data taking. For the time being, Double Chooz compares the far detector data to an adapted version of the neutrino spectrum originating from the former Bugey4-experiment [21]. As successor of the CHOOZ experiment, Double Chooz uses a new and optimized detector design composed of four concentrically nested detector vessels, each filled with a newly developed detector liquid. The neutrino target is filled with 10.3 m3 of gadolinium doped scintillator and exploits the distinct signature of the inverse beta decay produced by the positron and the delayed capture of the neutron on Gd. In order to increase the detection efficiency, the target is surrounded by 22.5 m3 of undoped scintillator, which is supposed to detect all gamma-emissions escaping the target. These scintillating volumes are surrounded by 110 m3 of non-scintillating buffer liquid, which is used as transparent shielding against external radiation mainly from the PMTs. The last layer surrounding these inner detector parts is composed of 90 m3 of muon veto scintillator, which is used to identify cosmogenic muons and muon correlated events. In addition, the entire setup is covered by an individual detection module composed of multiple layers of plastic scintillator stripes, which are used as outer muon veto. Liquid scintillators are sensitive detector materials, whose main properties as light yield, transparency or radio purity quickly suffer from wrong handling, improper storage or contact with incompatible materials. Thus, the employment of liquid scintillators in large scale experiments is challenging and requires, besides ideal storage- and handling-conditions, an ultra pure environment. Previous experiments2 suffered from a degradation of their detector liquids. The

2CHOOZ [93] and the Palo Verde experiment [94]

185 Summary & Outlook performance and stability of the detector liquids is therefore a major concern for all current liquid scintillator experiments and the main key for a high precision measurement of Θ13. Within this frame, the here presented work described the development and realization of all processes and hardware systems, which have been necessary for: 1. the clean production and storage of two detector liquids, 2. the clean transfer of these liquids to the underground laboratory, 3. the safe ﬁlling of the detector and 4. the stable handling of the detector during data taking.

11.5 Detector Liquid Production

Development and Installation of the Required Hardware

For the production of the detector liquids, a simple surface building close to the experiment was upgraded into a high-purity large scale mixing facility (LSA), which allowed to separately mix, handle and store 90 m3 of muon veto scintillator and 110 m3 of buffer liquid. In order to realize this task, the LSA was instrumented with: Two independent liquid handling systems, each composed of three large storage tanks, which are interconnected by a correlated pumping station. Each of these system provides all functions to receive, mix and handle the different components and to store the final mixtures for the later filling. In order to protect the detector liquids, all systems are exclusively made of compatible materials as stainless steel or fluorinated plastics. During the filling process, these systems were used to supply the underground laboratory with the different detector liquids. A gas handling system, which regulates the nitrogen flow through the LSA. This system is composed of two supply systems (HPN, LPN), different consumers (storage tanks, pumps) and a ventilation system (LPV). The high pressure system (HPN) was used as control-pressure to run the pneumatically driven membrane pumps and to purge the different liquids during the mixing process. The low pressure system (LPN) was used to supply all storage and transport tanks in the LSA with a constant low pressure nitrogen flow. The ventilation system, finally, collected and purified the “used”-nitrogen-flow and provided an artificial impedance to the outbound nitrogen-flow. This impedance was adjustable and could be used to set and maintain a constant low pressure nitrogen blanket on all storage- and transport-tanks in the LSA. A level monitoring system, which allows to observe all liquid-, gas- and temperature- levels in the different storage tanks of muon veto and buffer. The liquid levels and the temperature were measured by hydrostatic submersion sensors, the gas pressure levels were measured by an independent set of sensors. Using this system, all liquid handling processes (uploading or mixing), as well as gas handling processes (flushing or blanketing), could be monitored and consequently allowed a safe operation of these systems. After the installation of these systems, the entire liquid handling system in the LSA was manually cleaned, isolated and thoroughly flushed with nitrogen. Subsequently, the system could be used for the detector liquid production.

186 Summary & Outlook

Material Selection Process for the Detector Liquids

Prior to the on-site production of the detector liquids, a extensive material selection process was conducted at TUM. Considering the requirements on the final detector liquids, possible ingredients from different companies were compared and individually tested for their key properties: density, transparency, light yield and radio purity. Based on these measurements, the individual components of muon veto and buffer were selected and their composition determined. The results of this selection process are summarized in the following: Components and composition of the muon veto scintillator: For the muon veto scintillator finally selected was: LAB from Helm and n-paraffine from CBR, as well as PPO 3 and bis/MSB from Perkin Elmer. The selected LAB had a density of 0.806 ± 0.001 g/cm and showed an attenuation length of 9.73 ± 0.88 m at 430 nm. The lighter n-paraffine had a 3 density of 0.749 ± 0.001 g/cm and showed an excellent transparency with an attenuation length of 17.17 ± 1.55 m at 430 nm. In order to meet the required density and light yield, the muon veto scintillator is composed of 49.8 %vol. of LAB and 50.2 %vol. of n-paraffine, with an admixture of 2 g/l PPO and 20 mg/l bis/MSB. Due to the large amounts of PPO, a sample of PPO and a sample of the final muon veto scintillator was tested for radio purity, indicating only upper limits for Uranium, Thorium and Potassium (see [62, 72] or compare with table 5.4 and 11.2). Components and composition of the buffer liquid : The analog study for the buffer liquid selected two non-scintillating mineral oils: the already mentioned n-paraffine from CBR and a medical white oil with the trade name Ondina-917 from Shell. Ondina-917 had 3 a density of 0.854 ± 0.001 g/cm and demonstrated an attenuation length of 7.70 ± 0.70 m 3 at 430 nm. In order to meet the required density of 0.804 ± 0.008 g/cm , the buffer liquid is composed of 54 %vol of Ondina-917 and 46 %vol of n-paraffine.

Detector Liquid Production

For the production of the muon veto scintillator, a master solution composed of 4800 l LAB, 180 kg PPO and 1.8 kg of bis/MSB was produced in cooperation with Wacker Chemie. For the on-site production of the detector liquids, this master solution, as well as all other components of muon veto and buffer, were delivered to the LSA following a pre-defined delivery sequence. In order to promote the mixing process, each detector liquid was thoroughly blended by constant circulation and turbulent nitrogen purging in the respective storage tanks. After blending, the density of the liquids was fine-tuned and all liquids were stored in the LSA. In order to monitor the quality of the detector liquids and the cleanliness of the liquid handling system, both detector liquids were sampled. The analysis of these samples as well as the analysis of the first detector data indicated the successful and very clean production of 90 m3 of muon veto scintillator and of 110 m3 of non-scintillating buffer-liquid. The main properties of the produced detector liquids are summarized in table 11.2.

11.6 Liquid Transfer from the Surface to the Underground Laboratory

Development and Installation of a Trunk Line System

In order to transfer all detector liquids from their storage tanks to the ﬁlling system in the underground laboratory, TUM and MPIK installed a dedicated trunk line system (TLS). Due to the large surface being in contact with the liquids and the very low cleanliness conditions in

187 Summary & Outlook

Properties of the Produced Detector Liquids

Property Unit Requirement Muon Veto Buﬀer 3 Density g/cm 0.804 ± 0.008 0.804 ± 0.001 [95] 0.805 ± 0.001 [95] Transparency m@430 nm > 6 7.93 ± 0.73 [95] 14.57 ± 1.30 [95] Light Yield Ph/MeV > 6000 / 0 9000 ± 1000 [95] 0[95] Radio Purity 238 −10 −10 U g/g <10 < (457 ± 268)⋅10 [72] n.a. 232 −10 −10 Th g/g <10 < (74.2 ± 39.2)⋅10 [72] n.a. 40 −7 −10 K g/g <10 < (6.42 ± 0.34)⋅10 [72] n.a.

Table 11.2: Summary and comparison of the initially made requirements on the detector liquids and the actually measured values, using the results from liquid samples analysis and the first detector data analysis. The combined analysis shows that both detector liquids easily meet the requirements for Double Chooz and were consequently produced, handled and stored under high purity conditions. Due to the limited sensitivity of the measurement, only upper limits for the radio-purity of the muon veto can be given. the tunnel during the installation, the TLS was a possible source for re-contamination. In order to ensure the cleanliness of the TLS, all tubes were thoroughly flushed, using gaseous nitrogen and a batch of the final detector liquid. In addition, the tubes of gamma catcher and target were cleaned with an industrial detergent and a light acidic solution. Subsequently, this system has been used during filling to transfer all liquids from their storage tanks to the four different liquid handling systems in the underground laboratory. In order to monitor the cleanliness of this system, the transparency of all liquids was tested before and after the liquids passed the system. All samples showed stable optical properties, what allows to conclude that the TLS was cleanly installed and no source of contamination.

11.7 Filling of the Double Chooz Far Detector

During the filling process, the different detector vessels were subdued to liquid level as well as gas pressure differences. Due to the geometry and fragility of the detector vessels, such level differences can quickly lead to dangerous stress on the detector. The safety of the detector therefore depends on the capability of the filling and monitoring system to avoid or recognize dangerous level differences. In order to provide these capabilities, the Double Chooz far detector was equipped with two systems: The expansion tank operating system, which compensates thermal variations and volumetric changes of the detector liquids, and a gas handling system, which provides a homogeneous and stable nitrogen blanket for the detector and compensates for atmospheric pressure variations.

Development and Installation of all liquid- and gas-handling-systems in the underground laboratory

For the filling of the Double Chooz far detector, the underground laboratory was equipped with three main systems: A filling system (DFOS), composed of four separate liquid handling systems, all made of compatible materials as stainless steel, or in the case of the target system, exclusively fluorinated plastics. Each system provides three different filling modes, which allow to fill,

188 Summary & Outlook

handle and empty the individual detector vessels homogeneously and with the necessary precision to avoid critical liquid level differences above 30 mm. A gas handling system, composed of four sub-systems: three different supply systems (HPN, FPN, LPN), which provide different pressure levels as well as a ventilation system (LPV), which collects, purifies and regulates the outbound nitrogen flow. The HPN was used as supply or control pressure in order to actuate pneumatically driven pumps and valves. The FPN system allowed to flush the detector prior to the detector filling, the LPN system then was used to supply all four detector vessels with an individual nitrogen blanket. Together, these systems supplied the detector during all stages of detector life and allowed to flush the detector before the filling process and to fill and handle all detector liquids under the required nitrogen atmosphere. A detector monitoring system (DMS), which allows to monitor all liquid-levels and gas-pressure-levels in the detector. It is composed of two separate systems: Firstly, a liquid level monitoring system, which measures the absolute- and differential-liquid-levels in all detector vessels, and secondly, a gas pressure monitoring system, which measures the absolute and differential gas pressure-levels in and between the different detector vessels. Design goal for the DMS was the redundant monitoring of all levels with a minimum accuracy of 1 cm and 1 mbar, respectively. Using four independent systems (Laser, HPS, Tamago, XRS) for the liquid level measurement and two separate sets of pressure sensors to measure the gas pressure levels, the DMS demonstrated an accuracy for the absolute liquid level of ± 4 mm and the absolute gas pressure levels of ± 0.1 mbar. The relative differences could be measured with an accuracy of ± 2 mm and ± 0.05 mbar, respectively. Combining the different sensor information in a central level measurement-PC, this system was used to monitor the arising level differences and to supervise the filling process accordingly. After the installation of these systems in the underground laboratory, they were used to flush the detector with a constant nitrogen flow. Applied over several weeks, the oxygen content in the detector was reduced to less than 40 ppm, which was monitored by corresponding sensors mounted in the ventilation system.

Detector ﬁlling

For the detector filling, a dedicated filling process has been developed, composed of 22 individual filling steps, which considered critical filling points and avoided unnecessary stress on the different detector vessels. In order to protect the detector liquids from oxygen and a cross- contamination with the other detector vapors, each vessel was provided with an individual nitrogen blanket. The detector filling was finally realized with the Detector Fluid Operating System (DFOS), which allowed to increase all liquid levels homogeneously and with the necessary precision to avoid large liquid level differences. The filling process was supervised by the Detector Monitoring System (DMS), which allowed to monitor all liquid- and gas-pressure levels in the detector. The analysis of the corresponding level-data showed a homogeneous filling process, during which neither the sum nor the individual level differences exceeded the critical limits. The maximal observed liquid level differences were: 23 ± 2 mm in the buffer vessel, 15 ± 2 mm in the gamma catcher vessel and 14 ± 2 mm in the target vessel. The maximal observed gas pressure differences were significantly lower and reached a maximum at 0.35 ± 0.05 mbar on the buffer vessel, 0.43 ± 0.05 mbar on the gamma catcher vessel and 0.31 ± 0.05 mbar on the target vessel. The stress induced by density differences was only of minor order, as the detector liquids were already matched in density to the per-mil-level during the production process. Thermal variations in the detector could be observed but did not lead to any significant density variations during the filling process. Based on these measurements, it can be stated that none of the

189 Summary & Outlook detector vessels was subdued to any critical level difference and was consequently not harmed during the filling process. The integrity of the different detector vessels could additionally be confirmed by an analysis of the first detector data, where the event reconstruction demonstrated a clean separation between the different detector liquids.

11.8 Detector Handling During Data Taking

During the data taking phase, the Double Chooz detector is subdued to thermal variations, atmospheric pressure changes as well as manual operations as the submersion of calibration tools. Due to the fragility of the detector, these unavoidable inﬂuences can quickly lead to dangerous stress levels and, in the worst case, to a fracturing of the detector vessels. The safety of the detector therefore depends on the capability of the detector handling system to compensate these ﬂuctuations. In order to provide this capability, the Double Chooz far detector was equipped with two systems: The expansion tank operating system, which compensates thermal variations and volumetric changes of the detector liquids, and a gas handling system, which provides a homogeneous and stable nitrogen blanket for the detector and compensates for atmospheric pressure variations.

