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Data Reconstruction and Analysis for the GERDA Experiment Zurich Open Repository and Archive University of Zurich Main Library Strickhofstrasse 39 CH-8057 Zurich www.zora.uzh.ch Year: 2015 Data Reconstruction and Analysis for the GERDA Experiment Benato, G Posted at the Zurich Open Repository and Archive, University of Zurich ZORA URL: https://doi.org/10.5167/uzh-119515 Dissertation Published Version Originally published at: Benato, G. Data Reconstruction and Analysis for the GERDA Experiment. 2015, University of Zurich, Faculty of Science. Data Reconstruction and Analysis for the GERDA Experiment Dissertation zur Erlangung der naturwissenschaftlichen Doktorwurde¨ (Dr. sc. nat.) vorgelegt der Mathematisch-naturwissenschaftlichen Fakultat¨ der Universitat¨ Zurich¨ von Giovanni Benato aus Italien Promotionskomitee Prof. Dr. Laura Baudis (Vorsitz) Prof. Dr. Ulrich Straumann Dr. Alexander Kish Zurich,¨ 2015 ZUSAMMENFASSUNG Der neutrinolose Doppelbetazerfall (0νββ) ist ein nuklearer Prozess, der durch verschiedene Erweiterungen des Standardmodells der Teilchenphysik vorherge- sagt wird. Die Beobachtung dieses Prozesses wurde¨ beweisen, dass die Lepton- zahl nicht erhalten ist und dass ein nichtverschwindender Majorana-Massenterm fur¨ Neutrinos existiert. Im Fall, dass ein leichtes Neutrino ausgetauscht wird, wurde¨ diese Beobachtung es ermoglichen,¨ die effektive Neutrinomasse zu bestim- men und letztendlich zwischen normaler und invertierter Massenhierarchie zu unterscheiden. Das “Germanium Detector Array” (Gerda) ist ein Experiment, das nach dem 0νββ-Zerfall in 76Ge sucht. Es befindet sich im Laboratori Nazionali del Gran Sasso (LNGS) in Italien. In Gerda fungieren 18 kg auf 86% 76Ge angereicherte high-purity Germaniumdetektoren (HPGe) gleichzeitig als Quelle, als auch als Detektor fur¨ diesen Prozess. Diese hangen¨ innerhalb von flussigem¨ Argon (LAr), das sowohl fur¨ die Kuhlung¨ der Detektoren verwendet wird, als auch eine Abschir- mung gegen externe Strahlung darstellt. Phase I wurde im Jahr 2013 abgeschlossen mit einem Produkt aus Detektormasse und Messzeit von 21.6 kg yr und einen · Background-Index BI 10-2 counts/(keV kg yr) bei 2039 keV, dem Q-Wert dieser ≃ · · Reaktion (Q ). Sie fuhrten¨ zu einem Limit von T 0ν > 2.1 1025 yr, bei 90% confi- ββ 1/2 · dence level. Fur¨ Phase II des Gerda-Experimentes wurden weitere 20 kg Broad En- ergy Germanium Detektoren (BEGes) hergestellt und charakterisiert. Zusatzlich¨ wurde das LAr-Volumen mit Lichtsensoren ausgestattet, die das Szintillationslicht von LAr detektieren sollen. Wird der angestrebte BI von 10-3 counts/(keV kg yr) · · erreicht, dann kann Gerda mit Phase II eine mediane Sensitivitat¨ von 1.4 1026 yr · erreichen, bei einem Produkt aus Detektormasse und Messzeit von 100 kg yr. · In dieser Arbeit wird eine komplette Rekonstruktion der Daten von Phase I vorgestellt, sowie die darauf folgende erneute Auswertung. Es wurde ein neuer digitaler Shapingfilter fur¨ die Energiebestimmung entwickelt und optimiert. Da- durch konnte eine Verbesserung der Energieauflosung¨ von etwa 12% bei Qββ erreicht werden. Ausserdem wurde eine vollautomatische Energiekalibrierung er- stellt, die auf alle Daten von Phase I angewendet wurde. Das Ergebnis dieses Ver- fahrens wird benutzt, um die systematischen Unsicherheiten fur¨ die Energiebes- timmung jedes einzelnen Events und der Energieauflosung¨ bei Qββ zu bestimmen. Zusatzlich¨ dazu wird die Analyse des 0νββ-Zerfalls auf die reprozessierten Daten angewendet. Dabei wurden mogliche¨ Verbesserungen des statistischen Ansatzes 0ν genau uberpr¨ uft.