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Doctoral Thesis
Design and construction of the Prototype synchrotron radiation detector
Author(s): Grimm, Oliver
Publication Date: 2002
Permanent Link: https://doi.org/10.3929/ethz-a-004322330
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ETH Library EIH Dissertation ETH No. 14576 Eidgenosslsche Technische Hochschuie Zurich Swiss Federal Institute ofTechnology Zurich
Design and Construction of the Prototype Synchrotron Radiation Detector
A dissertation submitted to the Swiss Federal Institute of Technology, Ziirich for the degree of
Doctor of Natural Sciences
presented by Oliver Grimm Dipl. Phys., University of Hamburg, born 24 May 1972 in Buchholz, Germany
Accepted on the recommendation of Prof. A. Rubbia, examiner Prof. H. Hofer, co-examiner Prof. G. Viertel, co-examiner
April 2002 Design and Construction of the Prototype Synchrotron Radiation Detector
Oliver Grimm
PhD Thesis
Eidgenossische Technische Hochschule Zurich Labor fur Hochenergiephysik 8093 Zurich
April 2002 Abstract
The Prototype Synchrotron Radiation Detector (PSRD) is a small-scale experiment designed to measure the rate of low-energy charged particles and photons in a near-earth orbit. It is a precursor to the Synchrotron Radiation Detector (SRD), a proposed addition to the upgraded second version of the Alpha Magnetic Spectrometer (AMS-02). The SRD utilises the earth's magnetic field to identify electrons and positrons with energies above 1TeV by detecting the synchrotron radiation they emit in this field. This is an astrophysically interesting energy range not well covered by the remaining components of AMS-02. The SRD can discriminate against protons as they radiate only weakly. Electrons and positrons of such high energy efficiently lose energy by inverse Compton scattering off cosmic microwave background and starlight photons and by synchrotron radiation in galactic magnetic fields. For a particle of 1TeV, a lifetime of 2.1.105 years is estimated, during which a distance less than 1kpc from the creation point can be travelled by diffusion. Therefore, information on the acceleration mechanism (or else, the creation) of these particles within our relative galactic neighbourhood can be extracted from precise spectra and ratios of electrons and positrons. As an example, the mechanism conventionally assumed to be responsible for the acceleration of cosmic rays up to tens of TeV, supernova shock fronts, accelerates the material of the interstellar gas that is practically devoid of positrons. In that case, only positrons created as secondary reaction products should be present in the spectra. The number and energy of the synchrotron photons that the SRD needs to detect are both small. A typical electron event around 1 TeV will only result in 2 to 3 synchrotron photons in the keV energy range hitting a detector of several square metre in area. These few photons need to be discriminated against a large background consisting of electrons and photons also in the keY range. This can be achieved with a good time resolution, as the synchrotron photons will arrive in coincidence with the high-energy charged particle, uncorrelated with the background. Measurements on the photon background exist, by themselves calling for a time resolution of the order of 10 ns. Data on low-energy electrons, on the other hand, are very scarce and incomplete. Since a sufficiently precise knowledge of these rates is essential for the construction of the large-scale SRD, a measurement in space was indispensable. The main objective of the PSRD is, therefore, this background measurement. The detector employs components resembling, as far as practical, the current design ideas of the SRD, which allows, beyond the main goal, a realistic test in space of these components. The detector was designed to fly as a secondary payload on a Space Shuttle, within the Shuttle Small Payloads Project. The flight on board the Space Shuttle Endeavour took place 5 -17 December 2001, with a total running time of about 110 hours. The main focus of this work is the detailed description of the design and construction of the PSRD, including specific experimental studies carried out in support of this. This main part is preceded by a concise account on the current status of cosmic-ray physics, and by a brief overview of AMS-02 and the SRD. Zusammenfassung
Der "Prototype Synchrotron Radiation Detector" (PSRD) ist ein kleines Experiment mit der Aufgabe, die Rate von niederenergetischen geladenen Teilchen und Photonen in einer erdnahen Umlaufbahn zu messen. Es ist ein Vorlaufer des "Synchrotron Radiation Detector" (SRD), einer neuen Komponente, die fur die erweiterte, zweite Version des "Alpha Magnetic Spectrometer" (AMS-02) vorgeschlagen ist. Der SRD nutzt das Erdmagnetfeld, um Elektronen und Positronen mit Energien oberhalb von 1 TeV anhand der Synchrotronstrahlung nachzuweisen, die sie in diesem Feld emittieren, und um sie von Pro tonen zu unterscheiden, die kaum emittieren. Dieser Energiebereich ist durch die ubrigen Komponenten des AMS-02 ungenugend abgedeckt und astrophysikalisch interessant. Elektronen und Positronen mit derart hoher Energie verlieren Energie auf effiziente Weise durch inverse Compton-Streuung an der kosmischen Mikrowellen-Hintergrundstrahlung und an Sternenlicht und durch Synchrotronstrahlung in galaktischen Magnetfeldern. Fur ein 1 TeV Teilchen wird eine Lebensdauer von 2.1.105 Jahren abgeschatzt, wahrend derer eine Entfernung von weniger als 1kpc durch Diffusion vom Ursprungsort zuruckgelegt werden kann. Daher konnen Aussagen uber den Beschleunigungsmechanismus oder die anderweitige Erzeugung dieser Teilchen in unserer galaktischen Umgebung aus genauen Spektren und dem Ratenverhaltnis von Elektronen und Positronen gewonnen werden. Der Mechanismus beispielsweise, der ublicherweise fUr die Beschleunigung von kosmischen Teilchen bis zu einigen zehn TeV herangezogen wird, Schockfronten von Supernovae, beschleunigt Material des interstellaren Gases, das nahezu keine Positronen enthalt. In diesem Fall sollten nur sekundare Positronen (Reaktionsprodukte) zu den Spektren beitragen. Sowohl die Anzahl als auch die Energie der Synchrotronphotonen, die der SRD nachweisen solI, sind klein: Bei einem typischen Elektronenereignis von etwa 1 TeV werden lediglich 2 bis 3 Synchrotronpho tonen mit Energien von wenigen keV einen Detektor von einigen Quadratmetern Flache treffen. Diese geringe Anzahl Photonen muB gegen einen groBen Untergrund aus Elektronen und Photonen, ebenso im keY Bereich, unterschieden werden. Dies kann durch eine gute Zeitauflosung erreicht werden, da die Syn chrotronphotonen in Koinzidenz mit dem hochenergetischen geladenen Teilchen eintreffen, unkorreliert mit dem Untergrund. Messungen des Photonuntergrundes existieren, die allein eine Zeitauflosung von etwa 10 ns fordern. Daten uber niederenergetische Elektronen sind allerdings kaum vorhanden, und da eine hinreichend ge naue Kenntnis dieser Raten von grundlegender Bedeutung fur die Realisierung des umfangreichen SRD Projektes ist, war eine Messung im Weltraum unumganglich. Die hauptsachliche Zielsetzung des PSRD umfaBt daher die Messung dieses Untergrundes. Der Detek tor wurde unter Verwendung von Komponenten gebaut, die denen des derzeitigen Entwurfes fur den SRD so nahe wie moglich kommen, um neben dem Hauptziel auch einen realistischen Test dieser Komponenten im Weltraum zu erlauben. Die Konzeption ist darauf ausgerichtet, den Detektor als sekundare Nutzlast an Bord des Space Shuttles im Rahmen des "Shuttle Small Payloads Project" zu fliegen. Der Flug mit der Raumfahre Endeavour fand zwischen dem 5. und 17. Dezember 2001 statt, wobei die gesamte Datennahmezeit etwa 110 Stunden betrug. Das Hauptanliegen dieser Arbeit ist die detaillierte Beschreibung des Entwurfs und der Konstrukti on des PSRD, unter EinschluB von einzelnen unterstutzenden experimentellen Studien. Dem Hauptteil vorangestellt ist ein kurzer AbriB des derzeitigen Status der Physik der kosmischen Teilchen und ein Uberblick uber AMS-02 und den SRD. Contents
Introduction 9
Chapter 1 Cosmic Rays 11 1.1 Measurements...... 12 1.1.1 Energy spectrum ...... 12 1.1.2 Elemental and isotopic composition 12 1.2 Models . 14 1.2.1 Creation and acceleration . 14 1.2.2 Propagation through space 15 1. 2.3 Local influences ...... 17 1.3 High energy electrons and positrons 17
Chapter 2 The Alpha Magnetic Spectrometer 21 2.1 Scientific objective . 21 2.2 Detector description ...... 22
Chapter 3 The Synchrotron Radiation Detector 26 3.1 Principle considerations . 26 3.1.1 General . 26 3.1.2 Synchrotron radiation 27 3.1.3 Backgrounds 29 3.2 Detection principle ..... 32 3.2.1 Scintillators ..... 32 3.2.2 The YAP scintillator 34 3.2.3 Photomultiplier 35 3.3 Tentative design ... 39 3.3.1 General lay-out 39 3.3.2 Light shield 41 3.3.3 Electronics 41
Chapter 4 The Prototype Synchrotron Radiation Detector 44 4.1 Motivation . 44 4.2 Design overview . 45 4.3 Material and structural issues 48
5 6 CONTENTS
4.4 X-ray cassette . 49 4.4.1 Small YAP array . 49 4.4.2 Large YAP crystals. 52 4.4.3 Solar cells ...... 59 4.5 Silicon macrostrip detector 60 4.6 'frigger detector 61 4.7 Electronics 61 4.7.1 SRDYAP 63 4.7.2 SRDAPV 64 4.7.3 SRDSAB+TRG. 65 4.7.4 SRDSOL .... 67 4.7.5 SRDHVC+SLO. 67 4.7.6 SRDPWR .... 68 4.7.7 SRDPOW+LED 68 4.7.8 Grounding requirements 69 4.8 PCj104 computers . 69 4.9 Hard disks . 71 4.10 Hitchhiker data link . 74 4.11 Flight control program 77 4.12 Data rates . 80 4.13 Provisions for ground testing 80 4.14 Space-qualification tests ... 81
Chapter 5 Experimental Studies for PSRD and SRD 87 5.1 Time resolution . 87 5.1.1 Simplified model for the expected time resolution. 87 5.1.2 Measured time resolution ..... 90 5.2 Light yield of different YAP(Ce) crystals 90 5.3 X-ray absorption of wrapping materials 92 5.4 Detection efficiency for 511 keV photons 94 5.5 Afterpulse measurements ..... 97
Chapter 6 Summary and Conclusion 102 6.1 Experimental results .. 103 6.2 PSRD mission summary 104 6.3 Concluding remarks .. 106
Appendix A SRD: Effective Flight Distance and Granularity 109 A.l Effective flight distance 109 A.2 Granularity ...... 110
Appendix B Simulation of Light Propagation 112 B.l General . 112 B.2 Some simulation results . 113 CONTENTS 7
B.3 Principle deductions 114 BA Technical details 115 8.5 Flow diagram 117 B.6 Source code ... 118
Appendix C Laboratory Background Measurements 124
Appendix D Chronology of PSRD Development 126 D.1 Chronological description 126 D.2 Photographs . 130
Appendix E Collaboration Members 135
Appendix F NASA Flow Schedule 136
List of Figures 139 List of Tables . 142 References ... 143 Contact Information 146 Personae Gratae .. 149 Seite Leer / Bla,nk leaf
L..------' Introd uction
In the realm of large-scale astrophysics no laboratory-style experiments are possible. All theo retical models that are conceived to explain observed features in or of the universe - and even predictions of unobserved events like the big-bang or inflation - can be tested only against observational information that is received on earth from space. This information comes in the form of radiation and particles of all sorts, and using every bit of it and extracting the maximum from it is essential in understanding the true nature of many distant and unfamiliar phenomena. The first known and most obvious source of information on the universe is visible light, the close examination of which giving already insights on, for example, the distribution of stars and their surface chemical composition. When other regions of the electromagnetic spectrum became also available for examination due to new detection techniques and the elimination of the disturbing effects of the earth's atmosphere by taking detectors to space, much progress was made in the whole field of astronomy. Finally, with nowadays virtually the whole spectrum available and precision devices in operation, even cosmology, long branded as being highly speculative, has changed into an exact science. A further source of information are cosmic rays, energetic particles like protons, nuclei, elec trons, neutrinos and photons! arriving at earth from outer space. The energy spectra, elemental and isotopic composition, arrival directions and arrival times of these particles yield many more clues on the structure of, and processes in, our galaxy and the more distant universe. An experiment to measure these quantities of cosmic rays with high precision is the Al pha Magnetic Spectrometer (AMS), the first large particle spectrometer designed for space. A test flight on board a Space Shuttle was completed in June 1998 and a significantly improved version of the experiment is currently under construction and scheduled to be installed on the International Space Station (ISS) in 2004, for a duration of three years. To extend the energy range accessible to AMS, a new detector component that measures the synchrotron radiation emitted by electrons or positrons in the earth's magnetic field was conceived. The design of this Synchrotron Radiation Detector (SRD), essentially a large X-ray detector with good timing capability, requires information on the X-ray and low-energy particle background on the ISS orbit, data that is only scarcely available and incomplete. A precursor experiment, the PSRD, was therefore built to measure this background and also as a proof-of principle of some of the concepts for the SRD. This work is mainly concerned with the design and construction of the PSRD and with measurements in support of the PSRD and SRD. In Chapter 1, the basics of cosmic ray physics are laid out, with a summary of the known Characteristics, possible explanations and models of the available data and a description of the
lWhether one dubs a high-energy photon a cosmic ray or assigns it simply to the electromagnetic spectrum is purely conventional.
9 10 INTRODUCTION next steps necessary for improving or solidifying the understanding. The role the AMS and the SRD can have here is also sketched. The AMS experiment, as planned for the ISS, is briefly reviewed in Chapter 2. The physical and detection principles, and the current design status of the SRD, are described in Chapter 3. A detailed account on the PSRD, including its motivation and hardware and software realization, can be found in Chapter 4. Various experimental studies in support of the PSRD and SRD are detailed in Chapter 5. A summary of this work and of the PSRD shuttle flight, together with final conclusions, is given in Chapter 6. Some deductions from the bending radius of high-energy charged particles in a magnetic field relevant for the SRD are made in Appendix A. A simple Monte-Carlo program simulating light propagation in, for example, scintillators, that is used to investigate some general features of the light yield measurements, is presented in Appendix B. Measurements detailing the particle background found in the laboratory environment are reported in Appendix C. A chronological review of the PSRD development is given in Appendix D, followed by a list of all collaboration members in Appendix E. Finally, an outline of the general manifestation process for a Space Shuttle experiment in given in Appendix F. Contact informations for companies that are mentioned in the text are listed on pages 146/147, following the references.
Note on physical constants and units Calculations are done in SI units throughout this work. The physical constants used are as follows [PDGOO]:
Symbol Quantity Value c velocity of light in vacuum 2.998· 108 m/s h Planck constant 6.626· 10-34 Js e electron charge magnitude 1.602.10-19 C me electron mass 9.109.10-31 kg EO permittivity of free space 8.854.10-12 C/(Vm) 6 J.L0 permeability of free space 1.257.10- Vs/(Am) NA Avogadro constant 6.022.1023 l/mol k Boltzmann constant 1.381.10-23 J/K u unified atomic mass unit 1.661 . 10-27 kg
For computer storage units, the convention 1kByte = 1024 Byte, 1 MByte = 1024 kByte and 1 GByte = 1024 MByte is used. CHAPTER 1
Cosmic Rays
The existence ofcosmic rays, i. e. particles impinging on the earth from outer space, was originally established by observing the discharging of electroscopes. Victor F. Hess was around 1912 one of the first who carried these devices high into the atmosphere on board balloons and who inferred from the changing discharge rate that penetrating particles from above the atmosphere must be responsible. Their nature was subsequently scrutinised with a variety of methods, using ground or moun tain based observatories, stratospheric balloon flights and, finally, satellites orbiting the earth. These detailed measurements have already quite severely constrained models on the origin and propagation, so the understanding of the physics of cosmic rays has reached some level, though being far from complete. Many experiments are underway or planned to refine the data on cosmic ray characteristics. Interest in these particles is especially high since they represent material brought to earth from large distances where elemental and isotopic abundances can otherwise be inferred only indirectly. Specifically, antimatter nuclei heavier than helium in the cosmic radiation would give a strong hint that antimatter regions exist somewhere in the universe where the antinuclei were created by nuclear fusion in an antimatter star (antihelium, while also created by fusion in an antistar, could also be of primordial origin). Such regions are likely ruled out to some distance from earth as they would show the telltale sign of annihilation radiation from the contact area with normal matter regions. This radiation should have already been detected. For larger distances, the question of these regions has not been answered unanimously, though in [Coh98] it has been argued that they are excluded essentially up to distances that are comparable to the size of the visible universe if it is baryon-symmetric (permitting only small 'pockets' of antimatter at large distances). In [KhloOO], the case of a baryon-asymmetric universe containing small amounts of antimatter, locked in globular clusters in the halo of galaxies, has been discussed. These clusters would emit sufficiently small amounts of annihilation radiation to have remained undetected so far, but should leave a measurable imprint on the antihelium flux. On another level, cosmic rays also play a role in interstellar chemistry. They heat and ionise not only the general interstellar gas, but also the inside of dense molecular clouds that are otherwise rather well shielded against radiation by enclosing dust. This way, free ions and elec trons are generated such that chemical reactions can take place even in these extremely rarefied regions. Complex molecules consisting of up to 13 atoms have indeed been identified. A comprehensive overview not only of cosmic ray physics, but of the whole subject of high energy astrophysics, can be found in [Long92]. Up-to-date information is collected in [CRC99] and [SchI02].
11 12 CHAPTER 1. COSMIC RAYS
lI\ ~ 104 ·llJ -.. &t'I )( ...: .)( > :::J Cb 3 -1..1... 10 -I 0 '- -'-.... '"11) c; I -11) 2 Cb • Proton-4 '- C't 10 Cb I A n.n Shan ~ IS o Akeno '- '- • HaverahPark Q D Yakutsk 10 ..··Sydn.y
E: Energy .0F Nucleus (eV)
Figure 1.1 Differential energy spectrum of cosmic rays (from [Sok89])
1.1 Measurements
1.1.1 Energy spectrum The differential cosmic ray energy spectrum including all particles is shown in Fig. 1.1. It is found that the flux has a power law energy dependence, i. e. the flux is proportional to E-a , with the spectral index a. As the flux in this figure is multiplied by E 2.5 , a spectral index of 2.5 would result in a horizontal line. A clear feature is the changing of the spectral index from about 2.7 to 3.0 at around 3.1015 eV (3 PeV), the so-called "knee". The behaviour at the highest measured energies is not obvious from this compilation, but more recent measurements, shown in Fig. 1.2, indicate a rather complex spectrum (note that here the flux was multiplied by E 3 ). It is rather intriguing that, within the limited statistics available at these highest energies, the arrival directions seems to to distributed isotropically, just as at lower energies.
1.1.2 Elemental and isotopic composition Another key characteristic of cosmic rays is their composition. The nature of a cosmic ray at lower energies can be determined with standard techniques also used for particle identification in the laboratory, though the minimisation of secondary effects from the atmosphere requires that these detectors are brought to high altitude or into earth orbit. At higher energies, the decreasing flux requires such detectors to become forbiddingly large, so that other methods are used. The most successful, with which the highest cosmic ray energies of over 1020 eV have been measured, are air shower experiments. Here, the particle content of the showers that develop 1.1. MEASUREMENTS 13
25.2
N-> GJ 25.0
I s.. ...l7.I I 24.8 (J GJ l7.I. N I e 24;6
M- .,rz) 24.4 bO -0 24.21L7--...... ----,---1J,.;8----...... ~--=-~19---'-...... &-WoL-IL.L..I1-2,L.O--''--~· log Energy (eV)
Figure 1.2 Energy spectrum of cosmic rays at highest energy (from [CronOl]) when high energy primaries strike the atmosphere are sampled on the ground with an array of detectors and through detailed shower modelling one tries to reconstruct the energy and particle type of the primary. On average, roughly 85% of all cosmic rays are protons, 12% helium nuclei, 1% nuclei heavier than helium and 2% electrons. Photons are present on the per mil level. The elemental composition of cosmic rays, compared to that in our solar system, is shown in Fig. 1.3. As the total flux falls steeply with energy, these average values are overwhelmingly determined by the region where sensitive identification techniques can be applied. Generally, a similar composition for cosmic ray and solar system matter is found, with the specific exception that the group consisting of lithium, beryllium and boron at low atomic numbers and the one between scandium and manganese at medium atomic numbers, together with some additional individual elements, are a lot more common in cosmic rays. These elements are rather hard to make in nuclear fusion processes in stars and are therefore rare. In cosmic rays, they are though to be produced by fragmentation of heavier nuclei upon collisions with interstellar gas during their long distant travel. In fact, using radioactive isotopes produced this way as clocks, the average age of cosmic rays can be determined to be around 107 years. Looking closely at the isotopic distribution of individual elements, an overabundance of neutron rich isotopes as compared with solar system matter is found. The general higher abundance of elements with even atomic numbers both in cosmic ray and solar system matter can easily be understood from nuclear physics, as the binding energy of even nuclei is higher compared to the next odd nuclei due to the pairing interaction of nucleons. Elements with a first ionisation potential above 9 eV have systematically a lower abundance in cosmic rays, as seen for H, He, 0, Ne and S. This possibly reflects the first step in the acceleration process, which requires ionisation of neutral particles. 14 CHAPTER 1. COSMIC RAYS
H Li 8 N F No AI P Cl K Se V Mn Co Cu Co A'3 Br Rb Y • '1 • ! oliCrFeNi Zn Ge Se Kr Sr 106 He 8e' CONe Mg Si 5 Ar C - . '"'0 0 104 - - . 11 2 ~ in 10 1\ - '--' IV'" I'" \~ ID ill' IV ~ U E:: 10° - C "[J t: 2 :::l 10- - .0 Figure 1.3 Relative elemental abundances in the solar system (columns) and in cosmic rays (nne) (fromhttp://imagine.gsfc.nasa.gov/docs/science/know_12/cosmic_rays.html) 1.2 Models Interpretation of the cosmic ray data that is extracted from measurements on or near earth requires considerations of three distinctive ingredients: creation and acceleration of the cosmic ray particles, propagation to the solar system through interstellar and possibly intergalactic space and finally influences of the solar wind and earth's magnetic field. An outline of the processes that are generally believed to be in action is given in the following sections. Although much progress was made by the availability of precision data from space experiments, some aspects are still controversial and no attempt is made here to give a complete overview of all models (and trends). 1.2.1 Creation and acceleration A variety of particles is continually injected into interstellar space, at low energies by rather qui escent processes like stellar winds, and at higher energies by expulsions of outer stellar envelopes during the later stages in the life cycles of stars (resulting, e. g., in the creation of planetary neb ulae) or, still more violently, by supernova explosions. In fact, these particles, together with the primordial hydrogen and helium content left over from the (on-going) processes ofstar formation, 3 form the interstellar gas, the average density of which is about 1 particle per cm . However, the power-law energy spectra obtained over large energy ranges cannot be explained this way, also the very high energy that cosmic rays can attain remains a mystery ifone assumes only such 'direct' processes. The widely accepted explanation is to assume a distinct acceleration step that endows the particles with energy according to a power law. A stochastic acceleration in supernova shock waves is the mechanism believed to be mainly responsible. 1.2. MODELS 15 Fermi proposed already in 1949 a stochastic acceleration process that naturally results in a power-law spectrum. The essential idea is that charged particles can gain or lose energy when they interact with large magnetic irregularities (which can be assumed to act like a magnetic mirror), but that head-on collisions resulting in a gain of energy are slightly more probable than following collisions. The original mechanism that was put forward resulted in energy gains being proportional to (Vlc)2, where V is the typical velocity of a magnetic irregularity. As their velocities are generally believed to be low, the acceleration turns out to be too slow for the deduced lifetimes of cosmic rays. Also, no obvious reason comes out from this general theory why the spectral indices of the power-law spectra are around 2.5. More convincing results are attained when this idea is applied to the strong shocks that occur at the boundary between a high-velocity, expanding shell from a supernova explosion and the interstellar gas. In this case, an energy gain proportional to VI c is found, i. e. the mechanism is much more effective because it is both of first order in Vlc and V is much larger. Also, a definitive, natural spectral index results, though precise agreement with the measured values is still not easy to get. An overabundance of neutron-rich isotopes could also find a natural explanation by the supernova influence. The energy up to which this process is effective for supernova shock fronts is believed to be in the tens of TeV region, significantly lower than the highest energies that have already been measured. A similar mechanism can however also be envisaged to occur at other sites, possibly resulting in the same or a similar spectral index. The most fundamental limitation for this acceleration process is that the gyroradii of the particles must be smaller than the extension of the acceleration region. Compact objects with very strong magnetic fields (pulsars, for example) or very extended objects with weak fields (radio galaxies) can in principle provide the means to accelerate particles up to the highest measured energies. 1.2.2 Propagation through space The estimated average lifetime of cosmic rays of 107 years and the high isotropy of their arrival directions excludes that they followed straight lines from their acceleration sites to earth. In fact, one deduces from measurements of effects like Faraday rotation and Zeeman splitting that an irregular magnetic field with an average strength of around 2 . 10-10 T permeates the galaxy. Charged particles will follow curved tracks in these fields. For a particle of charge q, velocity v and mass m in a magnetic field B, one can calculate the radius of curvature R (assuming the particle moves perpendicular to the field lines) from the equality of Lorentz and centripetal forces, i. e. from 1 with ,= --;==== VI - (vlc)2' For a highly relativistic particle with v ~ c, energy E and, = EI(mc2 ), R _ m,c _ E (1.1) - qB - qBc' For a proton of 1018 eV and the magnetic field given above, R = 1.67.1019 m = MOpc. This is of the same order as the thickness of the galactic disc (to which the magnetic field is thought to be mainly confined) of about 300 pc. Thus, protons of energy below this (and higher charged particles even beyond) are expected to be curling rather randomly in the irregular magnetic 16 CHAPTER 1. COSMIC RAYS fields, resulting in a mostly isotropic distribution of their arrival directions and also in long distances travelled through the interstellar medium. Support for this model ofpropagation by diffusion comes from the fact that a weak anisotropy in the arrival direction distribution has been observed. Within the model, this can result from the net streaming of cosmic rays out of the galaxy. At the position of the solar system in the outer region of the galactic disc, at approximately 8.5 kpc distance from the centre, this streaming velocity is about 1O-4c. The detailed models suggest that the scattering by magnetic irregularities becomes progres sively less efficient above 1015 eV, as the gyroradii become similar to the size ofthe irregularities. The net streaming and consequently the anisotropy would become stronger, which is indeed ob served. This feature of stronger 'leakage' from the galaxy might explain the change of spectral index at the knee in the energy spectrum. As mentioned above, the overabundance of certain elements in cosmic rays is produced by spallation of heavier nuclei. The amount of material that must have been traversed to yield the 2 observed abundances can be calculated to be around 5 g/cm • Assuming an average density of the interstellar gas of 1 particle per cm3 and that it is composed mainly of hydrogen, a flight time of some 3· 106 years to accumulate this mass is found for highly relativistic particles. This is lower than their estimated average age of 107 years. A possible explanation is that the cosmic ray particles also spend a significant amount of time in lower density regions, e. g. in the halo of the galaxy. The highest energy cosmic rays that have been found can not be confined to our galaxy by magnetic fields, as, for example, a proton of 1020 eV would have a bending radius of 54 kpc in a field of 2.10-10 T, almost double the diameter of the galaxy. This suggests that they are of extragalactic origin. The flattening of the energy spectrum beyond 1019 eV is then possibly a result of this change from galactic to extragalactic origin. The distance that these highest energy cosmic rays can travel is limited by interactions with the cosmic microwave background radiation. The photons of this isotropic blackbody radiation of temperature 2.73 K have an average energy of 9.10-4 eV. 1 In the rest frame of an ultra relativistic proton, the photon energy is larger by a factor of 21' for head-on collisions [Sch102, Sect. 5.1.2]. For a proton with 7.1019 eV (-y = 7.5 .1O lD ), this results in a photon energy of 0 2 134 MeV, close to the threshold of 145 MeV for pion production via the process p +l' -+ P+ 7r • A detailed calculation, taking into account the spectral distribution of the microwave background and the cross-sections for the various possible reactions, shows that beyond 5· 1019 eV cosmic rays cannot have travelled significantly further than 30 Mpc (the so-called Greisen cut-off). It is, therefore, possible that these highest energy cosmic rays originate outside of our own galaxy, but an anisotropy should be observed in the arrival direction as the number of sources (galaxies) within the allowed volume is limited. As an example, the Virgo supercluster lies at a distance of some 15 Mpc. Cosmic ray arrival directions should point back to this region if they 1The spectral energy density u(v, T) of blackbody radiation (Planck spectrum) and the average photon energy < E> of this radiation are 3 00 81r hv 10 hv u(v, T) dv u(v, T) ~ ehvj(kT) _ 1 and Numerical evaluation of the last expression for T = 2.73 K gives 9.10-4 eV. In [Lan59, §57], an analytical solution 00 for the involved integrals is given, Z",-1 Z -1) = r(x)«(x) for x 1, with the Gamma function and 10 /(e dz > r(x) «(x) the Riemann-( function. Evaluation yields predominantly come from there. Other, exotic, explanations for these highest energies include the decays of very massive, so far unknown particles that might be relics of the big bang. These particles might, just as ordinary matter particles, cluster within the galaxy. In that case, the arrival directions of cosmic rays resulting from such decays would point predominantly to the galactic centre or to the galactic disc. If they are distributed homogeneously, the arrival directions would be isotropic. 1.2.3 Local influences The sun is subject to a steady mass loss, termed the solar wind, which carries particles away with a speed of some 350 km/so Magnetic fields are essentially frozen into this plasma because of its high electrical conductivity. Since the dynamics are dominated by the solar wind - its energy density is higher compared to the magnetic fields - the field configuration takes a spiral form as a result of the rotation of the sun and the radial particle flux. The strength of these fields varies with the ll-year solar activity cycle, and a clear effect can be observed on the cosmic ray particle fluxes below about 1 GeV per nucleon, the flux being suppressed at high activity. Detailed models of this effect are needed to get credible statements about the interstellar cosmic ray flux at these low energies. The same is applicable to the earth's magnetic field, though here detailed calculations are easier since the field is well known and measurements by interplanetary space probes outside the field exist. 1.3 High energy electrons and positrons A limit similar to the Greisen cut-off exists for high-energy electrons and positrons but at a much lower energy, due to inverse Compton scattering off the 2.7 K cosmic microwave background radiation and due to the emission of synchrotron radiation in galactic magnetic fields. The energy loss rate due to inverse Compton scattering for a highly relativistic electron of energy E is [Long92, Chapter 4] 4 ( didE) = --3aTC ' Urad' --2E )2 ' where ( Comp mec Urad is the energy density of low energy photons (from the microwave background radiation and starlight), aT the Thomson scattering cross-section. The losses due to synchrotron radiation in an irregular magnetic field of average energy 2 density Umag = B /(2/-Lo) can be written as [Long92, Chapter 18] -dE) =--aTC'U4 '--( E )2 mag 2 ( dt Synch 3 m ec The total energy loss rate is then (~) dE) = (dE) + (dE) = _iaTc ' (Urad + U mag) . 2 (1.2) ( dt total dt Comp dt Synch 3 mec A lifetime T can be defined according to E T= - (dE) . dI total 18 CHAPTER 1. COSMIC RAYS 1000 29 0.3 2 D=x·10 (E/1 TeV) cm Is 800 u- s Q) Cl 600 c ro a: 400 200 2 4 6 8 10 12 14 16 18 20 E (reV) Figure 1.4 Range of cosmic-ray electrons according to Sect. 1.3. D is the diffusion constant, the lifetime is taken from [Kob99] as 2.1· 105 years . (Ell TeV)-I. Writing (1.2) as dE) _ -bE2 'th b = 40'T(Urad + Umag) ( dt total -, Wl 3mec2 3 ' one has r = (bE)-1 and the solution 1 )-1 E(t) = ( bt + Eo ' so that E(O) = Eo. Then, E(t - r) = 00, so one meaning of r is that an electron or positron with current energy E must have been accelerated to (or created with) some higher, but unknown energy within time r before now. Also, E(t + r) = E(t)/2. 