Development and Installation of an Expansion Tank Operating System (XTOS)

XTOS is composed of three individually sized expansion tanks (with maximized surface), all mounted at the final liquid level in a pit next to the detector. The XTOS-tanks for buffer and gamma catcher are made of stainless steel, the tank of the target is made of PVDF. Each of these tanks is equipped with a side-glass and an additional low pressure nitrogen supply. Each vessel chimney is connected to XTOS by a steady upward going tube that enters at the bottom of the respective expansion tank. Once the liquid level in the detector reaches the XTOS-tanks, the expanding liquid can use the larger surface of the tank, which significantly reduces the level increase per degree. In addition, all tanks are individually sized in order to compensate for the different expansion volumes, which are produced by differently large liquid volumes. Using the critical liquid level of ± 3 cm as baseline, XTOS increases the thermal tolerance of the detector from ± 0.06 K to ± 1.5 K. The performance of XTOS could be demonstrated by monitoring the temperature development in the detector and the related development of liquid levels. The maximal observed thermal variation during 18 month of data taking was 0.9 K. The level variation in the same time frame was monitored and showed the expected variation for a properly working expansion tank system (e.g. 17 ± 4 mm in the buffer, where 2.0 cm were expected, for the other values see table 10.1). The maximal observed liquid level difference was measured to be 18 ± 4 mm between muon veto and buffer. Hence, it can be stated, that the expansion tank operating system works as anticipated and successfully increases the tolerance of the Double Chooz far detector to thermal variations.

Performance of the Gas Handling System during Data Taking

Apart from liquid level diﬀerences, the detector is also exposed to pressure variations, mostly coming from atmospheric pressure changes or variations in the gas blanket system. The safety of the detector therefore depends also on the reliability of the gas handling system and its capability to compensate internal or external pressure variations.

190 Summary & Outlook

The LPN-system supplies the hermetically closed detector with a permanent nitrogen flow. The LPV-system receives the nitrogen and provides an adjustable impedance to the outbound gas flow. As the impedance can be adjusted, it is also possible to regulate the back pressure and to apply a constant low pressure nitrogen blanket in the detector. In order to maintain this blanket pressure, the gas handling system has to compensate for atmospheric pressure changes and react flexible to the arising change. Monitoring the absolute and the differential pressures in the detector during the first 18 month of data taking showed an reliable working gas handling system. Although the gas handling system adjusted to atmospheric pressure changes up to 40 mbar, the gas handling provided a stable nitrogen blanket of 1.6 ± 0.18 mbar, limiting the differential pressures between the vessels normally below 0.1 ± 0.05 mbar and at peak times below 0.4± 0.05 mbar. Based on these results, it can be stated, that the gas handling system in the underground worked successfully and provided the necessary flexibility to compensate for atmospheric pressure changes.

11.9 Conclusion and Outlook

The here presented work described the successful production of two detector liquids, their clean transfer to the underground lab, as well as the successful filling and handling of the detector. Based on this work, the Double Chooz could successfully initialize the first phase of the experiment and start data taking on April 13th of 2011. The analysis of the first detector data showed a stably and well performing detector with cleanly separated detector liquids. −1 An analysis of the accidental background rate indicated only 0.261 ± 0.002 d [22], which is a factor three below the anticipated value, and a compelling evidence for a radio-chemically clean detector assembly and most important, clean detector liquids. The subsequent analysis, including 227 days of data as part of the second publication [22], indicated a best-fit 2 value of sin (2Θ13) = 0.109 ± 0.030(stat.) ± 0.025(syst.), excluding the non-oscillation hypothesis with 2.9 σ [22]. A combined analysis including the results of Daya Bay and RENO excludes 2 sin (2Θ13) = 0 even with 7.7 σ [38]. Considering that Double Chooz currently measures with only one detector (Phase I), which takes data since about one and a half years, the already clear measurement of the oscillation effect is a great success and expression of an ideally performing and clean detector. At present, arrangements for the building of the second (near) detector are in progress and will allow a start of phase II early in 2014. With the implementation of a second detector, Double Chooz will be able to further reduce the reactor based uncertainties, what will allow to minimize the systematic errors of the experiment. Assuming equally good performance also for the near detector, Double Chooz will be able to measure Θ13 with unique accuracy. This accuracy, combined with the unexpectedly large value of Θ13, cleared the way for future experiments to investigate the CP-violation in the leptonic sector as well as the yet unknown mass hierarchy.

Due to the good functionality of the liquid-handling, gas-handling and monitoring systems and the good results, which could be achieved by using these systems, equally designed systems shall be installed and utilized in the near detector also. In addition, the same production process for the detector liquids will be used for the near detector liquids. The gas handling system also does not have to be changed, whereas there is the need of only small modifications within the liquid handling and the monitoring system. For example, the author of this work suggests to exchange the membrane-pumps within the liquid handling systems because of their limited efficiency and noncontinuous flow-pattern. Within the Detector Monitoring System, it is suggested to investigate, if the hydrostatic pressure sensor used in the gamma catcher could be replaced by one with less radioactivity, to further improve the purity of the detector. In addition the level measurement system of the expansion tank demonstrated not the necessary stability,

191 Summary & Outlook because of which it is suggested to exchange this measurement technique by weight-sensors. Furthermore, the monitoring system should be secured against electrical power outage to avoid data loss. With the exception of these changes, there is no further need for improvements of the mentioned systems. This is a great success, regarding the required planning of the near detector, and states again the excellent performance of the realized processes and systems as well as the outstanding quality of the muon veto scintillator and the buﬀer oil, developed and executed by the author of this thesis.

192 Part V

Appendix

193 Appendix A

Double Chooz Experiment

A.1 Detector Design

194 Double Chooz Experiment

Figure A.1: Technical drawing of the DC-far detector: vertical cut through inner and outer detector indicating the structure and the dimensions of the vessels. The vessels are, from inside out, target, gamma catcher, buﬀer and muon veto.Picture taken from [59]

195 Double Chooz Experiment

(a) muon veto, equipped with PMT’s and Level (b) buﬀer vessel equipped PMT’s. Picture from [81] measurement system. Picture from [81]

(c) buﬀer vessel with GC and NT vessels installed. (d) Top-lid of the buﬀer vessel during closure of Picture from [81] the detector. Picture from [81]

(e) Delivery of the ﬁrst liquid components for the (f) Outer muon veto installed after ﬁlling was com- scintillator production in Chooz. Picture from [81] pleted. Picture from [81]

Figure A.2: Pictures a,b,c and d indicate the installation of the different detector vessel. Picture e indicates the delivery of the first liquids while f shows a picture of the outer muon veto after the detector has been filled.

196 Appendix B

Surface Installations

B.1 Liquid Storage Area

B.1.1 Pumping Station

Technical Details of the pumping stations of muon veto & buﬀer)

Name Type Purge Conn. BF Valve ID Nr. MU Valve ID Nr. Diameter Connection Iso.Valve m 0 BU V01 MU V01 1.0 inch welded Iso.Valve m 2 BU V02 MU V02 1.0 inch welded Pneu.Pump Mega 960 0 BF-Pump MU-Pump 1.0 inch flare Iso.Valve m 3 BU V03 MU V03 1.0 inch welded Iso.Valve m 0 BU V04 MU V04 1.0 inch welded Particle Filter 0.5µm 3 BF-Filter MU-Filter 1.0 inch flange Flow meter Coriolis 0 BF-FM MU-FM 1.0 inch flange Iso.Valve m 0 BU V05 MU V05 1.0 inch welded Iso.Valve m 0 BU V06 MU V06 1.0 inch welded Iso.Valve m 0 BU V07 MU V07 1.0 inch VCR male Iso.Valve m 0 BU V08 MU V08 1.0 inch VCR male Iso.Valve m 0 BU V09 MU V09 1.0 inch VCR male Iso.Valve m 0 BU V010 MU V10 1.0 inch VCR male Iso.Valve m 0 BU V011 MU V11 1.0 inch VCR male Iso.Valve m 0 BU V012 MU V12 1.0 inch VCR male Iso.Valve m 2 BU V013 MU V13 1.0 inch welded Sys. Bypass m 2 BU V014 MU V14 1.0 inch welded Pump Bypass m 0 BU V015 MU V15 1.0 inch welded Filter Bypass m 0 BU V016 MU V16 1.0 inch welded Iso.Valve m 0 BU V017 MU V17 1.0 inch welded

Table B.1: pumping station Details: Indicating the name of the system parts, valve type:(m=membrane), number of purge connections, Valve ID-number, valve diameter, valve connection

Liquid Flow Path Description

All valves have identification numbers allowing to describe flow patterns through the system by naming valves and active parts in order of the liquid flow. For instance would the unloading of a delivery truck into the BU storage tank Nr. 6 (using pump, but no filter) be described by: long: [SCP I, BU V01, BU V02, P, BU V03, BU V016, FM, BU V06, BU V07, ST-6 ] Meaning the liquid would enter at system connection point (SCP I) and flow through the valves BF V01, BF V02 passing the pump (P) valve BF V03, the bypass BF V16 and the flow meter

197 Surface Installations

(FM), the last main valve BF V06 before the liquid is routed into the ST via valve BF V07. In the following will for reasons of practicality the description be shortened and consistently used for this document describing the above mentioned way as:

short: [SCP I, 1, 2, P, 3, 16, FM, 6, 7, ST-6]

Table 4.2 summarizes the anticipated flow patterns for the different liquid handling tasks additionally indicating the different options provided by the bypasses.

B.1.2 Storage Tank Instrumentation

The following item describe the storage tank instrumentation in more detail concentrating on top and bottom flange Top Flange : Each tank has DN 200 top flange, which indicates a 24 mm thick stain less steel flange with a diameter of 340 mm, which offering on 200 mm all necessary connections and feed through’s. This flange is fixed with twelve 22 mm bolts to the storage tank and sealed with a flat PTFE gasket. The flange offers one connection for filling tube, three for the gas handling and 2 for the monitoring system. The one inch filling tube coming from the pumping station connects to the upper side of the flange using a welded VCR connector. A 12 cm long and one inch steel tube is running through the flange (10 cm). The lower end will then be used to connect the inner part of the filling tube, which is a 1-inch PFA-tube. For the HPN line the flange offers a welded 1/4-inch Swagelok connector as feed through, which is used for nitrogen purging. For the LPN line the flange offers a welded 3/4-inch VCR connector, which is used for blanketing the storage tanks. For the LPV line the flange offers a welded 3/4-inch ball valve, which can be used to isolate or pressurize the tanks. The last to connections are for the monitoring system. One 1/2-inch thread for the blanket gas pressure sensor and a second bigger one for the liquid level sensor. This bigger thread with a diameter of 5 cm can be closed by a PTFE sealed stain less steel cap. This cap however has at the center a smaller 1/4-inch sliding seal feed through. This cap-sliding-seal-solution allows exchange the sensors in case of a malfunction.

Filling Tube : This 1 inch PFA tube is connected to the inner side of the top flange using a one inch Swagelok straight connector. It runs to the bottom of the tank where it is fixed to the inner side of the bottom flange. Bottom Flange: Each tank has DN 150 bottom flange, which indicates a 22 mm thick stain less steel flange with a diameter of 285 mm, which offering on 150 mm all necessary connections. This flange is fixed with eight 20 mm bolts to the storage tank and sealed with a flat PTFE gasket. The flange offers two 1-inch bottom outlets. The first is directly welded to a one inch pneumatic ball valve the second is a 1-inch VCR connection, with two purge connections as backup in case of a malfunction of primary outlet.

Nitrogen Purge Tube : This 1/4-inch PFA tube enters the tank through the top flange and runs to the bottom of the tank where it is fixed to the inner side of the bottom flange. From this central point the tube is connected via a 1/4-inch Swagelok T-piece to another 1/4-inch PFA tube that is perforated and connects with both ends to the T-piece: This tube is about 10 m long and bended in a way that it has the shape of a flower with four leaves, which is covering the bottom of the tank. This flower shape is held together with stainless steel cable ties, which are interconnecting the bended tube. This flower shaped purging tube is connected to the high pressure nitrogen system and can be controlled by a needle valve and monitored by flow meter both mounted at the HPN panel in

198 Surface Installations

Figure B.1: Technical drawing of the buﬀer and muon veto storage tanks

199 Surface Installations the pumping station. Since this flower-shaped tube is only fixed at the center the outer parts of the tube will move while the HPN gas flow is moving trough it, which is helping to purge the liquid more efficient and homogeneously.

Monitoring Sensors : The gas pressure sensor is mounted (screwed) directly into the top flange and can (if necessary) easily be exchanged since it is accessible from outside. The liquid level measurement uses and hydrostatic submersion sensor, which enter the tank through the top lid and which runs directly down to the lowest point in the tank. The sensor cable of this sensor (black-teflon 5 mm) is entering the top flange through a sliding seal and runs down to the bottom of the tank. The sensor head uses a relative measurement technique, which compares the pressure on a ss-membrane with the pressure in a small capillary (hidden in the sensor cable). This capillary was extracted from the cable and cut open before it exists the sliding seal near the top flange- This ensures a pure measurement of the liquid level independent from the blanket pressure. For the installation it has to be taken care that the sensor head is neither to close to the filling tube nor to the tank exit due to the high flows, which affect the hydro statical pressure measurement. The sensor-head is not fixed to the bottom flange, which allows an exchange in case of a malfunction. For such an exchange it was anticipated to remove the sliding seal completely to offer enough space for the sensor head to pass through. It has to be mentioned that the sensor cable is running through lose cable tie, which could complicate the extraction of the sensor.

Adjustable Tank Feet : Each Tank is supported by four feed as indicated in detail A of figure 4.11. They offer a minimum height of 500 mm to the bottom flange. As indicated in the technical drawing each of this feed has a thread that can be extended. This allows level out the storage tanks independently to the underground.