¨ Insgesamt konnte eine Verbesserung von & 5% auf die T1/2- Sensitivitat¨ erreicht werden. Letztendlich wurde dann die Optimierung uber¨ den digitalen Filter auf die Rekonstruktion von Pulsen fur¨ die Pulse Shape Dis- 0ν crimination angewendet. Dabei wurden Hinweise gefunden, dass sich die T1/2- Sensitivitat¨ um & 10% verbessert. Fur¨ die Phase II von Gerda wurden vier 228Th Kalibrierungsquellen mit re- duziertem Neutronenhintergrund hergestellt. Die γ- und Neutronenaktivitat¨ wur- iii de durch geeignete Messungen in einer Low-Background-Umgebung charakter- isiert. Daruber¨ hinaus wurde eine neu entwickelte Technik verwendet, um ein mogliches¨ Entweichen radioaktiven Materials aus der Quellenummantellung auf- zuspuren.¨ Dadurch wurde die Starke¨ der Neutronenquelle um eine Grossen-¨ ordnung unterdruckt¨ im Vergleich zu kommerziell erhaltlichen¨ Quellen und die Existenz von Lecks konnte bis zu 10 mBq ausgeschlossen. Gleichzeitig zu den experimentellen Arbeiten fur¨ Gerda, wird eine Studie uber¨ die Aussichten der Suche nach dem 0νββ-Zerfall vorgestellt. Fur¨ die Wahrschein- lichkeitsverteilung der effektiven Majorana-Masse im Fall des Austausches leichter Neutrinos wurden Zufallsereignisse der Neutrino-Mischungswinkel und der qua- drierten Massendifferenzen erzeugt unter Berucksichtigung¨ der kosmologischen Grenzen auf der Summe der Neutrinomassen. Darauf beruhend wird die benotigte¨ 0ν 76 Sensitivitat¨ auf T1/2 fur¨ Experimente angegeben, die den 0νββ-Zerfall mit Ge untersuchen. iv ABSTRACT Neutrinoless double beta (0νββ) decay is a nuclear process predicted by several extensions of the Standard model of particle physics. Its observation would prove the non-conservation of total lepton number and the existence of a non-vanishing Majorana mass term for neutrinos. In case of light neutrino exchange, it would also provide a measurement of the effective Majorana mass and eventually allow to disentangle between the normal and inverted neutrino mass schemes. The Germanium Detector Array (Gerda) is an experiment for the search of 0νββ decay in 76Ge, located at the Laboratori Nazionali del Gran Sasso (LNGS), Italy. In Gerda, 18 kg of high-purity germanium crystals (HPGe) with ∼ 86% enrichment in 76Ge are simultaneously operated as source and detector of the process. They are directly inserted in liquid argon (LAr), acting as cooling medium and shielding against external radiation. The first data collection (Phase I) was completed in 2013 with a total exposure of 21.6 kg yr. It was characterized by a background index · BI 10-2 counts/(keV kg yr) at 2039 keV, the the Q-value of the reaction (Q ). ≃ · · ββ This led to a lower limit on the 0νββ decay half life (T0ν ) of T 0ν > 2.1 1025 yr at 1/2 1/2 · 90% confidence level. In view of Gerda Phase II, additional 20 kg of Broad Energy Germanium (BEGe) detectors were successfully produced and characterized, and the LAr volume was instrumented with light sensors to detect the LAr scintillation light induced by background radiation. If the aimed BI of 10-3 counts/(keV kg yr) · · is achieved, Gerda Phase II will reach a median sensitivity on the 0νββ decay half life of 1.4 1026 yr with an exposure of 100 kg yr. · · In this work, a complete reconstruction of the Phase I data and