10 4 Evaluating using the typical values B = 2.10- T, i. e. Umag = 9.9.10 eVIm3, and Urad = 8.7 .105 eV1m3 (with 6.105 eV1m3 coming from starlight and 2.7 .105 eV1m3 from the cosmic microwave background, see [Long92, Chapter 19]), one gets b = 6.1.10-7 (JS)-I. The lifetime can then be written as E r = 3.2· 105 years· -)-1 (1.3) ( I TeV In [Kob99], results from a calculation with more realistic parameters are reported. The authors use more appropriately an average of the square of the magnetic field, given by them as < B 2 >1/2= 7.10-10 T, and arrive at a lifetime of 2.1.105 years· (Ell TeV)-I. They use a diffusion equation and translate this lifetime into a distance r that an electron can diffuse of r ~ 2vDr, with D the diffusion constant. The probable range of values for D around TeV energies is given as (1- 4) .1029 . (Ell TeV)0.3 cm2/s, deduced from the observed weak anisotropies in the cosmic ray arrival directions. They then calculate that the range of a 1TeV electron is between 500 pc and 1000 pc. The range r is plotted in Fig. 1.4 between 1TeV 35 and 20TeV for three values of D (the energy dependence is r '" E-O. ). As it is generally believed that also these high-energy electrons are accelerated in supernova shocks, the number of possible sources within this distance is limited. Table 1.1 lists known 1.3. HIGH ENERGY ELECTRONS AND POSITRONS 19 Table 1.1 Supernova remnants within 1 kpc and age below 4.105 years (from [Kob99]) Name Distance (pc) Age (103 years) Emax (TeV) SN 185 950 1.8 116 S 147 800 4.6 46 G65.3+5.7 800 20 10 Cygnus Loop 770 20 10 Vela 250 12 - 16 13 - 18 Monogem 300 86 2.4 Loop 1 170 200 1.0 Geminga 400 340 0.6 nearby supernova remnants with their distance, age (time since supernova explosion) and max imum electron energy they are believed to generate. Following (1.1), a 1 TeV electron has a gyroradius of 1.67.1013 m = 111 AV in a magnetic field of 2.10-10 T, comparable with the diameter of the solar system and much smaller than the source distance. Since the typical size of magnetic irregularities that are responsible for the diffusive movement of cosmic rays is thought to be of the order of 0.1 pc ~ 2· 104 AV, also much smaller than the source distance, no highly anisotropic distribution of the arrival directions for electrons of this energy is expected. However, the small number of possible sources will likely impose structure onto the energy spectrum, reflecting the individual source spectra. Precision measurements allowing such de ductions are so far not available, giving justification to the Synchrotron Radiation Detector (Chapter 3). Measurements of the electron spectrum and calculations assuming contributions from the nearby sources listed in Table 1.1, adopting a specific but not unique set of parameters, are shown in Fig. 1.5. Since there is some uncertainty on the true distance of the Vela source (its estimated distance changed recently from 500 pc to 250 pc, see [Kob99]), the inferred contribution for three distances is indicated. Clearly, there is disagreement at the upper energy limit. The conventional acceleration process as described in Sect. 1.2.1, accelerating essentially material of the interstellar gas, will not inject positrons into the cosmic ray population, although they are created in pairs with electrons by collisions of high-energy protons with other particles. If, on the other hand, some other mechanism for their creation is additionally or exclusively at work, for example the decay of some so far undetected heavy particle, this might reveal itself if one would measure separately the electron and positron spectrum above 1 TeV. Further details on the aspect of the electron/positron ratio and general theoretical calculations referring to cosmic ray electron and positron diffusion are given in [Atoy95]. 20 CHAPTER 1. COSMIC RAYS y=2.4 Q.=lxI04Ier, • Ourdata 16 + Tool et al. 1997 b =1.5 x 1O- (GeV.lec)-1 ... Muller et al. 1997 for B > 500 GeV .. Barblelllnl etal. 1997 o Golden et al. 1994 D =1.0 x 1029(PJITeV)O.3 cm2'lec o Tang 1984 • Golden et al. 1984 h=3 C 111111 Webber et al. (Radio) 1980 1 VeJa 111I1J/1J/1f1'~~. O.2kpc _II~ 'LWf»I-}~'~I: I , ... "'0• • ...D contribudon from Vela allloureel 0.3kpc ..0 101 ...... a.....I..J...&..L..L.W._...I...-I-J".UJow-...... u..J&I,I...... oIoI-l""""'-iooWoJol,_...... 5 10° 10· 102 10' 104 10 ENERGY ( GeV ) Figure 1.5 Measured and calculated electron spectrum (from [Kob99]). Qe is the assumed average energy output in electrons beyond 1GeV by a supernova with a supernova rate of one every 30 years, , the source spectral index and h the galactic halo thickness; the parameters b and D as in the text. CHAPTER 2 The Alpha Magnetic Spectrometer 2.1 Scientific objective Measurements of cosmic ray characteristics have been undertaken since their discovery around 1912. Data on the energy spectrum and composition are available today over large ranges and already with some precision. However, so far no particle detectors of the type, complexity and precision usually found in high-energy physics have been used for studying cosmic rays outside the atmosphere. Somewhat comparable instruments were employed during balloon flights, but even at altitudes of some 35 km that were reached, the residual atmosphere above the instrument 2 amounts to about 4gjcm2 . This is comparable with the 5gjcm of matter that cosmic rays traverse during their whole lifetime of 107 years (see Sect. 1.2.2), so the precision of the results is necessarily limited by corrections that need to be applied to the measured data. Also, the exposure time is usually only several days, with the most advanced projects aiming at around 100 days. The basic justifications for the large-scale project that is undertaken with the Alpha Magnetic Spectrometer (AMS) experiment is the precision data that it will yield. The first version of AMS flew on Space Shuttle mission STS-91 on 2-12 June 1998. This flight was basically intended to be an engineering test but nevertheless yielded interesting science data. Details on the construction of AMS-01 are summarised in [Vier98] and results from the flight in [AlcaOO] and references therein. Here, the new version AMS-02 that is planned to be installed on the International Space Station (ISS) for a three-year mission in 2004 will be briefly reviewed. The energy range accessible to the upgraded AMS-02 extends from some 500 MeV to about 1 TeV, depending on the particular quantity under study (see Table 2.1 for some details). Com paring with Fig. 1.1 shows that these limits are many orders of magnitude below the knee region where precise compositional measurements, especially of the iron fraction, would be important to discriminate between different cosmic ray models. Because of the low rates at these energies, this is the realm for large, ground-based air-shower detectors. The high precision that AMS-02 can achieve within its energy range, however, will allow sensitive tests and adjustments of the models for generation, acceleration and propagation of these abundant lower energy particles. This precision has already proved fruitful in accelerator based experiments, for example by giving indications of new particles through their effects on spectral shapes even before they could be produced directly. In the context of astrophysics and as an example, a candidate for dark matter is the proposed supersymmetric partner of the neutrino, the neutralino X. Annihilations of two such particles are believed to produce electrons 21 22 CHAPTER 2. THE ALPHA MAGNETIC SPECTROMETER Table 2.1 Particle identification momentum ranges for AMS-02 [Bue01] (ranges for antiprotons and antinuclei are the same as for protons and nuclei, momenta per nucleon for ions) Particle Momentum range (GeV/ c) Elementary particles Electrons 0.3 - 3000 Positrons 0.3 - 300 Protons 0.3 - 3000 Charge identification of elements Ions Z .:s 4 0.3 - 1500 Mass identification of isotopes Ions A.:s 4 1 - 20 Ions 4 .:s A .:s 20 1 - 12 and positrons in the 30 GeV to 100 GeV region, possibly giving rise to a detectable excess in their spectra over what would be expected conventionally. Of course, any number of unexpected features might appear is such spectra are precisely measured. Apart from this, one fundamental question to be addressed by AMS is the existence of antimatter. Its charge and mass identification capabilities, and the long duration and large geometric acceptance (~ 0.5 m2 sr), will allow a highly sensitive search for antimatter nuclei. 2.2 Detector description The AMS-02 detector resembles in many aspects a conventional, accelerator-based general pur pose particle spectrometer as apparent from Fig. 2.1. The shuttle transport and space environ ment impose stringent requirements on almost every detail of the hardware realization, though. The overall weight (6000 kg) and dimensions are limited by the Space Shuttle, the power con sumption (2 kW) by the ISS. In the following, a brief description of the components and sub detectors of AMS-02 is given. Details on the SRD can be found in Chapter 3. Superconducting Magnet To achieve a significant higher magnetic field than in AMS-01, the permanent magnet is replaced by a superconducting magnet, cooled by slow evaporation ofliquid helium to 1.8 K. The heat load on the helium is reduced by employing electrically powered cryocoolers, so that an endurance of around 2.5 years is estimated (time until the helium supply of 2600 I is used up and the magnet will have to be switched off). The coils of the magnet consist of two dipole coils and 12 racetrack coils that return the magnetic flux and reduce the stray field to acceptable limits (below 15.2 mT at a distance of 2.3 m). The dipole moment of the whole magnet has to be very close to zero, so as not to exert forces on the ISS, the magnetic torque limit in the earth's magnetic field being 0.272 Nm. The coils are made of aluminium stabilised NbTi wire. The magnet will be charged after launch during approximately 1 hour to a final current of 459 A. The field in the magnet centre has a magnitude of about 0.87 T, with a bending power BL2 of 0.78 Tm2. Silicon Tracker To track the particles in the magnetic field 8 layers of silicon microstrip detectors are arranged in 3 double-sided and 2 single-sided layers, with a total area of some 6 m2. The detectors are 2.2. DETECTOR DESCRIPTION 23 SRD USS 11 ...... / . Veto Vacuum Counter Case ...... :::::..... EMC .' .' (a) Simplified cross section Radiators! Debris shield Radiators ~----7 (ram side) (wake side) Upper crate TOF column (51,52) (wake side) USS Veto counter m~rr----..L TOF (s3,s4) Lower crate column (wake side) Lower crate column (ram side) (b) 3d view Figure 2.1 Layout of the AMS-02 experiment (as of June 2001) 24 CHAPTER 2. THE ALPHA MAGNETIC SPECTROMETER mounted on a rigid, light honeycomb structure. The strip pitch is 27.5 pm in the bending and 52 pm in the non-bending plane. Since the radius of curvature in a magnetic field B for a particle of charge q and momentum pis r = pl(qB), the tracker actually measures plq, conventionally expressed in terms of the rigidity R = pclq with unit [R] = V. A resolution of 10 pm in the bending plane (30 pm in the non-bending plane) is expected, giving a rigidity resolution of 2% for 1 GV particles. The sign of the charge is determined from the bending direction and, additionally, a mea surement of the energy loss dEIdx will yield information (together with the same measurement from the time-of-flight system) on the absolute value of the particle's charge. The power dissipation of the tracker detectors within the magnet amount to some 200 W, this heat being transported away and dissipated by a heat pipe system and radiators. A laser alignment system will be employed to monitor the detector orientation. Transition Radiation Detector (TRD) This sub-detector will increase the energy limit for lepton/hadron discrimination by using the fact that the emitted energy in transition radiation in a medium with changing refractive index is dependent on the gamma factor of the charged particle, not on the velocity like the radiation in a Cerenkov detector (which is practically the velocity of light for all particles at high energy). A discrimination up to 300 GeV is expected between positrons and protons, up to some 100 GeV between pions and protons. A 23 mm thick fleece radiator is employed together with 6 mm diameter straw tubes up to 2 m long. They are filled with a Xe/C02 (80:20) gas mixture for the detection of the transition radiation photons (mean energy around 10 keV). The system contains about 300 1 of gas at 1200 mbar pressure, circulated and purified with a rate of 11/h. Electromagnetic Calorimeter (EMC) The three-dimensional electromagnetic sampling calorimeter consists ofalternating layers ofscin tillating fibres and lead absorbers with in total 16.5 radiation lengths, dimension 66 x 66 x 16 cm3 and weight 480 kg. It is supposed to discriminate between electrons and hadrons at a level of 10-4 between 1 GeV and 1000 GeV and to yield an energy resolution for leptons of 4.4% at 10 GeV and 1.5% at 100 GeV. It will also allow a detection of high energy photons with good angular and energy resolution. A measurable difference in the signal signature between nuclei and antinuclei is expected. Ring Imaging Cerenkov Detector (RICH) The RICH will measure the particle velocity by determining the opening angle of the cone of Cerenkov radiation emitted by fast charged particles. It uses the proximity focusing technique: a 20 mm thick Aerogel radiator with refractive index n = 1.14 is separated from the detection layer by a free space of 414 mm where the Cerenkov rings can expand. An array of photomultipliers and light guides with a pixel size of 7.3x7.3 mm2 is used to detect the Cerenkov photons. The detector will cover the momentum range from 1.7 GeVIc to 7.3 GeVIc per nucleon with a velocity resolution of some 0.15% and will reject upward moving particles with a momentum above the lower limit - a very good rejection is important to get a high antimatter sensitivity, since an upward moving particle will bend in the same direction as a downward moving an tiparticle. An isotope identification (by combining velocity and momentum information) up to mass numbers around 25 and an elemental identification (using also the fact that the number ------ 2.2. DETECTOR DESCRIPTION 25 of photons emitted per unit distance depends on z2) up to charge number 26 is expected to be possible. Time-or-Flight Detector (TOF) The transit time of particles through the detector is measured to 120 ps precision by two double layers ofplastic scintillators, one above and one below the silicon tracker. The resulting resolution in (3 = vje is (7(3 ~ 2.8% for highly relativistic particles with (3 ~ 1. The scintillator is arranged in bars 11 cm wide and 1 cm thick, read out by two fine-mesh photomultipliers on either side. Each photomultiplier is oriented individually to minimise adverse effects of the magnetic stray field, resulting in a complicated shape of the light guides connecting the scintillator bars to the photomultipliers. This sub-detector will, apart from the velocity measurement, also deliver a fast trigger to the experiment and will yield a value for the absolute charge by measuring the energy loss dEjdx, as is also done by the tracker. Veto Counter To veto against particles that suffer large scattering inside AMS or traverse it at large angles, the inner side of the magnet case is covered by a layer ofplastic scintillator counters. Only events that have no coinciding hit in the veto counters will be accepted. Other components The whole experiment is mounted to a so-called Universal Support Structure (USS). It is this structure that is fastened in the Space Shuttle payload bay during transport and that will be attached to the payload site on the ISS. On the ram side, i. e. the side in the flight direction of the ISS, debris shields are installed that protect specifically the helium vessel and the electronic crates from being punctured by small debris particles. The power that is used by the experiment must finally be rejected as heat through the radiators. Very good thermal insulation of the magnet is required to limit the consumption of helium. Most of the massive support structure is only needed during construction and launch, but in space acts merely as an unnecessary heat-conducting path. A unique support for the magnet has therefore been devised that will effectively decouple from the cold magnet parts when the whole structure relaxes in zero gravity, thus reducing heat transport into the magnet. Data Acquisition System The data acquisition system of AMS-02 is designed to handle an maximum event rate of 2000 s-1 with an average of 2kByte of data per event (peak data rate 4MBytejs). Read-out of the detectors, data reduction and further processing and control of the communication link to earth is planned to be done using a radiation-hard version of the PowerPC 750 processor, running at some 200 MHz and delivering about 300 MIPS computing power. All internal communication within AMS-02 is standardised to use the IEEE1355 (SpaceWire) standard, with also a large fraction of the electronic boards being standardised. Finally, data will be downlinked to earth by the NASA Ku-band communication system. CHAPTER 3 The Synchrotron Radiation Detector To extend the energy range accessible to AMS-02, a new detector component that is sensitive to the synchrotron radiation emitted by high energy electrons and positrons is foreseen. This Synchrotron Radiation Detector (SRD) effectively uses the weak but extended magnetic field of the earth as a bending magnet. The principle has been described by [Step83] and, in the context of AMS, by [Hof98]. The basic idea is that electrons and positrons above some 500 GeV radiate synchrotron radiation, whereas protons of even several 1000 TeV practically don't radiate at all. This feature allows to discriminate against the proton flux that is several orders of magnitude larger than the electron flux. And since electrons and positrons bend in opposite directions in the earth's magnetic field, a measurement of the arrival position of the synchrotron radiation photons on the detector with respect to the magnetic field and the direction of the charged particle allows a discrimination between particle and antiparticle. 3.1 Principle considerations 3.1.1 General As considerations of the energy loss of high-energy electrons and positrons indicate (see Sect. 1.3), the range of such particles in the TeV region should be limited to around 1kpc. Effectively, no electrons beyond some 10 TeV are expected to exist. No precise measurements of the spectrum in this energy range exist, especially also none with separation between electrons and positrons. As the proton flux is expected to be much higher and the particle identification at energies beyond 1 TeV gets uncertain with the standard components of AMS-02, a new sub-detector was conceived to fill the gap and to subsequently allow a test of models of the local characteristics (meaning within kiloparsec distances) of very high-energy charged leptonic cosmic rays. The concept of a detector that will allow such measurements is shown in Fig. 3.1. The elec tromagnetic calorimeter will give a trigger upon detecting a high-energy particle and the tracker will give its flight direction, though no measurable bending of the track will be seen for energies beyond 1 TeV. The large-area SRD will then look for synchrotron photons arriving in coinci dence with the charged particle. Protons will not radiate, for an electron the photons would arrive on the side indicated on the figure, for a positron on the other side. Also indicated is the fact that the identification is not so straightforward due to random background photons in the same energy range as the synchrotron photons (Sect. 3.1.3). 26 3.1. PRINCIPLE CONSIDERATIONS 27 -5 Bearth (-3xlO T) ~o~s -1------, ~e,<:J\.~., /' : ~ -20km\ SRD -3m Tracker t~~~~~~~~~~ ECAL Figure 3.1 Principle of the SRD 3.1.2 Synchrotron radiation The emission spectrum of synchrotron radiation is calculated in [Long92]. For an electron or positron with energy E e in a uniform magnetic field B and a pitch angle a (angle between the field lines and the flight direction of the electron),the emission spectrum is 00 .( ) = 2 v'3e3BsinaF( ) (3.1) JII 11"8 2 x, with F(x) = x / Ki(z) dz 11" Eocm e x and (E) E 411"me x = 3,2ehB sin a . 2 K 5/ 3 (Z) is a modified Bessel function, , = Eel(mec ) and E = hv with E the energy of the emitted radiation. The unit is [j(11)] = W1Hz = J. The shape of the emission spectrum is deter mined only by the universal function F(x) that is plotted in Fig. 3.2(a). A good approximation to this function, accurate to within 10% in the range plotted, is F(x) ~ 1.78xo.3 e-x [Sch102, Sect.4.I]. The resulting emission spectrum for an electron or positron of 1TeV in a magnetic field of 3.10-5 T moving perpendicular to the field lines, i. e. a = 90·, is shown in Fig. 3.2(b).1 A photon 2 3 2 rate spectrum as shown in Fig. 3.2(c) follows. It diverges towards zero as E- / • lWith these values, one finds x(2.5 keV)=0.125 and x(100 keV)=5.0. 2The modified Bessel functions KI/(z) can be written as 1r KI/(z) = 2 . ( ) (LI/(z) - II/(z)) , where II/(z) = exp (-~V) JI/ (zexp C;)) , SIll 1rV and a series expansion of the Bessel function JI/(z) is JI/(z) = L:~=o((-lt(z/2)"+2n)/(n!r(v + n + 1)) (see [Bron91]). For 0 < z « 1, it is sufficient to keep the first term, Le. JI/(z) ~ (z/2)" /r(v + 1). Then one easily 2 3 finds KI/(z) '" z-I/. Since the photon rate spectrum goes proportional to F(x)/x = ["''''' K S/ 3 (z) dz, it has a X- / behaviour at low values of x. KI/(z) is one solution of the differential equation d2R/dz +1/zdR/dz-(I+v2/z2)R = 0, and one finds indeed that the above approximation solves this in the given limit, for one can then neglect the constant term in the bracket. 28 CHAPTER 3. THE SYNCHROTRON RADIATION DETECTOR 1.4e-12 0.8 1.2e-12 1e-12 0.6 :>Q) 8e-13 ~ LL. 0.4 ~ 6e-13 4e-13 0.2 2e-13 0 0 0 2 3 4 5 6 20 40 60 80 100 x E (keV) (a) Universal function F(x) (b) Energy spectrum 0.4 6000 0.35 :> ~5000 ~ 0.3 UJ g 0.25 ~4000 '0 0 .J::. '0 e:. 0.2 ~3000 il 0.15 $ E ~2000 :::] z 0.1 1000 0.05 2 4 6 8 10 2 4 6 8 10 E (keV) E (keV) (c) Photon rate spectrum (d) Photon number spectrum on SRD Figure 3.2 Plots illustrating the synchrotron mechanism for a 1 TeV electron in a magnetic field of 3 . 10-5 T. As shown in Appendix A, the effective flight distance S for those synchrotron photons that still reach a detector of width 2L is S = R arccos{RI(L + R)), with R the bending radius of the charged particle. In the case above and with 2L = 3 m, R = 108 m and S = 18.3 km. To get the average photon number that would hit this detector per event, the photon rate spectrum needs to be multiplied by Sic, giving the photon number spectrum shown in Fig. 3.2{d). Numerical evaluation of the formulae with the parameters given above yields a photon rate in the energy range from 2.5 keY to 100 keY of 20200 photons per second and an average number hitting the detector of 1.16 per charged particle. Since the probability for the emission of n photons is Poisson distributed according to (3.2) with Jl. the average photon number calculated above, the values Po = 31 %, PI = 36%, P2 = 21% and P3 = 8% follow. These numbers are given for four energy ranges and three charged particle energies in Table 3.1. They do not indicate a very strong dependence on the detection energy range, but do make clear that higher energy electrons are detected much easier. However, the 3.1. PRINCIPLE CONSIDERATIONS 29 Table 3.1 Photon number distributions Pn after Sect. 3.1.2 for four detection energy ranges and three charged particle energies. Detection energy range (keV) / Charged particle energy (TeV) Photon 1.0-100 2.5-100 5.0-100 2.5-500 number n 1 2 5 1 2 5 1 2 5 1 2 5 0 21% 7% 4% 31% 11% 6% 44% 15% 8% 31% 9% 1% 1 33% 19% 13% 36% 24% 16% 36% 29% 20% 36% 21% 5% 2 26% 25% 20% 21% 27% 23% 15% 27% 25% 21% 26% 11% 3 13% 22% 22% 8% 20% 22% 4% 17% 22% 8% 21% 17% 4 5% 14% 18% 2% 11% 16% 1% 8% 14% 2% 13% 19% Average 1.56 2.65 3.25 1.16 2.25 2.89 0.82 1.88 2.54 1.16 2.45 4.51 number ..- 10 ;;.. II j"... I -"', N"'" -J COMPTEL ',£ 10 EGRET !;l -, i:i:: 10 § -3 ~/O Cl., -4 ASCA 10 -8~ HEAO-1 -, 10 10... L-.._----'--,----~ _ _L_~_----'--_~_ _L_~__.J 10 -.lL.---<--.L.~...... L~~~..L-~~~ 1~ 1~ 1~ 1~ Energy [MeV] 10 10' 10 Photon Energy (keV) (a) Summary of measurements (b) Photon flux in SRD energy range Figure 3.3 Spectrum of the diffuse photon background in a near-earth orbit [Kdi.01]. Note the different scales. background needs to be taken into consideration for precise deductions, see the next section and [Kra02] for full Monte-Carlo simulations. 3.1.3 Backgrounds The discrimination between electrons, positrons and protons is made more difficult because of the presence of backgrounds that could fake synchrotron photon hits. • Diffuse photon background A summary of the existing measurements of this background is shown in Fig.3.3(a) and an extract from this for energies relevant to the SRD in Fig. 3.3(b). The indicated flux between 2.5 keV and 100 keV corresponds to a rate of some 3· 106 photons per second on a 6m2 detector with 27r solid angle acceptance (typical SRD parameters). • Charged particle background Low-energy charged particles can also mimic synchrotron photon hits when they pass through the light shielding window and hit the scintillator with a residual energy in the 30 CHAPTER 3. THE SYNCHROTRON RADIATION DETECTOR 0W.M/TIROS Ener-qetic PCJrticI e:=: /] -Year Bo:se'line Plot 10<3 o QJ Ul "-. Ul~ 10<2 ~ :Jo Cl o n 10<1 /\ Figure 3.4 Flux spectrum of electrons above 30 keV at 878 km altitude [Che99]. The detector acceptance was 0.01 cm2 sr, so 1 count/s corresponds to a rate of 100 electrons/(cm2 s sr). keV region. Measurements of these rates are scarce and incomplete for SRD purposes. In Fig. 3.4, the average electron flux above 30 keV in the vertical direction is shown as measured by the NOAA-12 earth observing satellite. The lowest rates around the equator (except in the South-Atlantic magnetic anomaly) are about ten times the photon rate between 2.5 keY and 100 keY. However, the orbit altitude was 878 km, much higher than the 400 km altitude the SRD will have on the ISS and, moreover, electrons of 30 keV will be fully absorbed even in thin beryllium entrance windows (see Sect. 3.3.2 and Fig. 3.16(a)). No other useful data set was found, emphasizing the need for a realistic measurement under SRD conditions. With a background rate R within some energy range, the average number of events J1 during a time T is J1 = R· T. (3.3) Since the actual number of events during this time is Poisson distributed according to (3.2), the probability P:::.. n to have n or more hits can be calculated as n > O. (3.4) To a first approximation, this probability gives the chance to misidentify a proton that does not radiate synchrotron radiation as either an electron or positron if one requires n or more synchrotron photon hits as a signature. Existing measurements show that protons are expected to be several orders of magnitude more abundant than electrons, so this probability can be called 3.1. PRINCIPLE CONSIDERATIONS 31 Table 3.2 Required time resolutions for suppression against background after (3.4) Time resolution (ns) for proton suppression factor Background rate (cm 2ssr)-1 P>l P>2 P>3 10-6 10--=-7 10-8 10-6 10--=-7 10-8 10-6 10--=-7 10-8 8 2.10 -:.I 2.10 ·3 2.10 -4 28 8.9 2.8 370 170 78 16 1.10-2 1.10-3 1.10-4 14 4.5 1.4 180 85 39 24 7.10-3 7.10-4 7.10-5 9.4 3.0 0.9 120 56 26 Table 3.3 Required time resolutions for suppression against background after (3.5) Time resolution (ns) for proton suppression factor Background rate (cm2 ssr)-l P;l P;2 P;3 10-6 10-=-7 10-8 10-6 10-=-7 10-8 10-6 10-=-7 10-8 8 2.10 -;.l 2.10 -3 2.10 -4 40 13 4.0 580 270 125 16 1.10-2 1.10-3 1.10-4 20 6.3 2.0 290 134 62 24 7.10-3 7.10-4 7.10-5 13 4.2 1.3 190 90 42 proton suppression factor. For a given suppression factor, (3.4) then allows to calculate /-L and, for a specified background rate, the required time resolution follows from (3.3). The results from numerical calculations for n equalling 1, 2 and 3 are shown in Table 3.2 for three background rates: the already known photon background given above and two and three times that, reflecting the situation of the unknown charged background being similar or twice the photon background. Following the granularity discussion in Appendix A, is was assumed that only a 5 cm wide strip of the detector needs to be scanned for synchrotron hits, reducing the effective background accordingly. It can be seen that realistic values for the required time resolution are achieved only when demanding at least two synchrotron photons hits. However, a better approach to a valid signature is the requirement to have n or more than n hits more on one side of the detector than on the other side. Because the background will affect, on average, both sides equally, a reduction in the required timing precision will follow. The probability P~n to have n or more background hits more on one side than on the other is - 00 P~n = 2· L PI· PI+n+j' (3.5) i,j=O The individual probabilities PI reflect the chance of a hit on one side of the detector only, so the effective background rate used for the calculation is half of the value above (assuming a central hit of the leading charged particle). For the same parameters as above, the resulting time resolutions are shown in Table 3.3: for n=l the result is unchanged (to the precision given in the table), for n=2 the required time resolution is eased by some 40%, for n=3 by some 60%. Some further improvement will be possible by taking into account the difference in energy spectrum between background and synchrotron photons and by a finer determination of the area that needs to be scanned for hits. Details of results from comprehensive Monte-Carlo studies taking into account all these effects can be found in [Kra02], but it is clear from the results presented here that a time resolution of some 10 ns will be required to achieve background suppression on a level of 10-7 if the background rate including charged particles is not too large. Apart from the misidentification of a proton as either an electron or positron, the low-energy background can also result in an electron being falsely identified as a positron or vice versa. In a simulation in [Kra01], a detector sensitive from 2.5keV to 100keV and a time resolution of 10 ns was assumed, and two synchrotron photon hits where required as valid signature. A 50% 32 CHAPTER 3. THE SYNCHROTRON RADIATION DETECTOR detection efficiency for electrons between 1TeV and 2 TeV was found, together with a probability of 4 . 10-8 for misidentification as a positron and of 8 . 10-8 for identifying a proton as either a positron and electron. Only the known diffuse photon background was taken into account. 3.2 Detection principle The requirement of photon detection for energies between 2.5 keY and 100 keY with timing capabilities better than 10 ns and a detector size that measures in square meters leaves essentially only one option for the detector: fast inorganic scintillators read out by photomultipliers. This section will deal with these components in detail, especially also with the scintillator material that was chosen for the SRD. 3.2.1 Scintillators Many substances emit tiny flashes of light when energetic particles hit them. One of the first 3 instruments that made use of this effect in zinc sulphide was called spinthariscope , invented by William Crookes in 1903 and described in [Cro03]. This device consisted simply of a small zinc sulphide screen and a lens. With a well dark-adapted eye, it was possible to discern indi vidual flashes of light that occur when alpha particles emitted by radium hit the screen. Today, scintillators coupled with photomultipliers are widely used in all areas of particle detection. The large number of scintillating materials can be divided into three main classes: organic, inorganic and gaseous scintillators. These classes are distinguished, among other characteristics, by their scintillation mechanism. The basic process is that energy lost by a charged particle is converted into excitation or ionisation of the scintillator base material, then a fraction of this is emitted as visible or ultra violet light, either by the base material itself or through the action of one or several additional compounds in the scintillator. One of the best general and most comprehensive accounts on scintillators can still be found in [Bir64]. Here, only a brief overview will be given. Some general properties of several scintillators are collected in Table 3.4. Some of these values, especially light yield and decay time, can vary significantly from one measurement and scintillator to another. The actual behaviour of a scintillator is often also rather complex, e. g. exhibiting several emission lines with different and temperature dependent decay constants, so this is to be understood as a broad comparison only. More details on the YAP(Ce) scintillator are given in Sect. 3.2.2. Organic Scintillators These scintillators contain in their base material aromatic or otherwise conjugated molecules (having delocalised 1r electron orbitals), mostly benzene-ring structures, e. g. anthracene, poly styrene or polyvinyltoluene. The primary scintillation process is related to the excitation of 1r electronic states in the molecules of the base material. Excitation of other electronic states or ionisation leads to either thermal dissipation (i. e. scintillation inefficiency) or possibly delayed scintillation. In a unitary scintillator like anthracene, the base material is sufficiently transparent to the light emitted by the decay from the lowest lying excited state to the ground state. If this is not the case, like in polystyrene, one adds a second component, e. g. p-terphenyl, that takes 3Derived from the Greek spinthiras, meaning spark. 