B.1.3 Gas Handling System

Gas Filter Station

Name Type Purge Valve Number Diameter Connection Main Isolation Valve (pneu.) ball 0 XN V03 1.00 inch welded Manometer 0-9 bar 0 FIPI01 0.75 inch welded Isolation Valve membrane 2 FI V01A 0.75 inch welded Isolation Valve membrane 2 FI V02A 0.75 inch welded Filter 3 nm nominal 0 A&B 0.75 inch welded Isolation Valve membrane 2 FI V01B 0.75 inch welded Isolation Valve membrane 2 FI V02B 0.75 inch welded Manometer 0-9 bar 0 FIPI02 0.75 inch welded Purge Valve ball 0 FI V02Ap 0.25 inch VCR male Purge Valve ball 0 FI V02Bp 0.25 inch VCR male

Table B.2: Gas ﬁlter station details: giving the name of the connection, valve type and number, number of purge connections, valve diameter, valve connection

200 Surface Installations

HPN-High Pressure Nitrogen Manifold + Sub Manifolds

High Pressure Nitrogen Manifolds in the LSA

High Pressure Nitrogen Distributor

Name Type Purge Valve Number Diameter Connection Isolation Valve m 2 HPN V01 0.75 inch welded Pressure Reducer 0-8 bar 0 HPN PR 0.75 inch welded NT & GC supply m 0 HPN V02 0.5 inch VCR male BU supply m 0 HPN V03 0.5 inch VCR male MU supply m 0 HPN V04 0.5 inch VCR male Spare (Safety Valve) 5.5 bar 0 HPN V05 0.5 inch VCR male Spare(free) m 0 HPN V06 0.5 inch VCR male

Muon Veto PS HPN Sub-manifold

Name Type Purge Valve Number Diameter Connection HPN supply ST1 n 0 MU HPN V01 0.25 inch VCR male HPN supply ST2 n 0 MU HPN V02 0.25 inch VCR male HPN supply ST3 n 0 MU HPN V03 0.25 inch VCR male HPN supply Pump n 0 MU HPN V04 0.5 inch Swagelok Flow Meter ST1 float 0 MU ST 1 0.25 inch VCR male Flow Meter ST2 float 0 MU ST 2 0.25 inch VCR male Flow Meter ST3 float 0 MU ST 3 0.25 inch VCR male

buﬀer PS HPN Sub-manifold

Name Type Purge Valve Number Diameter Connection HPN supply ST4 n 0 BF HPN V04 0.25 inch VCR male HPN supply ST5 n 0 BF HPN V05 0.25 inch VCR male HPN supply ST6 n 0 BF HPN V06 0.25 inch VCR male HPN supply Pump n 0 BF HPN V04 0.5 inch Swagelok Flow Meter ST4 float 0 BF ST 4 0.25 inch VCR male Flow Meter ST5 float 0 BF ST 5 0.25 inch VCR male Flow Meter ST6 float 0 BF ST 6 0.25 inch VCR male

Table B.3: high pressure nitrogen manifold: HPN-Manifold and Sub-manifold details: description, valve type(b=ball, m=membrane, n=needle), number of purge connections, Valve ID-Number, valve Diameter, valve connection

Low Pressure Nitrogen Manifolds + Sub Manifolds

B.2 Trunk Line System

201 Surface Installations

Low Pressure Nitrogen Manifolds in the LSA

Low Pressure Nitrogen Distributor

Name Type Purge Valve Number Diameter Connection Isolation Valve m 0 LPN V01 0.75 inch welded Pressure Gauge 0-1 bar 0 LPN PR 0.75 inch welded NT & GC supply m 0 HPN V02 0.75 inch VCR male Separation Valve m 0 LPN V03 0.75 inch welded BU supply m 0 HPN V04 0.75 inch VCR male MU supply m 0 HPN V05 0.75 inch VCR male Spare (Safety Valve) m 0 HPN V06 0.75 inch VCR male

Muon Veto PS LPN sub-manifold

Name Type Purge Valve Number Diameter Connection LPN supply ST1 m 0 MU LPN V01 0.75 inch VCR male LPN supply ST2 m 0 MU LPN V02 0.75 inch VCR male LPN supply ST3 m 0 MU LPN V03 0.75 inch VCR male LPV Connection m 2 MU LPN V04 0.75 inch VCR male Spare capped 0 MU LPN V05 0.75 inch VCR male

buﬀer PS LPN Sub-manifold

Name Type Purge Valve Number Diameter Connection LPN supply ST4 m 0 BF HPN V04 0.75 inch VCR male LPN supply ST5 m 0 BF HPN V05 0.75 inch VCR male LPN supply ST6 m 0 BF HPN V06 0.75 inch VCR male LPV Connection m 2 MU LPN V07 0.75 inch VCR male Spare capped 0 MU LPN V05 0.75 inch VCR male

Table B.4: Low Pressure Manifold Details: Description, Valve Type(b=ball, m=membrane, n=needle),number of purge connections, Valve ID-Number, valve diameter,valve connection

202 Surface Installations

Trunk Line System

Trunk Line Module

Name Type Purge conn. Valve Number Diameter Connection

N2 Isolation Valve pneu.m TL N2 V02 0.75 inch VCR male N2 Isolation Valve m TL N2 V03 0.75 inch VCR male GC Isolation Valve pneu.m TL GC V01 0.75 inch VCR male GC Isolation Valve m TL GC V02 0.75 inch VCR male NT Isolation Valve pneu.m TL NT V01 0.75 inch ﬂare NT Isolation Valve m TL NT V02 0.75 inch ﬂare BF Isolation Valve pneu.m TL NT V01 0.75 inch VCR male BF Isolation Valve m TL NT V02 0.75 inch VCR male MU Isolation Valve pneu.m TL NT V01 0.75 inch VCR male MU Isolation Valve m TL NT V02 0.75 inch VCR male

1st Isolation Valve Box

Name Type Box Nr. Valve Number Diameter Connection

N2 Isolation Valve m TL N2 V04 0.75 inch VCR female GC Isolation Valve m TL GC V03 0.75 inch Swagelok NT Isolation Valve m TL NT V03 0.75 inch Swagelok

2nd Isolation Valve Box Name Type Box Nr. Valve Number Diameter connection MU Isolation Valve m TL MU V03 0.75 inch welded BF Isolation Valve m TL BF V03 0.75 inch welded

3rd Isolation Valve Box Name Type Box Nr. Valve Number Diameter connection MU Isolation Valve m 3 Tunnel TL MU V04 0.75 inch welded BF Isolation Valve m 3 Tunnel TL BF V04 0.75 inch welded N2 Isolation Valve m 3 Tunnel TL N2 V05 0.75 inch welded GC Isolation Valve m 3 Tunnel TL GC V04 0.75 inch ﬂare NT Isolation Valve m 3 Tunnel TL NT V04 0.75 inch ﬂare

Table B.5: Technical details of the trunk line system: description, valve type (b=ball, m=membrane, n=needle, pneu.m=pneumatic membrane), Valve ID-number, valve diameter, valve connection

203 Appendix C

Scintillator Production

C.1 Liquid Composition

Detector Liquid Composition Amount Ingredient CAS-Number

3 Neutrino target 80 %vol 8.2 m n-dodecane 112-40-3 3 3 10.3 m NT-LS 20 %vol 2.1 m PXE 6196-95-8 4.5 g/l 45 kg Gd-(thd)3 14768-15-1 0.5 %wt. 5 kg Oxolane, THF 1099-99-9 7 g/l 7 kg PPO 92-71-7 20 mg/l 0.2 kg bis-MSB 13280-61-0

3 gamma catcher 66 %vol. 14.8 m Mineral oil/Ondina 909 8042-47-5 3 3 22.5 m GC-LS 30 %vol 6.75 m n-dodecane 112-40-3 3 4 %vol. 0.9 m PXE 6196-95-8 2 g/l 45 kg PPO 92-71-7 20 mg/l 0.4 kg bis-MSB 13280-61-0

3 buﬀer Liquid 53.5 %vol. 53.5 m Mineral oil/Ondina 917 8042-47-5 3 3 100 m BF-Oil 46.5 %vol. 46.5 m n-paraﬃn 64771-72-8

3 muon veto 50 %vol. 45 m n-paraﬃn 64771-72-8 3 3 90 m MU-LS 50 %vol. 46 m LAB 68890-99-3 2 g/l 180 kg PPO 92-71-7 20 mg/l 1.8 kg bis-MSB 13280-61-0

Table C.1: Composition of the diﬀerent detector liquids and necessary amounts for the DC-far detector

204 Scintillator Production

Figure C.1: Comparison between a new and used ﬁlter cartridges used in the particle ﬁlter of the pumping stations in the LSA for the unloading of the 26 m3 LAB.

Figure C.2: Experimental setup for the determination of the absolute light yield, indicating a standard back scattering method using 137 Cs. A vertically mounted PMT (left) observes the light emission of the above mounted scintillator sample. The 137 Cs-gammas-radiation,, which is back scattered (180 degree) from the scintillator sample is detected by the second PMT(right). The back scattered gammas are identiﬁed by their residual and lower energy compared to the radiation coming directly from the source. The second PMT and the detection of back scattered gammas allows to setup a coincidence measurement and to deﬁne the deposited energy. The entire setup is situated within a light tight box (black box) in order to facilitate the measurement. Picture taken from [95]. Further information regarding the back scattering mehtod can be found in [95, 135]

205 Appendix D

Underground Installations

D.1 Liquid Handling System

206 Underground Installations

Target nch, PE, gas handling, Lab main exhaust line inch inch liquid lines, 3/4 inch, SS, gas handling, LPN 1/2 PFA, 1/2 inch, stainless steel, liquid lines, Muon veto 1/2 inch, stainless steel,liquid lines, Buffer 1/2 inch, stainless steel,liquid lines, gamma catcher 3/4 gas handling, exhaust line , PFA, 7-10 i Valve 0/I Valve incl bleeding valves 0/I Valve Regulating valve Regulating valve incl. bleeding valves Pressure regulator valve Height adjustable bubbler Pressure indicator / Manometer Oxygen-Monitor Flow meter Filter Myon veto Buffer Y-catcher Target Drawing: P.Pfahler, TUM Drawing: P.Pfahler, PN-U: Flushing pressure Nitrogen Underground Double Chooz Far Lab: Detector Liquid Handling Plan LPN-U: Low pressure Nitrogen Underground LPV-U: Low pressure Ventilation Underground Low pressure Ventilation LPV-U: HPN-U: High pressure Nitrogen Underground F DFOS: Detector Fluid Operating System Operating System Tank Expansion XTOS: XTOS Buffer Expansion 33 m³ Tank 1 Tank MU V 10

MU V 13 MU V 11 V MU MU Y-catcher Expansion MU V 12 Tank 2 Tank Pumping station TUM Target Expansion Weighing Tank Tank Weighing 10 m^3 Tank 3 Tank MU V 09

MU V 06 MU V 08 V MU V6.03 MU V 07 V6.20 V6.61 fine filling 250mL 40 m³ Tank 4 Tank BU V 10 V6.17 1/4”

BU V 13 BU V 11 V BU ½” BF

V6.26 V6.11

1/4” V6.03 V6.06 1/4” Tank 5 Tank BU V 12 V6.60 V6.55 V6.22 V6.22 V6.59

V6.16

V6.09 1/4”

1/4”

Pumping station TUM

V6.00

1/4” V6.18

V6.51

1/4”

V6.53 1/4”

V6.54

1/4” V6.56 Target V6.21 V6.07 V6.02 V6.04 V3.05 V6.08 Tank 6 Tank 1/4” 1/4” PFA 85 L 85 L PFA -40mBar--400mBar BU V 09 Flow meter Cooling BU V 06

PTFE Pump BU V 08 V BU V6.19 Filter 0.5µm Heating V6.14 V6.15 V6.13 V6.56 V6.23 V6.30 V6.31 V6.12 V6.01 BU V 07 V6.32 5 m³ Target Target Pump station (MPIK) 25 m³ Pump station (MPIK) Gamma Catcher Target NT-DFOS V5.20 V5.61

fine filling 4L

1/4” V5.17 1/4” ½”

V5.11

V5.26

1/4” V5.03 V5.06 1/4” V5.60 V5.55 TL N2 V02 TL pneumatic Valve V5.22 V5.22 V5.59

V5.16

V5.09 1/4”

1/4”

V5.00

Trunk Line Trunk Module

1/4” V5.18

V5.51 TL N2 V03 TL

1/4”

V5.53 1/4”

V5.54

SS 100 L SS 100 L -1--3 Bar 1/4” V5.56 LSA LSA V5.21 V5.07 V5.02 V5.04 V5.05 V5.08 Gamma catcher 1/4” 1/4” Flow meter Cooling PTFE Pump V5.19 Filter 0.2µm Heating V5.14 V5.15 V5.13 V5.56 V5.23 V5.30 V5.31 V5.12 TL N2 V04 TL V5.01 Black Box Gamma Catcher GC-DFOS Street V3.20 fine filling 4L V4.61 V4.17 1/4” ½”

V3.26 V4.11

1/4” V4.03 V4.06 1/4” V4.60 V4.55 V3.22 V4.22 V4.59 V4.16 V4.09 1/4”

TL N2 V05 TL

1/4”

V3.00 Tunnel Tunnel

1/4” V4.18

V4.51

1/4”

V4.53

Buffer Buffer 1/4”

V4.54

1/4” V4.56 V3.21 V4.07 V4.02 V4.04 V4.05 V4.08 SS 300 L SS 300 L -1--3 Bar 1/4” 1/4” Flow meter Neutrino Laboratory Cooling PTFE Pump V3.19 Filter 0.2µm Heating V4.14 V4.15 V3.13 V4.56 V4.23 V4.30 V4.31 V4.12 V4.01 Buffer BF-DFOS V3.61 V3.17 1/4” ½”

V3.11

1/4” V3.03 V3.06 1/4” V3.60 V3.55 V3.22 V3.22 V3.59

V3.16

V3.09 1/4”

1/4”

V3.00

1/4” V3.18

V3.51

1/4”

V3.53

1/4”

V3.54

SS 185 L SS 185 L -1--3 Bar

1/4” V3.56 V3.21 V3.07 V3.02 V3.04 V3.05 V3.08 1/4” 1/4” Concrete Channel Muon Veto Muon Veto Flow meter Cooling PTFE Pump V3.19 Filter 0.2µm Heating V3.14 V3.15 V3.13 V3.56 V3.23 V3.30 V3.31 V3.12 V3.01 Myonveto MU-DFOS Detector

Figure D.1: Global overview about the liquid handling system in the underground lab; (top): indicates the TLS from the LSA to the underground lab; (center): indicates DFOS and the four individual modules for MU, BF, GC and NT; (bottom, right): presents XTOS and the three tanks for BF,GC and NT, (bottom, center): presents the DFOS connections to the detector indicating short and long ﬁlling tube.