a re-analysis of 0νββ decay are reported. A new digital shaping filter for energy reconstruction is developed and optimized, yielding to a ∼ 12% improvement in energy resolution at Qββ. Moreover, an automatic procedure for the energy calibration is developed and applied to all Phase I data. The output of this procedure is used for computing the systematic uncertainty on the energy reconstruction of each single event, as well as the energy resolution at Qββ. In addition to this, the 0νββ decay analysis is applied on the reprocessed data, and possible improvements of the statistical 0ν approach are scrutinized, which can yield a & 5% improvement in T1/2 sensitivity. Finally, the optimization of digital filters is applied to the reconstruction of current pulses for pulse shape discrimination, and an indication for a & 10% improvement 0ν in T1/2 sensitivity is found. In view of Gerda Phase II, four 228Th calibration sources with reduced neutron emission rate were produced. Their γ and neutron activities were characterized with dedicated measurements performed in low background environment, and a newly developed technique was employed to detect possible leaks of radioactive material from the source encapsulation. As a result, the neutron source strength is suppressed by one order of magnitude with respect to commercially available sources, and the presence of leaks is excluded down to a 10 mBq level. v In parallel to the experimental work for Gerda, a study of the perspectives for 0νββ decay search is described. The probability distribution for the effective Ma- jorana mass in case of light neutrino exchange is extracted through a random sam- pling of the involved neutrino mixing angles and squared mass differences, with the inclusion of the cosmological bound on the sum of neutrino masses. Based 0ν on this, the required discovery sensitivity on T1/2 for experiments investigating 0νββ decay in 76Ge is reported. vi CONTENTS 1 introduction 1 2 neutrinos and double beta decay 3 2.1 Historical Overview . 3 2.2 Dirac and Majorana Neutrinos . 6 2.2.1 Seesaw Mechanism . 8 2.3 Neutrinoless Double Beta Decay . 8 2.4 Effective Majorana Mass . 11 2.4.1 How Does Cosmology Affect 0νββ Decay Search? . 16 2.5 Double Beta Decay Search . 22 2.5.1 State of the Art in Double Beta Decay Search . 25 2.6 Double Beta Decay Search in 76Ge.................... 26 2.6.1 2νββ Decay Search in 76Ge.................... 29 3 germanium detectors 31 3.1 Interaction of Particles with Matter . 31 3.1.1 α Particles . 31 3.1.2 Electrons and Positrons . 32 3.1.3 γ Radiation . 32 3.1.4 Compton Scattering . 33 3.2 Semiconductor Detectors . 35 3.3 Germanium Detectors . 39 3.3.1 Signal Formation in Germanium Detectors . 40 3.4 Signal Readout with Germanium Detectors . 41 3.5 Energy Resolution . 43 4 the gerda experiment 45 4.1 Experimental Setup . 46 4.1.1 Gerda Detectors . 48 4.1.2 Readout Electronics . 50 4.1.3 LAr Veto . 50 4.2 Gerda Data Structure . 51 4.3 Gerda Phase I . 52 4.3.1 The Background in Gerda Phase I . 56 4.3.2 Analysis of 0νββ Decay . 57 4.3.3 Analysis of 2νββ Decay . 58 5 calibration of the energy scale 61 5.1 228Th Calibration Sources . 61 5.2 The Gerda Calibration Software . 64 5.3 Quality Cuts for the Analysis of Calibration Spectra . 65 5.4 Peak Search and Identification . 72 5.5 Fitting of the Spectral Lines . 74 vii Contents 5.6 Calibration and Resolution Curves .
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