3.2. DETECTION PRINCIPLE 33 Table 3.4 Properties of scintillators at room temperature Density dE/dx Light yield Wavelength Refract. Decay (~:hV (Photons) (nm) index const. Ref. Scintillator (C&3 ) ) MeV at maximum emission (ns) Organic NE102A 1.03 2.0 11000 423 1.61 2.4 3,4 Anthracene 1.25 2.2 17000 447 1.62 30 3,5 Inorganic NaI(Tl) 3.67 4.8 40000 415 1.85 230 1,2 CsI(Tl) 4.51 5.6 11000 550 1.79 1000 1,2 CsI(pure) 4.51 5.6 1600 315 1.95 16 1 BaF2 4.88 6.0 6500 310 1.50 630 1,2 BGO 7.13 9.2 2800 480 2.15 300 1,2 YAP (Ce) 5.37 7.5 17000 370 1.95 27 5,6,9 LSO(Ce) 7.35 9.0 27300 420 1.82 47 5,7 Gaseous Xe (1 atm) 0.0055 0.0065 10000 325 < 20 5,8,10 Kr (1 atm) 0.0035 0.0044 5200 318 < 20 5,8,10 References 1 Harshaw/QS, Scintillation Detectors Catalogue 6 [Mosz98] 2 [Gru93], Table 5.1 7 [Mosz97] 3 Nuclear Enterprises, Inc., Scintillators for the physical sciences 8 [Bir64], Table 14.8 and 14.9 4 [PDGOO] 9 [Bac95] 5 dE/ dx calculated from http: / /physics.nist .gov/PhysRefData/ 10 [CRC92] Star/Text/contents.html over the excitation energy of the base material and emits at a longer wavelength where the attenuation length is more favourable. Also ternary or even higher systems are used. The energy transfer between base material and secondary component works mainly radiatively at low and non-radiatively at high concentration of this component. Since scintillation is an inherently molecular property in organic scintillators, they are avail able in a wide variety of forms, e. g. crystalline, liquid or plastic. They exhibit a fast light decay, mostly in the range 2 ns to 5 ns for the main scintillation component, and their emission wave length can be easily adjusted to fit a particular photocathode sensitivity. Their densities are close to 1 gjcm3 and their average atomic numbers are low, making them unsuitable for the detection of higher energy photons. Inorganic Scintillators Inorganic scintillators are usually crystals, often doped with low concentrations in the per mil range of an activator material. The scintillation is connected to the crystalline properties of the base material (i. e. the Bloch band structure consisting of valence, exciton, conduction and forbidden bands) and the crystals need to be, apart from deliberate doping, as perfect and pure as possible to give maximum light yield. The primary process is the generation of excitons in the exciton band that lies just below the conduction band, either directly or by recombination of electrons and holes in the conduction band. These excitons consist of bound electron-hole pairs that are free to move in the crystal as entities, but do not give rise to electrical conductivity since they are neutral. Scintillation occurs when such an exciton excites a luminescence centre that arises from local deformations by an impurity like the dopant or by other crystal defects. This luminescence centre might subsequently decay radiatively and emit visible or ultraviolet light, normally at 34 CHAPTER 3. THE SYNCHROTRON RADIATION DETECTOR a wavelength at which the bulk crystal itself is already transparent. Competing processes like exciton trapping or quenching (non-radiative decays) need to be reduced by very good crystal quality to obtain good performance. Despite this, scintillation is also observed in glasses, albeit with very low efficiency. Fluoride glasses doped with high cerium concentrations (5% to 10%), for example, show light yields of some per cent compared with usual crystal scintillators like NaI(TI), but have the advantage of being cheaper to produce, easier to cast in any form and of having a high radiation resistance (see [Daf94]). Gaseous Scintillators These usually consist of noble gases, where energy absorption and scintillation is occurring in the individual gas atoms, normally without adding a further component.4 The response is rapid, with decay times below 20 ns. The emission wavelength lies in the ultraviolet region, but self absorption is limited by the comparatively low density and the fact that even if self-absorption occurs, chances are that the photon will simply be reradiated, giving rise only to some extra delay. 5 Gaseous scintillators are often used in experiments dealing with highly ionising particles, e. g. heavy fission fragments, at pressures of several hundred atmospheres. Their response is much more linear with dE/ dx than that of solid scintillators, even for heavily ionising particles. One can also amplify the scintillation signal yield by applying an electric field (gas scintillation pro portional counter). This works even if the field strength is below that for electron multiplication since excitation, not ionisation, is necessary for scintillation, and if there is also an electron avalanche, the light multiplication factor is larger than that for electrons [Gru93, Sect. 5.2]. 3.2.2 The YAP scintillator The requirements that a scintillator for the SRD has to fulfil are, recalling the discussion in this chapter and in roughly decreasing order of importance: • Fast decay time, allowing sufficient background suppression due to good timing precision. • High light yield, since synchrotron photons of low energy are most abundant. Also, a high light yield improves the timing. • High density and high average atomic number to allow detection of higher energy photons.6 • Ruggedness to survive launch vibrations. • Available in rather large quantities at an affordable price. A comparatively new cerium doped inorganic scintillator named YAP (yttrium aluminium 7 perovskite , chemical formula YAI03 ) was found to suit best these requirements. LSO also has 4Nitrogen has been used, though, as a kind of wavelength shifter in inert gas scintillators. 5Resonance self-absorption is not limited by the energy shift due to the recoil of the gas atom: For DV emission 11 around 300 nm from a krypton atom, the relative energy shift due to recoil is approx. 2· 10- , whereas the shift due to thermal Doppler broadening at room temperature is of the order of 10-6. 6Since the total absorption coefficient J.L for photons is J.L = aNapiA, with cross section a, density p and atomic weight A, it's actually piA that should be large for given a. However, since the photo effect cross section rises approximately with Z5, a high Z material (with consequently high A) is still favourable. 7Perovskite is the name of the mineral CaTi03 and also the general term for its crystal structure. It is named after the Russian mineralogist Lev Aleksevich von Perovski (1792-1856). It is the most abundant crystal structure 3.2. DETECTION PRINCIPLE 35 Table 3.5 Properties of YAP(Ce) (References from Table 3.4 and [DeI97]) il Density (g/cm ) 5.37 Hardness (Moh) 8.6 Refractive Index 1.95 Crystal Structure orthorhombic Melting Point CC) 1875 Hygroscopic no Linear coefficient of Therm. conduct. (W/(K cm)) 0.11 Thermal expansionl (I/K) (4 -11) .10-5 Cleavage none Wavelength of maximum Decay constants (ns) 27 (89%), emission (nm) 370 140 (11%) Afterglow after 6 ms < 5.10-5 Radiation length (cm) 2.67 Photon yield (Photons/MeV) 17000 Moliere radius (cm) 2.82 dE/dx for MIPS (MeV/cm) 7.5 Attenuation length2 (cm) 6.8 1 depends on crystal axis 2 for scintillation light favourable properties, but is excluded not only because of its price but also because its natural radioactivity would present background problems. An extensive list of YAP properties can be found in Table 3.5. The photon yield of 17000 photons per MeV translates into a conversion efficiency of about 6% (assuming all scintillation photons are emitted at 370nm). YAP crystals are grown by the Czochralski method: they are drawn vertically up from molten material in a molybdenum crucible by help of a seed crystal to form meter-long ingots of up to 20 cm diameter. They are doped with about 0.6 mol% of cerium to give optimum scintillation properties. The major scintillation light component around 370 nm has its origin in a 5d to 4f transition in Ce3+ ions that have taken lattice sites of yttrium. Weak emissions at blue and green wavelengths have their origin at otherwise perturbed lattice regions, e. g. cerium at non-lattice positions. The scintillation emission spectrum of YAP(Ce) at room temperature is plotted in Fig. 3.5. As with many scintillators, the emission spectrum, decay times and absolute light yield change significantly with changing temperature. In [Tsu97], an increase of the absolute light yield of YAP(Ce) of 30% was found when decreasing the temperature from +20·C to -20·C. The decay constant of the fast component decreased to 19.3 ns, but the fraction of the total light emitted in the fast component decreased from 88% to 44%, making the effective decay constant longer at lower temperatures. The orthorhombic crystal structure of YAP is shown in Fig. 3.6. The structure is distorted only very slightly and therefore looks cubic. The YAP crystals used in this work were all produced by Crytur. 3.2.3 Photomultiplier The faint scintillation light pulses that are generated inside a scintillator are converted into an electrical signal and amplified by a photomultiplier. The schematic structure of such a device is shown in Fig. 3.7. It uses two basic effects: • The photoelectric effect of the cathode material to convert the scintillation light into electrons. This material contains usually alkali metals because of their low work function. A typical example is the bialkali photocathode made of Sb, Rb and Cs. The spectral response of the photomultiplier is determined by the photocathode and by the transmission properties of the window on which it is deposited. in the earth (about 75% of the lower earth mantle is thought to be composed of it) and several perovskites are currently being studied for their superconducting behaviour. 36 CHAPTER 3. THE SYNCHROTRON RADIATION DETECTOR 1.2 1 ~ ,,-i·-I-,-I-- ...... ' t, ....' 'c ~.~1:····· .~ ~ :J 0.8 ...... : : : ··;···········t········· .0.... T .: :: : ~ .--. --i --; --.- ~ .. ; -.--._..: -.-.: -- ~ ------: ---:- -.- >. 0.6 +' • . '", 'en...... : \: :::: c +.~ ~ ...... Q) 0.4 ... ·1·· .: ; : ; -' _. -: _. -'" c .' . . : . ' ' 0.2 t---, ------\ -----i ------L ------.------, ------.------' , ' ..'T : .. • .: : o 300 350 400 450 500 550 600 650 700 nm Figure 3.5 Emission spectrum of YAP(Ce) at room temperature [BlaOl] Yttrium •@ Aluminium • Oxygen Figure 3.6 The perovskite structure of YAP PHOTOCATHODE FOCUSING ELECTRODES STEM INCIDENT LIGHT INPUT WINDOW • ELECTRON MULTIPLIER ANODE PHOTOELECTRON (DYNODES) Figure 3.7 Schematic structure of a photomultiplier [HamOO] 3.2. DETECTION PRINCIPLE 37 PHOTOCATHODE ANODE -HV Figure 3.8 Voltage divider circuit for a photomultiplier with the anode grounded [HamOO] • The property of certain materials, e. g. SbCs, to emit more than one secondary electron when hit by a primary electron of suitable energy. 8 These materials are applied to a number of cascaded electrodes, then called dynodes . An electric field between them accelerates the emitted electrons to an energy that gives optimum secondary electron emission at the next stage. Very large amplification factors of 108 and above are achieved by successive multiplication in devices with up to 14 stages. Since the multiplication process operates in high vacuum, light enters through a sealed, gas tight window. Its transmission characteristic usually determines the sensitivity limit at short wavelengths. It can also reduce the total scintillation light transmission significantly in case there is a large difference in refractive index between the window and the scintillator by trapping light in the crystal. Focusing electrodes between the photocathode and the first dynode are used to collect all photoelectrons. A large part of the transit time uncertainty in larger photomultipliers comes from varying path lengths to the first dynode. The electron avalanche will finally be collected by the anode and produces a negative current pulse. The principle electric circuit to provide the various potentials to the photocathode and dynodes is a simple voltage divider chain as shown in Fig. 3.8. To ensure linearity even for high peak currents that occur in short pulses when high amplifications are used, capacitors are connected to the last few dynodes that will provide a charge supply and so keep potentials stable. For the same reason, sometimes a non-equal division of voltages is used with increased potentials between the last dynodes. Most measurements in this work were done with R5900U photomultipliers from Hamamatsu. This type employs a different geometry for the dynodes than the conventional type, as shown schematically in Fig. 3.9. This results in very compact dimensions - in the case of the 10 stage R5900U, the height of the photomultiplier is only about 2.5 cm (see Fig. 3.10). The photocathode 2 2 area is 18 x 18 mm , 36% of the device cross-section of 30 x 30 mm . Typical cathode radiant sensitivity and quantum efficiency spectral response curves are shown in Fig.3.11(a) (bialkali photocathode and borosilicate glass window), and the gain and dark current as a function of applied high-voltage in Fig. 3.11(b). One gets about 20% quantum efficiency for the YAP scintillation light and a gain of 2 . 106 at 800 V bias voltage. Due to the complicated internal structure of the R5900U, the response over its photocathode area is not uniform, as shown in Fig. 3.12 for two orthogonal directions through the centre of the photomultiplier. This will impose some variations on the measurement results presented in Chapter 5, depending on the exact scintillator and radioactive source position. To reduce these, either a collimator was used to have a well defined irradiation position or the source was placed at some distance from the scintillator to achieve a uniform irradiation of the whole area, i. e. to get average values. 8Derived from the Greek words dynamis, force, and hod6s, path. ------~------ 38 CHAPTER 3. THE SYNCHROTRON RADIATION DETECTOR 22.0±0.5 4MAX. 1 MAX. 4.4 ± 0.7 PHOTOCATHODE EFFECTIVE AREA INSULATION COVER Top View Side View Figure 3.9 Dynode structure of a metal Figure 3.10 Dimensions of the R5900U channel photomultiplier [HamOO]. photomultiplier (in mm) [HamOO] 100 CATHODE = RADIANT - \ SENSITIVITY ~ , "1-. \ 10 V .s ~ ~ ~ f :> >- f-QUANTUM Z f= (,) f-- w Cii z f--f-EFFICIENCY 1/ er: z w er: w 13 , ::;) en u:: (,) u. ~ f- w er: Z <:: <:: ::!: o 15 ::;) <:: f- w er: z , o w <:: o 0 ::;) , Z 0 0 0,1 2 :I: ~ (,) 10 ANODE DARK CURRENT 10·'0 <:: , 11 10' i••• 10- , 1 \ 100 L-__.L..._---L_--...JL---l_...L---l 10·'2 0.01 100 200 300 400 500 600 700 800 900 400 600 800 1000 WAVELENGTH (nm) SUPPLY VOLTAGE (V) (a) Spectral response (b) Gain and dark current Figure 3.11 Typical spectral response, gain and dark current curves for the R5900U photo multiplier [HamOO] 3.3. TENTATIVE DESIGN 39 100 r---~-:-:-~,":i""7"'-----=::;;;:-""""-~,--.,...:------, 100 ~ _-l:--";'~-_.:...:-1-: I I\ I I 1 _ : ': : 1 ,,, @ 80 ----- ~-l ~ I ~_J::~~.--'!-1-:------~ : I /~~_ ~ :: , :, : : ~ 80 ------~ :.-r ---- -t,-'-- -r-'~- -i ------~ :I: \,.:t : I : U t/ .\ ,. t I 1 ~ I I I _, I I I .- , :, I : : I: ~ 60 ----- ~-f----~------~------~-----~i------~ 60 ------1- - ,- -- -~ --lJ--+------:- --- t--:------o : I: :: I: 5 : I::: I: ~: ~ I~ Q) : : : I: :I:: : > :,,::: I: .~ 30 ---- -~~-----~------~------~----_Jt-----. '.g 30 ----- ~-1-----:------~------_:----l-lI-:------:,::: I~ ~ a3 I I I I I I I 1 I 11 ~ I I I t I '11 :" Cathode: : Il ,I I t I 1 20 ---- ~~t-----~------~------~------~----_. ~ -~-::-----f~~~;~f~-~-~-i-~---'~~--t--- I;' :'Anod'e: --- -;, -,I".... 20 J-- 1 I 1 1 , 1\ ' ,,, ,Il.....- ~ l I I I I ~~ , r : :::, o .. -: I I I I ...... o _ 0#" I : I I I" " -15 -10 -5 o 5 10 15 -15 -10 -5 o 5 10 15 Position (mm) Position (mm) (a) along dynode slits (b) perpendicular to dynode slits Figure 3.12 Response of the R5900U for light of 400 nm over its photocathode area. The solid lines refer to the photocathode response alone, the dashed lines include the effects of the multiplier chain [Metz99]. A point of special importance for the application in the SRD is the magnetic field tolerance of the photomultiplier, which is usually rather limited since electrons will easily deviate from their foreseen trajectories under magnetic influence. For the R5900U, the output signal stays constant to within 10% for fields up to about 5 mT orthogonal to the photocathode or parallel to the strip-like dynodes and up to 15 mT perpendicular to them. The dark count rate at room temperature is generally between 30 s-1 and 50 s-1 for the R5900U. This is less than one third of the diffuse photon rate of 160 s-1 between 2.5 keY and 100keV on the photocathode area (cf. Fig. 3.3(b)), so is negligible. 3.3 Tentative design 3.3.1 General lay-out The most straightforward design for the SRD would be to cover its whole area with scintil lating crystals, coupled individually to photomultipliers and protected against sunlight by thin windows, e. g. by beryllium foils of 2511m. As an example, using standard Hamamatsu R5900U photomultipliers of outer dimension 30 x 30 mm2 and assuming a detector area of 6 m2 , some 6600 tubes would be necessary, weighting around 170 kg and costing about five million Swiss francs (assuming a 50% price reduction with respect to individual tubes). YAP crystals of 2 mm thickness covering the same area would weight an additional 64 kg, with a price of the same order as the tubes. As noted above, 36% of the total detector area would be covered with photocathodes. A second proposed idea uses wavelength shifters and light guides to reduce the size and number of photomultipliers needed, as sketched in Fig. 3.13. Here, four crystals of 25x25 mm2 are coupled to a wavelength shifter that in turn is coupled to a small photomultiplier with circular photocathode. Using a shifter of matching spectral response, one can shift the ultraviolet YAP emission light to longer wavelengths around 420 nm where the photocathode has slightly 40 CHAPTER 3. THE SYNCHROTRON RADIATION DETECTOR Window: 25.0 flm Beryllium + 200 nm Aluminium Honeycomb Support Staggered Detector Modules 4 YAP(Ce) Crystals \. \ Wavelength Shifter I Light Guide E E 120mm o on Channel Photomultiplier I Front-end Electronics High Voltage (a) Subunit (b) Photomultiplier module Figure 3.13 Tentative design of the SRD scintillator lay-out 1.0 ,\ I\ ""' BC-484 fa I\ '2 0.8 J\ ;:j 11 I \ .0 .... I \ ~ / I .., 0.6 l '- '"0 '\ I I .E \ ! 0.. I : I I E 0.4 / \ Figure 3.14 Absorption and emission spectrum of the BC-484 wavelength shifter [Kni99] better response and, at the same time, one can collect the light onto a smaller photocathode. A candidate wavelength shifter is the type BC-484 by CrismatecjBicron. Its absorption and emission spectra, shown in Fig. 3.14, show a rather good overlap with the emission spectrum of YAP(Ce) (Fig. 3.5) and with the spectral response of a typical photocathode (Fig. 3.11(a)). The signal yield when using this wavelength shifter is rather limited, as briefly reported in Sect. 6.1, requiring further studies to determine if this option is really feasible. No better suited wavelength shifter has been identified thus far, however. To minimise losses. the cross section of the wavelength shifter should be constant, so, as suming a 25 mm wide and 2 mm thick bar at the crystal end, a photocathode area of 50 mm2 is needed (diameter 8 mm for a circular area). The number of photomultipliers in the example lay-out would be reduced to some 2400, and using a Hamamatsu R7400U that has the right photocathode diameter, the total weight would drop to 13 kg and the cost to around one million Swiss francs (50% price reduction assumed) for the same detector area. Another photomultiplier type called channel photomultiplier, recently introduced by PerkinElmer Optoelectronics, has also been considered as an option, as in Fig. 3.13. 3.3. TENTATIVE DESIGN 41 ...... r--,.-~---.------r:::=::::s====E::::::::==~=1 /'/v=-~ CD ~--!-I-/--+~/-----,q-//_-+--_-+--_ o 25~m C /11/1/ ~~m .~ OD f--__-I-+f--l-__l-l_O_O-'--~_m____1I__--_+_--- .~ l 0 It / / ~ 1 1 £ :=1:/:==:==:==:== o JJJ o'---...... LL..L--L..._...L-----J'------'-_-'-_-'------'_--L------.J o 2000 4000 6000 8000 104 Photon Energy (eV) Figure 3.15 X-ray transmission of beryllium foils of 25pm, 50 pm and 100 pm thickness (data fromhttp://cindy.lbl.gov/optical_constants/filter2.html) This device would result in a higher weight compared to the R7400U, but, due to its different construction principle employing little metal, in a lower radiation length. An advantage of it is the absence ofdynode-related noise, making this type well suited for photoelectron counting and small signals. One problem, on the other hand, is that it resistively dissipates the bias current, so a power-efficient high-voltage supply as used for the PSRD is not possible (see Sect. 4.4.1 and Fig. 4.9 on page 53). The wavelength shifter option, however, would coarsen the detector granularity to or beyond the point where the background rate increases markedly over the signal (see Sect. 3.1.3 and Appendix A). 3.3.2 Light shield The scintillating crystal/photomultiplier combination needs to be shielded against sun-light. A window completely opaque to optical light but as transparent as possible to X-rays is therefore needed. The best option identified is a thin beryllium foil coated with aluminium on the side facing the crystal. The coating acts as a better reflector for the ultraviolet scintillation light from the YAP crystal than the beryllium itself (see Fig. 4.7). The X-ray transmission for beryllium foils of three thicknesses is shown in Fig. 3.15. For me chanical reasons, 25pm is about the thinnest foil possible and it can be seen that the transmission at 2keV of such a foil is still 70%, well suited for the SRD. A possible reason to use thicker foils would be their advantageous absorption of the low energy charged particle background. As can be seen in Fig. 3.16, a 100 pm thick foil absorbs electrons with up to 100 keY and protons with up to 10 MeV kinetic energy. Indeed, as this contribution to the total background is not yet known with any confidence (see Sect. 3.1.3), data from the PSRD measurements are crucial for the choice of window. 3.3.3 Electronics The electromagnetic calorimeter of AMS-02 will deliver a trigger to the SRD if a particle with energy above some threshold was detected. Due to the trigger processing delay, it will arrive 42 CHAPTER 3. THE SYNCHROTRON RADIATION DETECTOR I,~ ',I.!,~ E :.: :,: t,l '.1 i j j liii ,l ,l ,! ,I ,I ::, !!l j i!: u : ! ! : ! :: ., ." . I 'Q;' 10 2 t Cl 1,lf,:···i,!··+,I·t,l·r,:;,fi,i1,1~,i j c ',It,1 +,1i,!+,i···+,I··+;[·',I· ,(,(,! t,i i,i··-·····-·r···· ···;··;·il·lli······· I1l cc j i : : iiiil ! iIII t!! ! ! j ll! i! j i l!! it .!:! ! ill! 10 .+Ht··········t·····j····I-··j-+·j-tH···········i-··-+···t··j-·j-'!-i·H-···· ··-l·_··_·i-··~·-~-~-;·~··~j·_··_··_·· : ; i:: :ll: :: J : 111:: ::,:" i'i, ,I:.. I:.. ..i.: i':.,::,·::.l::,'.!:·:•. I i Ii iilli [i i i li!i; i.. t.Ll.t l l I l..l..llJ.l -.l- .1._.1 .~.~lj.ll- .. _.. _.. -I.- .. _.l--j.-i-i-i.j.U-.._ . -1 IV i:IJ: l;::: :l: ::,: 1:::: :::: ::1:: 10 1o- ..j ...... •...... 1...... •.• • ;1I':..•.·I··•.·•.·•··.•·· •.·.·.·+····················--!- ...... •-.•..•.•+ .. 1 10 -1 i! ll! lOO 1. i ! ! i 111 ..'..t.l.HJJlt.. J.!.lJl.iJil... -2 lOOIll1l ~ ~ ~ ~ 10 H-....:....--2i'·······-··-··-····-·+···················-l····-···.-I --+.-- -..--.-..-+.j I t j i! i l i i 1 i i i: i 1 ; ; ; 1 ,l ,I ,1,1,1,1,1,1 !!!! !!!!! ' i 25Jlml i 1 i i iil11 1O~V1~ ii~i _iij~ ~HmltJJ1I[1[1~ 10 -, illI 10 -4 '.._. __ :'5. _.._ _~ _ _._ ___.._. __..__ _.._. __._.._ _._ _.._.__.. _. -2 -1 2 3 2 3 10 10 10 10 10 10 10 10 Kinetic Energy (MeV) Kinetic Energy (MeV) (a) Electrons (b) Protons Figure 3.16 Charged particle ranges III beryllium (data from http://physics.nist.gov/ PhysRefData/Star/Text/) several microseconds after the passage of the charged particle, requiring a storage of the photo multiplier output signals for that amount of time. The present design foresees an analog pipeline chip called APV9 for that purpose. This chip samples its 128 input channels continuously with 40 MHZ frequency and stores the data in a 160 column deep analog pipeline, i. e. the last 411S of the input waveform are available in integrated time slices of 25 ns. Upon a trigger, up to 18 consecutive time slices may be read out multiplexed over one line, which takes about 13011S with a read-out clock of 20 MHz (there is some overhead since digital control data is also transferred). The power consumption is about 2.4 mW per channel. A sketch of the operation principle is shown in Fig. 3.17. The multiplexed analog output of the APV will be digitised by flash analog-to-digital con verts (FADC), the data buffered, possibly compressed by a digital signal processor (DSP) and eventually fed into the DAQ system of AMS. The tentative block diagram for this design is shown in Fig. 3.18. It refers to the lay-out employing wavelength shifters that was previously described. The detector area is broken down into 40 subunits, containing 60 wavelength shifters with 4 YAP(Ce) crystals each. 90riginally developed for the read-out of the eMS tracker. Its design is still being improved, now by IBM in 0.25 pm technology under the name APV25. The parameters given here refer to the version that was also used in the PSRD, called APV6M. 3.3. TENTATIVE DESIGN 43 ADB ANALOG DATA BUFFER ~ 11 11 11 lV l ~. OUTPUT ... TO FADC CAPACITOR ARRAY ...... ~.J 31?OLAR 11 11 11 11 11 11 T ANALOG ~~ • •• ..... MUX BUnR X •• = :::====:::::;;iMi...,;:======-----=::::~~;;;:::: ( BIAS DAC's ADB CONTROL LOGIC J( READOUT eOGIC ~& CAL LOGIC & EVENT Bur FER 1( 1 : ... / " = COMMAND DECODER Figure 3.17 APV read-out design 40x subunits 1x APV board +/-2V +I_5V APV subunlt01 "'-J._J---l=-+-,.- inp119 Inp120 !np127··-.par,,· DSP In 128 charm~ 10x Controller 4xHV High Voltage Low Voltage Power Supply Power Supply: DC - DC Converter Controller for: Monitor functions: switch on/off LV bias voltage 10r +SV +1-5V +/-2V temperature switch onioff HV CPM943 1.SkV 2kV pressure to control monitor function monitor: voltage monitor: voltage current 3x CAN BUS current TRG Figure 3.18 Tentative block diagram of the SRD electronics using an APV read-out CHAPTER 4 The Prototype Synchrotron Radiation Detector During the preliminary studies for the SRD, it became clear that not enough information on the background situation in a near-earth orbit was available. This is indispensable not only because all efficiency and background rejection calculations need this as input to make reliable predictions about the detector performance, but also because too high a background would render the whole SRD principle not feasible. Also, as has been described in the previous chapter, some essential design characteristics (like the window thickness) depend on the background rate. Therefore, it was found necessary to measure this crucial parameter in space. To this end, a small precursor experiment, called Prototype Synchrotron Radiation Detector (PSRD), was designed and constructed. In this chapter, details on the objective of this detector and on its realization are given. The PSRD flew on board the Space Shuttle Endeavour on the mission STS-108, 5 -17 De cember 2001. A chronological description and several photographs illustrating the construction, testing and flight are given in Appendix D. 4.1 Motivation Following the description of the SRD concept, the scientific motivations for the PSRD are, in order of decreasing importance, as follows: • The X-ray photon background that the SRD has to expect is reasonably well measured, see Fig.3.3(b). It amounts to 50 Photons/(s cm2 ) between 2.5 keY and 100 keY on a 27r detector. With this background alone, a time resolution of some 10 ns was found to be necessary in Sect. 3.1.3 to sufficiently suppress the large proton background. It is known that low-energy charged particles are present in the near-earth environment (see Fig. 3.4), but almost no information on their energy distribution is available. No clear idea exists thus on the effect they will have on the SRD background situation, making the measurement of this rate the most important objective of the PSRD. • The principle design features of the SRD - a scintillator/photomultiplier combination, shielded with a beryllium window against sunlight and read out with an APV analog pipeline chip - are also employed in the PSRD, though not all combined together in the same way as with the SRD. A validation of these elements in a space environment is thus an important objective of the PSRD. 44 4.2. DESIGN OVERVIEW 45 Figure 4.1 Hitchhiker cross-bridge in the Space Shuttle payload bay. The PSRD was flown on a similar structure. • Gaining insight into the general requirements and procedures for the design of a space qualified experiment, specifically from the mechanical point of view, will be valuable for the construction of the larger scale SRD. • Since data on low-energy charged particle rates in a near-earth orbit are scarce, they may be of interest in their own right and be useful for other, future space experiments that are sensitive in this energy range. • Separate from the SHD, a collaboration with the Institute of Quantum Electronics at ETH Zurich enables a world-wide first test of CdTejCdS-based solar cells under actual space conditions. Experience gained with the electronic design of the APV read-out and its support electronics will also be of help in the SRD development, although this purpose alone would not justify a space experiment. 4.2 Design overVIeW The PSRD was designed to fly as a secondary payload on a Space Shuttle, participating in the so-called NASA Hitchhiker program that is part of the Shuttle Small Payloads Project. Within this program, electrical power and basic up- and downlink capabilities are provided to a number of individual, small payloads. They are mechanically attached to a support structure mounted cross-bay in the shuttle or they are directly mounted to the walls of the payload bay. As can be seen in Fig.4.1, a typical Hitchhiker payload is mounted in a standardised cylindrical can, though a unique mechanical mounting is used for the PSRD. The Hitchhiker that the detector was flown on was called MACH-I]. The basic configuration of the PSRD is shown in Fig. 4.2. Detector and electronic components are mounted in individual cassettes that can be inserted into their side-plate supports in a 1Multiple Application Customized Hitchhiker-l 46 CHAPTER 4. THE PROTOTYPE SYNCHROTRON RADIATION DETECTOR Small YAP Array (Below Beryllium Foils) Non-Flight Patch-Panel + Repeater Box Flight Patch-Panel + Cover Jtag Port + Cover Adapter Plate \ Data Storage Containers Filler Box Battery Container Large Light Trap / Venting Figure 4.2 Overview of the PSRD (Cabling not shown) drawer-like fashion, facilitating construction. Cassettes are interconnected by cabling that runs along the front side and the whole experiment is interfaced with the Hitchhiker avionics by three cables at the bottom end. The PSRD is mounted on a wedge-shaped adapter plate that tilts the detector away from a large experiment on top of the Hitchhiker cross-bridge. This would otherwise partly block the field-of-view (see the photographs in Sect. D.2). To provide system-level redundancy, the whole experiment is split electrically into two indi vidual but identical parts. Except at the very first stage of power input (since there is only one power supply cable coming from the Hitchhiker avionics) and in the trigger detector (Sect. 4.6)' there is no cross-connection between the two sides. Mechanically, the components for both sides are for the most part mounted in one cassette. Since only electrical power and a simple up-/ downlink is offered by the Hitchhiker program, the experiment must function mostly autonomously, in particular the data storage must be local. Four hard disks with a total of 100 GByte are used for this purpose. The computer program that controls the PSRD is designed to run smoothly if there is no ground control and monitoring possible. One general design consideration is to provide adequate venting of all volumes inside the PSRD. During the ascent and descent of the Space Shuttle, the ambient pressure will change by one atrnosphere within two minutes. Especially over fragile parts, such as the thin entrance windows, no significant pressure differential must build up. Several venting channels are thus used, designed to prevent light from entering the experiment (light traps). Also, the operation environment in space requires rather tough components. The environmental limits for several parts of the PSRD are collected in Table 4.1. 4.2. DESIGN OVERVIEW 47 Table 4.1 Environmental requirements for PSRD components Operating Survival PC/104 CPU module -40°C - +85 °C, < 95% humidity -55°C - +85 °C, < 95% humidity PC/104 VGA module -40°C - +85°C, < 95% humidity -65°C - +125°C, < 95% humidity PC/104 bus extender -40°C - +85 °C same as operating Lithium battery -45°C - +75°C < +1000C / < +120°C (leakage) 1 Hard disk Travelstar +5°C - +55°C, 8% - 90% humid- -40°C - +65°C, 5% - 95% humid- ity, 0.7 atm - 1.05 atm pressure ity, 0.25 atm - 1.05 atm pressure Cooling fan -10°C - +70°C -40°C - +70°C DiskOnChip -40°C - +85 °C -55°C - +85 °C Photomultiplier -80°C - +50°C, < 3°C/min gradi- same as operating ent Plastic scintillator < + 70°C (softening point) same as operating Macrostrip detector2 -20°C - +40°C same as operating Solar cell -100°C - +100°C same as operating SRDxxx board -40°C - +85 °C same as operating DC/DC converter -55°C - +105°C -55°C - +125°C Printed-circuit board < +200°C (solder melts) same as operating 1 Underwriters Lab limit / Technical limit from manufacturer 2 Limit from AMS-Ol silicon detector The weight of the detector (without adapter plate) is 93.5 kg, see Table 4.2, the total power consumption 140 W. A detailed account of the individual components can be found in the fol lowing sections; a brief description, from top to bottom, is given here. • X-ray cassette The scintillators and photomultipliers for the background rate mea surements are mounted in this cassette. 12 YAP(Ce) crystals of 18x18x1 mm3 and 4 of 30 x 30 x 30 mm3 are used, the latter being enclosed in veto counters made of plastic scin tillator to accept only photons. The small YAP crystals are shielded against sun light with beryllium windows, the large ones with an aluminised plastic foil called TOR-LM. Also, four CdTe/CdS solar cells are installed here. • Silicon macrostrip detectors Two macrostrip detectors with strip pitch 194 pm and dimension 107x64 mm2 are mounted orthogonal to each other in two individual cassettes. They are read out by the APV6M chips that are planned to be used for the SRD. • Trigger detector A charged-particle trigger is generated by the detection of a coinci dence between the two plastic scintillators that are mounted in this cassette. • SRDAPV Read-out electronic, including flash ADCs, for the APV chips. • SRDSOL Read-out logic for the solar cells. • SRDYAP Flash ADCs and support circuitry for the read-out of the 12 small YAP crystals. • SRDSAB+TRG Scaler and support electronics for the 4 large YAP crystals and logic to generate the trigger from the two plastic scintillators. • SRDHVC+SLO Control logic for the high voltage generators and electronics to measure temperatures and pressures within the PSRD. 48 CHAPTER 4. THE PROTOTYPE SYNCHROTRON RADIATION DETECTOR Table 4.2 Weight of individual PSRD components Component Weight (kg) X-Ray Cassette 9.5 Silicon Macrostrip Detectors 5.4 Trigger Detector 5.4 SRDxxx Cassettes (APV,SOL,YAP,SAB,HVC) 9.0 SRDPOW, SRDPWR 12.7 Data Storage 8.6 PC/104, Fuse Box, V-Beam 7.4 Filter Box 2.7 Main Frame 23.3 Connectors, Cables 7.8 Fasteners 1.7 Total 93.5 • SRDPOW+LED Control of the DC/DC voltage converters and the LED system. • SRDPWR DC/DC converters that produce +5 V, -5 V and +45 V from the shuttle supplied +28 V are mounted on a support made from a single piece of aluminium. Radiators on both sides help in dissipating the heat that is generated due to conversion inefficiencies. • Data storage In total 4 hard disks of 25 GByte each are mounted in two separate com partments pressurised to 1 atm. All experiment data are stored on these disks. • PC/104 All electronic boards and the hard disks are interfaced to two 80x486 personal computers with PC/104 industry standard form factor. 