207 Underground Installations

D.1.1 DFOS Instrumentation of MU, BF, and GC

Intermediate Tank (IMT) The individual IMT sizes (MU=185 l, BF=300 l,GC=100 l) account for the liquid level change they would induce in the detector if a full IMT would be drained (accidentally) into the detector. Calculations of the vessel group suggest that a liquid level difference of more than 3 cm would be critical. The IMT’s and their volumes comply to those restriction by holding an equivalent of only 2 cm per IMT. This level increase clearly depends on the available surface,, which was calculated at mid z- level of the detector. Figure D.3 is illustrating the available surface area when the liquid is still in the main bodies (mid-z-level) and when the liquid is in the chimney. The IMT’s for MU,BF,GC are made of stainless steel and were designed to tolerate a pressure range between -1 and +3 bar. Each IMT is equipped with different monitoring sensors displaying temperature, gas pressure. The liquid level is normally visually monitored using a side glass but special levels (full, mid-full and empty) are monitored by capacitive sensors mounted on the side glass. These levels are additionally transferred to the PLC, what allows to control automated re-filling- or emptying-processes. Further information about the intermediate tanks, their gas- as well as liquid connections are summarized in table D.2. The gas connections allow to pressurize, to ventilate each IMT individually. Three liquid connections, one entering from the top and going to the bottom (allowing also to use this connection backwards), a and two entering from the bottom. One is a standard bottom outlet and one overflow tube, which provides the MU-DFOS option to recirculation liquid from and to the detector. See also table D.2 for a detailed technical summary, including all connection points..

Heat Exchanger Since the liquids of MU, BF, GC are stored in the LSA (which has no environmental control) they are exposed to large thermal variations. For the detector filling these liquids have to have the same ○ temperature as naturally observed in the underground lab (∼14.5 C ). Consequently all liquids stored in the LSA have to be thermalized during filling by heating in winter, or cooling in summer. The thermal control system consist of a heating-cooling unit situated in front of the lab able to send hot or cold water through a heat exchanger. Cooling and heating This stain less steel piece offers a big surface and prevents any mixture unit between the heating medium (water) and the scintillator. It is installed within the DFOS tubing right before the entrance of the IMT. The PLC offers a individual operation mode called IMT-circulation,, which allows to circulate and thermalize the detector liquids within each DFOS-module. A thermometer (installed directly on the IMT) is monitoring the scintillator temperature and supplies the heating and cooling unit (H&C unit) as well as the PLC with feed back information,, which is initializing or terminating the circulation process once the anticipated scintillator temperature is reached. When the scintillator temperature is leaving an user defined range the circulation is restarted automatically. For general safety of the scintillator the heating and cooling capacity of this system is limited between +9 C○ and +25 C○ for laboratory and environmental safety reason coolant was chosen to be water instead of more efficient liquids.

208 Underground Installations

Intermediate Tank Details

Dimensions Unit MU BF GC NT Height mm 1000 1038 1020 995 Diameter mm 550 700 550 400 Wall thickness mm 2.5 3 2.5 15 Norm.Volume l 185 300 100 85 Max. Volume l 200 330 110 100

Pressure Unit MU BF GC NT Max. Pressure bar +3 +3 +3 +0.4 Min. Pressure bar -1 -1 -1 0

Material Unit MU BF GC NT Walls SS SS SS PVDF Top/Bottom SS SS SS PVDF Joints PTFE PTFE PTFE PVDF

Instrumentation Unit MU BF GC NT Bottom Flange mm DN100 DN100 D100 DN100 1-NPT-Side Glass inch 0.5 0.5 0.5 0.5 2-NPT-Liquid Outlet inch 0.5 0.5 0.5 0.5 3-NPT-Liquid Outlet inch 0.5 0.5 0.5 0.5 4-G-Side Temp.sensor inch 0.5 0.5 0.5 0.5

Top Flange mm DN150 DN150 DN150 DN370 5-NPT-Side Glass inch 0.5 0.5 0.5 0.5 6-NPT-Liquid Inlet inch 0.5 0.5 0.5 0.5 7-G-Press. Gauge inch 0.5 0.5 0.5 0.5 8-NPT-FPN- U inch 0.5 0.5 0.5 0.5 9-NPT-LPN-U inch 0.5 0.5 0.5 0.5 10-NPT-LPV-U inch 0.75 0.75 0.75 0.75 11-NPT-Fine Fill. inch 0.75 0.75 0.75 0.75

Figure D.2: The figure presents a technical drawing of the buffer IMT as well as a table summarizing the technical details of the four different IMT’s and its connections

Particle Filter The particle filter is located before the IMT and defines the clean area after, which any pollution would reach the detector. This part of the system is therefore only opened or disconnected when absolutely necessary. The installed cartridge is made of PE and has a nominal pore size of 2 µm in contrast to the cartridge used in the pumping station in the LSA,, which has 5 µm. The filter housing is made of PVDF and offers five standard connections points. Two at the top for venting accumulated nitrogen gas, a third one at the bottom for drainage and two for the liquid flow. Filter

Membrane Pump The Pyrus 20 from Terbor is pneumatically driven membrane pump. It is fully made of PFA and offers a theoretical capacity of 20 l/min but demonstrated not more than 10 l/min when implemented and used in DFOS. The pump offers in total three flaretek connections. One 3/8-inch connection for the gas supply connected to a additional air controller with, which the pump speed can be regulated and two 1/2-inch liquid connections, which are both at the same side. The pump is the anticipated driving mechanism to all standard functions like IMT circulation, Pyrus 20 filling or emptying processes and can be set into function automatically by the PLC or manually by the user who has additional the possibility to regulate the gas flow to the pump by a needle valve installed in the supply line. Membrane pumps have only one

209 Underground Installations direction of ﬂow,, which made it necessary to adjust the tubing in a way to realize bi-directional pumping with only one pump.

Flow meter offers with the E&H, Promass 83a a coriolis mass flow meter normally used in high purity industrial applications. The flow meter is made of stain less steel and uses also an 0.5 inch flange connections sealed by PTFE-flat gaskets. It offers the measurement and integration of mass flow the measurement of temperature and density with an accuracy of 0.25 percent of the measured value. kg Density is measured with an accuracy of ±0, 02 l and Temperature with ±0.5C. A Promass 83atouch panel mounted on the flow meter housing allows to display values or change settings manually. Additionally exists an interface to the PLC, which is using the Flow meter Together with the PLC, which is opening valves and starting the pump the Flow meter allows to dose a preset amount of liquid either to the IMT or directly into the detector. The same function can also be used to re-fill or re-circulate an IMT automatically.

Fine Filling Tanks (FFT) The Fine Filling Tanks are a smaller version of the intermediate tanks and are used only in the Chimney area of each detector vessel. They are made of PVDF, which is transparent enough to see the liquid level through the tanks walls. The tank offer two 0.5 inch connection, one at the top and one at the bottom additionally allows a scale along the side wall to read volume changes in the range of 0.1L. All tanks are resistant to vacuum, which allows to suck liquid either out of the IMT to fill it up or out of the detector to actively control the liquid level in the chimney area. The anticipated operation mode is gravity induced empty and filling but the tanks are also resistant to overpressure, which allows to Fine Filling push liquid to IMT or detector. Tank The needed Volume to induce a dangerous liquid level difference in the chimney are way smaller than in the main detector body as indicated in Figure D.3. Further more is each Tank adapted in size to the related chimney surface. The muon veto vessel has no chimney and in consequence also no fine filling system.

The Tubing used for MU,BF and GC is electro polished stain less steel. Within each frame the number of connection was reduced by bending the the 0.5 inch tube rather than using unnecessary connectors. When ever a connection was unavoidable Swagelok connections have been used. The tubing includes three kinds of valves manual membrane and ball valves and normally closed pneumatic ball valves. The manual valves are used to set the liquid flow through the system. The pneumatic valves, which are mounted in strategic position are used to support automated standard filling functions, which can be realized by the PLC. Due to the positions of the pneumatic valves requires each liquid flow a combination of pneumatic and manual valves: This insures that the PLC can not start a process (like detector filling ) accidentally since the flow path Tubing is always interrupted by manual valves. A detailed presentation of all vales and connection points is summarized in table D.1

210 Underground Installations

Chimney Cross Section

BFBF GC

NT 0.15L/cm 0.9L/cm 1.6L/cm MU BF GC Vessel Cross Section NT MU BFBF GC

NT 42L/cm 50L/cm 146L/cm

92L/cm

Figure D.3: Scheme of a top view of the detector vessels indicating the cross sections and available surface area in two different z-levels: The red lines indicate two z-levels. The first in the center and the second at the level of all chimneys. These level have been taken to calculate the necessary (and safe) size of the IMT’s and the fine filling tanks (FFT’s). Each of these tanks holds the equivalent of 2 cm level increase in the corresponding vessel (3 cm would be critical if a IMT would drain accidentally into the detector). The drawing to the right indicate the cross sections at these levels and indicate the necessary volume to increase the liquid level by 1 cm.

211 Underground Installations

Detector Fluid Operating system for MU, BF and GC MU,BF,GC modules

Name Type Nr. Valve ID Diameter connection Tubing Iso.Valve pneu.m, SS 4 VX.00, 14, 15, 23 0.5 inch Swagelok Tubing Iso.Valve b, SS 19 VX.01-05, 07-13, 16-19, 21, 22, 24 0.5 inch Swagelok Tubing Iso.Valve m, SS 1 VX.06 0.5 inch Swagelok Tubing Drain Valves b, SS 6 VX.52-55, 57, 60 0.5 inch Swagelok FF Iso. Valve n, SS 1 VX.20 0.5 inch Swagelok FF reg. Valve b, SS 2 VX.26,56 0.5 inch Swagelok LPV-U Valve b, SS 1 VX.27 0.75 inch Swagelok LPN-U Valve b, SS 1 VX.28 0.5 inch Swagelok FPN-U Valve b, SS 1 VX.29 0.5 inch Swagelok Filter vent. b, SS 1 VX.30 0.25 inch Swagelok Filter drain b, SS 1 VX.31 0.5 inch Swagelok Frame drain m, PFA 1 V6.61 0.5 inch Swagelok HPN-Pump Iso. Valve pneu. m, SS 1 VX.90 12mm Festo HPN-Pump pneu. m, SS 1 VX.91 12mm Festo HPN-Pump reg. Valve n, SS 1 VX.92 12mm Festo Particle Filter 0.2µm, SS 1 FX.01 0.5 inch Swagelok Membrane Pump Pyrus 20, PTFE 1 PX.01 0.5 inch Flare Flow Meter Coriolis 1 EF X.201 0.5 inch Flange

Detector Valve Station for MU, BF, GC

Name Type Nr. Valve Number Diameter connection Long Filling Line Iso. Valve m, SS 1 VX.58 0.5 inch welded Short Filling Line Iso. Valve m, SS 1 VX.59 0.5 inch welded Bypass Valve m, SS 1 VX.51 0.5 inch welded Drain Valve m, SS 1 VX.62 0.5 inch welded

Table D.1: Technical details of the DFOS and the valve station: description, valve type(b=ball, m=membrane, n=needle, pneu.m=pneumatic membrane), valve ID-number, valve diameter, valve connection

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D.1.2 DFOS Valve Identiﬁcation

For DFOS with 4 almost equal system it will be necessary to introduce a nomenclature to identify the individual valves, which have to be distinguished and yet to be quickly compared. This was done by deﬁning a module number as well as a valve number of, which the last one would be equal through all modules.

system Module nr. Valve nr. Valve-ID Valve Description muon veto 3 14 V3.14 MU-IMT-Entrance buﬀer 4 14 V4.14 BU-IMT-Entrance gamma catcher 5 14 V5.14 GC-IMT-Entrance target 6 14 V6.14 NT-IMT-Entrance Gas system 1 17 V1.17 N2-Detector Isolation

The “main line” indicated in red in Figure D.4 will serve as example for a description of the flow path. It is starting at the system Connection point (SCP) going over manual and pneumatic valves before the pump (P) is reached. After the pump follows particle filter (F), flow meter (FM), heat exchanger (HE) until it the liquid enters the IMT. The filling of an IMT could therefore be described as

Figure D.4: Technical drawing of the buﬀer module high lighting the ﬂow path indicating the main line in DFOS

Long Version SCP (system conncetion point), Vx.00, Vx.02, Vx.04, Px.01 (pump), Vx.05, Vx.07, Fx.01 (ﬁlter), Vx.08, EFx.201 (ﬂow meter), Vx.14, HEx.01 (heat exchanger), IMT (intermediate tank) Short Version EP, 0, 2, 4, P, 5, 7, F, 8, FM, 14, HE, IMT

For future ﬂow path descriptions in this document but also in all tables this short version will be used.

D.1.3 DFOS P&ID’s

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Figure D.5: Piping and instrumentation diagram of DFOS indicating the muon veto module [111]

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Figure D.6: Piping and instrumentation diagram of DFOS indicating the gamma catcher module [111]

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Figure D.7: Piping and instrumentation diagram of DFOS indicating the target module [111]

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D.1.4 DFOS Instrumentation of Target Module

Detector Fluid Operating System for NT

Target Module

Name Type Nr. Valve Number Diameter connection Tubing Iso.Valve pneu.m, PFA 4 V6.00, 14, 15, 23 0.5 inch Flare Tubing Iso.Valve m, PFA 20 V6.01-13, 16-19, 21-22, 24, 32 0.5 inch Flare Tubing Drain Valves m, PFA 6 V6.52-55, 57, 60 0.5 inch Flare FF Iso. Valve m, PFA 1 V6.20 0.5 inch Flare FF Reg. Valve n, PFA 2 V6.26,56,25 0.5 inch Flare LPV-U Valve m, PFA 1 V6.27 0.75 inch Flare LPN-U Valve m, PFA 1 V6.28 0.5 inch Flare FPN-U Valve m, PFA 1 V6.29 0.5 inch Flare Filter vent. m, PFA 1 V6.30 0.25 inch Flare Filter drain m, PFA 1 V6.31 0.5 inch Flare Frame drain m, PFA 1 V6.61 0.5 inch Swagelok HPN-Pump Iso. Valve pneu. m, PFA 1 V6.90 12mm Festo HPN-Pump pneu. m, PFA 1 V6.91 12mm Festo HPN-Pump reg. Valve n, PFA 1 V6.92 12mm Festo Particle Filter 0.2µm, PFA 1 F6.01 0.5 inch Flare Membrane Pump Pyrus 20, PTFE 1 P6.01 0.5 inch Flare Flow Meter Coriolis, PFA 1 EF 6.201 0.5 inch Flange

Detector Valve Station for NT

Name Type Nr. Valve Number Diameter connection Long Filling Line Iso. Valve m, PFA 1 V6.58 0.5 inch flare Short Filling Line Iso. Valve m, PFA 1 V6.59 0.5 inch flare Bypass Valve m, PFA 1 V6.51 0.5 inch flare Drain Valve m, PFA 1 V6.62 0.5 inch flare

Table D.2: Technical details of the target module of DFOS and the target valve station: description, valve type (b=ball, m=membrane, n=needle, pneu.m=pneumatic membrane), valve ID-number, valve diameter, valve connection

Intermediate Tank (IMT) The target IMT is in contrast to the previously described IMT’s for MU, BF and GC fully made of PVDF to account for material compatibility with the target scintillator that on his part does not allow the usage of metals. The target IMT has a different working pressure range that is between -50 mbar and +400 mbar, which is sufficient for the target module since it has different requirements. For example is the target scintillator stored in the weighing tank situated in the laboratory. The IMT has therefore not to tolerate a high hydrostatic pressure. On the other hand is the necessary suction lift to empty the target vessel achievable with the installed pump a supportive vacuum on the IMT is therefore not necessary. The target-IMT is equipped with different sets of sensors, which are in- terfaced with the PLC that is monitoring, collecting and displaying these information for all modules. The sensors are measuring temperature, gas pressure and three liquid levels (informing the user) and providing sensor data for the PLC that can be used to control automated processes. The liquid temperature is monitored by a sensor mounted at the lower NT-DFOS side wall of the IMT. The gas pressure inside the IMT is measured by a digital pressure gauge mounted at the top lid displaying the current pressure also at the gauge and allowing a notification in case of overpressure by the PLC. The liquid level is normally visually monitored using a side glass but special liquid level like full, mid-full and empty are

217 Underground Installations monitored by capacitive sensors mounted on the side glass allowing automated re-filling or emptying. Additionally offers each IMT three gas connections for FPN-U, LPV-U and LPN-U to pressurize, to ventilate or to apply a low pressure gas blanket. Three liquid connections, one entering from the top and going to the bottom of the tank allowing also to use this connection backwards if necessary (additionally avoids mounting foaming as a result of rippling during IMT-filling) the second a standard bottom out let and the third is a overflow tube, which would allow a pressure free recirculation between detector and IMT. The individual IMT size accounts again for the liquid level change that would be induce in the target vessel if the full IMT would be drained (accidentally) into the detector. Calculations of the vessel group (CEA) suggest that a liquid level difference of more than 3 cm would be critical for the acrylic vessels. The IMT and its volume is complying to those restriction by holding an equivalent of only 2 cm per IMT. This clearly is dependent on the available surface, which was calculated at mid z-level. Figure D.3 is illustrating the available surface area and its dependence on the z-level. For the target IMT neither the lower limit is necessary since the target pump can over come the height difference between target bottom and IMT (about 5m) nor the higher limit is needed for this Tanks since it is not connected to the LSA but to the weighing tank, which is situated in the same lab.

Particle Filter The particle filter is situated before the IMT and defines the clean area after, which any pollution would reach the detector. This part of the system is therefore only opened or disconnected when absolutely necessary. The installed cartridge is made of PVDF and has a nominal pore size of 5 µm. The filter housing is made of PVDF and offers three connections points. Two at the top for venting accumulated nitrogen gas and a third one at the bottom for drainage.

filter Membrane Pump The Maxim 110 from Terbor is pneumatically driven membrane pump. It is fully made of PFA and offers a theoretical capacity of 35 l/min but demonstrated not more than 20 l/min when implemented and used in DFOS. The pump offers in total three flaretek connections. One 3/8- inch connection for the gas supply and two 3/4-inch liquid connections, which are both at the same side. The pump is the anticipated driving mechanism to Magnum 610 all standard functions like IMT circulation, filling or emptying processes and can be set into function automatically by the PLC or manually by the user who has additional the possibility to regulate the gas flow to the pump by a needle valve installed in the supply line. Membrane pumps have only one direction of flow, which made it necessary to adjust the tubing in a way to realize bi-directional pumping.

Mass Flow Meter from Siemens with the name Mass 2100 is a coriolis mass flow meter normally used in high purity industrial applications. The flow meter is made of stain less steel and uses an 1/2-inch threaded connection. It offers the measurement and integration of mass flow the measurement of temperature and density with an accuracy of 0.1 percent of the measured value. A touch panel mounted on the flow meter housing allows to display values or change settings manually. Additionally exists an interface to the PLC, which is using the flow meter to regulate liquid flows and allows to dose a preset amount of liquid either to the IMT or directly into the detector. The same function can NT-DFOS also be used to re-fill or re-circulate an IMT automatically.

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Fine Filling The fine filling tanks are a smaller version of the intermediate tanks and are used only in the Chimney area of each detector vessel. They are made of PVDF, which is transparent enough to see the liquid level through the tanks walls. The tank offer two 1/2-inch connection, one at the top and one at the bottom additionally allows a scale along the side wall to read volume changes in the range of 0.1 l. The Tank is resistant to vacuum, which allows to suck liquid either out of the IMT to fill it up or out of the detector to actively control the liquid level in the target chimney. The anticipated operation mode is gravity induced empty and filling but the tanks are also resistant to overpressure, which allows to push liquid NT-DFOS to IMT or detector. The needed Volume to induce a dangerous liquid level difference in the chimney is way smaller than in the main target body as indicated in Figure D.3. The Volume necessary to raise the liquid level by 1cm is in the target Chimney 0.1 l. The fine filling tank therefore has 0.2 l volume to stay below the critical value of 3 cm. Tubing the tubing in the neutrino target module is fully made of 1/2-inch PFA-tubing to avoid any metal in the liquid path of the neutrino target. The number of connections was reduced by bending the tubes with heat to route them with in the frame. The implementation of valves and tanks were realized by flaretek connection, which avoid metal packings. The target scintillator and the tubing are both not conductant, which leads to an accumulation of static charge on the tubing or instruments as the particle filter. A potential dangerous discharge is antagonized by a grounded meal mesh that is enveloping the whole PFA-tubing with in the frame but also along the complete liquid handling system. The tubing layout is slightly different to the other modules since it is connecting the weighing tank (WT) to the trunk line system. During filling the scintillator is supplied from the weighing tank and not form the TL-system, which applies only a small hydrostatic pressures to the target module. After filling the liquid in the tubing is pushed back to the WT and the tubing is disconnected. The weight of the isolated WT is measured again to calculate the amount of liquid, which has been put into the detector. The target module tubing includes manual as well as pneumatic membrane valves. Each valve has visual position indicators at the valve top. The valve position indication of pneumatic valves is additionally realized by checking the opening pressure as a indicator. These pneumatic valves are strategically positioned support the user with the handling of standard filling functions, which will be explained in the next section.

D.1.5 Programmable Logic Controller, DFOS-PLC

The automated modes shall help the user to simplify the detector filling by overtaking standard filling steps as, which have to be repeated continuously during detector. The three filling steps are: 1. Step: Fill IMT 2. Step: Circulate / Thermalize the liquid in the IMT 3. Step: Empty IMT (Detector Filling) The user has to set manual valves before he can start a certain step, which avoids accidental steps and the PLC is additionally monitoring certain values, which have to be in range before a new step can be started. Detector filling for instance can only be started if the temperature of the liquid or the gas pressure in the IMT is within range. These values are shown (among others) in the software menu shown in figure D.8a and would be underlayed in orange if these values would be out of range.

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A part of these automated support function allows a second software menu to control all pneumatic valves and pumps manually by providing overview schemes of each subsystem meaning the four DFOS modules and the gas handling system. These interactive tubing scheme includes all valves and indicate sensor readings and pneumatic valve positions in different colors. In figure D.8 are exemplarily shown two manual mode screens demonstrating the interface and the monitoring values. The first is D.8c showing the gas handling panel including the pressure gauge readings of HPN-U and LPN-U as well as the status of the pneumatic valve (V1.17) in green, which corresponds to an open valve. Figure D.8d is exemplarily showing the tubing scheme for the target DFOS module. Indicating the pneumatic valves and pump in white color, which corresponds to a closed valve or a not running pump. Additionally indicates the green box at the bottom of this screen the response of an leakage sensor, which is installed in each DFOS- module’s retention pan. The user administration forms the third part of the software menu and is restricted to filling coordination and the system administrator. It allows to configure the system itself (time, language or programming tools). The access organization is necessary to prevent accidental misuse and allows to implement users secured by password and equipped with access level. A lower access level allows the standard user to monitor the system, a higher level allows the filling shifters to work. The highest level allows to change sensor ranges and is only given to filling experts. This restricted menu allows also to reset counters as the number of filling cycles or the integrated volume measured by the flow meters.

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(a) Overview screen for automated ﬁlling (b) Administration screen

(e) Setting screen for alarm and critical values (f) Software main page

Figure D.8: Software menu used in the control unit (touch screen) of the PLC mounted to the muon veto frame. The software has diﬀerent menus for each DFOS module, the gas handling system as well as administrating menus.

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D.1.6 DFOS Connections

The following section is dedicated to the tubing installed in the underground laboratory and connection between DFOS, XTOS and detector. The tube routing scheme below is indicating the liquid lines installed in the laboratory including the color code to differentiate between different liquids. Tubes in whole lines are marking the long filling tubes where as the dashed lines are indicating the short filling tubes. The long dashed lines are marking the trunk line system.

Trunk Line Connection : The ﬁrst liquid connection is connecting the trunk line to DFOS and provides all DFOS-Modules with detector liquid from the trunk line system or respectively the weighing tank. The TLS enters the laboratory with in the concrete channel below the laboratory ﬂoor level what marks the start of TLS-DFOS-Part. All tubes have to cross the room in order to connect to the DFOS-modules. These tubes are equal for all DFOS-modules and are made out of of 3/4-inch PFA tubes. The tubes are encased in PVC-tubes and additionally hidden under a metal plate to shield them from mechanical forces before they reach the individual DFOS-Module. The entrance point is marked by a pneumatic valve (Vx.00), which marks the end of the DFOS-TLS and the start of DFOS itself.

Long Filling Tube : The long filling tubes are running from the pneumatic valve (Vx.23 within each DFOS module) along the laboratory wall where they pass a valve station before they enter finally into the detector where they run down to the bottom of the related detector vessel. The tubes are supported and fixed by a rail system as can be seen in figure D.1.6. The half inch PFA target line is running the same path but is hidden in a U-shaped rail to be supported and protected from mechanical forces an additional metal sleeve around the PFA-Tube avoids electrostatic charge. The tubes are running to different locations at the detector top lid where they enter the detector. Short Filling Tube : The short filling tubes are running from the pneumatic valve (Vx.24 within each DFOS module) along the laboratory wall where they pass a valve station before they enter finally into the detector. The short filling tubes are reaching only roughly 20 cm below the final liquid level. The tubes are supported and fixed by a rail system as can be seen in figure D.1.6. The half inch PFA target line is running the same path but is hidden in a U-shaped rail to be supported and protected from mechanical forces an additional metal sleeve around the PFA-Tube avoids electrostatic charge.

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Figure D.9: The scheme shows the tube routing in the far-lab and indicates the connection between DFOS, XTOS and TLS to the detector. In order to provide a better overview the tubing is presented in three diﬀerent pictures separating the the installations into: liquid tubing, gas supply tubing and exhaust lines.

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Valve Station : This valve station is the last barrier before the detector and allows to isolate the detector from DFOS. A single valve station is composed of three manual valves (Vx.58, Vx.59, and the bypass Vx.51), which allow to isolate the detector. Valve Vx.51 interconnects the short and long filling tube, which and provides a detector bypass. This bypass can be used to start detector circulation without the detector, slowly including the detector by closing the bypass valve, which is increasing process safety. Ad- ditionally can this bypass be used to clean the tubing between DFOS and the valve station. The tubing used for the detector connection is the same as already used in DFOS. Meaning Figure D.10: Picture showing the last valves (valve half inch stain less steel tubing for MU, BF station) before the liquids enter the detector. The and GC. The target tubing is again PFA and figure presents the short and long filling lines as well as a bypass between, which allows to circulate (and additional enveloped in metal mesh in order clean) the section of the tubes without affecting the to avoid static charge. The picture is showing detector. the four valve stations (target in PFA) and the rail system, which holds all tubing.