4.3 Material and structural issues All structural parts of the PSRD are made from surface-treated aluminium, types AA5083 and AA6082. In case electrical conductivity over the contact areas is required, the alodine treatment was used, yielding a yellowish colour of the surface. All other surfaces were black-anodised, as this gives a much harder and more durable surface than alodine.2 The black surface also favours heat transport by radiation within the PSRD cassettes due to its good infrared emission and absorption characteristic. The outer surface, facing space, is covered with Silver Teflon tape, type G401902 by Sheldahl, to limit the heating in sunlight. This tape has both a high reflectivity for visible light (avoiding heat influx from sunlight) and a high emissivity for infrared radiation (dissipating heat efficiently into space). All large, structural bolts were acquired directly from NASA to comply with their require ments. The bolts were secured by safety wiring, as shown in Fig. 4.3. Smaller structural bolts were bought locally, but strength tested before use. They are secured using Vibra-Tite by ND Industries, a locking coating for threaded fasteners. Most threads are fitted with HeliCoil in serts to increase the strength and to prevent jamming of the bolts under load in the rather soft aluminium. Airex R82.60 foam by Alcan was used as shock-absorbing support for the macrostrip detector and the solar cells. The electronic boards were made from G10 and a conformal coating (Solithane 113) was applied to them to avoid outgassing and for protection reasons. 2Both surface treatments were done by the Anox company. 4.4. X-RAY CASSETTE 49 Figure 4.3 Safety wiring of the mounting bolts A full list of all materials used in the PSRD was supplied to NASA for concurrence. The finite-element math model, structural calculations and documentations for the PSRD (especially also the Str-uctuml Vcr-ification Repor-t) were done by ISATEC. The simulations indicated positive margins-or-safety in all cases, so that no testing beyond that described in Sect. 4.14 was necessary. Load factors of ±11 9 in all three axis simultaneously and an ultimate factor-of-safety of 2.6 were required; the maximum actual loads expected for the x/y/z axis are 3.2 g/1.0 g/2.5 9 for ascent and 2.0 g/1.3 g/4.2 9 for descent (x axis along payload bay, y axis along wings, z axis perpendicular to wings). 4.4 X-ray cassette The detector components of the PSRD to measure the background rate of photons and charged particles are located in the so-called X-ray cassette. It contains 12 YAP(Ce) crystals of dimension 18 x 18 xI mm3 that are read out in a digital-scope manner via flash ADCs and 4 larger crystals of dimension 30 x 30 x 30 mm3 that employ somewhat simpler electronics, namely scalers that count events above three different thresholds. Additionally, 4 solar cells based on CdTe/CdS are installed to verify their operation characteristics in the space environment. The external appearance and internal structure of the X-ray cassette are shown in Fig. 4.4. As with all other cassettes, guiding rails on both sides help insertion into the side plates that provide mechanical support. Cable bars run along the front side to fix the extensive cable tree. 4.4.1 Small YAP array The central part of the X-ray cassette is used for 12 photomultipliers with their scintillators for addressing main objective of the PSRD, the background measurement. The YAP(Ce) crystals of 18x18xl mm3 match exactly the photocathode size of the R5900U photomultipliers from Hamamatsu. Crystals of this thickness will fully absorb photons up to about 30 keY, as can been seen from Fig. 4.5. The face of the crystal coupled to the photomul tiplier is polished to optical flatness, the other five faces are fine-grinded. This surface finish is both cheaper and delivers a higher light yield compared to fully polished crystals due to reduced trapping of light inside the scintillator (cf. Sect. 5.2). The crystals are glued to the photomultiplier windows with BC-600 optical glue (an epoxy resin made by Crismatec/Bicron). For a 125]lm thick layer, the transmission of this glue is above 95% between 340 nm and 400 nm, and above 98% at longer wavelengths. Its refractive index3 3 As long as the refractive index of the glue is not higher than that of the YAP(Ce) scintillator, the light transmission is unaffected by the precise value of the index. Important is a good transparency around 370 nm. 50 CHAPTER 4. THE PRDTOTYPE SYNCHROTRON RADIATION DETECTOR Mounting holes to side plate Guiding rail (a) External appearance (b) Internal structure (c) View of the top side (d) View of the interior Figure 4.4 Layout of the X-ray cassette. In (c), silver Teflon tape has already been applied, still being partly covered by its yellowish protection layer. The four corner crystals are temporarily protected in (d) with a cover. is 1.56 and it retains a certain flexibility when cured, advantageous with the rapidly changing temperatures the PSRD will experience in orbit. Each crystal is individually shielded against sun light by one or two beryllium windows (bought from Metorex). As stated in Sect. 3.3.2, the thickness of the windows could be used to control the charged particle background. To investigate into this under actual space conditions, four crystals are covered by one 25 pm foil, four by two 25pm foils and four by two 50 pm foils. With multiple foils, the light flooding in the case of a punctuation of the foils by a micrometeorite might also be limited sufficiently to allow further operation of this unit. The foils are mounted between two black-anodised aluminium frames that have a thin zigzag groove machined into them to allow venting of the volume between the foils without letting light in, see Fig. 4.6. The inner sides of the beryllium foils are coated with a 200 nm thick layer of aluminium 4.4. X-RAY CASSETTE 51 1 rnrnxO.3rnrn 0 25 50 75 100 125 150 175 200 Energy (keV) Figure 4.5 Absorption of X-rays in Figure 4.6 Design of the frames holding the YAP crystals of 1 mm, 2 mm and 5 mm beryllium windows thickness (data from [Hub95]) to act as a reflector for the scintillation light. Empirically, the signal yield was higher with the aluminium coating and indeed, as shown in Fig. 4.7, aluminium is a much better reflector than beryllium. Around 370 nm it is also better than silver, which is often used as a mirror material at longer optical wavelengths. It should be noted, however, that these reflectivity values depend significantly on the surface quality and preparation method of the examined samples. As a reflector for YAP scintillation light, though, these differences are of little consequence because of the large total reflection angle that confines most light to the crystal, even in the absence of a reflector. Since a certain distance between the window and the crystal could not be avoided for con struction reasons, the effect of this distance on the light yield was measured. As seen in Fig. 4.8, no significant light loss occurs for distances up to several millimetres. Because the number of reflections that is necessary to return light to the photocathode should not increase markedly as long as the distance is smaller than the cross-dimensions (about 18 mm in this case), this is expected. The distance in the final design is about 4 mm. Like with all other photomultipliers in the PSRD, a green light emitting diode (LED) is mounted on the small YAP photomultipliers to allow functional verifications without using radioactive sources or rare cosmic rays. The LEDs are connected to the SRDPOW+ LED board that allows control of brightness and duration of the LED pulses. Additionally, four of the photomultipliers have PT-lOOO temperature sensors mounted on their base. The high-voltage of around 800 V is generated individually for each photomultiplier by a voltage-elevator circuit (Cockroft-Walton generator) built around a LeCroy MHVI00 control chip. The circuitry4 is located on three small printed-circuit boards stacked underneath the base of each photomultiplier. The basic circuit diagrams of the control and high-voltage generator sections are shown in Fig. 4.9. The control circuit in (a) essentially provides a square-wave pump voltage with 100 kHz frequency between +45 V and ground. The pump voltage is enabled when the output voltage of the generator section is below the set value and disabled otherwise. Fast comparators will also disable the output when preset current or voltage limits are exceeded (in that case, the output needs to be re-enabled manually). 4Designed by Vladimir Koutsenko, Massachusetts Institute of Technology, Cambridge, USA. ------ 52 CHAPTER 4. THE PROTOTYPE SYNCHROTRON RADIATION DETECTOR ~101 ~ ~ ~100 ~ ~ 100 ~ ">' "S; E 99 ·u g ID 80 Ql ~ > 98 ID ~ U C ID ID a: 97 "0 ·0 60 .s 96 (ij E 0 95 z 40 • 94 20 93 92 • ~-----'------l.-_L L.~L_---'_...L ----'------'------'---~----'-~_-'--'-~-~'---,- 91 L J L_. 400 450 500 0 2 4 6 8 10 12 Wavelength (nm) Distance PMT-Window (mm) Figure 4.7 Normal incidence reflectivity Figure 4.8 Light yield as a function of of aluminium, silver and beryllium (Data distance between YAP crystal 18x18 mm2 from [CRC92, Al and Ag] and [PaI98, Be]; and an aluminised Kapton window. Dis the curves are spline interpolations) tance pieces made from aluminium, inner 2 cut-out 19x19mm . The pump voltage is fed to the elevator chain in (b). The maximum theoretically attainable output voltage is the number of stages multiplied by the pump voltage, 24· 45 V = 1080 V in this case, but in practice it is lower and also unstable at the upper limit. The nominal voltage for all photomultipliers in the PSRD is 800 V, with some variability due to inherent gain differences between individual tubes. The maximum voltage used is 840 V. The voltage partition between the electrodes is given by the number of stages between their taps, here it is 1:2:2:2:2:2:2:2:2:3:4. The lower chain of capacitors provides stabilisation of the dynode potentials against the high transient currents that occur when pulses pass through the photomultiplier, especially at the last stages. The output voltage is kept constant to about 0.5 V and the maximum average output current is 100 pA. This circuit has the advantage over the conventional bias scheme shown in Fig. 3.8 of having no constant current flowing through a resistor chain. Current is in principle only flowing to replace charges on the dynodes that have been taken away by the signal current of the photo multiplier. The anode output is split into two signals of ratio 1:10 by the circuit shown in Fig. 4.10 and the signals are then routed via 50 n impedance cables to the SRDYAP cassette for digitisation (Sect. 4.7.1). Typical signals of the non-attenuated output are shown in Fig. 4.11 for, in this case, 900 V bias voltage. Examples from the final energy calibration for one particular photomultiplier are shown in Fig. 4.12 (non-attenuated output). The energy resolution at 5.9 keY is about 35%, at 22.6keV about 21%. 4.4.2 Large YAP crystals Four large YAP crystals of dimension 30x30x30 mm3 are mounted in the corners of the X-ray cassette to allow measurements of the background rate at higher energies (see Fig. 4.13). On three sides, the crystal is enclosed with a 5 mm thick plastic scintillator (NE102A) that 4.4. X-RAY CASSETTE 53 +5V +45 V \00 8 bit O.22uF .------+-..----1H f---ioND select code ,.....; OCLDAC 0 ,.....; t:l 0 ~ OVLDAC t:l VFB l::::: l::::: h=-.f!PUMP~~:'p > 0 0 tMON IMON Q GND 8 U g~ ,.....; U > ro ;.... GND ::I: ..... ~ GND .8 .-b1) .... ~ Q) ~ 0 .- Q) 0 ~ .;;J0.. Data :; ::E Push- Pull Output GND (a) Control circuit to HV control circuit I~~~~~~~I WJ I~ ------{=----=--=-=---=--=~::J------, .1 .1 .1".1 ".1.1 ".1 .I. J ".1" "I "11 11 ., ,,"I ,,'I 'I'""""" T 'I - " " " " " R5900U Socket Signal Aoodo Out ~ 1 (b) High-voltage converter Figure 4.9 Circuits for high-voltage generation 54 CHAPTER 4. THE PROTOTYPE SYNCHROTRON RADIATION DETECTOR Figure 4.10 Circuit for reading out the anode signals (two outputs with 1:10 ratio) acts as a veto against charged particles; on the other sides, the YAP crystal is shielded by the aluminium support structure, see Fig. 4.4(b). The veto counter is made from three pieces glued together with BC-600 optical glue and it is read out with one photomultiplier via a flat face, see Fig. 4.14. The transmission of the plastic scintillator for X-ray photons is shown in Fig. 4.15 35% at 10 keY and 83% at 20 keY. The lowest counting threshold is set to 10 keY, see Sect. 4.7.3. Both crystal and veto counter are individually wrapped in several layers of Teflon tape (as X ray absorption is negligible at the energies of interest for this component), while light cross-talk5 is eliminated by an extra layer of aluminium foil between them. The opening in the X-ray cassette for this counter assembly is shielded against sun light with two stacked windows made from aluminised TOR-LM foil. This foil, produced by Triton Systems and based on phosphine oxides and polymers, is especially resistant against atomic oxygen occurring in significant amounts near earth. Atomic oxygen is aggressive against standard polymers and can etch away thin films. Since the TOR-LM foil itself is not opaque, it was coated on both sides with 200 nm of aluminium. Because, as said above, X-ray absorption is no issue, there was no need to use expensive and fragile beryllium windows. The photomultiplier base is the same as for the small YAP array, except that the amplitude ratio between the two outputs is 1:3. The signals are routed to the discriminators in the SRDSAB board. Each photomultiplier has a PT-lOOO temperature sensor mounted on its base. Output signals from this assembly (non-attenuated output) for l09Cd and 54Mn sources are shown in Fig. 4.16. These display good linearity from 22.6 keY to 835 keY. A spectrum of l09Cd taken with a conventional photomultiplier base at 900 V bias and a standard charge integrating ADC is shown in Fig.4.17 (gate length 150 ns, electronic amplifi cation 12 times). The broad plateau below the 22.6 keY line is due to the unresolved yttrium escape peak at 7.7 keV. Since no shaping was used, a few entries of spiky, low-charge events are found below the pedestal at 52 counts. By comparing the 22.6 keY peak position with the single photoelectron peak that becomes visible at electronic amplifications of about 100, a signal yield of approximately 1 photoelectron per keV of deposited energy is found. The variation of the signal yield when moving the source to different positions on the crystal amounts to some 30%. Even without a source and after a long time in darkness, a significant count rate exists above 5This cross-talk could be an issue without aluminium foil because of the large amount of light generated in the YAP crystal for MeV photons and the finite transmission even of several layers of Teflon tape. Cross-talk might trigger the veto counter which has a low threshold. 4.4. X-RAY CASSETTE 55 Tek Stop: Sing le Seq 500M5/S Tek Stop: Single Seq 500M5/S I [ T ] I I [ T ] I 1-> ,...... [,.....r'" ./ f/ V 4- 1/ 4- / I \.. Ln "uum 10 M luuns Lr " 17m er <,uum/o M luuns er \. I,mv (a) 109Cd source, 22.6 keY (b) 109Cd source, 88keV Tek Stop: 200MS/s 50 Acqs I [ T 1 I -> /~ l/ 4- u "uum 'ID M t~uns Lt \. ',umv (c) Single photoelectron Figure 4.11 Output signals from one small YAP photomultiplier at 900 V bias voltage. Time base is 100 ns for 109Cd and 250 ns for the single photoelectron signal. 56 CHAPTER 4. THE PROTOTYPE SYNCHROTRON RADIATION DETECTOR 400 100 200 ~------~ pos. = 19.8± 0.1 pos. = 76.2 ± 0.5 ,-.. Ell keV = (O.30± 0.(0) A c (j 6.9 ± 0.1 (j = 16.1 ± 0.5 t:l 11)'" = 'El + ( 0.03 :1: 0.06 ) .;:: res. = 34.6 % 11)'" res. = 21.1 % ..... ::l 150 t=: Jtilt =3889.1 S-1 ".5 Jtilt =2070.0 S-1 i:I.:l t=: U i:I.:l ~ 200 50 11) 100 "0 .E } 50 (a) 55Fe source, 5.9 keY (b) 109Cd source, 22.6keV (c) Calibration curve Figure 4.12 Energy calibration of the small YAP array. In (a) and (b), the histograms for two radioactive sources are shown, in (c) the calibration curve that results (l33Ba was also used). ~ 100 1-----..... c o :;Je- o «Jg 80 60 40 20 10 10 Energy (MeV) Figure 4.13 Absorption of X-rays in YAP crystals of 30 mm thickness (data from [Hub95]) 4.4. X-RAY CASSETTE 57 Veto counter Photomultiplier base / High voltage generator Figure 4.14 Detail of the design of the large YAP/veto counter combination. The YAP crystal is hidden by the three faces of the veto counter. ~ co 0 1/ ---- co / 0 l:: .9 '" '8'" ..q< / a'" 0 1/ ~ C\I / 0 J 5 10 15 20 25 Photon Energy (keV) Figure 4.15 Transmission of X-ray photons through 5 mm of plastic scintillator NEI02A (base polyvinyltoluene, Cg HlQ; from http://cindy.lbl.goy/optical_constants) 58 CHAPTER 4. THE PROTOTYPE SYNCHROTRON RADIATION DETECTOR Tek Stop: 200MS/s Tek Stop: 200MS/S I 1 I I 1 I ",' _m ns m m (a) 109Cd source, 22_6keV and 88keV lines (b) 54Mn source, 835keV and Compton edge Figure 4.16 Output signals from a large YAP at 800 V bias voltage. Horizontal time base is 250 ns, vertical scale 20 mV and 200 mV per division, respectively. C/l ; , .5!l Peddtal 52 couhts ··_~_··_··_··_··_;·_··_··_··_··_··_··_··_··-t-··_··_··_··_··_··_··_··_·t··_··_··_··_··_··_··_··_··t··_··_··-.. _.. _.. _.. _.. _.. w-E 600 i : 500 ------[------1------r------t------ : i i , 400 ·-··-··-··-··-··-r·_··_··_··_··_··_··_··_··-t-··_··_··_··_··_··_··_··_·Ji··_··_··_··_··_··_··_··_··t··_··_··_.. _.. _,,_ .. _.. _.. ! : i _.._.. _.. _.. _.. _~._ .. _.._.._.. _.. _.. _.. _.. _l_ .. _.._.._.. _.. _.. _.. _.. _.l .. _.. _.. _.. _.. _.. _.. _.. _.. L._ .. _.. _.. _.. __ ._.. _."_ .. 300 : ! : : ! 22.6 ~eV ! ··-··-··-··-··-··-··-··t··----··-··-··-··-··-··-·· g 200 , 0 ; N : P") 100 200 400 600 800 100e ADC Count Figure 4.17 l09Cd spectrum ofa large YAP crystal at 900 V bias voltage. The energy resolution a/J..L is 33%. 4.4. X-RAY CASSETTE 59 Tek Run: 500MS/s Sample I[ T ." "'.:' Figure 4.18 Signals from cosmic particles passing through a veto counter. Bias voltage was 800 V, vertical scale is 50 mV per division. high thresholds, as the numbers below show: Threshold (keV) 10 20 50 170 Count rate with YAP crystal (s ) 46 41 35 14 Count rate without YAP crystal (s-l) 200 o The contribution of the photomultiplier to the rate is clearly negligible. The measurements were made using signal shaping of 200 ns integration and 100 ns differentiation to have a well defined energy threshold. These numbers reflect the laboratory environment of cosmic rays and radioactive decay photons from building materials. Indeed, in another series of measurements, a clear peak at 1461 keY from radioactive 4oK, known to be present in concrete, was found, together with a steeply increasing rate towards lower energies (see Appendix C for more details). This background rate is dependent on the location of the detector, most explicitly, of course, in the case of operation in space - the measurement of this rate being the principle goal of the PSRD. Typical signals from cosmics passing through the top face of the veto counter are shown in Fig. 4.18. The three-fold cosmic ray trigger that was used allowed particles also to traverse at somewhat oblique angles or along one of the side faces, resulting in several large events. The thresholds for the veto counters in the PSRD were set to 40 mV. 4.4.3 Solar cells The Thin Films Physics Group at the Institute of Quantum Electronics (IQE) at ETH Zurich has developed a novel process to make flexible solar cells, based on layers of CdTe and CdS films deposited on polymer sheets. They report a conversion efficiency of up to 8.6%. These solar cells are of high interest for both space and terrestrial applications because they can offer a high power-to-weight ratio around 3 kW/kg, more than ten times better than conventional, silicon-based cells [RomOl]. 60 CHAPTER 4. THE PROTOTYPE SYNCHROTRON RADIATION DETECTOR Silicon macrostrip detector (a) Sketch (b) Photography. The four black, circu lar extensions on the cassette frame are venting baffles. Figure 4.19 Silicon macrostrip detector cassette One factor of key importance for space applications is the performance after irradiation by high particle fluences, as occur especially in near-earth orbits due to trapped particles. Ground based irradiation studies have already shown a good resistance against high proton fluxes around la MeV (see also [Rom01]), but measurements with actual particle fluxes are desirable. In collaboration with the IQE, the PSRD offers the possibility for a very first test of the performance of CdTejCdS solar cells in space. Rigid cells deposited on standard soda lime glass are used, as the behaviour of a polymer film under irradiation, where aging is also an issue, would give unwanted complications at this stage of the investigations. The cells are also used as a solar light monitor for the main PSRD measurements. This helps to filter out contaminated data in the case of a light leak in one of the window assemblies. Four solar cell arrays, each containing four sub-cells, are mounted on top of the X-ray cassette as shown in Fig. 4.4(a). One calibrated photodiode per array (mounted in the circular opening) serves as a light irradiation reference and a PT-lOOO sensor measures the temperature. The output of each individual sub-cell and the sensors are connected to the electronics on the SRDSOL board. 4.5 Silicon macrostrip detector One of the objectives of the PSRD is to test the APV read-out chip foreseen for the SRD (see Sect. 3.3.3) under actual space-flight conditions. As the chip is not suited for the implementation of the primary goal of the PSRD, the background measurement, a separate component was included, the silicon macrostrip detector shown in Fig. 4.19. A silicon strip detector that was designed for the GLAST experiment6 was found as a con venient off-the-shelf detector to interface with the chip. Because of its rather large strip pitch of 194 pm, it is referred to as a macrostrip detector. The total number of strips is 320, bias resistors are 30 MD, the area is 107 x 64 mm2 and the silicon thickness 400 pm. 6The Gamma-Ray Large Area Space Telescope (GLAST) shall examine the photon spectrum between 10 keY and 300 GeV in space. Launch is currently scheduled for 2006. ~~ -~~---~------ 4.6. TRIGGER DETECTOR 61 Alternately, strips are DC-coupled to one APV chip over a Kapton cable on either side, with some strips close to the edge of the detector connected together in pairs since only 128 input channels per APV are available. The chip is then connected, again with a Kapton cable, to the front-end electronics, containing, among others, two DC/DC converters that generate the bias voltage of approximately 100 V from 5 V and some support electronics, specifically for the I2C bus to interface with the APV chip. The front-end electronics are then connected to the SRDAPV board, where the FADCs and control circuitry are located. Two silicon detector cassettes are used, with the detectors being mounted orthogonal to each other. Both APV chips in each cassette are connected to one SRDAPV board. Two PT-WOO temperature sensors and two red emitting LEDs are used per cassette (red instead of green, as used for the photomultipliers, since the response of silicon is better at longer wavelengths). The LEDs are mounted close to the edge of the silicon. In Fig.4.20 some output signals of the silicon macrostrip detector are shown. In (a), one time slice of an LED pulse7 is plotted for both APV chips (even and odd channels, respectively). The first four data items contain, in digital coding, a four-bit header, followed by an eight-bit number identifying the first column (time slice) and then the analog data of the 128 channels. Fifteen time slices of one channel of the same LED signal are plotted in (b), while in (c) one time slice of a cosmic signal is shown. Since the APV was designed to yield outputs linear up to energy depositions of several times that of a minimum ionising particle, the cosmic signal is rather small, approximately 1/18 of the full scale. 4.6 Trigger detector The read-out of the APV chip is triggered by the detection of the passage of a charged particle through the trigger detector. As shown in Fig. 4.21, it consists of two NE102A plastic scintillators mounted individually in aluminium holders and read out on both sides by R5900U photomul tipliers. A 17 mm thick absorber made of Delrin (a polyacetal resin manufactured by DuPont) between them filters out low-energy particles. Each photomultiplier has an LED attached. The photomultiplier base is the same as for the small YAP array, except that both outputs deliver the same amplitude. Each photomultiplier is then connected to the SRDSAB+TRG board on both sides of the PSRD. Since the trigger requirement is a coincidence between at least one photomultiplier of the top scintillator with at least one of the bottom scintillator, this cross connection ensures that triggering on both sides still works if one photomultiplier per scintillator should fail. PT-WOO temperature sensors are mounted on the base of each photomultiplier. The output signals have the same shape as those from the veto counter (see Fig. 4.18, page 59), with amplitudes varying with the inherent photomultiplier gain, as all bias voltages were set to 800 V. The individual thresholds were set proportional to this gain to ensure identical trigger timing for all four photomultipliers. 4.7 Electronics The custom-made electronics8 for the PSRD are mostly built around XILINX programmable gate array chips that perform all control and sequencing functions. The boards are connected via an extension of the standard PC-ISA bus to the computers at the bottom of the experiment 7 An external LED was used, not the one mounted on the board, therefore channels in the middle of the detector were illuminated. BDesigned by Volker Commichau, Klaus Hangarter and Clemens Camps, Ill. Physikalisches Institut, RWTH Aachen, Germany. 62 CHAPTER 4. THE PROTOTYPE SYNCHROTRON RADIATION DETECTOR --, "5250 "5250 a. a. "5 "5 o o Cl Cl o o ~ 200 ~ 200 e:t: e:t: APV3 150 ..~-"""' ..·-...... ~I 150 , ."0 . i ~.~ :~ ...... ~.••,.:""--.• ...---....._..io...... • ...... : .... __••....__.._' __ - .... /.",-.~....,...I APV 1 100 100 .ii-- -...-' . . .. 50 50 o 20 40 60 80 100 120 140 o 2 4 6 8 10 12 14 Data item from APV Time Slice (a) One time slice of an LED signal. Due to a (b) Time evolution of the LED signal on channel bonding problem, 2 adjacent strips on APV 3 are 73, APV 1. One time slice corresponds to 25 ns. always interchanged. "5250 .9- :::3 Cl o Cl o ! Cii 200 ~ I 150 f- r...... -...:...... ··"---·...·.;,.,...-..:...·..·~.,..--..---...... ·-."..--..·I 100 ". I i 50 I I "~_L'-~~" _~~ L~ 20 40 60 80 100 120 140 Data item from APV (c) One time slice of a cosmic event in channel 22 (data item 34). Figure 4.20 Signals from the silicon macrostrip detector. Bias voltage is 105 V. Note that in (a) and (c) the first 12 data items are digital control data from the APV. 4.7. ELECTRONICS 63 Top scintillalor Photomultiplier holder I assembly High·voltagc generator High voltage generator (a) Cassette lay-out (b) Scintillator assembly Figure 4.21 Layout of the trigger detector cassette and are controlled by a program written in the C language running under DOS (the DOS core of Windows 95 was used). They appear as standard components on the I/O bus of the PC in the address range Ox300 to Ox37e, so communication is through writing and reading of registers. The XILINX chips can be programmed through a connector accessible from the outside of the PSRD (the Jtag port in Fig. 4.2), allowing updates until late into the construction. The electronics of each sub-detector for both sides of the PSRD are mechanically located in one cassette, but electrically separated, as can be seen in Fig. 4.22 where two examples, the SRDAPV and SRDYAP boards, are shown. In (a), the large, square XILINX chips are visible on the top, in (b) copper cooling bars have been glued to the XILINX of the SRDYAP. These bars are only used for those XILINX chips that dissipate too much power for cooling through the printed circuit board. The space-qualified AirBorn, ITT-Cannon and Radiall connectors that are used for interconnection of the boards are also visible. The printed-circuit boards were produced at CSIST, Taiwan, components were mounted by Elfab and a conformal coating was applied at Oerlikon Contraves. A simplified diagram of the interconnections between the various detectors and electronic parts of the PSRD is shown in Fig. 4.23. The +5 V voltages for the two disks and the +45 V for the high-voltage generators are switched under software control, all others come up as soon as shuttle power is applied to the detector. Individual boards have a power consumption between 4W and 14W. The general read-out timing is sketched in Fig. 4.24. Note that, as will be explained below, the scintillator triggers might be complemented by software-generated triggers to guarantee a constant average read-out rate of the small YAP array. In the following sections, the features and characteristics of the boards relevant to the actual PSRD flight are described, but generally not those that are used only during the testing phase. 4.7.1 SRDYAP The outputs of the two ranges from the small YAP array in the X-ray cassette are connected to 12 eight-bit FADCs per side of the PSRD. They sample the outputs with 20 MHz frequency and store the data in 8 kByte memories. The sampling is continuous and the memories are written in a circular buffer-like fashion, i. e. they always hold the last 409.6 ps of data. Upon a trigger, the sampling is stopped, a data available flag is set and the data is finally read out by the control 64 CHAPTER 4. THE PROTOTYPE SYNCHROTRON RADIATION DETECTOR (a) SRDAPV board (b) SRDYAP board with cooling bars attached Figure 4.22 Photographs of two electronic cassettes. Dimensions are 358 mm width and 374mm depth. program and stored on hard disk. The trigger might either be an actual scintillator trigger or can be generated by the software. To record the required amount of data for the primary goal of the experiment, the background rate measurement, independent of the actual scintillator trigger rate, the software normally assures that the total number of triggers is 22 per 10 seconds. This number can be reduced to 10 per 10 seconds by ground command (see Sect. 4.11). Upon reception of a trigger, a BUSY signal that is connected to the SRDTRG board is set, inhibiting further scintillator triggers until it is reset by the control software. Since the sampling operates in stop mode, the signatures of charged particles passing through the crystals and generating scintillator triggers will be recorded. This gives the possibility of using this data to monitor changes in the YAPjphotomultiplier combination through the pulse-height distribution of such events, independent of the LED system. Since this system was designed for the purpose of functional verification, not calibration, it is not very stable against temperature or other environmental changes. 4.7.2 SRDAPV The two APV chips of one silicon macrostrip detector cassette are connected to one SRDAPV board, such that side A of the PSRD controls the top detector, side B the bottom one. The APV operational parameters are set through the SRDAPV board over the 12 C bus.9 This includes, apart from general settings affecting the signal shaping, specifically the correct latency value, such that the 18 time slices of interest are retrieved upon a trigger. A trigger from the SRDSAB+TRG board is routed to the XILINX which then initiates the read-out of the APV. The multiplexed data from one APV is digitised by one FADC at 20 MHz sampling frequency and stored in an 8 kByte memory. One event has (128 + 12) x 18 = 2520 9This is a bi-directional 2-wire bus for communication between integrated circuits developed by Philips Semi conductors. 4.7. ELECTRONICS 65 Figure 4.23 Simplified connection diagram of the PSRD electronics Byte (18 time slices, 128 channels and 12 Byte header), but 3200 Byte are read, since some idle data preceding the event data is included. In idle mode, the APV outputs a regular sequence of levels corresponding to digital zero and one, so the idle data is used to determine these levels such that the digital header can be extracted from the analog data sequence. Upon read-out by the XILINX, a flag is set in the SRDAPV registers to inform the control program that data is available. A BUSY signal is set and routed to the SRDTRG board, in hibiting further triggers until the read-out is complete. The data in the memory is eventually read out by the program and stored on hard disk. 4.7.3 SRDSAB+TRG Scaler section The two outputs of the large YAP photomultipliers and the signals from the plastic veto counters (cf. Sect. 4.4.2) are fed to the SRDSAB section of this board. The already attenuated signals are again split into two with a ratio of 1:3 on this board, so that in total three signals with an amplitude ratio of 1:3:9 are available from each large YAP crystal. All signals are then fed to adjustable discriminators, with the thresholds being calibrated to correspond to 10keV, 40keV and 120 keV photon energy. The three discriminator outputs of one large YAP crystals are 66 CHAPTER 4. THE PROTOTYPE SYNCHROTRON RADIATION DETECTOR Scintillator Trigger Dead time per event due to computer read-out of detectors, data packing and storage approx. 300 ms 1200 ns Small YAP Array 8I92x50 ns = 409.6 us I Trigger marks stop of sampling I -I Trigger delay 1200 ns .-.__300 ns_ . MACRO I 18x25 ns = 450 ns I Trigger marks first read-out channel I -I Trigger delay 1200 ns, latency 1500 ns 1.-611s Large YAP Asynchronous of trigger, data stored to disk approx. every 30 sec I I Open-circuit (100 kn) voltage, short-t:ircuit (200 mn) current and temperature measuredlonce per second Solar Cells I Asynchronous of trigger I Temperatures and pressures measurctd once per sec, HV parameters every 5 min I Slow Control Asynchronous of trigger LED Verification 20s I Asynchronous of trigger ---" Initiated at start-up, every 30 min l and by reception of pulse on BLCMD4. Figure 4.24 Schematic of the read-out timing (timing not to scale) connected to 16-bit scalers with the plastic veto counter acting as an inhibit. Additionally, the logic AND of the veto counter and the 10 keV-threshold signal is fed to another scaler. This logic, illustrated in Fig. 4.25, effectively counts photon events above the three thresholds and, separately, charged particle events that pass through the veto counter and deposit a least 10 keV energy in the YAP crystal. The scalers are gated such that they count events during 10 ms, then their contents are transferred to a 64 kByte memory (organised as 32k x 2 Byte) and they are cleared and restarted, so the photon and charged particle rates are measured with 10 ms resolution. The dead time between gates is 1.611S. The memories would be filled in 41 s, but are read out by the control program approximately every 30 s. Trigger section Both sides of the PSRD receive the outputs of all four trigger photomultipliers in the trigger cassette (for redundancy reasons, see Sect. 4.6). The signals are connected to discriminators with adjustable thresholds and then a logic OR of the two signals from the top and of the two signals from the bottom scintillator is generated, then a logic AND between these two. Effectively, a 4.7. ELECTRONICS 67 Veto PMT SRDSAB Figure 4.25 Principle design of the SRDSAB logic (D=Discriminator, S=Scaler) trigger signal is routed to the SRDYAP and SRDAPV boards when at least one top and one bottom photomultiplier give out a coincident signal. Trigger generation is normally inhibited if any BUSY signal is present (from SRDYAP or SRDAPV). A trigger or BUSY is also accepted from external inputs, while both generated trigger and actual BUSY are also given out to external connectors for testing purposes. All BUSY and trigger sources can be individually disabled via software. 