D.1.7 Expansion Tank Operating System (XTOS)

Once the detector is full and the final liquid level is reached the MU-scintillator level is two 2 centimeters below the MU-top-lid. The remaining gas volume (660≈ l on 33 m ) provides a comfortable expansion volume for the 90 m3-MU-LS because of, which the MU-level rises only 2 mm by a variation of one degree ○K. Given such a ratio it is obvious that the muon veto ○ can easily tolerate thermal variations even up to ± 15 degree K until the top-lid would be reached. The other liquids however are limited to the cross section of their chimney, even a small thermal variation would lead to a dangerous increase of the liquid level. These vessels are additionally equipped with separate expansion tanks. The general design is equal for all tanks only the cross sections and materials are changing. All tanks offers a big top-flange, which is holding all connections gas connections (LPN and LPV) as well as all connections for liquid and gas pressure measurement. At the tank itself are additional connections for liquid connection (detector connection: 3/4-inch connection horizontally mounted at the bottom of the tank) and a side-glass that allows to check the liquid level by eye and independent from electronic measurements. Each tank provides in addition a outlet valve at the side glass, which allows to remove liquid from the tanks. Inside the tank are baffle boards mounted in such a way that the connection to the detector (T3) is separated from the two connection points of the side-glass (T1, T4). This prevents the introduction of light into the detector as the light has to get reflected around the baffle boards before light could enter the connection tube to the detector. Table D.3 below is summarizing details and the connections of the XTOS-tank.

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XTOS Tank Details

Dimensions Unit MU BF GC NT Height mm – 150 150 150 Width mm – 1850 540 450 Depth mm – 2000 2000 2000 Volume L – 555 162 135

Pressure Unit MU BF GC NT Max. Pressure bar – n.a n.a +1.0 Min. Pressure bar – 0 0 0

Material Unit MU BF GC NT Walls SS SS SS PVDF Top/Bottom SS SS SS PVDF Joints PTFE PTFE PTFE PVDF

Instrumentation Unit MU BF GC NT T1-NPT Bottom side glass inch – 3/8 3/8 3/8 T2-NPT Bottom spare inch – 0.5 0.5 0.5 T3-NPT Side liquid inlet inch – 0.75 0.75 0.75 T4-NPT Top side glass inch – 3/8 3/8 3/8 T5-NPT Top spare inch – 0.5 0.5 0.5

Top Flange mm – DN150 DN150 DN150 1-Swagelok LPN-U conn. inch – 0.75 0.75 0.75 2-Swagelok LPV-U conn. inch – 1.0 1.0 1.0 3-Swagelok Pressure Sens. conn. inch – 0.25 0.25 0.25 4-Swagelok Level Meas. inch – 0.25 0.25 0.25 5-SwagelokLevel Meas. inch – 0.25 0.25 0.25

Table D.3: The ﬁgure presents a technical drawing of the NT-XTOS-Tank and the BF-Tank as well as a table summarizing the technical details of the three diﬀerent XTOS-Tanks and its equal connections

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Figure D.11: Technical drawing of the expansion tanks: (top) drawing of the target-tanks; (bottom): drawing of the buﬀer tank. Pictures made by [111]

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Figure D.12: (left) Underground laboratory during the installation phase, indicating the detector chimney, LPN-supply lines and the LPV-lines,, which connect to the O2-panel; (center): LPN-supply lines coming from the LPN-distributor and going to the detector chimney; (right): detector chimney, indicating the diﬀerent connections for LPN- & LPV-lines

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D.2 Gas Handling System

nch, gas handling, Lab main exhaust line inch inch inch 1/2 stainless steel, gas handling, High pressure 1/2 , stainless steel, gas handling, Flushing pressure 1/2 inch, stainless steel, gas handling, Low pressure 3/4 , gas handling, exhaust line 7-10 i Valve 0/I Valve incl bleeding valves 0/I Valve Regulating valve Regulating valve incl. bleeding valves Pressure regulator valve Height adjustable bubbler Pressure indicator / Manometer Oxygen-Monitor Flow meter Filter Myon veto Buffer Y-catcher Target PN-U: Flushing pressure Nitrogen Underground Drawing: P.Pfahler, TUM Drawing: P.Pfahler, LPV-U: Low pressure Ventilation Underground Low pressure Ventilation LPV-U: HPN-U: High pressure Nitrogen Underground F LPN-U: Low pressure Nitrogen Underground DFOS: Detector Fluid Operating System Operating System Tank Expansion XTOS: Double Chooz Far Lab: Detector Gas Handling Plan

H LPV-BOX 0xygen FILTER--BOX Supply pressure 8.5 bar Measurement Back Flow protection Active Char Coal Filter XN V01 Liquid Nitrogen Plant (LN 2 ) 3000 L XN Purge Valve 3/4 inch Street XN V02 XN V03 FI PI 01

VENTILATION VENTILATION LPN-BOX Active Char Coal Filter H 3/4 inch FI V01 B FI V01 A FI V01 A B FI V02 A FI V02 FI V02 B Filter Station Neutrino Laboratory Back Flow protection FI PI 02 3/4 inch PLC TL N2 V02 TL pneumatic Valve DFOS Neutrino Lab Exhaust line XTOS 1/2 inch 10m^3 NT Trunk Line Module Trunk TL N2 V03 TL LSA LSA DFOS-VALVES PVDF 85L 0--0.4 Bar Target Detector 1/2 inch NT Pumpe GC 22,5m^3 Y-catcher Black Box SS 100L SS 100L -1--3 Bar TL N2 V04 TL Buffer LPN-Ring Buffer gas-outlet for Laser & PMT-Flanges 1/2 inch GC Pumpe 3/4 inch Glove Box 1/2 inch BF Weighing Tank Tank Weighing 110m^3 Buffer SS 300L -1--3 Bar BF Pump Street 1/2 inch Myon veto MU Pump SS 185L -1--3 Bar CONSUMER 90m^3 MU TL N2 V05 TL Concrete Channel 1/2 inch LPN-Distributor 1/2 inch LPN-U FPN-U 1/2 inch Flow meter 3/4 inch 3/4 inch 3/4 inch 50L 50L @200bar -2,4mBar H Tunnel Tunnel Safety valve 600mB Pressure Indicator TL N2 PR TL Emergency N2 Supply +7mBar TL N2 V06 TL V000 Laboratory Main Shut Valve Laboratory Main Shut Valve Over & under Pressure Protection 3/4 inch SUPPLY SUPPLY Pressure Gage 4Bar --> 3-9mBar Gas Filter 4nm Safety valve (opening @ 5.5Bar) PLC N2 Valve supply 1/2 inch 1/2 inch Pressure gage 8Bar -> 4 Bar 1/2 inch 3/4 Zoll Spare HPN-U

Figure D.13: Overview scheme of the gas handling system in the far laboratory

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Figure D.14: Picture of the isolation valves at the top of the IMT-tanks, indicating the LPN, FPN, and LPV-connections of each IMT-tank. The isolation valves allow to produce diﬀerent pressure scenarios, from totally isolated over ﬂowing or steady nitrogen blanket until over- and under-pressurized.

Figure D.15: LPN-distributor, indicating the two LPN-supply lines, the common volume to equalize the LPN-pressure and the four outlet lines, which supply the diﬀerent detector vessels with low pressure nitrogen for blanketing

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LPV-U

Figure D.16: The O2-panel is the ﬁrst station after the nitrogen of XTOS and detector is merged in order to guarantee the same back pressure on both systems. The panel leads each gas ﬂow individually through a inlet valve (, which can be used to isolate the detector) and a O2-sensor. The sensors can detect oxygen down to 40 ppm and are interfaces with the DFOS-PLC. The PLC indicates the O2-levels 100 ppm and 500 ppm and triggers an alarm above 1000 ppm. Drawing made by [111]

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Figure D.17: Technical drawing and picture of the over- & under-pressure situations composed of 2×2 oil bubblers. The bubblers are connected to the LPN-manifold and used as relive valve in case the dangerous pressure variations in the detector. The box is connected in such way that the detector can vent pressures, which are above +7 mbar or below -2.5 mbar. This systems was installed to enable the detector to adjust for pressure diﬀerences even in the case it is isolated by the LPN-manifold as the bubblers are connected behind the main-LPN-isolation valve (V017). Drawing made by [111]

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D.3 Monitoring System

D.3.1 Liquid Level Monitoring System

Hydrostatic Pressure Sensors

Figure D.18: Radio assays of the HPS-sensor [120] and the guide tube box both installed in the gamma catcher vessel [136]

D.3.2 Gas Pressure Monitoring

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Figure D.19: Technical drawing of the diﬀerential pressure sensor AP-47 from the Keyence used for the blanket monitoring of the detector and the XTOS-tanks. Picture taken from [117]

233 Appendix E

Detector Filling

Figure E.1: Filling curves in the detector showing the individual curves of MU, BF, GC, NT by indicating the level increase in dependance of the introduced liquid; (top left): Indicates the filling curve in muon veto. The reach of the buffer vessel as well as the flooding of the BF-top-lid can clearly be recognized and was used as independent indication for the reach of a vessels; (top right): Indicates the filling curve in buffer. The reach of the GC vessel as well as the flooding of the GC-top-lid can clearly be recognized. Furthermore, indicates the steep increase the reach of the chimney and the chimney filling phase; (bottom left): Indicates the filling curve in GC. The reach of the NT vessel as well as the flooding of the NT-top-lid can clearly be recognized. Furthermore indicates the steep increase above the reach of the chimney and the chimney filling phase; (bottom right): Indicates the filling curve in NT. The level increase homogeneously until the level reaches the target chimney and the start of the chimney filling phase.

234 Detector Filling

E.1 Filling Modes

Automated Filling Mode IMT-Filling Mode (Standard) Manual Filling Mode Continuous Filling Mode Fine Filling Mode Automated Filling Mode (standard running mode) Using the DFOS to fill the detector means to fill up an IMT from the LSA, to thermalize the liquid within the IMT and to empty it safely into the detector. These steps have to be repeated a couple of hundred times not only for one but for all of the four DFOS frames at the same time. These three running modes are the standard running modes. To ease the handling of the full system and to disburden the filling team these standard running modes are supported by a PLC, which opens or closes valves and starts or stops pumps. These standard (or automatic) modes can be started by pushing a button on the touch screen of the PLC, which automatically will open all needed valves and start the pump in order to start the wanted process.

1. Step: Filling the IMT (using gravity or via Pump) The liquid will be supplied by the TLS and enters the DFOS when the PLC opens a pneumatic valve (VX.00) and starts the pump. The liquid will enter and flow through a teflon pump, a filter, a flow meter and a heat exchanger before it enters the IMT itself (only when all manual valves have been opened). The flow meter has to be set in advance manually to the requested Volume and will supply a stop signal to the PLC to stop the fill up process once the requested volume has passed. The PLC will also stop the filling process when the sensors detect an overfilled IMT. The path for the liquid is depending on the emptying path since the liquid volume should only be counted once in the flow meter. Therefore it is necessary to coordinate filling and emptying process in order to avoid double counting. 2. Step: Circulating the IMT-Volume for heating or cooling The liquid will be circulated from IMT to the same IMT using the pump and passing the heat exchanger shortly before the liquid enters the IMT again. The temperature of the liquid is monitored by temperature sensors mounted on the side wall of the IMT. The PLC will continue circulating liquid through the heat exchanger as long as the temperature in ○ the IMT is not within the manual set value and its range of tolerance (i.e. 15±1 K). Once the liquid has the correct temperature the system will maintain it as long as the circulation running mode is kept. 3. Step: Emptying the IMT (Detector Filling) (via Pump or FPN-U) The PLC will open up the pneumatic valves (VX.15, VX.23) to allow the flow out of the DFOS and into the filling lines, which include also manual valves to avoid accidental filling. The PLC will only open up these valves when the system is: Fully thermalized and within temperature range No errors or alarms are noted on the PLC When the IMT is emptying, the PLC is monitoring the time needed to decrease the liquid level in the IMT between full and mid full. If the time is longer than a certain value (manually set; experience values can be set) an alarm will be given. This will help to recognize an unequal filling of one of the IMTS or a clogged filter within one DFOS-frame. The lower liquid sensor of

235 Detector Filling each IMT is giving a stop signal to the PLC once the level has decreased to it in order to stop the emptying process. The sensor is installed at a level that the IMT will not run empty but will remain with a volume of roughly 10 - 20 l to avoid pushing gas through the liquid lines. For each process it is necessary to set manual and pneumatic valves in order to successfully start the process meaning the system cannot accidentally jump into a diﬀerent automatic mode or start a process alone. Manual Filling Mode: In addition to this automatic mode we can run each DFOS-frame individually in a manual mode. In manual mode all pneumatic valves and pumps can be closed or opened individually. The manual mode shall cover all processes that are not used during standard running modes explained above. Those processes could be cleaning runs, test runs, alternative circulation runs or -continuous ﬁlling mode

Continuous Filling Mode: In this mode it is possible to bypass the IMT and to fill the liquid coming from the LSA directly into the detector. This mode does not allow a thermalization of the liquid but allows to save filling time where thermalization is not needed. This direct filling mode saves a factor of 3 to 4 in time, since the IMT has neither to be filled nor to be emptied by FPN-pressure. In this mode the liquid will be pumped or pressed through DFOS, filtered and its volume measured before it finally enters the detector. However, it is not possible to thermalize the liquid since the heat exchanger is directly in front of the bypassed IMT. This mode will only be used where a thermalization is not necessary and the geometry of the detector does not indicate otherwise. Fine Filling Mode The fine filling mode is specially designed to be used in the chimney area of the detector. The related system uses smaller intermediate tanks (B 4.02 in the drawing below, picture to the left), which are in volume adapted to the needs of the individual chimneys. Chimney filling follows the same idea as it is discussed above with the use of IMT during normal detector filling. A safe amount (for the chimney this time) of liquid is given from the IMT to the significant smaller fine-filling-tank (ff-tank) this can be done by gravity or by a little vacuum pump that is connected to the upper side of the ff-tank. Each fine-filling tank is slightly transparent and has a scale, which allows reading the volume that has been entered into it or drained from it. In order to fill the chimney the fine ff-tank will be filled from an IMT via gravity (or a small under pressure). The bottom connection between IMT and ff-tank will be closed and the ff-tank will be opened to the detector. The volume corresponding to a wanted liquid level raise will be added by gravity (optional: with the help of FPN; FPN-connection is available on the IMT) As mentioned above it is possible to use a little vacuum pump to under pressurize a fine-filling tank, which will allow drawing liquid out of the IMT or if routed differently out of the detector. It is the same type of vacuum pump as we use it for the Loris-tube. Due to the small volume of these tanks this can be done fairly accurate and in small amounts. This might be needed if we see unwanted liquid level due to liquid heating or overfilling. In this case this liquid drawing could help to secure the detector. In case it would be necessary to draw a bigger amount out of the detector it is also possible to start the DFOS pumps to pump liquid out.

236 Detector Filling

(a) IMT Filling

(b) Circulation Way A(F+FM)

Figure E.2: Overview of the different filling paths used during IMT-filling mode, indicating IMT-filling, circulation and filling

237 Detector Filling HE

(a) Detector Filling -with Pump- HE

(b) Detector Filling -with Gravity-

Figure E.3: Overview of the different filling paths used during continuous-filling mode.

238 Detector Filling

(a) Filling the FF-Tank (b) Detector Fine Filling

Figure E.4: Overview of the different filling paths used during fine-filling mode.

239 Appendix F

Results

1.0

0.8 trigger efficiency 0.6

0.4

0.2

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 energy [MeV]

Figure F.1: Trigger eﬃciency in the inner detector: the eﬃciency increases from 50% at 400 keV to +0 100−0.1% at 700 keV including total systematic uncertainty. Plot from [65]

240 List of Figures

1.1 Inﬂuence of the three individual parts in the survival probabilities for 3 MeV- neutrinos ...... 10 1.2 Survival probability ofν ¯e’s for 3 MeV ...... 11 1.3 Survival probability for reactor neutrinos for diﬀerent neutrino energies ...... 12 1.4 Mass-hierarchy for the three neutrino mass-eigenstates ...... 13

2.1 Schematic view of 235U-ﬁssion and subsequent following processes ...... 14 2.2 Energy spectrum of reactor neutrinos and reactor fuel composition and development over one year ...... 16

3.1 Picture of the Double Chooz Collaboration ...... 19 3.2 Picture of nuclear power plant (CNPE) in Chooz ...... 20 3.3 Overview scheme of CNPE de Chooz ...... 22 3.4 Disappearance eﬀect forν ¯e’s at 3 MeV ...... 23 3.5 Signature of the IBD in liquid scintillators ...... 24 3.6 Observable energy spectrum for the IBD in liquid scintillator detectors ...... 25 3.7 Comparison of the varying coincidence signature, following an IBD-signal on hydrogen, carbon and gadolinium ...... 26 2 3.8 Expected disappearance signal for Double Chooz, simulated with sin (2Θ13) = 0.1 2 −3 2 and ∆m = 2.5 ⋅ 10 eV ...... 28 3.9 Vertical cut through the DC-far-detector, showing the inner detector structure . . 30 3.10 Position of the three outer muon veto modules in correlation to the detector . . . 32 3.11 Data acquisition system for muon veto and inner detector ...... 34 3.12 Primary decay modes, line or end point energies (Emax) and half-life of diﬀerent radioisotopes (T1~2) ...... 35 3.13 Cosmogenic Induced Radioisotopes ...... 36

4.1 Pictures and drawing indicating the LSA and the installations therein ...... 42 4.2 Piping and instrumentation diagram of the liquid handling systems in the LSA . 44 4.3 Picture of the liquid handling system in the LSA ...... 45 4.4 Pumping and instrumentation diagram of the pumping stations for MU and BF. 46 4.5 membrane pump [73] ...... 46 4.6 particle filter [74] ...... 47 4.7 flow meter [75] ...... 47 4.8 membrane valve [76] ...... 47 4.9 Picture of the buffer pumping station module ...... 49 4.10 Pictures of the storage tanks for muon veto and buffer ...... 50 4.11 Technical drawing and details of the storage tanks of muon veto and buffer . . . . 52 4.12 Technical details about the hydrostatic- and gas-pressure-sensor installed in the storage tanks ...... 53 4.13 Pictures of the emergency closure system in the LSA ...... 54

241 LIST OF FIGURES

4.14 Overview drawing of the monitoring- and ECS-system installed in the LSA . . . . 55 4.15 Overview drawing of the gas handling system in the LSA ...... 57 4.16 The gas ﬁlter station and the liquid nitrogen plant ...... 58 4.17 N2-distribution system in the LSA ...... 59 4.18 Pictures of the ventilation system in the LSA ...... 60 4.19 Picture of the street part of the trunk line system ...... 62

5.1 Jablonski-diagram provides a simplified illustration of the different energy levels of the p-electrons in benzene as to find in organic liquid scintillators ...... 65 5.2 Chemical structure of LAB and PXE ...... 70 5.3 Attenuation length measurement of different LAB samples ...... 70 5.4 Chemical structure of tetradecane and n-paraffine ...... 71 5.5 Attenuation length measurement of different alkane-samples ...... 72 5.6 Overview scheme, indicating the stokes shift of LAB, PPO and bis/MSB . . . . . 73 5.7 Light yield of LAB-based scintillators for varying PPO concentrations ...... 74 5.8 Absorption- and emission-bands of PXE, PPO and bis/MSB ...... 75 5.9 Chemical structure of mineral oil and n-paraffine ...... 76 5.10 Absorbance and attenuation length comparison between Ondina-909 and -917 . . 76

6.1 Pictures of the liquid delivery to the LSA ...... 81

7.1 Global overview of the liquid handling chain in the DC-far detector ...... 85 7.2 Picture of Detector Fluid Operating System of the DC-far detector ...... 86 7.3 Picture of the DFOS modules of buffer and target ...... 88 7.4 Piping and instrumentation diagram of the buffer module ...... 91 7.5 Drawing of the expansion tank operating system (XTOS) ...... 95 7.6 Pictures of XTOS during the installation in the far laboratory ...... 97 7.7 Overview of the gas handling system in the far laboratory ...... 98 7.8 Picture of the nitrogen supply system in the underground laboratory ...... 99 7.9 Piping- and instrumentation-diagram of the nitrogen distribution system in the far lab ...... 100 7.10 Overview of the gas handling system in the far laboratory ...... 106 7.11 Technical drawing of the LPN-Box ...... 108 7.12 Picture of the LPV-system in the underground laboratory ...... 109 7.13 Technical drawing of the LPV-Box ...... 111 7.14 Illustration and position indication of the level measurement systems used in the DC far detector ...... 113 7.15 Overview of the laser level measurement system ...... 115 7.16 Picture of the HPS-Sensor ...... 116 7.17 Overview of the Tamago level measurement system ...... 117 7.18 Overview of the critical point sensor system ...... 118 7.19 Overview of the XRS-System ...... 120 7.20 Overview of the XTOS-level measurement system ...... 121 7.21 Overview of the level measurement PC and its connections ...... 122 7.22 Picture of the differential gas pressure sensor AP-47 and the related amplifier AP-V40 ...... 124 7.23 Connection Scheme for the gas pressure monitoring system of the far detector . . 125 7.24 Connection scheme for the gas pressure monitoring system of the expansion tank operating system (XTOS) ...... 126

8.1 Vertical cut through the center of Double Chooz Detector ...... 127

242 LIST OF FIGURES

8.2 Illustration of the detector indicating the different filling phases and the LM- systems in the detector ...... 132 8.3 Illustration of filling phases 1-3 ...... 133 8.4 Illustration of filling phases 4-5 ...... 134 8.5 Illustration of filling phases 6-8 ...... 135 8.6 Illustration of filling phases 9-11 ...... 137 8.7 Illustration of filling phase 12 ...... 138 8.8 Illustration of filling phases 13-14 ...... 139 8.9 Illustration of filling phases 15-16 ...... 140 8.10 Illustration of filling phases 17-18 ...... 142 8.11 Illustration of filling phases 19-20 ...... 143 8.12 Illustration of filling phases 21-22 ...... 145 8.13 Detector filling: Overview of the general liquid level increase in the DC far detector146

9.1 Absorption (A) and attenuation length (Λ) measurements of the three muon veto samples taken from the three different storage tanks...... 150 9.2 Absorption (A) and attenuation length (Λ) measurements of the muon veto sample taken from the intermediate tank in the underground laboratory ...... 150 9.3 Absorption (A) and attenuation length (Λ) measurements of the three buffer samples taken from the three different storage tanks...... 152 9.4 Absorption (A) and attenuation length (Λ) of the buffer sample taken from the intermediate tank in the underground laboratory ...... 153

10.1 Detector filling: Observed LPN-blanket pressures in MU, BF, GC, NT ...... 157 10.2 Detector filling: Observed liquid level differences between MU, BF, GC and NT . 158 10.3 Detector filling: differential pressures between MU, BF, GC and NT ...... 159 10.4 Detector filling: Observed temperature variations in MU, BF and GC ...... 161 10.5 Detector monitoring: Temperature development in MU, BF and GC after the detector has been filled ...... 163 10.6 Detector monitoring: Liquid level variations of BF, GC and NT during the first 1.5 years of data taking ...... 163 10.7 Detector Monitoring: Observed liquid level differences between MU, BF, GC and NT ...... 164 10.8 Atmospheric pressure in the underground lab during data taking ...... 165 10.9 Nitrogen blanket pressure during data taking ...... 166 10.10Detector monitoring: differential pressures between MU, BF, GC and NT . . . . . 167

11.1 Data taking statistics of the ﬁrst 398 days of data taking ...... 168 11.2 Muon rate and energy spectrum in the inner veto ...... 169 11.3 Variation of the energy deposition per unit of length of the inner muon veto . . . 170 11.4 Muon rate and energy spectrum in the inner detector ...... 171 11.5 Muon-correlated energy spectrum indicating neutron captures on H, C and Gd and the stability of these energy peaks over a time period of 300 days ...... 172 11.6 Single spectrum in the prompt energy window between 0.7 and 12.2 MeV . . . . . 173 11.7 Single event rate in the prompt and delayed energy window ...... 174 11.8 Single event reconstruction in the inner detector vessels ...... 176 11.9 Prompt and delayed signal after applying the neutrino selection cuts (4-7) . . . . 177 11.10Time and space correlation between prompt and delayed event ...... 178 11.11Vertex reconstruction of prompt (top row) and delayed (bottom row) IBD-events in the inner detector ...... 179 11.12Observed neutrino rate per day over a time period of one year ...... 180

243 LIST OF FIGURES

11.13Prompt energy-spectrum and combined background-spectrum for the ﬁrst 227 days of data taken by the Double Chooz experiment ...... 181 2 11.14Recent results on sin (2Θ13) measured by accelerator- and reactor-based experiments, ...... 182 11.15Prompt energy-spectrum and combined background-spectrum for the ﬁrst 227 days of data taken by the Double Chooz experiment using hydrogen analysis ...... 184

A.1 Technical drawing of the DC-far detector ...... 195 A.2 Pictures of the DC far laboratory during the installation of the diﬀerent detector vessels ...... 196

B.1 Technical drawing of the buﬀer and muon veto storage tanks ...... 199

C.1 Picture of the new and used ﬁlter cartridges used in the particle ﬁlter of the muon veto pumping station in the LSA for the unloading of LAB ...... 205 C.2 Experimental setup for the determination of the absolute light yield, indicating a standard back scattering method using 137 Cs...... 205

D.1 Global overview about the liquid handling system in the underground lab . . . . . 207 D.2 Technical details of the intermediate tanks in the DFOS system ...... 209 D.3 Scheme of a top view of the detector vessels indicating the cross sections and available surface area in two different z-levels ...... 211 D.4 Technical drawing of the buffer module high lighting the flow path indicating the main line in DFOS ...... 213 D.5 Piping and instrumentation diagram of DFOS indicating the muon veto module . 214 D.6 Piping and instrumentation diagram of DFOS indicating the gamma catcher module215 D.7 Piping and instrumentation diagram of DFOS indicating the target module . . . 216 D.8 Picture of the software menu used in the control unit (touch screen) of the PLC mounted to the muon veto frame ...... 221 D.9 Overview scheme of the tubing in the DC-far laboratory ...... 223 D.10 Pictures of the valve station in the DF-far laboratory ...... 224 D.11 Technical drawing of the expansion tank used for the target ...... 226 D.12 Picture of the underground laboratory during the installation phase ...... 227 D.13 Overview scheme of the gas handling system in the far laboratory ...... 228 D.14 Picture of the isolation valves at the top of the IMT-tanks ...... 229 D.15 LPN-distributor ...... 229 D.16 Technical overview drawing of the O2-panel ...... 230 D.17 Technical drawing and picture of the over- and under-pressure protection box . . 231 D.18 Radio assays of the HPS-sensor and the guide tube box both installed in the gamma catcher vessel ...... 232 D.19 Technical drawing of the differential pressure sensor AP-47 from the Keyence . . 233

E.1 Filling curves in the detector showing the individual curves of MU, BF, GC, NT 234 E.2 Overview of the different filling paths used during IMT-filling mode ...... 237 E.3 Overview of the different filling paths used during Continuous-filling mode . . . . 238 E.4 Overview of the different filling paths used during fine-filling mode ...... 239

F.1 Plot of the trigger eﬃciency in the inner detector ...... 240

244 List of Tables

1.1 Currently known best ﬁt values for the oscillation parameters ...... 7 1.2 Oscillation length (L0), the survival probabilities for near and far detector, as well as their diﬀerence for neutrino energies between 2 and 10 MeV ...... 12

2.1 Mean values for the energy release per fission of the four main fission isotopes . . 15 2.2 Reactor fuel composition at the beginning of a burning cycle and the energy release per fission for the four main fission isotopes ...... 17

3.1 Composition of the Double Chooz Collaboration ...... 19 3.2 Inverse beta decay channels, including their necessary energy threshold Qth . . . 23 3.3 Natural abundance of H, C and Gd in percent, the absorption cross-section for thermal neutrons, gamma emission and capture time for n-captures on H, C, Gd 26 3.4 Summary of detector dimensions ...... 29 3.5 Composition and amounts of muon veto, buﬀer, gamma catcher and neutrino target used for the far detector ...... 33