4.7.4 SRDSOL Each of the solar cell assemblies on the X-ray cassette contains four individual cells. The short circuit current (over 200 mn) and open-circuit voltage (over 100 kn) are measured once per second for all cells, together with the output of one calibrated reference photodiode and one PT 1000 thermistor per side of PSRD. The voltages are sequentially switched to one 12-bit ADC and the digital data is stored as two-byte words in a 16 kByte memory. The memory pointer is increased by 64 Byte for every sample sequence, so the memory is filled in 4.3 minutes, though the control program writes the data every 60 seconds to disk. 4.7.5 SRDHVC+SLO High-voltage control section The LeCroy MHV100 chips that control the high-voltage generation (Sect. 4.4.1) are connected in one chain to the high-voltage control section of this board via two differential lines. One transmits a 100 kHz clock, the other bidirectional data (32 bits transmitted to the chip, then 32 bits received from the chip in one sequence). Individual chips are selected by an 8-bit unique address. The clock and data line levels are set and cleared manually with the correct timing by the control program itself via one register on this board. The settable parameters include the output voltages, and the overcurrent and overvoltage limits. The actual output voltages, currents and various status information can be read back. Temperature and pressure monitoring section A sequencer on the board reads all 14 PT-lOOO temperature sensors and the two pressure sensors once per second, converts the values to lO-bit digital data and stores these in a 2 kByte memory. The memory is read every 60 seconds by the control program that stores the data to hard disks 68 CHAPTER 4. THE PROTOTYPE SYNCHROTRON RADIATION DETECTOR and checks if disk temperatures and pressures are within operation limits. If not, it will switch the disks off until conditions improve. Hitchhiker interface section The basic up- and downlink lines that are offered by the Hitchhiker avionics as described in Sect. 4.10 are interfaced to this board. Due to specific grounding requirements of NASA, all lines are isolated by optocouplers from the PSRD electronics. Via one register the control program can check the actual levels on the bilevellines and also if a pulse was seen since the last reading (the hardware recognises level changes with the correct sequence and timing as a pulse). The output level of the analog data line can be set to eight different values by connecting a variable number of diodes between the +28 V supply voltage and ground. The two thermistor lines that are used for data transmission can be connected to an array of eight parallel, individually switchable resistors with values 5.1 kO, 10 kO, 20 kO, 39 kO, 82 kO, 160 kO, 330 kO and 680 kO. An eight-bit number written to the appropriate register of the board will switch these resistors following the binary pattern of the number, with a zero resulting in 1MO. 4.7.6 SRDPWR The PSRD electronics are supplied with voltages of +5 V, -5 V and +45 V by an array ofDCjDC converters that are located on the SRDPWR board. Two converters are always used in parallel and connected together over diodes for redundancy. The converters are fed with power from the shuttle at +28 V through an external filterbox that provides filtering against conducted electro magnetic interference and one easily accessible fusebox per side of PSRD where individual power lines are fused. External power for testing purposes can be fed into the fusebox. Overvoltage protection is provided on the SRDPWR board. The total power consumption of the PSRD is about 140 W, the overall conversion efficiency 60% (including losses in the fuses and filters). The filterbox is the only component (apart from the trigger photomultipliers) that is common to both sides of the PSRD, being the single point-of-contact with the power and signal lines coming from the Hitchhiker avionics. The signal lines are routed directly and separately for both sides to the SRDHVC+SLO boards. The filterbox also contains a connection to a switch located in the Space Shuttle crew compartment that allows the astronauts to interrupt power to the experiment in case this proves impossible by ground commanding because of, for example, a link loss. 4.7.7 SRDPOW+LED The SRDPOW board performs controlling and monitoring of the circuitry on the SRDPWR. The control program can check if the generated voltages are above a minimum value and switch the supply voltages for the hard disk drives (Sect. 4.9) and for the high-voltage generators. The electronics used to control the LED system is located in the SRDLED section of this board. The pulse duration and intensity of individual LEDs can be adjusted and they can be pulsed one-shot or at 300 Hz frequency. The latter will be done for 20 seconds every 30 minutes by the control program and on request by ground command to allow off-line checks of proper detector functioning. 4.8. PC/104 COMPUTERS 69 Hitchhiker: PSRD Side AIB Side +28 V shuttle ower PSRD ower PSRD ower +28 V return PSRD si nal ound PSRD si nallines PSRD si nal round Frame round PSRD ower HH si nallines PSRD si nallines HH si nal round PSRD si nal round Figure 4.26 Simplified grounding scheme of the PSRD components 4.7.8 Grounding requirements The ground isolation requirements for the signal lines have been mentioned above in Sect. 4.7.5. Furthermore, all cassettes and conducting mechanical structures are connected to a specific frameground. Though this connection is normally provided through these structures themselves, an explicit ground connection cable, called fault bond wire, is required to ensure proper ground ing even in case conduction over the structure is interrupted. All electric grounds within the PSRD are connected to signal ground, this in turn being connected to frame ground only at a single point within the SRDPWR for each side of the detector. The return of the 28 V shuttle power is not connected to any ground within the PSRD, the isolation to frame ground provided by the DC/DC converters exceeding 10 MO. Specific minimum resistances are required by NASA between all ground, signal and power lines, as laid out in the interface control document ICD-2-19001. The grounding principles are sketched in Fig. 4.26. 4.8 PC/104 computers The experiment is controlled by one PC/104 computer per side. This is a standard 80x486 PC running at 133 MHz, but in a compact format as seen in Fig. 4.27. It consists of three modules interconnected over the ISA bus by an 104 pin connector (hence the computer's name) that is located inside the stack. All modules are designed to work within the extended industry standard temperature range between -40 cC and +85 cC, and use a total of 8 W power at +5 V. CPU module The CPU module, type CoreModule 4DXe made by Ampro Computers, contains, apart from the CPU itself, 16 MByte of memory and almost all ports and interfaces conventionally used: parallel, two serial, floppy disk drive, IDE hard disk and keyboard. A solid-state flash disk with 8MByte capacity, type DiskOnChip Millennium made by M-Systems, is also mounted on the module. The flight control software is stored on this disk and the computer boots from it if no hard disk is switched on. The CPU and the Chipset are thermally connected via copper cooling bars to the frame of the CPU module to conduct away the heat. The CPU is the hottest part of the PSRD and would otherwise easily overheat in vacuum. Two PT-lOOO temperature sensors and the Hitchhiker 70 CHAPTER 4. THE PROTOTYPE SYNCHROTRON RADIATION DETECTOR 168 mm ~ 130mm Lithium battery Secondary ballery ~__--- container ~~------ 152mm Chemizorb powder Araldite Battery PeB (a) Module stack (b) Battery container Figure 4.27 PC/I04 computer stack. One frame is empty for cabling reasons. 22kQ 3.6V Schottky diodes ~_I- Figure 4.28 Battery protection circuit inside the battery container thermistor (see Sect. 4.10) are glued to these cooling bars close to the CPU to provide redundant readings of this important temperature. A 3.6 V lithium/thionyl chloride battery, type SL-340 made by Sonnenschein Lithium, is used to power the real-time clock of the computer when no external power is applied. The battery is encapsulated several times as shown in Fig.4.27(b) following NASA safety requirements: the inner container is filled with Chemizorb powder, made by Merck, to absorb any leaking fluids, the outer container with Araldite glue (applied under vacuum to avoid air bubbles). A small printed-circuit board contains the battery itself and series resistors and diodes to prevent excessive currents and reverse-charging, as shown in Fig. 4.28. Reverse-charging is limited to the leakage current of the Schottky diodes (below IpA at 5 V and +75 QC) or, in case of complete failure of them, to 0.45 mA at 5 V by the parallel combination of the 22 kO resistors. Both values are well below the charging limit of 10 mA given on the safety data sheet by the manufacturer. To prove safe operation experimentally, the short-circuit current of the battery was measured without any protection circuit over an extended period of time. The current decayed to small values as shown in Fig. 4.29 without any mechanical change, e. g. deformations, visible on the battery. VGA module The VGA graphics card, type CMllOHR by Real Time Devices, supports standard SVGA graphic resolutions. The VGA chip on this card has a copper cooling bar attached like the CPU module. ISA bus extender module This card allows connection of the external electronic boards to the ISA bus of the PC/I04 using up to 2 m of cable. It detects access to addresses Ox300 to Ox37e on the I/O bus and routes the lowest 6 bits of these addresses and the corresponding 16 bits of data to the external boards. 4.9. HARD DISKS 71 "E ~ o::J 10 10 o 2.5 5 7.5 10 12.5 15 17.5 Time (hours) Figure 4.29 Short-circuit current of SL-340 battery 4.9 Hard disks Two 25 GByte, 2.5-inch IDE hard disks, type Travelstar 25GS made by IBM Corporation, are used per side of PSRD. Their environmental requirements for operation are +5 DC to +55 DC and an absolute pressure between 0.7 atm and 1.05 atm. A humidity between 8% and 90% is also required. Since they are designed for computer laptops, they can sustain rather high shock and vibration levels during operation, even higher levels when non-operating. One hard disk consumes on average 6 W at +5 V. The disks are mounted in a pressurised container as shown in Fig. 4.30, filled with nitrogen of 14% humidity at 21 DC.lO The container was filled at atmospheric pressure (about 1 atm) , so, as clear from Fig. 4.31 where the variation of pressure with temperature following the ideal gas law is shown, the pressure will rise above the data-sheet limit at temperatures that themselves are still within limits. After consultation with the manufacturer it became clear that no mechanical damage is expected at higher pressure but, possibly, a higher read/write error rate due to an increased distance between the read/write head of the drive and the magnetic disc (this distance is generated by a gas cushion between head and spinning disc, so it is dependent on the pressure). However, as also indicated in Fig. 4.31, no such errors occurred during a test even at 1.17 atm and 35 DC in 36 GByte of data. Since, in addition, individual rare bit errors in the science data would be insignificant, this exceeding of the pressure limit was deemed acceptable. Looking at the excerpt from a Mollier diagram for air in Fig. 4.32 (also usable for nitrogen with small error) that shows the variation ofrelative humidity ep with temperature, 14% humidity at 21 DC will give below 5% above 40 DC. However, it was judged more important to stay well away from condensation at the lowest operation temperature, thus determining the choice of humidity. It should be noted that the diagram is valid strictly only at constant pressure, since the relative lONitrogen was bubbled through water and mixed with dry gas to achieve the required humidity. The humidity was measured at the outlet of the two gas connections used for flushing. 72 CHAPTER 4. THE PROTOTYPE SYNCHROTRON RADIATION DETECTOR Fans Connector for power and data lines Mounting Bracket (a) Pressurised container (b) Inner lay-out Figure 4.30 Design of the disk container holding two 25 GByte hard disks E TC·C) :§. 1.2 -!--'-H~-di~-k-4~t;;d'~'t-T'i7-~-t~'i"~5-tkg'C- -"--:-'-"-i-"-"--'- 40 t=-~;:'--::;7-"-T---:;"~-T--"';':::; ;::---='r-:~'=-- i!' • No read/write error in 36 GBy~ data :J ~ 1.15 ---t--- : Cl. 1.1 D~~~~~Ft : i , : i 1.05 li~it--.,---:------t------:------~--~--- ·····H~,~t..~t-tl 10 0.95 : : ~ , . o ; , 09 : Hard diSk·~~;;;;rature range: ------~ - +--- +5 deg C 10 +55 deg C: ·10 0.85 L-~0'-JL->~---l20~~~-4-"-O~~-'--6LO~--.J Temperature (deg Cl Figure 4.31 Variation of pressure Figure 4.32 Mollier diagram for aIr at 1 atm with temperature in the hard disk (adapted from [Sta98, p. 683]) container 4.9. HARD DISKS 73 humidity changes proportional to the pressure at constant temperature. The sensitivity of the relative humidity to temperature changes is much stronger, though, than to the accompanying, relatively small changes in pressure. Two fans inside the container provide adequate air flow for cooling of the disks in the convection-free zero gravity environment. Thermostats with setpoints of + 15°C (switch on) and +30°C (switch off), together with a 5.3 W heater, keep the disks from becoming too cold. Two temperature and two pressure sensors are monitored by the control program that will switch the disks off in the case of violation of any limit. Due to a BIOS limitation of the PC/104 computers, hard disks above 8 GByte are not accessible via the standard operation system routines. To make the full disk space available, a disk-manager software called EZ-Drivell is used that installs some code in the master boot record of the hard drive to circumvent this limitation. This works only, however, if the computer is actually booted from that hard disk. Therefore, both IDE hard disks that are used per side of PSRD are jumpered as master and only one is powered at any time. The disks are divided into partitions of at most 2 GByte. Details on the boot operations are given in Sect. 4.11. The official specifications of the IDE hard disk interface do not foresee an unpowered disk being connected to the cable, like it is implemented, for example, in SCSI. Usually, either one (master) or two (master and slave) hard disks are connected to the IDE cable, always powered when the controller is powered. Initially, problems arose with the concept of two master disks: the hard disk was sometimes not recognised by the operating system during the boot process, or even permanent boot failures occurred. The problem was traced to parasitic powering of the nominally unpowered disk through the (always connected) signal lines and insufficient grounding of the power lines of the switched off disk - the DC/DC converters, when switched off, ground their output lines only over a significant resistance, unlike standard laboratory power supplies. With a 120 n resistor put permanently between power and ground lines of all disks, this problem was solved, at the additional but insignificant expense of 0.2 W of power for a switched on disk. It was found that the malfunctions occurred again if the resistance was increased. In that case, the voltage spikes that appear on the power lines of the switched-off disk when the other disk is operating started to reach beyond 800 mV. This supports the explanation of parasitic powering over protection diodes in the input circuitry of the disks (as this is about the voltage drop over a silicon diode) and subsequent bus confusion through erratic or undefined states. However, the disk-manager software still crashed when trying to repartition a hard disk due to an unknown reason. Only proper grounding of the unpowered disk over essentially 0 n solved this problem. Since repartitioning would be a rare operation, only needed in case of a serious disk problem, and since all other programs worked, a special connector was installed on the flight patch-panel (see Sect. 4.13) as a work-around, allowing explicit, manual grounding over an external switch if necessary. The hard disks are planned to be removed from their container and to be connected to an external computer for downloading the data after flight. Rotational energy 12 According to NASA requirements , rotating mechanical assemblies with stored energies above 19310 J are termed fracture critical and need special safety verification procedures. As shown llDistributed free of charge by Western Digital Corporation. Foreseen for hard disks by this manufacturer, it can also be used for other brands. A full-fledged, commercial version of the software is available from StorageSoft Solutions. 12Document NASA-STD-5003, Fracture Control Requirements for Payloads using the Space Shuttle 74 CHAPTER 4. THE PROTOTYPE SYNCHROTRON RADIATION DETECTOR Advanced Carrier Customer Customer Ground Equipment Support System Support Equipment Figure 4.33 Sketch of the data link between PSRD and ground below, both fans and hard disks in the PSRD stay well below that limit. The rotational kinetic energy E is where I is the moment of inertia and n the angular velocity. For a point mass m at a distance r 2 from the rotation axis, I = mr . For a rotation speed of x revolutions per minute, n = 27rx/60, so 2 E= --mr (27r-x )2 2 60 Performing the calculation for the fans inside the disk container and assuming that all of the fans mass of 8 g is located at the end of the fan blades at 12.5 mm distance to the axis, a conservative upper limit for the stored energy of 0.76 J follows for the rotation speed of 10500 rpm The exact weight of the actual discs inside the hard disks is not known, but it is not much larger than that of the fan. They spin at 5411 rpm and have a diameter of 2.5 inch (6.35 cm), so the stored energy is larger than compared to the fan, but only by a factor of a few, certainly also much below the limit given above. 4.10 Hitchhiker data link The Hitchhiker avionics allows some basic data link between the experiment and computers on the ground, as illustrated in Fig. 4.33. Lines to and from the PSRD are connected to the Hitchhiker avionics, electronics that controls all the payloads on the Hitchhiker cross-bay bridge. The avionics is interfaced with the Space Shuttle, and via a satellite network and ground stations data can be exchanged between it and a computer called ACCESS13 that is installed in the POCC14 . Computers are then connected to the ACCESS over RS-232 serial lines and provide an interface to send commands and display data. 13 Advanced Carrier Customer Equipment Support System 14payload Operations Control Center, located at the Goddard Space Flight Center of NASA in Washington, DC, from where the Hitchhiker payload is controlled. r------~------ 4.10. HITCHHIKER DATA LINK 75 Uplink For uplinking commands four lines, called bilevel lines or BLCMDx, are provided. One can set the level of a line to 0 V or +28 V or send a pulse that corresponds to a sequence 0 V -+ +28 V (for 50ms) -+ OV. As noted in Sect. 4.7.5, the SRDHVC+SLO board is able to distinguish between the levels and pulses, although either only pulse or level operation is used on one line in the commanding scheme used for the PSRD. The uplink is sketched in Fig. 4.34(a). • The level on BLCMDl adjusts the data taking rate. The standard rate (level low) has 22 triggers per 10 seconds, the reduced rate (level high) 10 triggers per 10 seconds (see Sect. 4.7.1 and Sect. 4.12). •A pulse on BLCMD3 announces to the PSRD an upcoming power down. Upon reception, the control program will stop writing to disk and spin the disks down. This way, an unlikely but possible corruption of the data structure of the active partition (due to, for example, an incomplete writing of the file allocation table due to the power cut) is prevented. The read/write head is parked even when power is interrupted unannounced, but it is mechanically less stressful if done so in a controlled manner.15 •A pulse on BLCMD4 will manually start an LED functional verification run, pulsing the LEDs for 20 seconds, as is also done automatically every 30 minutes by the control program. This is foreseen for off-line verification of a temporary problem that might have appeared on-line (in the downlinked data). Also, if the PSRD was prepared to power down by a pulse on BLCMD3, it can be reactivated by sending a pulse on BLCMD4. The power to the PSRD can be switched by ground command through the ACCESS system. If this fails, a switch in the crew compartment can be operated by the astronauts that will cut power in the filter box (Sect. 4.7.6). Downlink The PSRD uses three thermistor lines and one analog data line to transmit data to the laptop computers located on the ground, as shown in Fig. 4.34(b). • One thermistor line, THERl, is connected to an actual thermistor that is glued to the CPU of the PC/104 computer. This way the CPU temperature, the hottest part within the PSRD, can be read on the ground even if the experiment is not powered or in case the PSRD-specific part of the data link should fail. • The two thermistor lines THER2 and THER3 are connected to an array of switch able resistors in the SRDHVC electronic board (Sect. 4.7.5). Theoretically, 256 different resistance values can be generated through an 8-bit number written to the appropriate register of the SRDHVC and, through calibration, this number could be regenerated from the data received on ground. However, since these lines where not designed for binary data exchange, the transmission is not bit-precise and only 45 values are used for the PSRD to ensure safe transmission. • 8 different voltages between 0 V and +5 V can be switched to the analog data line PCMAD on the SRDHVC. This line is used as a "computer heartbeat" and for command acknowledgement: the voltage is increased or decreased by one step every 5 seconds and the 15In the case of a sudden power cut, the head is parked using the rotational energy of the spinning disk. This works inherently less smooth than software-controlled parking. 76 CHAPTER 4. THE PROTOTYPE SYNCHROTRON RADIATION DETECTOR BLCMD1 Level Data Taking Rate Level = 0 Standard Rate (2.2 Hz) Level = 1 Low Rate (1 Hz) BLCMD2 not used BLCMD3 Pulse Prepare for Power Off Terminate disk writing, close files, spin hard disk down BLCMD4 Pulse Start LED Functional Verification LEDs will be pulsed with 300 Hz for 20 seconds Level 0 =OV, Level 1 =+28V, Pulse =OV -> +28V (for 50 ms) -> OV ----~... +28 V Power Astronaut Controlled ACCESS Controlled (SSP 16) (a) Uplink Heartbeat Direction is reversed upon command reception on BLCMDx THERl Real thermistor measures CPU temperature Displayed on ACCESS, even if PSRD is off THER2 Thermistor lines used for downlinking data 8 bit can be transmitted through computer controlled parallel THER3 combination of 8 different resistors; (only 45 out of 256 values } -----1~ are used for safer transmission); THER2 is used as index, THER3 as data. Data items cycled every 10 seconds. (b) Downlink Figure 4.34 Details of the data link 4.11. FLIGHT CONTROL PROGRAM 77 Table 4.3 Data downlinked through the thermistor lines Index Description Range Res. 0 Calibration value: always 44 - - 1-14 Temperature sensors 1-14 (-50 - +96.7) °C 3.3°C 15-16 Pressure sensors 1, 2 (0.5 -1.5) atm 0.023 atm 17 Scintillator triggers in last interval1 0-44 1 18 Software triggers in last interval1 0-44 1 19-26 SAB count in last 10 ms bin, channels 0 - 7 0-44 1 27 Calibration value: always 27 - - 28 High-voltage status, channels 0 - 3 bitwise (1=OK) 29 High-voltage status, channels 4-7 bitwise (1=OK) 30 High-voltage status, channels 8-11 bitwise (1=OK) 31 SRDPOW power status 1:L bitwise (1=on, 0=0£1') 32 SRDPOW power status 23 bitwise (1=on, 0=0£1') 33-38 Computer real time bitwise (5 bits per item) 39 Disk power status (disk 1 & 2) bitwise (1=on, 0=0£1') 40 Disk active partition 0-12 (O=d:, 1=e:, ... )4 41 Space available on active partition 0-2.2 GByte 0.05 GByte 42 Flight program status 15 bitwise 43 Flight program status 26 bitwise 44 Calibration value: always 0 - - 1 22 s or 10 s, depending on data rate setting (BLCMD1) 4 see Table 4.4 2 ±5 V SRDYAP, -5 V SRDAPV 5 Data rate setting, LED pulser status 3 +5 V SRDAPV, +5 V PC/104, +45 V 6 Disk writing, disk full, disk environment direction is reversed upon reception of a command. Since it is expected that the computers will lock up repeatedly in space due to memory and CPU upsets triggered by the energy deposition of passing charged particles, this heartbeat provides a means for quick detection of such an incident; PSRD power will then be cycled to restart the computers. The downlinked information includes temperatures, pressures, trigger rates, high-voltage and power status, control program status, available disk space and computer real time (Table 4.3). Since, for the same reason as computer lock-ups, the real time clock might be upset (jump randomly) and since this time is the only correlation between shuttle position and the data on hard disk, it is important to follow this clock on ground. However, one downlink cycle takes 7.5 minutes, about 8% of the shuttle orbit period of 90 minutes, so the precision is limited. The positions and connections of the temperature and pressure sensors are shown in Fig. 4.35. 4.11 Flight control program The flight control program16 provides simultaneously a graphical user interface for testing pur poses and an autonomous running mode that is used during the flight. This way, the operations of the program can be monitored during ground verification without changing any parameters for the flight. The program is written in the C language and runs under the DOS core of Windows 95. Almost all operation parameters are set in a number of configuration files. The sequence of events after switching the PSRD on is as follows: 16Written by Alexei Lebedev, Massachusetts Institute of Technology, Cambridge, USA. 78 CHAPTER 4. THE PROTOTYPE SYNCHROTRON RADIATION DETECTOR back left right front SIDE SIDE A B Figure 4.35 Positions and connections of temperature and pressure sensors. The detector components are shown from top to bottom, with the parts of the X-ray cassette as seen from above. TS = PT-WOO temperature sensor, PS = pressure sensor, TH = Hitchhiker thermistor. Only one of the two PT-WOO solar cell sensors per side is actually connected. 4.11. FLIGHT CONTROL PROGRAM 79 Table 4.4 Drive letter assignment when one hard disk is on (without hard disk, the only drive c: will be the DiskOnChip) Size Size Drive letter Partition type Drive letter Partition type (MByte) (MByte) c: Primary (boot) 33 J: Logical 2147 d: DiskOnChip 8 k: Logical 2147 e: Logical 2147 1: Logical 2147 f: Logical 2147 m: Logical 2147 g: Logical 2147 n: Logical 2147 h: Logical 2147 0: Logical 2147 1: Logical 2147 p: Logical 1727 ----* After applying power to the PSRD, both hard disks are initially off. The PCj104 will boot from the DiskOnChip and start the flight control program. ----* The program will check if the disk environment parameters (temperature and pressure) are within limits and, if so, it will power the first hard disk and reboot the computer. ----* The PCj104 will then boot from the hard disk, which also installs the EZ-Drive software (see Sect. 4.9). The flight control program on the DiskOnChip will be started again. The drive letter assignment under DOS in this case is listed in Table 4.4. ----* The program will check if there is still storage space available on the disk and if it is accessible without errors. If not, it will switch to the second disk and reboot from this one. If the second disk is also full (or unusable), it will continue to work without disks, i. e. without reading out the detectors, but still servicing the data link. The power of the PSRD needs to be cycled in this case to restart the boot sequence. ----* Once an acceptable disk with sufficient free space is switched on, the control program will initialise all electronic boards, set the high-voltages and enter the normal operation loop. This loop consists of reading out data from the detectors if available and storing it to disk, lis tening to ground commands and reacting accordingly, generating the "heartbeat" and downlink data sequence, and monitoring of the disk environment, available storage space and high-voltage status. If environmental limits of a disk are exceeded, it will be switched off until the situation improves. Ifone hard disk is full or becomes inaccessible for any reason, the control program will switch to the next one. In case some high-voltage is deviating from its preset value, the program tries to reset it. It also logs all status information regularly to disk. Data is only written to the 12 large logical partitions listed in Table 4.4. Because of the high importance of the DiskOnChip and the first partition of the hard disk (the latter holding the boot code to start the control program and the BIOS extension software, the former also the control program itself), data is never written to these two partitions. This further reduces the chance of problems, for example by a corruption of the file structure due to a unforeseen power cut. If a serious last-minute problem arises with one hard disk, it can be disabled permanently by a setting in the control program's configuration files. 80 CHAPTER 4. THE PROTOTYPE SYNCHROTRON RADIATION DETECTOR 4.12 Data rates The data rates calculated in the following refer to the rate for both sides of the PSRD combined. When operating in the standard mode with 22 triggers per 10 seconds, 24·8 kByte· 2.2 S-l = 422.4kByte/s or 34.8 GByte per day are generated by the small YAP array (12 crystals, 2 ranges). In case all these triggers are scintillator triggers, additionally 4·3200 Byte· 2.2 s-l = 27.5kByte/s or 2.3 GByte per day would come from the macrostrip detector (4 APV chips). With a scaler width of 10 ms, 16·2Byte· 100 s-l = 3.1 kByte/s or 264 MByte per day come from the SRDSAB (16 scalers). The data rates from the solar cells of 128 Byte/s and from monitoring are negligible compared to those. So the raw data rate in standard mode will be approximately 37 GByte per day, while the reduced trigger rate of 10 triggers per 10 seconds produces some 17 GByte per day. A simple, non-loss compression algorithm is implemented for the small YAP data that reduces continuous sequences of equal numbers and continuous sequences of two alternating numbers. Since most of the data bytes usually have a constant value, reflecting the baseline, or slight fluctuations around it, high compression factors exceeding 10 were measured in the laboratory. Because the structure of the actual data in space is unknown, the precise efficiency of the compression cannot be predicted, but it is clear that the 100 GByte disk space is certainly sufficient for a flight of 12 days at the low trigger rate and likely also at the standard rate. Since the actual data rate written to disk will be monitored during the flight, the trigger rate can be adjusted accordingly. 4.13 Provisions for ground testing All internal connectors, especially also those for temporary connections to the PC/104 like for monitor and keyboard, are inaccessible once the various cover panels are attached. Since for testing and verification purposes it is necessary to have detailed insight into the PSRD operations beyond that offered by the data link, a number of connections are brought to the outside through two patch-panels. One flight patch-panel is attached on each side of the experiment as shown in Fig. 4.36(a). It brings the internal connections to the outside using space-qualified connectors as are also used within the PSRD and will stay in place during the flight, being then covered with a protective cap. The non-flight patch-panel of Fig. 4.36(b) is connected temporarily with cables to the flight patch-panel and attached to the experiment's mounting plate. It will be removed before flight. The non-flight patch-panel uses standard connectors for the various computer ports, LEMO connectors for the trigger and busy signals, and contains a floppy disk drive. The latter is especially important since booting from floppy disk would be the only option left to start the experiment in case the DiskOnChip fails, as the hard disks can only be turned on by the flight control program - located on the DiskOnChip. The parallel port can be used, among other things, to connect a Zip Drive, allowing the downloading of larger amounts of data from the detector (up to 250 MByte per Zip disk, which can be copied at a rate of some 11 MByte/min). Small files, for example flight software updates, can also be transferred through the serial port (transfer rate about 11 kByte/s). Since the area around the PSRD is not easily accessible once it is installed on the cross bay bridge, it is difficult to install monitors and keyboards due to their limited cable lengths. Therefore, a special extender, model LongView by Cybex17, is used that allows extension of the 17Now marketed by Avocent. 4.14. SPACE-QUALIFICATION TESTS 81 Head of mounting screw Power 3.5"' floppy External VGA disk drive trigger & busy signals PowerKeYboard~;;~f!;;~~~~~;r=:LED Reset Parallel Speaker Seria!2 Serial! PC/l04 EXl Busy OUT} connectors ExtBusyINP ExtTRGOUT ExtTRGINP (a) Flight patch-panel (b) Non-flight patch panel Figure 4.36 Design of the patch panels. The special short-circuit connector (Sect. 4.9) is not shown. monitor, keyboard and serial port connections to up to 150 m. It consists of two small boxes, one attached to the non-flight patch-panel and one at the remote location. They are interconnected by one standard, Category 5 UTP cable that carries all signals and the device operating power, so it is sufficient to connect a power supply to the remote box. Power can be fed to the PSRD over the non-flight patch-panels if stand-alone testing is required. This way, both sides of the detector can also be powered individually. The trigger input can be used to set-up an external cosmic ray trigger, also in coincidence with the internal scintillators of the PSRD. A setting in the SRDSAB+TRG board determines whether a logical AND or OR between the internal and external triggers is required. The flight control program can be started in a non-flight mode, then allowing manual ma nipulations of all detector component settings. 