4.1 Surface installations, realized by TUM and MPIK ...... 41 4.2 Flow path table for the muon veto and buﬀer pumping stations ...... 48 4.3 Individual sub-systems and their pressure range of the gas handling system in the LSA...... 56 4.4 Trunk line system ...... 63

5.1 Summary of all requirements on the detector liquids ...... 69 5.2 Attenuation length of the LAB samples from Helm, Cepsa and Wibarco ...... 71 5.3 Attenuation length and density of different alkane samples ...... 72 5.4 Radio chemical impurities of PPO ...... 74 5.5 Attenuation length and density of different Ondina samples ...... 77 5.6 Main properties of all components of the muon veto scintillator and the buffer liquid ...... 77

6.1 Composition of the muon veto scintillator and the buﬀer liquid ...... 78

7.1 Hardware systems installed in the underground laboratory realized by TUM and MPIK ...... 84 7.2 Instrumentation details of the diﬀerent DFOS-modules ...... 87 7.3 Flow paths summary of the DFOS-modules ...... 92 7.4 Thermal Expansion in the chimney with and without XTOS ...... 96 7.5 Summary of the nitrogen supply systems in the underground Laboratory . . . . . 99 7.6 Technical details of the HPN-U manifold ...... 101 7.7 Technical details of the FPN-U manifold ...... 102 7.8 Technical details of the LPN-U manifold ...... 103 7.9 Summary of nitrogen consumers in the underground laboratory ...... 104

245 LIST OF TABLES

7.10 LPV-U systems of DFOS and detector and sub-systems ...... 107 7.11 Summary of the detector monitoring system ...... 112 7.12 Technical details of the Laser level measurement system ...... 114 7.13 Technical details of the HPS-Sensors ...... 116 7.14 Technical details of the cross reference system (XRS) ...... 119 7.15 Overview of the pressure monitoring system ...... 124

8.1 Summary of the ﬁlling process of the DC-far detector ...... 130

9.1 Summary of all requirements for the detector liquids...... 148 9.2 Composition of the muon veto scintillator and buﬀer liquid ...... 149 9.3 Transparency, density and light yield of the muon veto scintillator ...... 151 9.4 Radio purity analysis of the muon veto sample ...... 151 9.5 Transparency, density and light yield of the buﬀer liquid ...... 152 9.6 Density, light yield, transparency and radio purity of the MU, BF, GC and NT . 155

10.1 Expected liquid level increase in the detector with and without XTOS for a thermal variation of 0.9 K ...... 164

11.1 Uranium and thorium concentration in target and gamma catcher ...... 174 11.2 Comparison between the minimum requirements on MU and BF with the actually measured values for density, light yield, transparency and radio purity...... 188

B.1 pumping station Details ...... 197 B.2 Technical details of the gas ﬁlter station ...... 200 B.3 Technical details of the high pressure nitrogen manifold in the LSA ...... 201 B.4 Technical details of the low pressure nitrogen manifold in the LSA ...... 202 B.5 Technical details of the trunk line system ...... 203

C.1 Composition of the diﬀerent detector liquids and necessary amounts for the DC- far detector ...... 204

D.1 Technical details of the detector ﬂuid operating system and the valve station . . . 212 D.2 Technical details of the target module of DFOS and the target valve station . . . 217 D.3 Technical details of the expansion tank operating system (XTOS) ...... 225

246 Glossary

A AAS Atomic Absorption Spectrometry B BF Buffer BF-DFOS Buffer Detector Fluid Operating System BF-FFT Buffer Fine Filling Tank BF-IMT Buffer Intermediate Tank Bis/MSB 1,4-bis(2-methylstyryl)-benzene BF-Oil Buffer Oil BF-PS Buffer Pumping Station C CM Continuous Mode CNPE Centre Nucléairede Production d’Electricité´ CPS Critical Point Sensor D DAQ Data Acquisition System DC Double Chooz DFOS Detector Fluid Operating System DMS Detector Monitoring System E ECS Emergency closure system EDF Electricitéde´ France F F Filter FADC Flash Analog to Digital Converter FD Far Detector FEEE Front End Electronics FF Fine Filling FFT Fine Filling Tank FEP Fluorinated ethylene propylene FM Flow Meter FPN-U Flushing Pressure Nitrogen Underground G GB Glove Box GC Gamma Catcher GC-DFOS Gamma Catcher Detector Fluid Operating System GC-FFT Gamma Catcher Fine Filling Tank GC-IMT Gamma Catcher Intermediate Tank GC-LS Gamma Catcher Liquid Scintillator H

247 Glossary

HE Heat Exchanger HPN High Pressure Nitrogen HPN-U High Pressure Nitrogen Underground HPS Hydrostatic Pressure Sensor HV High Voltage I IBC International Bulk Container IBD Inverse Beta Decay ID Inner Detector IMT Intermediate Tank IMT IMT-Mode Filling IV Inner Veto L LAB Linear Alkylbenzene LH Left Handed LI Light Injection System LL Liquid Level LM Level Measurement LM-PC Level Measurement Computer LMS Level Measurement System LN2 Liquid Nitrogen LPN Low Pressure Nitrogen LPN-U Low Pressure Nitrogen Underground LPV Low Pressure Ventilation LPV-U Low Pressure Ventilation Underground LS Liquid Scintillator LS-LM Laser Level Measurement System LSA Liquid Storage Area LSD Liquid Scintillator Detector M MPIK Max Planck Institute for Nuclear Physics MS Master Solution MU Muon Veto MU-DFOS Muon Veto Detector Fluid Operating System MU-IMT Muon Veto Intermediate Tank MU-LS Muon Veto Liquid Scintillator MU-PS Muon Veto Pumping Station N N2 Nitrogen Gas NAA Neutron Activation Analysis ND Near Detector NI National Instruments, Co. NT Neutrino Target NT-DFOS Neutrino Target Detector Fluid Operating System NT-FFT Neutrino Target Fine Filling Tank NT-IMT Neutrino Target Intermediate Tank NT-LS Neutrino Target Liquid Scintillator O OD Outer Detector OV Outer Veto

248 Glossary

P P Pump P&ID Piping and Instrumentation Diagram/Drawing PE Polyethylene PE-HD Polyethylene High Density PE-LD Polyethylene Low Density PFA Perfluoroalkoxy PLC Programmable Logic Controller PMNS Pontecorvo, Maki, Nakagawa, Sakata PMT Photo Multiplier Tube PPO 2,5 diphenyloxazol PS Pumping Station PTFE Polytetrafluoroethylene PVC Polyvinyl chloride PVDF Polyvinylidene fluoride PWR Pressurized Water Reactor PXE Phenylxylylethane PXI PCI eXtensions for Instrumentation S S Steel SEP System Entry Point SS Stainless Steel T TIG-welding Tungsten Inert Gas Welding TLS Trunk Line System TLM Trunk Line Module TT Transport Tank V VME Versa Module Eurocard W WT Weighing Tank X XRS Cross Reference System XTOS Expansion Tank Operating System

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256 Acknowledgement

Zum Abschluss meiner Arbeit möchte ich mich bei allen bedanken, die mir in den letzten Jahren zur Seite standen. Prof. Franz von Feilitzsch und Prof. Stefan Schönert sage ich zuerst vielen Dank fürdie Möglichkeit am E 15 zu promovieren und fürIhre damit verbundene stetige Unterstützungin allen Belangen. Darüber hinaus danke ich Ihnen fürdie vielen Vorschusslorbeeren, Geschichten aus der Jagd- & Forstwirtschaft und ein sehr motivierendes Arbeitsumfeld. Prof. Lothar Oberauer, meinem Doktorvater, danke ich fürseine Betreuung dieser Arbeit, sehr viel Vertrauen, und die Bereitstellung von politischem Gewicht, vor allem wenn man selbst nicht genug davon hat. Zudem möchte ich mich fürviel Menschlichkeit, Zeit sowie seine stets offene Bürotürbedanken. Dr. Marianne Göger-Neff gilt der Dank fürIhre große Hilfsbereitschaft, der Beantwortung vieler Fragen, ihre Offenheit, die gute Zusammenarbeit und fürdas Korrekturlesen dieser Arbeit. Dr. Frank Hartmann for good ideas and a critical mind, which made me reconsider more than once. Dr. Christian Buck und allen anderen Mitgliedern des MPIK danke ich fürdie tolle Zusammenarbeit, viel Hilfe und Unterstützungim Laufe der letzten Jahre, sowie fürdie gemein- schaftliche Lösungvon Problemen und viele lustige Abende in Chooz. Dr. Hong Hanh Trinh Thi hat meinen Dank fürs Vorbild sein - nicht nur was gewis- senhaftes und strukturiertes Arbeiten angeht, sondern vor allem, wenn es um Verhandlungen mit Verkäuferngeht. Hanh, ohne Deine wertvolle Arbeit wärevieles nicht so gut wie es jetzt ist. Danke ! Dr. Jean Come Lanfranchi danke ich fürseinen entspannenden Humor, seine immer guten Ratschläge,Spaziergänge,Bootsfahrten, gegrillten Mitternachtsfisch und die Möglichkeit, Pa- pageien tanzen zu lassen. Patrick Perrin for wordless understanding, French lessons, very good meals, insides, transla- tions, constant help, and a coffee break at the right time. Michael Franke, Judith Meyer und Anton Röckl verdienen großen Dank fürIhre Diplo- marbeiten, welche die hier vorliegende Arbeit unterstützen.Vielen Dank fürall eure die Arbeit, unermüdliche und sehr kompetente Hilfe, fürMarmorkuchen, Kerzen und die durchgestandenen Kalibrationsnachtschichten im Beschleuniger. Allen Borexinesen, Lenisten sowie Cryonisten: habt Dank fürviele Geschichten und Einsichten in eure Experimente und stets die richtige Ablenkung, wenn man gerade ein Pause braucht. Im Detail sind das: 5 l Rotwein, Latte Macchiato, Klettern, Mittagsplanschen, Helsinki, sowie, nicht zu vergessen, stetige Updates bezüglich der neuen Lieder von David Hasselhoff. Allen Double Chooz’lern gilt mein Dank fürihre stetige Unterstützungund tolles Ar- beitsklima, fürsehr viel Hilfe beim Planen, Bestellen, Abholen, Transportieren, Installieren, Analysieren und Simulieren. Vielen Dank füralles ! Nils Haag und Martin Hofmann danke ich fürdie ausführliche und geduldige Korrektur dieser Arbeit, ihre vielen Verbesserungsvorschläge,das Privatseminar zur Lösung der Dirac- Gleichung, viele Antworten auf noch mehr Fragen... und... Nilspferde. Dem alten Sekretariat und damit Beatrice van Bellen und Alexandra Füldner, die mir

257 Acknowledgement zu Beginn dieser Arbeit bei all meinen verwaltungstechnischen Problemen zu Seite standen und die nicht müdewurden mir zu erklären,dass das Original bei mir bleibt, der gelbe Durchschlag zu Wahrenannahme gehört,der rote zur Verwaltung muss und das man den dunkelweißen nie raus reißen darf! danke ich ganz herzlich. Dem neuen Sekretariat und damit Maria Bremberger, die mir in den letzten Jahren immer aus der Patsche half, auch wenn ich schon wieder vergessen hatte den Reiseantrag rechtzeitig abzugeben, gilt mein besonderer Dank. Darüber hinaus möchte ich mich noch fürgute Stim- mung, lustige Geschichten und die Erkenntnis bedanken, daß man besser drückt als zieht. Der Werkstatt und damit Harald Hess, Erich Seitz und Thomas Richter möchte ich für die an Kunst grenzende Verwirklichung meiner Ideen danken. Fürgute Vor- wie Ratschläge, immer freundlichen Empfang, selbst wenn man schon wieder kommt, um doch noch etwas zu ändern. Mein Dank gilt weiterhin der Firma Pürstinger und damit den Herren Kehrberg, Brauner, Deinböck, Gahn und, nicht zu vergessen, Tim-Bob, die Double Chooz auch zu ihrem Projekt gemacht haben. Ebenso bedanke ich mich bei Wacker-Chemie und damit Herrn Dr. Stöhrer,der mit viel En- gagement, professionellem Rat und der Bereitstellung von idealer Infrastruktur einen entschei- denden Beitrag zur Produktion des Muon-Veto-Szintillators leisten konnte und damit auch wesentlich zum Erfolg dieser Arbeit beigetragen hat. All meinen Freunden , und damit (glücklicherweise) sehr vielen lieben Menschen, die mich in den letzten Jahren begleitet haben. Ich freu mich schon sehr darauf wieder mehr Zeit mit Euch zu verbringen, und zwar genau ab jetzt! Der Band Waaazzzzuuup ! What can you say ...? You, me, them, everybody, everybody ! Meiner lieben Familie, ohne Eure Unterstützunghätteich es nie soweit geschafft. Vielen Dank, fürsDasein und Begleiten, fürsan mich Glauben und Aufbauen, fürsAblenken und in- teressiert sein. Einfach füralles !...ab jetzt hab ich auch wieder Zeit was an die Wand zu dübeln !

Dem besondersten aller Menschen, nämlich meiner Liebsten, Angelika Giglberger, danke ich von ganzem Herzen fürall das was Du in den letzten Jahren fürmich getan hast und damit für so viel mehr als man hier aufzählenkann. Vielen Dank fürDeine unermüdliche Unterstützung, Dein Durchhaltevermögen,Deinen unerschütterlichen Glauben an mich und daran, daß diese Arbeit doch irgendwann fertig wird. Für die Korrektur dieser Arbeit und Deine Fähigkeit Kom- mas zu setzen, fürsAufbauen, fürsAblenken, fürBrotzeitbretter, fürunendeckte Saftschorlen, fürKerschgeist... und fürso viel Geduld. Ich liebe Dich!

...and thanks for all the ﬁsh !

258