4.14 Space-qualification tests To qualify for space-flight, a suite of tests was performed on the PSRD according to NASA requirements. • Verification of natural frequencies and mechanical integrity Sine-sweep between 5 Hz and 2000 Hz to determine natural frequencies. They should all be above 100 Hz, otherwise more extensive testing is required.18 The lowest natural frequency of the PSRD was found to be around 200 Hz. Random vibrationl9 between 20 Hz and 2000 Hz at an rms level of 6.8 g to simulate the launch and landing environment of the Space Shuttle. The excitation spectrum and the response of the PSRD for a particular sensor location are shown in Fig. 4.37. Repetition of the sine-sweep to show that no frequency shift occurred due to, for example, loosening of fasteners during the vibration test. 18NASA document 740-SPEC-008, Hitchhiker Customer Accommodations fj Requirements Specifications 19NASA document NASA-STD-7001, Payload vibroacoustic test criteria 82 CHAPTER 4. THE PROTOTYPE SYNCHROTRON RADIATION DETECTOR 100 10.. g'lHz 0.1 0.001 + +--__-'-+--__+--+- ---,. _ 1000 20 100 Frequency (Hz) 2000 Figure 4.37 Random vibration excitation spectrum (dashed line) and response of the PSRD (Jullline). The first natural frequency at this particular acceleration sensor position is indicated. • Thermal-vacuum test to verify the thermal model of the detector The detector was mounted in a thermal-vacuum chamber upside-down on a hot/cold plate as shown in Fig. 4.38. The chamber was then evacuated to 9· 10-6 mbar and the cold-plate set to -30 cC. After the temperatures were somewhat stabilised, the detector was switched on. Due to the principle difference in the thermal environment - in space the detector will radiate heat away, while in this test heat was conducted to the thermal plate - it was not possible to get absolutely realistic results from this measurement. After being powered for 17.5 hours the hottest parts, the two PC/104 CPU chips, approached their upper temperature limit. The test was stopped at that point, but the temperature changes were already slow enough such that the relative temperature distribution within the PSRD could be inferred with sufficient detail. • Electromagnetic compatibility test to determine radiated and conducted emissions2o The detector was installed in a special test room as shown in Fig. 4.39, where the char acteristic cones to prevent reflections of radiation and one large antenna are visible. The conducted emission through the power lines between 30 Hz and 50 MHz was measured, as well as the radiated emission between 14 kHz and 10 GHz. A few exceedings of limits were found around 2 GHz, where the requirements are especially strict because Space Shuttle communication frequencies lie in this region. The exceedings were later deemed acceptable by NASA. Plots of the measured emission in this and in a lower frequency range are shown in Fig. 4.40, together with the NASA limits. 2°following NASA document ICD-2-1900l, revision L, Shuttle arbiter/cargo standard interfaces 4.14. SPACE-QUALIFICATION TESTS 83 Aluminium bars Thermal " " ~>,,~ (hot/cold) l . J plate ,.... ~/'~ ~~~ (a) Mounting to thermal plate (b) Photograph with external tempera ture sensors attached Figure 4.38 PSRD thermal-vacuum test set-up Figure 4.39 PSRD during the EMC test 84 CHAPTER 4. THE PROTOTYPE SYNCHROTRON RADIATION DETECTOR -10 .. 20L..J.-...I....J....l...l..-'_...I...J 14 k1lz 1 GBz (a) 14 kHz-1 GHz 130'r---,- 120,j--4- 110 100'I----1---} 90'1-'---1--+--1---+--+-..-4 lCj--4---+--I---!--+---+---+--l--+----j~_+--_i---~ c'-----'_--'-_...... _..l-_i...-.....l._-J.._..J-_..l.---I---'--l._-l.._..J-J 1." GHz 2.37 GEz (b) 1.7GHz-2.37GHz Figure 4.40 Emission curves measured in the EMC test with the NASA limits indicated. Vertical scale is in dBllVim. The lower curve in (a) is an outdated limit that was erroneously used in the beginning and that sparked a significant effort to improve the shielding. 4.14. SPACE-QUALIFICATION TESTS 85 • Acoustic test of the beryllium and TOR-LM windows21 The thin windows were tested at an overall sound pressure level of 141 dB and frequencies between 31.5 Hz and 2500 Hz for survival of the lift-off noise. The sine-sweep and vibration tests were done at Contraves Space, Zurich, the thermal-vacuum test at the Max Planck Institute for Extraterrestrial Physics22 in Munich, the electromagnetic compatibility test at the EMC-Testcenter, Zurich, and the acoustic test at the National Space Program Office23 (NSPO) in Taiwan. Leak rate estimation of disk container From the results of the thermal-vacuum test, namely from the vacuum chamber pressure curve, a sensitive estimation of the maximum leak rate of the disk container can be made, using the fact that even a small leakage would have affected the pressure in the vacuum chamber significantly. It is assumed for the purpose of the calculation of the limit, that the vacuum chamber pressure would have stayed constant at a value Pc uniquely due to leakage from the two disk containers while pumping with a constant volume flow dVp/dt. With the disk container volume Vd, the equation dPd(t) . V _. dVp dt d - Pc dt holds for the disk container pressure Pd at constant temperature because of conservation of particle number. The depressurization (leak rate) of the disk container is then (4.1) The volume flow of the vacuum pump is not known directly but can be calculated from the depressurization curve of the empty vacuum chamber. Using the ideal gas law, one finds for constant chamber volume Vc and temperature Tc and with ne being the number of mols of particles dPc(t) . V = dnc(t) . kN T.. (4.2) dt c dt A c Particles are removed from the chamber by the pump according to nc(t) . dVp (4.3) Vc dt . Putting (4.2) and (4.3) together, the (possibly time dependent) volume flow of the vacuum chamber pump is (4.4) 21 see footnote 19 above 22The support of Prof. Dr. J. Triimper, K. Dittrich and F. Heuschmann is gratefully acknowledged. 23With kind support from Shih-Chang Lee of the Academia Sinica, Taipei. 86 CHAPTER 4. THE PROTOTYPE SYNCHROTRON RADIATION DETECTOR At 1.2.10-5 mbar, the pressure curve of the empty vacuum chamber had a slope of ap 3 proximately 9.2· 10-8 mbarImino With the chamber volume of Vc ~ 0.67 m , a volume flow of dVp/dt ~ 5.11/min follows from (4.4). The combined volume of the two disk containers is Vd ~ 2 . 4.51, so the estimated upper limit for the leak rate from (4.1) is dpd(t) -6 . ~ ;S 6.8·10 mbar/mm = 0.01 mbar/day. This number is a conservative estimate: mass spectroscopy showed that the residual gas in the chamber with the PSRD installed was almost exclusively water evaporating from the detector surfaces and not nitrogen (the filling of the disk containers) and the vacuum chamber pressure was not constant at 1.2· 10-5 mbar, but was in fact dropping further. The disk container is therefore certainly tight enough for a stay in space vacuum during a 12 days flights. CHAPTER 5 Experimental Studies for PSRD and SRD 5.1 Time resolution A good time resolution is, as explained in Sect. 3.1.3, crucial to suppress sufficiently the diffuse photon and particle backgrounds. The YAP crystal is intrinsically a fast scintillator with a primary decay time around 30 ns, but the achievable time resolution also depends on the number of photoelectrons generated by a synchrotron photon hit. With a simple model for the expected time resolution and a few measurements, some insight into the timing aspects of the SRD shall be given here. The actual timing properties of the SRD will be more complicated, as the foreseen read-out employs a pulse-fitting method, not a simple threshold as discussed here, and statistical variations of, for example, the photoelectron number will occur. All these can be taken properly into account only by a Monte-Carlo approach. 5.1.1 Simplified model for the expected time resolution The timing precision of the correlation between the leading high-energy charged particle trigger and a synchrotron photon hit ~ the photons arriving coincident with the charged particle to within 1 ps ~ depends to a large extend on the number of photoelectrons generated by the synchrotron photon. Clearly, the probability for the first photoelectron to arrive early increases with their total number. For the SRD, it is planned to use a pulse fitting procedure to determine the arrival time of the synchrotron photon. This method will also depend on the number of photoelectrons, working best if this number is large and, consequently, the pulse shape is smooth. In the following a simpler case is treated analytically, namely the expected probability dis tribution of the time between the charged particle start trigger and the stop trigger that is generated when the nth photoelectron is created on the photocathode. Effects of photomulti plier rise time variations, electronic shaping, etc. are neglected in this simple approach. The total number of photoelectrons generated by the synchrotron photon is called N and is assumed to be fix, with no statistical variation taken into account. The excited electrons in the scintillator crystal lattice each decay according to an exponential distribution with time constant T under emission of an optical photon to their ground state and, determined by the quantum efficiency, collection efficiency and others, release photoelectrons. The infinitesimal probability dP(t) that a particular photoelectron is released between times t and t + dt is dP(t) = ~e-t/T dt. (5.1) T 87 ------ 88 CHAPTER 5. EXPERIMENTAL STUDIES FOR PSRD AND SRD ~ 0.3 ~ 0.3 "w ·w c c ~ 0.25 ~ 0.25 ~ 0=1 ~ 15 2 0.2 ~ 0.2 e e Cl.. Cl.. 0.15 0.15 0.1 0.1 0.05 0.05 o 5 10 15 20 25 30 o 5 10 15 25 30 Time (ns) Time (ns) (a) N = 22 (5.4keV), T = 32ns (b) N = 10 (2.5keV), T = 32ns Figure 5.1 Time probability distributions for different thresholds, following (5.2) The probability that this particular photoelectron is released any time after time t is, following t T t T integration of this expression, e- / , so the probability to be released before this time is 1-e- / • The combined probability that n - 1 photoelectrons come before t, one between t and t + dt and the remaining N - n after time t is This accounts only for one particular decay pattern of the excited electrons in the crystal. The missing combinatorial factor is N . (~~n, which accounts for the N possibilities to choose a photoelectron that should come between t and t + dt and the possibilities to choose the n - 1 photoelectrons from the remaining N - 1 that should come before t. The complete formula for the probability distribution dPn(t)/dt that the measured time of arrival ofthe nth photoelectron is between t and t + dt is, simplifying the above expressions slightly, dPn(t) --- ~ (~~:) (e_t/T)N-n+l (1- e-t/T)n-l dt 1 N! ( _t/T)N-n+l (1 _t/T)n-l (5.2) T (N - n)! (n _ I)! e - e oo The proof of the correct normalisation of this expression, Jo dPn(t) / dt dt = 1, is given below. This function is plotted in Fig. 5.1 for a number of parameters. Fig.5.1(a) corresponds ap proximately to a 5.4 keV photon in a YAP crystal (assuming 20 photons/keV, combined quantum and collection efficiency 20%), Fig. 5.1(b) to a 2.5keV photon, the low energy limit for the SRD. Clearly, triggering on the first photoelectron is most advantageous for the timing precision, as (5.2) becomes in that case (n = 1) dP1(t) = N (e-t/T)N. (5.3) dt T The integrated probabilities within the first 10 ns for the cases N = 1 and N = 2 and for T = 32 ns are 27% and 46%. These are the chances to have a trigger in a 10 ns window for the best time resolution (first-electron trigger) and the smallest possible number of photoelectrons (1 or 2). In Fig. 5.2, (5.3) is plotted for four values of N. 5.1. TIME RESOLUTION 89 ~0.18 '(jj a5 0.16 -0 ~0.14 j 0.12 £ 0.1 0.08 0.06 0.04 0.02 [==~=~:::::~~~;;;;~ o 5 10 15 20 25 30 Time (ns) Figure 5.2 Time probability distributions for first-electron timing, following (5.3) A similar expression as (5.2) has been stated in [Kuch68] in the context of pulse shape discrimination between neutrons and photons. Proof of correct normalisation The functional dependence of (5.2) on t is clear. To show that the combinatorial factors are chosen correctly, the proof that the equation integrates correctly to one is given here. 00 1 )N-n+l ( N! J(-Tt/ 1 - t/T )n-l dt :;:: (N - n)! (n _ I)! e - e o o 1 N! -7 N-n+l ( n-l t T J (with the substitution x e- / ) :;:: (N _ n)! (n _ I)! ---;-x 1 - x) dx = 1 1 N! N-n ( )n-l (N-n)!(n-l)! Jx I-x dx. o The last integral can be identified with the Beta function B (a, (3) (Euler's integral of the first kind, see for example [Bron91]), defined by 1 Q 1 1 B(a, (3) = Jx - (1 - x)f3- dx. o It is related to the Gamma function by B( (3) = r(a) r((3) a, r(a+,B) , and since for positive, integral arguments r(n) = (n - I)!, it follows N! I = (N _ n)! (n _ I)! B(N - n + 1, n) = 1. 90 CHAPTER 5. EXPERIMENTAL STUDIES FOR PSRD AND SRD lower upper threshold 5.4 keY I I I I I I5.4 keY (25 %) 54Mn ~ 1835 keY (lOO %) I threshold Figure 5.3 Schematic set-up for time resolution measurements 5.1.2 Measured time resolution The time resolution was measured with the coincidence set-up shown schematically in Fig. 5.3. The 54Mn radioactive source emits with a certain probability two photons in coincidence, one with 5.4 keY and one with 835 keY energy (with no angular correlation). Using the high-energy photon and a fast CsI scintillator to generate a precise start signal with very little time jitter, it is possible to measure the distribution of the arrival times of the stop trigger with a given discriminator threshold. Since numerous other photons with varying energies are also emitted by the source in co incidence, trigger thresholds were set for both crystals so to select only 835 keV and 5.4 keV coincidences. The trigger threshold for the CsI was set just below the 835 keV photo peak, and the upper (veto) threshold for the YAP was set such that the broad 5.4 keV line was fully accepted. The lower (trigger) threshold was set to about 3 photoelectrons. The measured time distributions are shown in Fig. 5.4. In (a) a fit following (5.2) with parameters N = 18, n = 3 and T = 32 ns is included, showing acceptable agreement with the simple model, albeit a somewhat wider base of the peak. The YAP scintillation indeed has a weak component with a long decay time (Table 3.5), possibly explaining this observation. Clearly, the time distribution is narrower without the upper veto, as expected since photons with larger energies are also allowed to coincide, resulting in a better time resolution. How ever, since the distribution of these photon energies is not known, this can not be compared quantitatively with the calculations in the previous section. 5.2 Light yield of different YAP(Ce) crystals With the intention of maximizing the light output of the YAP(Ce) scintillating crystals, several specimens differing in surface finish and size were compared. To this end, charge spectra were taken with 55Fe (5.9 keY) and l09Cd (22.6 keY) radioactive sources, using identical electronic 5.2. LIGHT YIELD OF DIFFERENT YAP(CE) CRYSTALS 91 (/) Q) "-E: 250 w Count rate 2 Hz 400 200 300 150 50 140 150 160 180 190 200 Q30 140 150 160 170 180 190 200 Time (ns) Time (ns) (a) with upper veto (b) without upper veto Figure 5.4 Measured time distributions. In (a) a fit following (5.2) with N = 18, n = 3 and T = 32 ns is superimposed. Table 5.1 Relative light yield of different YAP(Ce) crystals (wrapped with 6 layers of Teflon tape). Coupling to the photomultiplier was through a polished face. The measured values, cor rected by the inherent light yield given by the manufacturer, are listed in the last column. Relative light yield Crystal size and surface finish measured manufacturer corrected 18x18x1mm'S, fine-grinded on 5 faces, 1 face polished 100% 100 % 100% 3 18 x 18 xl mm , 2 large faces polished, fine-grinded otherwise (77±5)% 92% 84% 18 x 18 xl mm3 , all faces polished (68±5)% 90% 76% 3 9 x 9 x 1mm , all faces polished (122±5)% 110% 111% settings and the same R5900U photomultiplier at constant voltage in all cases. The pedestal corrected peak positions with the different crystals are then proportional, to first order, to the light yield. A LeCroy 2249A charge-integrating ADC with a gate of 200 ns was used. The results, normalised to the light yield for the crystal of 18 x 18 xl mm3 that is fine-grinded on 5 faces, are collected in Table 5.1. Polished means the respective surface is polished to optical flatness, jine-grinded that it is grinded by the manufacturer using boron carbide powder to a surface roughness Ra of (2-3) !lm. Also listed in the table is the relative light yield as given by the manufacturer for each crystal (measured against some internal standard). The advantage of fine-grinded over polished crystals is clear. An increase in light yield, albeit much less pronounced with improvements only between 5% and 8%, was also achieved by roughening polished crystals manually using sand paper. The likely explanation for this is the reduction of light trapped inside the crystal due to the random reflection direction at a rough surface (cf. the results from a light propagation simulation in Appendix B). The relatively higher light yield of the small crystal can be partly attributed to the inherent characteristic of that particular crystal as indicated by the manufacturer's figure for the light yield, but also to the reduction of the photocathode response of the R5900U towards the edges 92 CHAPTER 5. EXPERIMENTAL STUDIES FOR PSRD AND SRD of the larger crystal, as apparent from Fig. 3.12. The manufacturer does not measure the relative light light for every crystal individually, but for one crystal 010 mmX 1 mm, fine-grinded on five sides, per ingot. This explains the differences 3 between the measured and quoted values in Table 5.1 for the three crystals of 18 x 18 xl mm . 5.3 X-ray absorption of wrapping materials The absorption of X-ray photons in various materials can be computed easily with data pub lished by the Center for X-Ray Optics at the Lawrence Berkeley National Laboratory (see http://cindy.lbl.gov/optical_constants/filter2 .html). However, the chemical formula and density are needed, which are not easily obtained for several wrapping materials that are of interest due to their good reflection qualities for YAP scintillation light. The absorption at 5.9 keY was therefore measured for the following materials: • 80 pm thick Teflon tape, bought from the Swiss company Angst&Pfister; also offered by Crismatec/Bicron under the name BC-642 • 25.4pm thick TOR-LM foil, produced by Triton Systems • 180pm thick Tyvec paper, produced by DuPont The absorption was measured using a R5900U photomultiplier and a YAP scintillator. An 55Fe radioactive source emitting 5.9 keY photons, collimated to a spot size of 2 mm diameter, was placed centrally on top of the scintillator with a variable number of layers of the wrapping material in between. The counting rate was measured with a threshold just below the 55Fe peak. An exponential plus a constant was fitted to the resulting rate-versus-Iayer graphs as shown in Fig. 5.5. From these, an absorption per layer of 19% for Teflon, 13% for TOR-LM and 8% for Tyvec is deduced. The X2 values are too high because of variations beyond Poisson statistics (only this is taken into account for the error of individual data points) when adding extra layers of material. The positioning of the source is not identical every time, but the fit itself is still visibly good. Since the chemical formula of Teflon, C2F4, is known, a calculation with the data from the Berkeley web site can be performed, using the measured absorption value at 5.9 keY to adjust 3 the density such as to normalise the curve. With a density of 0.85 g/cm , about 40% of the density of solid Teflon, the curve shown in Fig. 5.6 results. It is clear that the transmission at the low end is insufficient for the purposes of the SRD, even more so because at least two layers of Teflon tape are required for good scintillation light reflection. Assuming a similar behaviour of the equivalent curves for the two other wrapping materials, the transmission also of these would not be adequate, even though one layer would be enough. Therefore, the likely candidate as a sun-shielding window for the SRD is a beryllium window with a thickness between 25pm and some 100 pm, as was used for the PSRD (Sect. 4.4.1 and Fig. 3.15). The efficiency as a reflector was measured with a small YAP crystal of 8.8 mm diameter and 1 mm thickness on a R5900U photomultiplier. The crystal was irradiated with 55Fe photons and the position of the peak, pedestal corrected, was taken as a measure of the light output. The results are listed in Table 5.2 (the light yields for aluminium foil and TOR-LM foil coated with aluminium are practically the same). Teflon is the best reflector, though the differences are rather small. Even without reflector, the light yield is still good, owing to the high refractive index of the scintillator, which makes it difficult for optical photons to escape into the surrounding. 5.3. X-RAY ABSORPTION OF WRAPPING MATERIALS 93 -165~61 ~ 2250 9.323 -0.1414 .El 2334. g 2000 ~ c "§12000 • :::J 0 o () 1750 () 13% 1500 10000 1250 , • 8000~ 1000 • • 750 6000 500 250 4000L 0 2 4 8 10 12 o 2 4 6 8 10 12 Layers of Teflon Layers of TOR-LM Film (a) Teflon tape, 80 pm thick (b) TOR-LM foil, 25.4 pm thick ~ 14.51 /5! 8.327 ' -O.7973E-01 ~ 4000 107:2: C • -, :::J 0 () Absorption per layer: 8% 3500 • I 3000 • 2500 • 2000 L ~~O." o 2 4 6 8 10 12 Layers of Tyvec (c) Tyvec paper, 180 pm thick Figure 5.5 Absorption measurements for different reflector materials at 5.9 keY photon energy. The fit function is ePl+P2n + P3, so the absorption per layer is 1 - e P2 . Table 5.2 Relative light yield of a YAP crystal 08.8 mmX 1 mm with different reflectors Teflon (6 layers) 100% Tyvec (95±2)% no wrapping (69±3)% Teflon (2 layers) (94±2)% Aluminium foil (91±2)% ------ 94 CHAPTER 5. EXPERIMENTAL STUDIES FOR PSRD AND SRD ~ co v---- c:i I:::: 0 'fii to / m 0 '8 1/ m I::::ro '<:!' / ~ 0 E-o C\l 0 / o l/ o 2000 4000 6000 8000 Photon Energy (eV) Figure 5.6 X-ray transmission through 80]lm of Teflon with density 0.85 g/cm3 . The density has been adjusted to give the same value at 5.9keV as the measurement shown in Fig.5.5(a). Coatings It would have been attractive from a mechanical perspective to replace the wrapping of the crys tal with a reflective coating. From the results above, aluminium seemed to be a good candidate as its reflectivity, when used as a wrapping material, is only slightly lower than that of Teflon tape. An aluminium layer of some 200 nm is already opaque to visible light, so this would have also yielded extremely high transparencies for X-rays. To investigate this, two small YAP (Ce) crystals of 9x 9xl mm3 , one polished on all sides, one fine-grinded on five sides, were coated with 230 nm aluminium on five sides. The peak positions of the 5.9 keY (55Fe) and 22.6 keY (109Cd) lines in the ADC spectrum were measured with identical electronic settings. The yield after coating was between 40% and 50% of the value with Teflon wrapping, below the result for a crystal without any wrapping. Removing the coating again from the crystal that was originally polished using sand paper and then wrapping it with Teflon gave a peak position 10% higher than before coating, consistent with the generally better light yield of rough crystals and confirming the detrimental effect of the coating. Even more severe reductions in signal yield for an aluminium coating were found for the complicated geometries of the PSRD plastic veto counters (Sect. 4.4.2). From these results, any external reflector (even none) is better than aluminium coating, so this option was not followed further. 5.4 Detection efficiency for 511 keV photons The detection efficiency for 511 keY photons of CsI(Tl) (3 cm thick) and YAP(Ce) (1 mm and 2 mm) was measured with the set-up sketched in Fig. 5.7, using a 22Na radioactive source that emits two photons back-to-back from electron/positron annihilations. The source was in the immediate vicinity of the crystal under investigation but at some distance to the CsI(Tl) crystal of dimension 1x 1x 1 cm3 that delivered the trigger. It was thus 5.4. DETECTION EFFICIENCY FOR 511 KEV PHOTONS 95 few mm distance varied 500 ns Int. two 511 keV photons back-ta-back In In Figure 5.7 Set-up for the detection efficiency measurements assured, due to the back-to-back emission, that a photon must have passed through the test crystal when a trigger was seen. R5900U photomultipliers from Hamamatsu were used, the crystals were coupled using optical grease and wrapped with several layers of Tefl.on tape. The amplifiers (Ortec 474) were set to 500 ns integration, the maximum value. Unfortunately, this is still a bit short for the long decay time of CsI(Tl) (IllS) to attain smooth signal shapes. To reduce the background, the upper and lower thresholds of the trigger photomultiplier were set tightly around the 511 keY photopeak, as apparent from the spectra with and without thresholds in Fig. 5.8. The threshold of the other photomultiplier was set to a corresponding energy of about 80 keY for the YAP(Ce) crystals, but was less well defined for the CsI(Tl). The trigger rates a and b of the upper and lower thresholds of the trigger scintillator and c and d of the coincidences of these two with signals from the crystal under investigation were determined, and the detection efficiency E calculated as d-c E=b_a' The background rate was measured by either removing the source altogether or by placing a 5 cm thick lead block between source and trigger crystal (both methods had the same effect). The four individual rates were corrected accordingly. There is further background present between the thresholds in the trigger crystal, unrelated to the 511 keY photopeak, when the 22Na source is in place, as see in Fig.5.8(a). From this spectrum, 15% of the entries between the thresholds belong to the constant background. The measured rate b - a was therefore additionally corrected by a factor 0.85 to account for this. The results for the three crystals investigated are summarised in Table 5.3. Measurements were done at various distances between the radioactive source and the trigger crystal. The larger the distance, the smaller the effect of variations in path length due to non-perpendicular impinging on the crystal under study. But this also lowered the count rate, increasing the statistical error. A first-order calculation of the path length variation, however, shows that the effect on the results is negligible within the errors. 96 CHAPTER 5. EXPERIMENTAL STUDIES FOR PSRD AND SRD VJ VJ I Q) .~ 160 r ".5 ~ ~ tij 100 u.J 140 120 80 100 60 80 40 - 60 20 goo 350 400 450 500 550 600 650 700 350 400 450 500 550 600 650 700 ADC Channel ADC Channel (a) without thresholds (b) with upper and lower threshold Figure 5.8 ADC spectra of the 511 keY photopeak in the CsI(Tl) trigger crystal Table 5.3 Measured detection efficiencies for 511 keY photons. Errors are statistical only. Distance source- Measured Average Investigated crystal trigger crystal (cm) efficiency (%) efficiency (%) CsI(Tl) 3x3x3 cm 1.0 75±2 2.5 73±2 2.5 78±3.5 74.6±1.3 8.0 74±9 10.0 76±31 20.0 80±54 YAP(Ce) 18x18x1 mm3 2.5 7.6±0.2 5.0 8.7±1.7 7.6±0.2 5.0 5.5±1.4 YAP(Ce) 25x25x2 mm3 3.0 16±0.8 15.8±0.8 6.0 13±2.9 For the large CsI(Tl) crystal, it was also possible to measure the photopeak efficiency to (75±3.5)%. For the thin crystals, no clear photopeak was visible in the spectrum. For the CsI(Tl) crystal, an absorption probability of 74% is found using literature data from [Hub95], in very good agreement with the measurement. However, for a 1 mm thick YAP(Ce) crystal, about 4.4% absorption at 511 keY is calculated using this data, and 8.7% in the case of a 2 mm crystal. As the measured detection efficiency of 7.6% for 1 mm implies an efficiency of 1 - (1 - 7.6%)2 = 14.6% for a crystal of 2 mm thickness, in acceptable agreement with the measured value of 15.8%, the results presented are consistent. An explanation for the discrepancy with the literature data needs to reproduce this. This could be provided by backscattering from material behind the scintillator, from, for example, the photomultiplier window. Indeed, photons backscattered into the crystal under 5.5. AFTERPULSE MEASUREMENTS 97 investigation would increase the measured efficiency. Furthermore, as these photons will have a lower energy, they will have a higher chance of absorption upon the second passage through the crystal. With the simplifying assumptions that of N impinging photons, a fraction Al are absorbed upon the first passage through the crystal, a fraction R of the remaining photons are backscat 0 tered through 180 and then absorbed with probability A2 , the equation would need to hold to explain the measured values, where x is the ratio between the measured efficiencies and the literature values given above. With Al = 4.4%/8.7% (literature values for the absorption of 511 keY photons in a 1 mm/ 2mm thick crystal), A 2 = 10.5%/20% (absorption of 170keV photons)\ and x = 1.727/1.816, values for R of 32% and 39% follow. These values should be identical within this model, but 0 taking into account that not all photons are backscattered through 180 , having therefore partly higher energies and longer path lengths upon the second passage and so modifying the simple relationship used above, the explanation seems reasonable. The problem is, however, the fact that no such effect of backscattering can be deduced from the results for the large CsI(TI) crystal, where about 25% of all photons are still transmitted and could undergo scattering. With the calculated value of R ~ 35%, Al = 75% and A2 ~ 1 (full absorption of the backscattered photons), a value of x = 1.1 follows from the equation above ~ an efficiency of 84% should have been measured, clearly inconsistent with the result of 74.6±1.3%. It is possible that the lower threshold for the CsI(TI) was indeed too high and the integration time too short for the long decay time. The non-observance of backscattering effects is then explainable if the backscattered photons did not trigger the discriminator due to their low energy. Unfortunately, the existing data do not allow a definitive statement on the validity of this explanation. An attempt to measure the true absorption efficiency by eliminating backscattering was not made, as these data are available. In fact, the set-up used here reproduces the situation in the SRD. Backscattering will occur and indeed help in increasing the detection efficiency at larger energies, an effect that needs to be accounted for in eventual detector simulations. 5.5 Afterpulse measurements The objective of the SRD requires significant reduction of backgrounds to discern the small and infrequent signal events. Apart from the diffuse particle background described in Sect. 3.1.3, which is unavoidable and requires good time resolution to counteract it, the photomultipliers envisioned as detectors will themselves contribute noise. One component to this noise, exclusively on a single photoelectron basis, are electrons emitted thermally from the photocathode that enter the multiplier chain. Another component, to be investigated here and possibly resulting in larger pulses, are afterpulses following large scintillation events. These afterpulses are due to feedback processes following large pulses (high current at the end of the multiplier chain). Residual gas in the photomultiplier may get ionised and ions will then travel backwards and might hit a dynode or the photocathode or, similarly, optical photons from electroluminescence occurring on a dynode might do so. The latter process will usually result in the emission of at most one electron, whereas ions will likely release several new electrons. IThe energy Eb of photons backscattered though 180' is related to their initial energy Eo by Eb = Eo(1 + 2Eo/511 keV)-l, giving 170 keY for Eo = 511 keY. The energy of the Compton edge is then Eo - Eb. 98 CHAPTER 5. EXPERIMENTAL STUDIES FOR PSRD AND SRD Count 1.5 ns FWHM NIM Inhibit Figure 5.9 Set-up for the afterpulse measurements The resulting amplitude of the afterpulse will then depend on where the electrons have been liberated - the further down the multiplier chain, the smaller the pulse will be. For the SRD, where the scintillators will be hit continually by higher energy particles and large pulses might pass through the photomultiplier frequently, these afterpulses become detri mental if their rate becomes comparable to the particle background. To investigate this, the set-up sketched in Fig. 5.9 was used: large scintillation events were simulated by shining short, bright LED pulses2 of 450 nm wavelength onto the window of a R5900U photomultiplier (oper 6 ated at 900 V bias voltage with a gain of approximately 5 x 10 ). Its output was routed through an amplifier to a discriminator with adjustable threshold, which was in turn connected to a LeCroy 2551 scaler capable of counting up to 100 MHz. The scaler was normally inhibited, but through a programmable delay a 20 ns window could be opened at a given time after the main LED pulse. Additional cable delays were used to synchronise the main pulse from the photomultiplier and the trigger signal from the LED controller. The LED was pulsed at 10 kHz or 100 kHz and afterpulse delay spectra were taken in com puter controlled 20 ns increments, with usually 20 s integration time per bin. The LED brightness is fixed, but the relative distance and orientation of photomultiplier and LED were arranged to yield a response of the photomultiplier equivalent to about 80 photoelectrons, with some varia tions between individual measurements. A typical response is shown in Fig. 5.10. Assuming 20% quantum efficiency, 80 photoelectrons would correspond to 400 photons or, in a YAP(Ce) scintillator that yields some 20 photons per keY of deposited energy, to 20 keY. However, the scintillator would emit these photons with a decay time around 30 ns, whereas here they are generated almost instantaneously, giving a much higher charge density at the end of the multiplier chain. The afterpulse probability depends on this density, so a direct translation of these measurements to the actual scintillator application is not possible. A series of delay spectra taken with different counting thresholds is shown in Fig. 5.11. No significant change in shape is apparent when increasing the threshold, with only the total rate decreasing, indicating that the afterpulses have a large spread in amplitudes. However, the broad maximum around 1300 ns seems to get suppressed at higher thresholds. The delay of the afterpulses stemming from electroluminescence is at most equivalent to the transit time of the pulse inside the photomultiplier, around 10 ns for the R5900U, or smaller. Clearly, these do not contribute to the spectra shown, so ion feedback must be responsible for the observed structure. Since the charge density is highest at the end of the multiplier chain, most ions are generated there. With the simple model that the ions travel back all the way to the photocathode under the action of the electric field and release a new afterpulse avalanche, their travel distance s between anode and photocathode is related to the time t by the equation for motion under 2FWHM 1.5 ns. The pulses were generated using the NanoLED pulsed diode system by IBH. 5.5. AFTERPULSE MEASUREMENTS 99 Tek Run: 200GS/s Sample I E T I 1-> V- I I ~ I V Ln I Luum IU M IU.uns Ln I ~ ::>Lmv Figure 5.10 Response of the R5900U photomultiplier to an LED pulse. Single photoelectron events have a pulse amplitude of about 10 mV. constant acceleration, a 2 F 2 Eq 2 S = -t = -t = -t . 2 2m 2m Here, a is the acceleration of a particle of mass m under a force F, with F = E . q for an ion of charge q in an electric field E. Writing the electric field simply as E=U s ' with U being the voltage between photocathode and anode, the expression for the delay time t is t = I¥!i. s. (5.4) With U = 900 V and s = 2 cm (length of R5900U), the following delay times for singly charged ions result (q = 1 e; the 10 ns transit time delay of the afterpulse is not included): Ion Species Nt jCO+ Co+2 Mass (u) 2 18 28 44 201 Delay Time (ns) 136 407 508 636 1361 Following [Can85] and [Bir64, Sect. 5.3.7], these are some of the ion species that are thought to be responsible for afterpulses. The upper two peaks in Fig. 5.11 are clearly compatible with the delay times of Nt and Hg+, giving credibility to the explanation as ion feedback. The lower peak could be explained if N2+ is also formed, as this would have half the delay time of Nt. It should be noted that the model of (5.4) is likely too simple to reproduce fine details of the delay spectra. A series ofdelay spectra with different photomultiplier bias voltages is shown in Fig. 5.12. The decrease in gain of the photomultiplier (a factor of 3 per 100 V) was approximately compensated 100 CHAPTER 5. EXPERIMENTAL STUDIES FOR PSRD AND SRD ...~ ~22.5 i c c III - " :a 20 r- :a (J) • I~ c . (J) ~ 10 f"r,, ~ 17.5 (\J Q; Q; i I 0. . 0. ~ 15,- ~ 8 t'~ :a :a I 12 5 ro I 0 2 . :: .0 ~ I' I e e 6 11 ,I \, I 0. 0. lo Ol 10 Ol ~,~ (J) (J) 'il• • .\, i I. :; :; 1 ~ ~ .1 .Ii i~ ~ 7.5 e- Ol 4 I ~~ f: ~ ~ ~ 5. t '\ : ~I/.Ntl~ ~ 2.5: f :j~ +M~;rJ I "'tIb lE OLcI~~ ~" , "~~ o 250 500 750 1000 1250 1500 1750 2000 o 250 500 750 1000 1250 1500 1750 2000 Time after main pulse (ns) Time after main pulse (ns) (a) 30mV (;::: 3 photoelectrons), 5.7% (b) 45mV (;::: 4.5 photoelectrons), 2.5% 6 :ac (J) c o 5 (\J I Q; I 0. , ~ 4 ~, :a I ro I .0e I 0. I Ol (J) ~ :; e- ~ Ol ~ ~ ~ .,~ ~:~~r~r~~~!.... ,...... • ...... • I 500 750 1000 1250 1500 1750 2000 Time after main pulse (ns) (c) 60mV (;::: 6 photoelectrons), 1.3% (d) gOmV (;::: g photoelectrons), 0.2% Figure 5.11 Afterpulse spectra with varying counting threshold. Main LED pulse corresponds to approx. 80 photoelectrons, total afterpulse probability within 211S of main pulse is indicated. (indeed somewhat overcompensated) by increasing the electronic amplification. Nevertheless, the afterpulse rate drops significantly and, very clearly, the relative proportions of the peaks change. Following (5.4), the delay times should change inversely with the square root of the bias voltage, that is by some 20% between 900 V and 600 V. This is not apparent from Fig. 5.12, but the model is likely too simple to be applied to changes of this order. The total afterpulse probability of around 6% for a threshold at 3 photoelectrons is by a large margin insignificant for the SRD application, as the rate of higher energy particles that generate afterpulses is certainly below the background rate of X-ray photons and low-energy electrons. ------ 5.5. AFTERPULSE MEASUREMENTS 101 ..~ "i 0 o ~ 14 c c :0 :0 (J) (J) c 12 c 0 o (\J (\J ID ID c- 10 a. ~ ~ :0 :0 8 .0e'" .0e'" C- c Ol Ol (J) 6 (J) :; :; e- e- Ol Ol «-== 4 ~ 2 250 500 750 1000 1250 1500 1750 2000 250 500 750 1000 1250 1500 1750 2000 Time after main pulse (ns) Time after main pulse (ns) Ca) 900 V, x 2, 4.0% Cb) 750V, x 12,1.7% ..-o 3 ~ r 1 :5 1,I ~ (J) 2.5, 1'\ j 2;1'11~ :0 :!T I, ~ ~ ! I rli ~ 1.5f11J1i :; L 11/ · " 1 I 1 FI1 .\ I~ ~.cl' ~~ !~ 0.5 ~ ~ w ~. -,.- ~ 0' ,Le ,...... :-.c~~~ o 250 500 750 1000 1250 1500 1750 2000 Time after main pulse (ns) Cc) 600 V, x 110, 0.4% Figure 5.12 Afterpulse spectra with varying photomultiplier bias voltage. Main LED pulse cor responds to approx. 80 photoelectrons, electronic amplification and total afterpulse probability within 211-8 of main pulse are indicated. Counting threshold is 30 mV (>:::J 3 photoelectrons). CHAPTER 6 Summary and Conclusion The main focus of this work was the detailed description of the design and the construction of the Prototype Synchrotron Radiation Detector (PSRD) and of its connection to the Synchrotron Radiation Detector (SRD). This proposed addition to the upgraded next version of the Alpha Magnetic Spectrometer has been detailed in Chapter 3 to the design level that it has currently reached. Its scientific background and the basics of cosmic ray physics were sketched in Chapter 1, stressing espe cially the role of high-energy electrons and positrons. Their energy loss due to inverse Compton scattering and synchrotron radiation emission in galactic magnetic field essentially limits the dis tance such particles of TeV energy can travel to below 1kpc, so information about the relative galactic neighbourhood can be deduced from their spectra. The aim of the SRD is the measurement of the energy spectrum of electrons and positrons beyond 1 TeV. The working principle relies on the detection of the small number of synchrotron photons that are emitted in the X-ray region by an electron or positron in the earth's magnetic field. To intercept some 2 to 3 photons with typical energies of a few keY, as emitted by a 1 TeV electron, requires a detector area of several square metres. The synchrotron photons are immersed in a diffuse background of photons and particles with similar energies. The basic means to counteract this background is a good time resolution, since the synchrotron radiation arrives in coincidence with the emitting high-energy particle, uncorrelated with the background. Detailed measurements of the energy distribution exist currently only of the photon compo nent of the background. From these alone, a necessary time resolution of some 10 ns is deduced. The available data on the charged particle background is very limited, with the best data set identified only giving particle rates above 30 keY, with no energy resolution. Knowledge of the full background rate and of its spectral distribution is essential for designing the SRD, determining first the principle feasibility (the realization is only possible if the rate is not too high), second the possible design options by setting the time resolution and third the detector performance (detection efficiencies, misidentification probabilities and alike). As a precursor to the SRD, the main objective and justification of the PSRD is the mea surement of the energy-resolved low-energy charged particle and photon background rate near earth. It employs similar techniques as foreseen for the SRD ~ scintillating YAP(Ce) crystals coupled to photomultipliers~,thereby providing also an opportunity to verify these techniques under realistic space conditions. The design of the PSRD is explained in detail in Chapter 4. A number of experimental studies were performed in support of the PSRD and SRD devel opment, concerning various aspects of photomultiplier and scintillator operation (Chapter 5). 102 6.1. EXPERIMENTAL RESULTS 103 The results are briefly summarised in the following section. Additionally, a few comments on measurements with wavelength shifters are made. A summary of the PSRD flight is given in the second section of this chapter. Finally, some concluding remarks are made. 6.1 Experimental results The timing measurements in Sect. 5.1 and the simple model presented indicate that timing precisions below 10 ns are possible with the YAP(Ce) scintillator, also for signals of only a few photoelectrons if the trigger point is set low enough, although the efficiency (probability that an event will fall into a given time window) will drop markedly for such small signals even if first-photoelectron timing is used. With respect to surface quality and wrapping of the YAP(Ce) crystals, the cheaper, rough surface finish was found to be advantageous over all-polished surfaces, with an all-polished crystal giving only 70% of the light of a rough one (Sect. 5.2). Teflon was found to be the best wrapping material, but its use is excluded for the SRD by its too high absorption for low-energy X-rays (Sect. 5.3). Beryllium foils of 2511m thickness coated with 200 nm of aluminium, on the other hand, are worse with respect to the reflectivity by only some 10% and have a much better transparency. Direct coating of the crystals with aluminium turned out to be very detrimental to the light yield. The measurements of the detection efficiency for 511 keY photons in Sect. 5.4 showed clearly higher values for thin YAP(Ce) crystals than expected from literature data. The likely explana tion is backscattering in the set-up, an effect that will also contribute in the SRD, though to an extend that will depend on the actual realization (material distribution). The studies on afterpulsing in Sect. 5.5 showed rather complex afterpulse delay spectra re sulting from ion feedback. The absolute rate of afterpulses was found to be insignificant with respect to the natural particle background that the SRD will experience. A few basic, mostly qualitative, measurements that are not reported in Chapter 5 were also performed with the BC-484 wavelength shifter, considered as an option for the SRD realization in Sect. 3.3.1. The signal yield of a 25x25x1 mm3 YAP(Ce) crystal with a wavelength shifter of 25x25x2 mm3 directly underneath, read out via a large face, was only about 40% compared to the crystal alone (a large XP2020Q photomultiplier was used, all couplings with optical grease, wrapping with Teflon tape). With a 100x25x2 mm3 shifter, read out via the small side furthest from the crystal, the yield dropped to 30% compared to the crystal alone. Several different wrapping methods were tried, but no significant improvements were attained. With the inherent light yield of YAP(Ce) of approximately 17 photons per keY (see Ta ble 3.5), about 2.5 photoelectrons for 2.5 keY of deposited energy are generated in the last case, assuming 20% quantum efficiency of the photomultiplier. The actual losses in a realization suit able for the SRD will depend largely on the position, being smaller for crystals closer to the photomultiplier. On the other hand, further losses are expected in a real, more complicated geometry. Certainly the wavelength shifter option is, if the losses cannot be reduced markedly, on the edge of feasibility for the SRD. Charged cosmic rays passing through the wavelength shifter were measured to deposit en ergies equivalent to 10 keY to 20 keY in the YAP(Ce) scintillator. The noise rate due to this response of the shifter is however insignificant, as long as the wavelength shifter total area is not much larger than the scintillator area. 104 CHAPTER 6. SUMMARY AND CONCLUSION Table 6.1 Summary of PSRD data taking periods during STS-108 mission (as inferred from the downlinked data). The duration of each data taking period is given in hours and minutes. Total for detector side A is 113:06, for side B 113:50. After activation, both sides did not always start data taking at the same time. Shuttle attitude MET1 (d/h:m) PSRD Side Deep-Space Bay-to-Earth Docked Other o/ 2:53 A&B 0:42 0/3:36 A&B 15:14 1 / 2:40 A&B 11:49 1 / 14:30 A&B 6:41 3 / 19:39 A&B 6:45 4 / 20:12 A&B 6:47 6 / 19:52 B 6:30 6 / 20:20 A 6:02 7 / 18:58 A&B 6:44 8 / 23:10 B 12:05 8 / 23:15 A 12:00 9/20:01 B 5:29 9 / 20:12 A 5:18 10 / 1:31 A&B 9:58 10 / 11:30 A&B 4:46 10 / 16:50 A&B 20:20 A 37:01 12:41 38:18 25:06 Total B 37:01 12:52 38:51 25:06 1 Mission Elapsed Time, given in days, hours and minutes since launch 6.2 PSRD mission summary The PSRD was launched on board the Space Shuttle Endeavour on mission STS-108 on 5 December 2001, 23:19 MET (17:19 EST local time in Florida) from launch pad 39B at the Kennedy Space Center. The activation times of the detector were at that point not yet fully known, as decisions were taken during the flight depending on the remaining energy reserves of the shuttle. A summary of the activation periods that were finally allocated is given in Table 6.1. The total running time is significantly above the initially guaranteed 34.5 hours of deep-space attitude. The data set recorded by both sides of the detector together amounts to 1 some 38 GByte , half of the available storage space and also much above the expectations. With a total recording time of about 110 hours, this corresponds to a data rate of 354 MByte/hour. As calculated in Sect.4.12, the raw data rate in standard mode, i. e. with a trigger rate of 22 triggers per 10 seconds that was always used during the flight, is 37 GByte per day or 1580 MByte/hour, so the compression algorithm reduced the amount of data by a factor of around 4.5. Altogether, 107 events were registered, each containing a 409.611S long background sample. The shuttle landed back at Kennedy Space Center on 17 December 2001, 18:55 MET (12:55 EST local time). After the PSRD was returned to ETH Zurich in February 2002, the science data was downloaded through the parallel port via Zip disks during four days. Data analysis lExactly 20609492534 Byte (19.19 GByte) were written to disk on side A, 20163639220 Byte (18.78 GByte) on side B. 6.2. PSRD MISSION SUMMARY 105 E 100 I, ,I, ,I,, ,I, ,I, ~-l-! ,~! g 0 E, ..l..-L'--.LI-.L....J'--1....'-L-I'LL'.....1-'.L...... LI I U 600 620 640 660 680 700 720 740 760 780 U Cl ~ 1~ E'~1~L,~"4~1~d~,L:J 600 620 640 660 680 700 720 740 760 780 Sample Bin (a) Non-saturated hits ..... ::: ;:l f88 ,,,I , , I ,, I ,, I , I , , I , , I , , I ,I 0 0 l I ,L, U 640 660 680 700 720 740 760 780 800 820 U Cl 20 (b) Signal saturated in non-attenuated output Figure 6.1 Two raw signals from the small YAP array. Top row non-attenuated, bottom row 1:10 attenuated output. One sample bin corresponds to 50 ns. ~ ~ggt··- ..-.. _.. _.. __...;:-.. _.. _.. _.. _.. _··_··_··_··_··-~f··_··_··_··_·· _.. _..-..-··_··_··_;:---~-I----- ·.;!I 10g _------··-··-··-··-··-,--t------,--[""------or·------··-··-r·-··------T------··-··lJ·----,··------·L IIII II ! I IIII III t o 100 200 300 us 400 Figure 6.2 One event of a small YAP crystal. Shown are the full 409.6 ps (8 kByte) data of the 1:10 attenuated output. This event resulted from a scintillator trigger. commenced immediately afterwards. The PSRD is now planned to be used as a working exhi bition object. A few preliminary results from the initial science data survey are shown in the following to illustrate the detector performance. Two raw signals from the small YAP array, as read from the FADe memory, are shown in Fig. 6.1. In (b), the signal saturated in the non-attenuated output, but was within scale of the 1:10 attenuated output. In Fig. 6.2, the full data recorded from the 1:10 attenuated output of one small YAP crystal in one event is shown (409.6 ps in 50 ns steps or 8 kByte, cf. the end of Sect. 4.4.1). The vertical scale is energy calibrated, with full-scale corresponding to about 1 MeV. One background rate spectrum as measured during the shuttle flight is shown in Fig. 6.3. It reflects the average rate between 2 keY and 90 keY seen in one particular channel during one orbit. The integral rate amounts to 8700 s-1. This is much above the photon rate of 160 s-1 in this 106 CHAPTER 6. SUMMARY AND CONCLUSION 1000 o 20 40 60 80 Energy (keV) Figure 6.3 Background rate spectrum as measured during the shuttle flight, averaged over one orbit (91 minutes). energy range (Fig.3.3(b)). However, no attempt was made in this early phase of data analysis to exclude the south-atlantic anomaly, where the rates are expected to be orders of magnitude above those at other locations. Further analysis will separate the various contributions to the spectrum. 6.3 Concluding remarks The measurements reported in Chapter 5 and the experience gained from the construction of the PSRD, regarding operation characteristics of photomultipliers, light yields of various crystals, beryllium windows as light shields, etc., render the basic approach towards the SRD, described in Chapter 3, feasible. An important point that remains to be studied is if the required time resolutions can also be achieved with the proposed APV read-out in the case of small signals that correspond only to a few photoelectrons. Both in the case of single photoelectron signals and large signals the pulse shape is well known (the response of the photomultiplier and that of the scintillator, respectively), and a pulse-fit method will clearly give good time resolutions even if large sample intervals are used (25 ns, for example). For a small number of photoelectrons, the shape of individual signals is largely random, making this method more challenging. This will be especially important for the read-out option employing wavelength shifters because of the large light losses that occur. The cooperation with NASA left some special impressions, as it is partly indeed an "agency", but partly also a research institution. This division also applies to their personnel. Many require ments unknown to ground-based physics experiments had to be met due to the space-borne 6.3. CONCLUDING REMARKS 107 nature of the detector, mostly dealing with safety issues. Considerable more time than usual in the field of particle physics went, in fact, into paperwork, comprising certifications and test re ports, reviews of a large number of NASA documents, and detailed descriptions of essentially all aspects of the PSRD. The noteworthy exception to this are issues that were deemed exclusively mission-success relevant by NASA. Regarding this aspect of technical realization and scientific objective, the influence that was taken by NASA was very limited. NASA or, more directly, the Hitchhiker management within the Shuttle Small Payloads Project took, in effect, the role of a "transportation company" with little attention on what they transported. Of course, the manifestation of the PSRD was eased by the fact that it is an essential part the large AMS-02 project, in which NASA itself is involved. The performance of the PSRD during the flight was, to a very large extend, as expected, without any significant malfunctions. The total amount of collected data, thought to be rather limited before launch because the allocated running time got more and more reduced the closer launch came, was finally quite satisfying, filling almost half the available storage space. The hard disks, one critical, mechanical part, were working well, and no measurable leakage from the hard disk container occurred. The temperature of the detector was generally lower than on earth, resulting several times in some delay until the hard disk temperatures were above the lower limit after applying power. A detailed description on the data analysis, detector performance and of the scientific out come of the PSRD will be presented in the PhD thesis by Barbara Zimmermann. "Am meisten Zulauf haben die Erzahler. Um sie bilden sich die dichte sten und auch die bestandigsten Kreise von Menschen. Ihre Darbietun gen dauern lange, [...]" Elias Canetti, Erziihler und Schreiber / I " APPENDIX A SRD: Effective Flight Distance and Granularity A.I Effective flight distance The bending radius R for a highly relativistic particle in a uniform magnetic field has been given in (1.1). For an electron or positron, this equation can be written as e Ee R = me,c = E since eB eBc' '=--2'mec where Ee is the energy of the particle and B the field strength. It is assumed that the particle moves perpendicular to the field lines. Synchrotron radiation is emitted tangentially to the path of a high-energy electron or positron with a very small opening angle. With reference to the following figure, it is clear that only synchrotron radiation that is emitted during some effective flight distance S can reach a detector of width 2Lo when the charged particle enters the detector centrally. y Magnetic field f(x) ® ®® R , a,\ I x For the bending circle, the equation x 2 + y2 = R 2 holds, so for the first quadrant dy(x) -x dx VR2 - x 2 109 110 APPENDIX A. SRD: EFFECTIVE FLIGHT DISTANCE AND GRANULARITY The straight line!(x) has, if it is tangent to the circle at Xo, therefore the equation 2 Since !(XI) = 0, Xl = R2/xo, and with Xl - R = Lo it follows that Xo = R /(Lo + R). Because cos a = xolRand 8 = aR, the final result for the arc length 8 is R 8 = R arccos L + R o The emission rate in photons per second needs to be multiplied by 81c to get the average number of photons that will hit the detector. As an example, for a 1 TeV electron in a magnetic field of 3 . 10-5 T (the typical strength of the magnetic field close to earth) and a detector width of 2Lo = 3 m, R = 1.1.108 m and 8 = 18.3 km. Over this distance, the magnetic field changesI by a maximum of about 1% and can indeed be assumed to be uniform. A.2 Granularity The synchrotron photons are predominantly emitted within a cone of full opening angle 2h [Long92, Chapter 18]. Over a distance of 20 km, this corresponds to a width of 2 cm for a 1 TeV electron. The detector area that needs to be searched for synchrotron photons is therefore reduced to a strip a few centimetres wide, its orientation being determined by the trajectory of the primary charged particle and by the earth's magnetic field direction. A minimum granularity of several centimetres is thus required to take advantage of this to effectively reduce the background by rejecting hits outside the region of interest. Furthermore, the distribution of synchrotron photons on the detector also suggests a fine granularity. Calling N(L) the number of synchrotron photons that hit the detector within dis tance L of the charged particle, one has for a given synchrotron emission rate A N(L) -8(L)AA(= -Rarccos -RR) and c c +L dN(L) A R 2 dL c (R + L)JL(2R + L)' because the number of synchrotron photons emitted per unit arc length is constant. Considerable simplification of these formulae is possible by realizing that for all practical cases R» L, for then one has N(L) ~Rarccos (~) = ~Rarcsin (Vl- (R~S) '" ~RV1-(R~S~~RJ1-(I+~r ~ !!:..J2LR, c 3 IThe radius dependence, being essentially that of a dipole, is B(r) ~ 1/r , so dB/B(r) = -3dr/r. ------~---- A.2. GRANULARITY 111 where the relation arccos x = arcsin()(1- x 2 )), valid for 0 ~ x ~ 1, and the Taylor's series for arcsinx and (1 + x)-2 to first order have been used. Within this approximation, dN(L) ~ ~ (If. dL cV 2i The fraction of photons F(x) that hit a detector of width Lo within a distance xLo from its centre, 0 ~ x ~ 1, is F(x) = N(xLo) = JXLo = r:: N(Lo)Lo yX, independent of the bending radius of the charged particle (hence also of its energy) and of the actual size of the detector. In the following figures, the functions F(x) and dF(x)jdx = (2v!X)-1 are plotted. The latter is simply the photon distribution function. 3 0.8 2.5 0.6 x x "'0 LL -; 1.5 LL 0.4 "'0 1- 0.2 0.5 0 0.2 0.4 0.6 0.8 0 0.2 0.4 0.6 0.8 x x The fractions of photons that hit within certain distance bins from the charged particle, for the case L o = 1.5 m, are given in the following table: Distance (cm) 0-5 5-15 15-30 30-50 50-100 100-150 x 0-0.033 0.033-0.1 0.1-0.2 0.2- 0.33 0.33-0.67 0.67 -1 Fraction (%) 18.3 13.4 13.1 13.0 23.9 18.3 From this, also the synchrotron photon distribution calls for a fine enough granularity: since the crystaljphotomultiplier combination through which the charged particle passed will be blinded to further synchrotron hits due to the large energy deposition by the charged particle (about 700 keV in 1 mm of YAP for a minimum ionising particle), a significant number of synchrotron photons are lost if the granularity exceeds some centimetres - almost 20% in the case above for a granularity around 5 cm. APPENDIX B Simulation of Light Propagation B.l General To study the characteristics of the propagation of scintillation light inside a crystal in more detail, and to achieve a better understanding of the main parameters affecting the amount of light that finally reaches the photocathode of a photomultiplier, a simple Monte-Carlo simulation was written. The flow diagram describing its operation is shown in Sect. B.5. It essentially simulates a rectangular volume of some given refractive index (the scintillator, for example) that is bounded by materials with different or identical refractive indices on five sides and the detector, also with some refractive index, on the sixth. A specular or diffuse reflector of given reflectivity is furthermore simulated immediately outside the boundary of the five sides. Simulated photons with isotropically distributed propagation directions are generated at some point inside the crystal. They are followed using geometrical optics through their paths, until they are either absorbed in the bulk material, characterised by some absorption length, or in the reflector, or until they are transmitted to the detector. At the end of the program, the number of photons lost and detected and statistics on the distances travelled, the number of reflections at each wall and some other data are printed out, as shown in the following output of a particular simulation where 100000 photons were simulated. Photons detected 40218 / 40.2% Photons absorbed in bulk 59000 / 59% and in reflector 782 / 0.782% Wall intersection statistics: Wall 0: 57980 / 3.6% Wall 1: 57761 / 3.58% Wall 2: 57851 / 3.59% Wall 3: 58000 / 3.6% Wall 4: 680239 / 42.2% Wall 5: 699948 / 43.4% Sttatistics for detected photons (diffuse reflections 38067 / 2.42%): o reflection(s): 16331/40.6% 1 reflection(s): 12617 / 31. 4% 2 reflection(s): 2110/5.25% 3 reflection(s): 2373 / 5.9% 4 reflection(s): 1643/4.09% 5 reflection(s): 1454 / 3.62% 6 reflection(s): 991/2.46% 7 reflection(s): 533 / 1.33% 8 reflection(s): 342/0.85% >=9 reflections: 1824 / 4.54% < 1 mm: 16331 / 40.6% < 2 mm: 7891 / 19.6% < 3 mm: 4383 / 10.9% < 4 mm: 829 / 2.06% < 5 mm: 389 / 0.967% < 6 mm: 431 / 1.07% 112 B.2. SOME SIMULATION RESULTS 113 < 7 mm: 150 / 0.373% < 8 mm: 35 / 0.087% < 9 mm: 129 / 0.321% >=9 mm: 9650 / 24% Statistics for photons lost in reflector: Wall 0: 162 / 20.7% Wall 1: 146 / 18.7/, Wall 2: 140 / 17.9% Wall 3: 163 / 20.8% Wall 4: 171 / 21.9% Wall 5: o / 0% Statistics for photons lost in bulk material: < 1 mm: 561 / 0.951% < 2 mm: 769 / 1.3% < 3 mm: 934 / 1.58% < 4 mm: 846 / 1.43% < 5 mm: 928 / 1.57% < 6 mm: 961 / 1. 63% < 7 mm: 877 / 1.49% < 8 mm: 961 / 1. 63% < 9 mm: 885 / 1.5% >=9 mm: 51278 / 86.9% Average length travelled for detected photons: 5.42 mm Average length travelled for absorbed photons: 57.1 mm Limitations of the simulation are as follows: • Refraction and possible multiple reflections between the crystal and the reflector, to the outside of the crystal, are not taken into account. • Possible scattering in the bulk material is neglected. •A very simple model is used for diffuse reflections. • Reflections back into the crystal occurring at the photocathode-side of the photomultiplier entrance window are ignored. • It is not possible to simulate rough crystal surfaces or coatings (essentially due to a lack of knowledge oftheir reflection characteristics and ofthe influence on total internal reflection). • Only rectangular shapes are supported. B.2 Some simulation results The fractions of photons detected, lost in bulk and lost in the reflector are given below for 3 two simulated YAP crystals (refractive index n=1.95) of 18x18x1 mm3 and 30x30x30mm , coupled to a borosilicate glass window of a photomultiplier (n=1.49) and wrapped with either Teflon (diffuse reflector) or aluminiurn foil (specular). A layer of air (n=1. 0) between crystal and reflector was assumed. The absorption length was set to 60 mm and the starting point for the simulation was in the centre of the crystal. Crystal18x18xl mm3 Type of reflector Detected Lost in bulk Lost in reflector Teflon (reflectivity 98%) 40% 59% 1% Teflon (reflectivity 90%) 39% 57% 4% Aluminium (reflectivity 98%) 34.4% 64.1% 1.5% Aluminium (reflectivity 90%) 34% 59% 7% None (reflectivity 0%) 28.4% 42.7% 28.9% 114 APPENDIX B. SIMULATION OF LIGHT PROPAGATION Crystal 30x30x30 mm3 Type of reflector Detected Lost in bulk Lost in reflector Teflon (reflectivity 98%) 21.6% 77.7% 0.7% Teflon (reflectivity 90%) 21% 76% 3% Aluminium (reflectivity 98%) 19% 80% 1% Aluminium (reflectivity 90%) 18.8% 76.7% 4.5% None (reflectivity 0%) 16.4% 59% 24.6% The absorption length used is comparable to the lower limit of 68 mm given in [De197]. In [Bac98], an absorption length of 140 mm is reported. Using that value for the case of Teflon with 98% reflectivity, the numbers would change to 42.4%, 56.7% and 0.9% for the small and to 31.4%, 67.8% and 0.8% for the large crystal. Replace the borosilicate window by one made itself of a YAP crystal, thereby eliminating reflections at this interface altogether, the following numbers follow (with absorption length 60mm): Crystal18x18xl mm3 Type of reflector Detected Lost in bulk Lost in reflector Teflon (reflectivity 98%) 93.3% 6.5% 0.2% Teflon (reflectivity 90%) 92.4% 6.4% 1.2% Aluminium (reflectivity 98%) 92.4% 7.3% 0.3% Aluminium (reflectivity 90%) 92% 7% 1% None (reflectivity 0%) 82.8% 5.6% 11.6% Crystal 30x30x30 mm3 Type of reflector Detected Lost in bulk Lost in reflector Teflon (reflectivity 98%) 38.6% 60.8% 0.6% Teflon (reflectivity 90%) 37.9% 59.2% 2.9% Aluminium (reflectivity 98%) 36.2% 63.0% 0.8% Aluminium (reflectivity 90%) 35.6% 60.5% 3.9% None (reflectivity 0%) 29.6% 46.3% 24.1% There is no significant change when, instead of starting the simulation in the centre of the crystal, random starting points within the crystal are used (uniform illumination with higher energy X-rays). B.3 Principle deductions The relative light output of a crystal of 18x18x1 mm3 for Teflon (reflectivity 98%), aluminium (90%) and no reflector (0%) is simulated as 1:0.85:0.7. This is in acceptable agreement with the ratios of 1:0.91:0.69 measured experimentally in Sect. 5.3 (see Table 5.2), giving credibility to the basic deductions from this simple simulation approach that are summarised in the following. 1 lIt should be noted, though, that the values in Table 5.2 were measured with a cylindrical crystal of dimension 08.8mmxlmm. B.4. TECHNICAL DETAILS 115 • The light output without any reflector (i. e. when all light leaving the crystal is lost except that going to the photocathode) is still significant because of the large total internal reflection angle, a consequence of the high refractive index. • With a borosilicate glass window (low refractive index), a large majority of the photons are lost by absorption in the bulk of the crystal, even when one assumes a long absorption length and even for a small crystal. • By eliminating the refractive index boundary to the photomultiplier by using a YAP entrance window, the light output is doubled for the large crystal and more than doubled for the small crystal. Looking at the detailed statistics, the number of reflections and average distances that photons travel are, as expected, smaller, since light is trapped less in the crystal. • The light output for a large crystal changes rather strong between the cases of short and long absorption length, since overall travel distances are longer. The light output of small crystals is not very sensitive to this parameter. • The actual reflectivity of aluminium is around 90% (see Fig. 4.7), that of Teflon tape about 98%. But the simulation results for two reflectivity values in each case show that the major advantage of Teflon over aluminium is its diffuse reflection behaviour, not its higher reflectivity. Indeed, Teflon of 90% reflectivity would give about the same fraction of detected photons, with only the fraction of photons lost in the reflector to all photons lost being larger; vice versa for aluminium of 98% reflectivity. Higher reflectivity means more chance to have a long travel distance inside the crystal, i. e. higher bulk absorption probability. Lower reflectivity increases the chance of absorption in the reflector. The benefit of the diffuse reflection of Teflon is to reduce the amount of trapped light. The better light yield of crystals with rough compared to polished surfaces has likely the same origin. B.4 Technical details The source code written in the C programming language is listed in Sect. B.6. The parameters of the simulation are set in the section 'User modifiable data' (in the listing, a YAP crystal of 18 x 18 x 1 mm3 in air, wrapped with Teflon tape and coupled to a borosilicate window is simulated). The C function randO returns a random integer number between 0 and RAND_MAX (above 2.109 in the compiler implementation used, but guaranteed only to be at least 32767), so the function RandomO return one of RAND_MAX+1 random numbers between 0 and 1. The unit vector CurrentDir [] is initialised as (cos(ep) sin('l9), sin(ep) sin('l9), cos('l9)), with ep being uniformly distributed between 0 and 21T and 'l9 being distributed as sin('l9) /2 ~ this yields unit vectors pointing randomly in all directions.2 Depending on the starting conditions, the initial position in CurrentPos [] can either be a fixed value or itself be distributed, for example randomly, within the crystal. The intersection with the nearest wall is then found by calculating the distances of all six walls along the unit vector CurrentDir [] and then choosing the smallest, non-negative value. The coordinates of the intersection point (origin for the next trace step) are then written to 21f X is a random number distributed uniformly between 0 and 1, then iJ == arccos(l - 2x) will be distributed as required (see [Bra92, Sect. 4.9]). This is how the function RandomAngle 0 is constructed. 116 APPENDIX B. SIMULATION OF LIGHT PROPAGATION ., 0.8 : , a: , •I ~"2 : ' 0.6 n, 5 10 15 20 25 30 a (Degree) Figure B.1 Plot of ri, r~ and R for nl = 1.00 and n2 = 1.95 CurrentPos [] and the angle between CurrentDir [] and the unit vector normal to the wall is computed. According to the given absorption length and the distance just calculated, provision for possible loss in the bulk material is made. If the angle is larger than the total internal reflection angle determined from the refractive indices, the component ofCurrentDir [] normal to the wall is reversed and the tracing continued. Even if the angle is smaller, reflection might occur. The formulae to calculate the fraction of the electric field amplitude of a wave reflected at a boundary between two media of refractive indices nl and n2 are given in [Jack75, Sect. 7.3] as n2 cos a - J ni - n§ sin2 a nl cos a - n2Jni - n§ sin2 a r -l = ------'--r======and rll = , n2 cos a + J ni - n§ sin2 a nl cos a + n2Jni - n§ sin2 a where r -l and rll refer to the electric field components perpendicular and parallel to the plane of incidence, respectively, and a is the angle between the incoming wave through the medium with n2 and the normal vector of the boundary. Assuming that the light is unpolarised, the reflection coefficient R for intensity (or, more appropriately in this context, the photon reflection probability) is 2 2 R = _r-l_+_r...:.:.11 (Fresnel formula for unpolarized light), 2 as implemented in the program. If reflection occurs, the component of CurrentDir [] normal to the wall is again reversed and the tracing continued. The values of ri, r~ and R for nl = 1.00 and n2 = 1.95 (i. e. for a YAP crystal/air boundary) and for angles up to the total reflection angle of 30.9° are plotted in Fig. B.1. Ifno reflection occurs at the boundary, the photon might be reflected at the reflector outside the boundary, with a chance given by the reflectivity. If reflection occurs and the reflector is of specular nature, the new angle is calculated as above. If it is a diffuse reflector, a random angle within the hemisphere that points back into the volume is generated and the tracing continued with this angle. Tracing is continued until either absorption or detection occurs. The latter is defined as transmission through Wall [5] (on the -z axis). B.5. FLOW DIAGRAM 117 B.5 Flow diagram Start simulation Yes Maximum ,------=.=-< number of loops reached? CurrentDir[] unit vector with spherical coordinates Phi = 2 Pi *RND Theta = acos(l-2*RND) Compute intersection with nearest wall along CurrentDirD No Compute new position Invert component of Yes Total reflection? CurrentDir[] perpendicular to wall No Reflection as R=ln(rsI\2+rp"2) with cs Reflection Invert component of and rp reflection coefficients for the two Yes even if no total polarizations of the electric field (see CurrentDir[] B reflection? Jackson, Classical Electrodynamics) perpendicular to wall No No No Invert component of CurrentDir[] perpendicular to wall Diffuse Generate random CurrentDir[] B but pointing back 118 APPENDIX B. SIMULATION OF LIGHT PROPAGATION B.6 Source code 1* Monte-Carlo simulation for light propagation in some crystal or similar medium A rectangular volume, enclosed by some other material and a reflector, is simulated, geometrical optics are used. Set parameters in section 'User modifiable data' below. Terminology: 'trace' simulated path of one photon 'leg' one straight section of a trace 'intersection' intersection between a leg and a wall Compile source code with 'cc -lm Light-Propagation.c' to bind in math functions. Coordinate system: I+z Theta counted against +z axis, 1 Phi against +x axis, +y axis is 1______+y Phi=Pi/2 I I+x Version 1.2 6/12/2001 Oliver Grimm, ETH Zurich *1 #include #define TRUE 1 #define FALSE 0 typedef char BOOL; II Function prototypes void RandomAngle(double *); double Random(void); 1* Structures defining walls; wall numbering: 0 = +x, 1 = -x, 2 = +y, 3 = -y, 4 = +z, 5 = -z Detector behind wall 5: all photons transmitted through will be counted *1 struct Wall { double Position; II Position in mm on coordinate axis of this wall double n_outside; II Refractive index outside of crystal double Reflectivity; II Reflectivity of reflector behind wall BOOL Diffuse; II TRUE: Diffuse reflector, FALSE: specular reflector }; double TotrefAngle[6J; II Total reflection angle, computed from refract. index 1* -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=- User modifiable data -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=- *1 #define MAXNUM 100000 II Number of runs #define ABSLEN 60 II Absorption length in millimeters #define N_INSIDE 1.95 II Refractive index outside of crystal #define X_START 0 II x,y and z coordinate of initial position #define Y_START 0 II within simulation volume; if RANDOM is B.6. SOURCE CODE 119 #define Z_START 0 II set to TRUE, than initial position will be #define RANDOM FALSE II distributed randomly within simulation volume struct Wall Wall[6] { II see definition of 'struct Wall' above { +9, 1.00, 0.98, TRUE}, { -9, 1.00, 0.98, TRUE}, { +9, 1.00, 0.98, TRUE}, { -9, 1.00, 0.98, TRUE}, { +0.5, 1.00, 0.98, TRUE}, { -0.5, 1.49, 0.98, TRUE} }; 1* ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ Main program ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ *1 1* Definition of variables CurrentPos [] Current 3d position (x,y,z) CurrentDir[] Unit vector giving current flight direction (x,y,z) IntersectWal1 Current number of wall that will be hit next LastWall Number of last wall that was hit CurrentReflections Number of reflections in current trace TotalReflections Total number of reflections occured in simulation DiffuseReflections Total number of diffuse reflections in simulations TotalIntersections Number of intersections in simulation Photons_detected Number of photons detected Photons_absorbed Bulk Number of photons absorbed in bulk Photons_absorbed_Reflector Number of photons lost in reflector LostHist [] Number of photons lost at each wall IntersectionHist[] Number of intersections at each wall r Distance to next wall Length_in_Trace Total length travelled in current trace Length_detected Total length in simulation of detected photons Length_absorbed Total length in simulation of absorbed photons Count Counter for simulation round mainO { double CurrentPos[3] , CurrentDir[3]; double r,Length_in_Trace, Temp,Temp1,Temp2,Temp3; double Length_detected=O, Length_absorbed=O; long Count=O, i; long Photons_detected=O,Photons_absorbed_Bulk=O,Photons_absorbed_Reflector=O; long CurrentReflections, ReflectionHist[10]={O,O,O,O,O,O,O,O,O,O}; long DiffuseReflections=O, LengthHist[10]={O,O,O,O,O,O,O,O,O,O}; long TotaIReflections=O; long TotalIntersections=O, IntersectionHist[6]={O,O,O,O,O,O}; long LostHist[6]={O,O,O,O,O,O}, BulklostHist[10]={O,O,O,O,O,O,O,O,O,O}; int IntersectWall, LastWall; II Wall number of last intersection II Initialize random number generator 120 APPENDIX B. SIMULATION OF LIGHT PROPAGATION srand«unsigned int) time(NULL»; II Compute total reflection angles from refractive indices (if existing) for(i=O; i<6; i++) if(Wall[i] .n_outside < N_INSIDE) TotrefAngle[i] = asin(Wall[i] .n_outside/N_INSIDE); else TotrefAngle[i] = DBL_MAX; 1* ************************************************************************ ************************************************************************ Main simulation loop ************************************************************************ ************************************************************************ *1 for(Count=O;Count if(Count'l.1000 == 0) { printf("Event number: 'l.d of 'l.d ('l.d'l.'l.)\r", Count+1,MAXNUM,Count*100/MAXNUM); fflush(stdout); } Initialize start position and angle and counters if(RANDOM==FALSE) { CurrentPos[O] X_START; CurrentPos[l] Y_START; CurrentPos[2] Z_START; } else { CurrentPos[O] Random()*(Wall[O] .Position-Wall[l] .Position)+Wall[l] .Position; CurrentPos [1] Random()*(Wall[2] .Position-Wall[3] .Position)+Wall[3] .Position; CurrentPos [2] Random()*(Wall[4] .Position-Wall[5] .Position)+Wall[5] .Position; } II Check that initial position is within simulation volume if (CurrentPos [0] RandomAngle(CurrentDir); CurrentReflections = 0; Length_in_Trace 0; LastWall = -1; 1* ********************** Tracing Loop ********************** *1 NextLeg: 1* ===== Compute intersection with nearest wall Formula: CurrentPos[] + r*CurrentDir[] = Wallpositon *1 r = DBL_MAX; IntersectWal1 = -1; for(i=O; i<3; i++) { Temp = (Wall [2*i] .Position - CurrentPos[i) I CurrentDir[i]; if(Temp r = Temp; IntersectWal1 2*i; } Temp = (Wall [2*i+l) .Position - CurrentPos[i)) I CurrentDir[i); if(Temp if(IntersectWall == -1) { printf("Error: Could not find intersection with wall !\n\n"); exit(EXIT_FAILURE); } LastWal1 = IntersectWall; 1* ===== Did absorption occur over length r ? if(Random() > exp(-r/ABSLEN)) { if((int) Length_in_Trace<9) BulklostHist[(int) Length_in_Trace)++; else BulklostHist[9)++; Photons_absorbed_Bulk++; Length_absorbed += Length_in_Trace; continuej } Compute new position and update counters ===== *1 for(i=O; i<3; i++) CurrentPos[i) = CurrentPos[i)+r*CurrentDir[i); IntersectionHist[IntersectWall)++; TotaIIntersections++; Did total reflection occur ? ===== *1 II Temp is angle between trajectory and wall normal vector Temp = acos(CurrentDir[IntersectWaII/2) * ((IntersectWall%2==0) ? 1 -1)) ; if(Temp > TotrefAngle[IntersectWall)) { CurrentDir[IntersectWaII/2) *= -1.0; CurrentReflections++; TotaIReflections++; goto NextLeg; } 1* ===== If not, maybe still reflection? Reflection of electromagnetic waves according to Jackson, Classical electrodynamics, formulas (7.39) and (7.41) *1 Templ = sqrt (pow(Wall [IntersectWall) .n_outside,2) pow(N_INSIDE*sin(Temp),2)); Temp2 = pow((N_INSIDE*cos(Temp) - Templ) I (N_INSIDE*cos(Temp) + Templ), 2); Templ *= N_INSIDE; Temp3 = pow((pow(Wall[IntersectWall) .n_outside,2)*cos(Temp) - Templ) I (pow(Wall[IntersectWall) .n_outside,2)*cos(Temp) + Templ), 2); if (Random() < (Temp2+Temp3)/2) { CurrentDir[IntersectWaII/2) *= -1.0; CurrentReflections++; TotaIReflections++; 122 APPENDIX B. SIMULATION OF LIGHT PROPAGATION goto NextLeg; } 1* ===== Transmitted through wall 5. i.e. detection? if (IntersectWall==5) { if(CurrentReflections<9) ReflectionHist[CurrentReflections]++; else ReflectionHist[9]++; if«int) Length_in_Trace<9) LengthHist[(int) Length_in_Trace]++; else LengthHist[9]++; Photons_detected++; Length_detected += Length_in_Trace; continue; } Transmission through wall to reflector II Absorption in reflector? if (Random() > Wall [IntersectWall] .Reflectivity) { Photons_absorbed_Reflector++; LostHist[IntersectWall]++; Length_absorbed += Length_in_Trace; continue; } II Specular or diffuse reflection, if diffuse generate random angle in II hemisphere pointing to the inside from current wall if (Wall [IntersectWall] .Diffuse == TRUE) { do { RandomAngle(CurrentDir); } while(CurrentDir[IntersectWaII/2] * (IntersectWall'lo2==0 ? 1 -1) >= 0); DiffuseReflections++; } else { CurrentDir[IntersectWaII/2] *= -1; } CurrentReflections++; TotaIReflections++; goto NextLeg; } 1* for *1 II Output of statistics printf("Photons detected 'lod I 'lo.3g'lo'lo \n", Photons_detected. (double) Photons_detected/MAXNUM*100); printf("Photons absorbed in bulk 'lod I 'lo.3g'lo'lo and in reflector 'lod I 'lo.3g'lo'lo\n\n". Photons_absorbed_Bulk. (double) Photons_absorbed_Bulk/MAXNUM*100, Photons_absorbed_Reflector. (double) Photons_absorbed_Reflector/MAXNUM*100); printf("Wall intersection statistics:\n"); for(i=O; i<6; i++) printf(II Wall 'lod: 'lo8d I 'lo7.3g'lo'lo\t\t", i. IntersectionHist[i], (double) IntersectionHist[i]/TotaIIntersections*100); printf("\n\nStatistics for detected photons 11 "(diffuse reflections 'lod I 'lo.3g'lo'lo):\n". DiffuseReflections. (double) DiffuseReflections/TotaIReflections*100); for(i=O; i<9; i++) printf(" 'lod reflection(s): 'lo8d /'l.7.3g'lo'lo\t". i. ReflectionHist[i], (double) ReflectionHist[i]/Photons_detected*100); printf(" >=9 reflections: 'lo8d I 'lo7.3g'lo'lo\n\n", ReflectionHist[9], (double) ReflectionHist[9]/Photons_detected*100); for(i=O; i<9; i++) printf(" < 'lod mm: 'lo8d I 'lo7.3g'lo'lo\t\t". i+1, LengthHist[i]. B.6. SOURCE CODE 123 (double) LengthHist[i]/Photons_detected*100); printf(" >=9 mm: %8d / %7.3g%%\n", LengthHist[9] , (double) LengthHist[9]/Photons_detected*100); printf("\nStatistics for photons lost in reflector:\n"); for(i=O; i<6; i++) printf(" Wall %d: %8d / %.3g%%\t\t", i, LostHist[i], (double) LostHist[i]/Photons_absorbed_Reflector*100); printf("\n"); printf("\nStatistics for photons lost in bulk material:\n"); for(i=O; i<9; i++) printf(" < %d mm: %8d / %7.3g%%\t\t", i+l, BulklostHist[i], (double) BulklostHist[i]/Photons_absorbed_Bulk*100); printf(" >=9 mm: %8d / %7.3g%%\n", BulklostHist[9] , (double) BulklostHist[9]/Photons_absorbed_Bulk*100); printf("\n Average length travelled for detected photons: %.3g mm\n", Length_detected/Photons_detected); printf(" Average length travelled for absorbed photons: %.3g mm\n", Length_absorbed/(Photons_absorbed_Bulk+Photons_absorbed_Reflector)); return(EXIT_SUCCESS); } /* ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ Sub routines ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ */ /* Generates random phi (0 .. 2*Pi) and theta (0 ..Pi) with probability distribution of theta as 1/2*sin(theta) (see S.Brandt, Datenanalyse, p.83) and fills given vector */ void RandomAngle(double *AngleVector) { double Phi, Theta; /* Generate random Phi (0 ..2*Pi) and Theta (0 ..Pi) with probability distribution of Theta as 1/2*sin(Theta) (see S.Brandt, Datenanalyse, p.83) */ Phi = Random() * 2 * M_PI; Theta = acos(1.0 - 2*Random()); *AngleVector = cos (Phi) * sin(Theta); *(AngleVector+l) sin(Phi) * sin(Theta); *(AngleVector+2) = cos(Theta); } /* Generates random number between 0 and 1 */ double Random(void) { return (double) rand()/RAND_MAX; } ------ APPENDIX C Laboratory Background Measurements While making measurements with the large YAP scintillators used in the PSRD, a significant trigger rate was found without radioactive source even with rather high thresholds. That rate was not due to the photomultiplier, but was made up of actual signals from the crystal (see the end of Sect. 4.4.2). To investigate this background, spectra were taken with a 3x3x3 cm3 CsI(TI) crystal coupled to a R5900U photomultiplier operated at 800 V bias (CsI(TI) was used instead of YAP(Ce) because the former was already set-up for other reasons at that time). Spectra were taken with a charge integrating ADC using a gate width of IllS. The energy calibration was performed using the 835 keY line of 54 Mn and the 1116 keY line of 65Zn. The accuracy of this calibration can be estimated from Fig. C.1 where the energies of the additional 511 keY annihilation line of 65Zn and the lines at 1173 keY and 1333 keY of 60Co were measured, yielding a good agreement with the nominal values within the indicated calibration errors. After this calibration, spectra without radioactive source were taken for two energy ranges (/) Q) 'C Jjaoo 700 400 600 aoo 1000 1200 200 400 600 aoo 1000 1200 1400 Energy (keV) Energy (keV) (a) 65Zn, annihilation peak position nominally (b) 60 Co, lines nominally at 1173 keV and at 511 keY 1333keV Figure C.l Spectra showing the accuracy of the energy calibration 124 LABORATORY BACKGROUND MEASUREMENTS 125 (/) --, (/) ID ID ~80 1 ~3500 w Event rate (full histogram): 0.23 5. 70 3000 (18 ±O.4)MeV 2500 2000 1500 1000 500 (2650 ± 50) keY 0 1000 1500 2000 2500 3000 10 15 20 25 30 35 Energy (keV) Energy (MeV) (a) Lower energy. Nominal energy of 4°K line (b) Higher energy. dE/dx for MIPS in CsI(Tl) is 1461 keY. is 5.6 MeV/em. Figure C.2 Background spectra taken with a CsI(TI) scintillator as shown in Fig. C.2. In (a) the 40K line at 1461 keY, coming from the radioactive potassium that is present in concrete, is clearly visible, plus a line at 2650 keV and some more structure. A photon line at 2650 keY would have its Compton edge at 2420 keY (see footnote 1 on page 97) and an indication of this is present in the spectrum, although it is hard to determine the energy from it. Due to the low trigger rate of 6 s-l pile-up from the 40K line is excluded, also the energy of the line would be too low for this explanation, so this is likely a real photon line. Indeed 208TI, a short-lived part of the decay chain of naturally occurring radioactive 232Th, emits photons of 2615 keY. Furthermore, 214Bi, part of the decay chain of 238U, has significant lines at 1120keV and 1764keV and 208TI another, weak line at 860keV [CRC92]. All these emission lines are discernable in the spectrum, though not easily deduced from it. Some com plication results from the Compton edge of the 40K line at 1244 keY. It is superimposed on the steep shoulder and not far from the 1120 keY line. In Fig. C.2(b) a peak due to charged cosmic rays passing through the crystal is visible. The stopping power dEjdx for MIPS in CsI(TI) is 5.6 MeVjcm, so 16.8 MeV should be lost in a 3 cm thick crystal. As no special trigger was used, cosmic rays could also pass at oblique angles through the crystal, with consequently longer path lengths and higher energy losses. From [PDGOO, Sect. 20], the total flux of hard cosmic rays passing through a horizontal area from above is 0.009cm-2s-1, Le. 0.08s-1 on an area of 3x3cm2. This is compatible with the trigger rate within the peak in the spectrum, which is about 30% of the full trigger rate of 0.23 s-l , that is 0.07 s-l. APPENDIX D Chronology of PSRD Development In the following a chronological review of the initial conception, design, construction and flight of the PSRD is given, together with a collection of photographs. Many details on the shuttle flight can be found on the NASA web site at http://spaceflight.nasa.gov/shuttle/archives/ sts-l08/index.html. The PSRD web site is located at http://psrd.home . cern. ch/psrd. D.l Chronological description 1998 October Publication of a paper in Nuclear Instruments and Methods laying out the principle of the SRD in the context of AMS [Hof98]. December First experimental studies towards the SRD design, initially using gas-filled drift chambers, then switching to scintillators read out by photomultipliers. 1999 March - April Realization that not enough data on the background situation that will be encountered by the SRD is available, thus giving the initial impetus for the PSRD development. 31 May-1 June The first meeting dealing with the PSRD was held in Zurich. It was followed by 8 more meetings, the last one on 27/28 November 2000. August First integral mechanical design ideas using the standard GAS canl offered by NASA for small payloads. September Development of the electronic boards for the PSRD started at RWTH Aachen. Also, the development of the flight software was begun by Alexei Lebedev (MIT). 1 December Letter from S. Ting, head of the AMS collaboration, to J. O'Fallon, director of the Division of High-Energy Physics of the US Department of Energy (that largely finances the AMS project), requesting authorisation for a small payload accommodation on a Space Shuttle flight, initiating the formal manifestation of the PSRD. 2000 6 January Letter from J. O'Fallon to M. Sistilli at NASA headquarters forwarding the request and asking for a shuttle flight "before 2001". The detector at this point was called PSRDC, for IGet Away Special, a canister-like, standardised enclosure 126 D.l. CHRONOLOGICAL DESCRIPTION 127 "Prototype Synchrotron Radiation Detector Component". January As it became apparent that the approach of using a standard or modified GAS can is inappropriate, the design was changed to a custom-made support structure, with a modular, cassette-like approach. By the beginning of April, the principle design features had essentially reached their final state. 22 February A Request for Space Shuttle Flight Assignment (NASA form 1628) was officially filed, referring to a 77 kg, 250 W detector and asking for a flight in January 2001 as a Hitchhiker payload and requesting "1.5 days cumulative minimum duration of deep space data taking". March The decision was taken to add solar cells and the respective read-out boards to the PSRD. 14 April A technical interchange meeting was held at the Goddard Space Flight Center, marking the manifestation of the PSRD on a Hitchhiker bridge on mission STS-I07, scheduled at that point to launch on 17 May 2001. The bridge was initially called LONESTAR (Launch On Need Enabling Science, Technology and Research), with the name changed to FREESTAR (Fast Reaction Experiments Enabling Science, Technology, Applications and Research) by July. 26 May A preliminary random vibration test with the PC/104 computer and one hard disk was successful, clearing especially the mechanically delicate disk for use in the experiment. June The production of the final versions of the PSRD electronic boards started at CSIST in Taiwan. 21 July The Phase 0/1 Flight Safety Data Package was delivered to NASA. 10 August The Structural Verification Plan, detailing the structural verification requirements and the verification procedures, and the Fracture Control Plan were officially filed by J. Bates, PSRD Project Manager, to T. Dixon, FREESTAR Mission Manager. September Acceptance tests of the final electronic boards at RWTH, Aachen, with all compo nents passing, a few after minor modifications. Development of omine analysis software started in parallel, initially concentrating on the rather complex APV data. 19 September Acoustic test of PSRD windows at NSPO, Taiwan, successfully completed. 10 October The updates necessary for the Phase 2/3 Flight Safety Data Package were deliv ered to NASA. 25 October Sine-sweep and random vibration tests at Contraves, Zurich. The mounting of the hard disk container was found to be too weak as it became loose during the vibration test. It was later improved by using more, and larger, bolts. 4 December Final measurement of the response of the trigger detector scintillators to cosmic rays. 8 December A report on temperature simulations from C. Clark was filed to J. Bates, stating that the worst-case maximum temperatures of any PSRD component lie well within NASA safety limits, clearing concerns about failures of power interruption circuits. 2001 8 -12 January Thermal-vacuum test at the Max Planck Institute for Extraterrestrial Physics, Munich. 19 January Calibration of small YAP(Ce) array. 24 January First electromagnetic compatibility test. As several exceedings of limits were found, the detector was retested several times afterwards with progressively improved shielding. However, most of these exceedings vanished when it became clear that an outdated version of 128 APPENDIX D. CHRONOLOGY OF PSRD DEVELOPMENT the document detailing the emission limits was used. The few remaining exceedings were later deemed acceptable by NASA. 26 January Measurement of the response of the completed veto counters to cosmic rays and final adjustment of the threshold values. 31 January Calibration of the thresholds for the large YAP(Ce) crystals were done. 2 February Final version of the Structural Verification Report was submitted. 28 February Fracture Control Summary Report filed to T. Dixon. 9 March The PSRD and all support equipment was shipped from Zurich to Washington, DC, arriving there on the 16 March. Testing of the detector at the Goddard Space Flight Center (GSFC) started three days later. Crews of the PSRD team were repeatedly working at GSFC and later at the Kennedy Space Center (KSC), until the flight. 2 April The PSRD was officially moved from FREESTAR to MACH-1 (Multiple Application Customized Hitchhiker-1), a similar Hitchhiker bridge scheduled to fly on STS-108 in November 2001. The launch delay of STS-107 (until April 2002 at that point; at the time of writing until at least June 2002) became unacceptable in the view of support of the PSRD to the SRD development. Since a large radiator, part of the CAPL-3 experiment, is located on top of the MACH-1 bridge, it was necessary to tilt the PSRD to avoid obstruction of the field-of-view; a wedge-like support was constructed for this purpose. 21 September All MACH-1 components were shipped to the Kennedy Space Center for final testing and integration onto the cross-bay bridge. PSRD stand-alone testing at KSC commenced four days later, followed by testing after integration. 26 November The integration of the MACH-1 bridge with the Space Shuttle was completed. 29 November The planned launch of the shuttle on this day was delayed first by one day, then to 4 December, because of incomplete docking of an unmanned Progress cargo ship to the International Space Station a few days earlier. It was found later that a stray rubber sealing ring, possibly left behind by an earlier Progress vehicle, prevented a solid docking. This in turn would have made docking of the Space Shuttle (to another port) dangerous, as the structural stress during docking might have ruptured the weak connection of the cargo ship. 3 December Two of the current space station crew members cleared away the stray sealing ring during a space-walk. After that, the cargo ship could be docked firmly. 4 December The launch scheduled for 23:45 MET (17:45 EST local time) was cancelled a few minutes before lift-off due to unacceptable weather conditions, both over the launch pad and at the runway that would be used for landing in the case of an emergency abort after lift-off. The forecast for the following day claimed again "70% chance" of fine weather, as also for this day. 5 December At 23:19 MET (17:19 EST local time), the Space Shuttle Endeavour finally launched from pad 39B at Kennedy Space Center, Florida. 2 hours, 53 minutes after launch the PSRD was activated and a few minutes later confirmation of operation was received through the downlinked data. 42 minutes later, the shuttle was turning into PSRD (deep-space) attitude. 7 December The shuttle docked to the International Space Station at 21:03 MET. PSRD took data for more than 38 hours while docked. 15 December Undoeking occurred at 18:28 MET. 17 December At 18:55 MET (12:55 EST local time), the Space Shuttle touched down on runway 15 at KSC and came to a stop about 1 minute later. PSRD running in total was 37:01 hours in deep-space and 76:29 hours in other attitudes of the shuttle. 19.2 GByte of data were written to disk on side A, 18.8 GByte on side B. D.l. CHRONOLOGICAL DESCRIPTION 129 2002 January The MACH-1 Hitchhiker bridge was de-integrated from the shuttle, preprocessed at KSC and then shipped to GSFC were it was taken apart. The PSRD was shipped back to Zurich on 1 February. 5 February The PSRD returned to ETH Zurich on truck from Luxembourg, where is arrived by air-freight. The non-flight patch-panels were attached and the detector was successfully pow ered. Downloading of the science data through Zip disks started the same day and was completed on 8 February. 11 February A detailed health check was performed. One photomultiplier showed no response to LED signals and the SRDSOL board on side A did not respond anymore. Otherwise, all functions were normal. First signals from the small YAP array were extracted from the data. Data analysis started, during which investigations of the two problems will also be done. March Preliminary analysis showed that no photomultiplier or SRDSOL malfunction is ap parent in the data. First spectra showed proper detector operation for the the small YAP array. Detailed results will be presented in the PhD thesis of Barbara Zimmermann. 130 APPENDIX D. CHRONOLOGY OF PSRD DEVELOPMENT D.2 Photographs The photographs in this section illustrate the construction and testing of the PSRD, the integra tion with the MACH-1 Hitchhiker bridge, and finally the flight. Some photographs of hardware components are given in the relevant sections of Chapter 4. Aluminium structure after machining, One of the first tests after cabling was completed. showing the individual cassettes and Individual cassettes are now more difficult to the support for the DC/DC remove due to the cable harness. The monitors converters with its radiator wings. display one page of the flight control program. A functional test of the detector at Kennedy Space After the PSRD was integrated on Center in Florida. The orange colour comes from the Hitchhiker bridge, the protective the protection layer of the silver Teflon tape that layer of the silver Teflon tape was was applied to the detector for thermal reasons. removed, giving a shiny appearance. Visible at the bottom are the three cables that connect the detector to the Hitchhiker avionics. ------ D.2. PHOTOGRAPHS 131 The completed Hitchhiker bridge is hoisted into a so-called Transcan container for shipment to the launch pad, where it will be mounted into the shuttle payload bay. The tilting of the PSRD is apparent, conceived to avoid obstruction of the field-of-view by the large, white radiator structure on the top of the bridge. One of the final photos taken after the Hitchhiker bridge was mounted in the shuttle, just before closing the payload bay doors prior to launch. 132 APPENDIX D. CHRONOLOGY OF PSRD DEVELOPMENT The logo of the PSRD mission. The Space Shuttle Endeavour after roll-out from the OPF (arbiter Processing Facility) on its way to the large VAB (Vertical Assembly Building) where it will be mated with the two solid rocket boosters and the external tank. The Hitchhiker bridge was not yet integrated with the shuttle at this point. D.2. PHOTOGRAPHS 133 The shuttle was moved to the launch pad on The major part of the thrust for lift-off is a crawler-like vehicle. Next, the structure coming from the two solid rocket boosters, extending to the left will rotate over the while steering is performed through the shuttle and then the payload bay doors are three main engines of the shuttle that glow opened under clean-room conditions. The only faintly. Hitchhiker bridge is then installed. ISS003E8328 2001/12/0719:22:38 This photo was taken from the International Space Station just prior to docking. The top side of the PSRD is visible on the front-left of the Hitchhiker bridge. The Ku-band communication antenna of the shuttle is extended to the front-right of the payload bay. 134 APPENDIX D. CHRONOLOGY OF PSRD DEVELOPMENT A snapshot of the display on one laptop computer showing the status of the PSRD as downlinked two days after launch. About 6 GByte of data had been already taken per side at this point. Temperatures were stable, while one high-voltage channel failed permanently. After 11 days and 20 hours, the shuttle landed at Kennedy Space Center. About 38 GByte of data were taken by the PSRD in total. APPENDIX E Collaboration Members RWTH Aachen, Ill. Physikalisches Institut B, Germany Clemens Camps, Volker Commichau, Giinter Fliigge, Klaus Hangarter Lockheed Martin Company, Houston, USA Ken Bollweg, Craig Clark, '!rent Martin, Jim R. Pettis, Ernie Weeks Johnson Space Center, NASA, Houston, USA James R. Bates MIT, Laboratory for Nuclear Science, Cambridge, USA Michael Capell, Vladimir Koutsenko, Alexei Lebedev, Samuel C. C. Ting ETH Ziirich, Institut fiir Quantenelektronik, Switzerland Derk Batzner, Ayodhya N. Tiwari ETH Ziirich, Labor fiir Hochenergiephysik, Switzerland Hans Anderhub, Simon Baumgartner, Adrian Biland, Lubomir Djambazov, Oliver Grimm, Hans Hofer, Richard Kan, Gerald P. Kenney, Michael Kraber, Jos Kuipers, Werner Lustermann, Daning Ren, Ulf Roser, Gert M. Viertel, Hanspeter von Gunten, Silvia Waldmeier Wicki, Barbara Zimmermann Institute of Physics, Academia Sinica, Taipei, Taiwan Shih-Chang Lee, Zhong Liang Ren CSIST, Lung-Tan, Tao Yuan 325, Taiwan Yung-Jing Fanchiang, Te-Shing Wang Institute of High-Energy Physics, Kyungpook National University, Taegu, Korea Guinyun N. Kim, Manwoo W. Lee 135 APPENDIX F NASA Flow Schedule The typical schedule for supplying information, documents and the flight hardware to NASA is given here for reference purposes. The actual schedule for the PSRD was more compact, with especially the early items being shifted closer to launch and several items being joined together (see Appendix D for the PSRD chronology). The process is initiated by filing NASA form 1628 (Request for Space Shuttle Flight Assign ment). Most dates afterwards are given relative to launch date. General Customer Payload Requirements Launch - 24 months (1628 approval + 1 month) Customer accommodation meeting L- 23 months Public Affairs Office input L - 6 months Crew familiarization inputs L - 6 months Flight hardware delivery to GSFC L - 6 months Payload functional testing L - 5 months De-integration L + 14 days Mechanical Documentation Fracture Control Plan L- 26 months Fracture Control Plan Update L- 18 months Structural Verification Plan L- 18 months Mechanical drawings L- 18 months Fracture Control Summary L - 6 months Structural Verification Report L - 6 months Integration procedures for GSFC GSFC delivery - 1 month Integration procedures for KSC KSC delivery - 3 months Electrical Electrical schematics L- 18 months Test procedures for GSFC GSFC delivery - 1 month EMC test report at GSFC delivery Command bit pattern for orbiter IVT L- 17 months Preliminary power requirements L- 17 months Final power requirements L - 7 months Test procedures for KSC KSC delivery - 3 months Interface Verification Test at KSC ~L-3weeks 136 NASA FLOW SCHEDULE 137 Safety Phase 0/1 Safety Data Package (Ground & Flight) L- 18 months Phase 2 Safety Data Package (Ground & Flight) L- 12 months Phase 3 Safety Data Package (Ground & Flight) L - 8 months Final launch site safety information KSC delivery - 75 days Thermal Reduced thermal model L- 18 months Thermal report L - 6 months A number of Goddard Internal Simulations (GIS) and Joint Integrated Simulations (JIS) are normally conducted until several weeks before launch, practising operations that are used during the flight. Due to the rather simple commanding of the PSRD, a "paper simulation" without involvement of personnel was sufficient in this case. Many other documents are generated by NASA, using partly this supplied information and also by requesting further details. Additionally, several reviews are held during the manifestation process, dealing especially with safety issues. The following figure outlines the verification and documentation procedures necessary for flying a payload with the Space Shuttle. The relevant NASA documents are given ([DankOO], AIR = as required). ISafety Pol~y and Requirements for Payloads Using the Space Transportation System & ISS Addendum NSTS 1700.B Payload Safety Review and Data Submittal Requirements NSTSIISS 13830 l Mechanical Syolama Stroaa Corroalon TA·93·037 IIIPayload VerificationStructuralRequirements,QuallllcationSpace Shuttle ProgramI I IDesign Criteria for Controlling Stress Corrosion Cracking 1 TA·94·041 NSTS 14046 MSFC·SPEC·522 I MSFC·HDBK·527 M_Inlcal Syetama Structural V.llIcotlon Plan Part8 end M.te,lall Lilt Varlftcatlon Plan I H I H Mechanical Syatama Matartala Uaaga AgrHmanto AIR Verification Report St..... AnalyalsITaot ~ I Positive Safety Margins from Combined Loading (Launch, Operational, Thermal, Pressure, Etc) tl Flnlta Elamant Modal AIR I Strongth Taato AIR I Dynamic AnalyalaITaot Structural Frequencies Determination HModal Toot Corralatlon AIR I H Random Vibration Taot I L.J Acoustic Tilt AIR I Pressure Profile Analysis AIR for Equipment Sensitive to Pressure Profile Loads Tharmal _Iyala Thermal Model Required Ma.. ProparUaa 4 Structurel V.riflcellon Report I Seite Leer / Bla,nk leaf List of Figu res 1.1 Differential energy spectrum of cosmic rays ...... 12 1.2 Energy spectrum of cosmic rays at highest energy...... 13 1.3 Relative elemental abundances in the solar system and in cosmic rays 14 1.4 Range of cosmic-ray electrons ...... 18 1.5 Measured and calculated electron spectrum 20 2.1 Layout of the AMS-02 experiment 23 3.1 Principle of the SRD ...... 27 3.2 Plots illustrating the synchrotron mechanism 28 3.3 Spectrum of the diffuse photon background in a near-earth orbit 29 3.4 Flux spectrum of electrons above 30 keV...... 30 3.5 Emission spectrum of YAP(Ce) at room temperature. 36 3.6 The perovskite structure of YAP ...... 36 3.7 Schematic structure of a photomultiplier ...... 36 3.8 Voltage divider circuit for a photomultiplier ..... 37 3.9 Dynode structure of a metal channel photomultiplier 38 3.10 Dimensions of the R5900U photomultiplier ..... 38 3.11 Spectral response, gain and dark current curves for the R5900U photomultiplier. 38 3.12 Response of the R5900U for light of 400 nm over its photocathode area. 39 3.13 Tentative design of the SRD scintillator lay-out ...... 40 3.14 Absorption and emission spectrum of the BC-484 wavelength shifter 40 3.15 X-ray transmission of beryllium foils 41 3.16 Charged particle ranges in beryllium ...... 42 3.17 APV read-out design...... 43 3.18 Tentative block diagram of the SRD electronics 43 4.1 Hitchhiker cross-bridge in the Space Shuttle payload bay. 45 4.2 Overview of the PSRD ...... 46 4.3 Safety wiring of the mounting bolts. . 49 4.4 Layout of the X-ray cassette...... 50 4.5 Absorption of X-rays in YAP crystals 51 4.6 Design of the frames holding the beryllium windows 51 4.7 Normal incidence reflectivity of aluminium, silver and beryllium. 52 4.8 Light yield as function of distance between crystal and window 52 139 140 LIST OF FIGURES 4.9 Circuits for high-voltage generation...... 53 4.10 Circuit for reading out the anode signals ...... 54 4.11 Output signals from one small YAP photomultiplier at 900 V bias voltage 55 4.12 Energy calibration of the small YAP array ...... 56 4.13 Absorption of X-rays in YAP crystals of 30 mm thickness ..... 56 4.14 Detail of the design of the large YAP/veto counter combination. . 57 4.15 Transmission of X-ray photons through 5 mm of plastic scintillator 57 4.16 Output signals from a large YAP at 800 V bias voltage. ... 58 4.17 109Cd spectrum of a large YAP crystal...... 58 4.18 Signals from cosmic particles passing through a veto counters 59 4.19 Silicon macrostrip detector cassette. .... 60 4.20 Signals from the silicon macrostrip detector 62 4.21 Layout of the trigger detector cassette ... 63 4.22 Photographs of two electronic cassettes. .. 64 4.23 Simplified connection diagram of the PSRD electronics . 65 4.24 Schematic of the read-out timing ...... 66 4.25 Principle design of the SRDSAB logic ...... 67 4.26 Simplified grounding scheme of the PSRD components 69 4.27 PC/104 computer stack ...... 70 4.28 Battery protection circuit inside the battery container 70 4.29 Short-circuit current of SL-340 battery...... 71 4.30 Design of the disk container ...... 72 4.31 Variation of pressure with temperature in the hard disk container. 72 4.32 Mollier diagram for air at 1atm ...... 72 4.33 Sketch of the data link between PSRD and ground ...... 74 4.34 Details of the data link...... 76 4.35 Positions and connections of temperature and pressure sensors 78 4.36 Design of the patch panels...... 81 4.37 Random vibration excitation spectrum and response of the PSRD 82 4.38 PSRD thermal-vacuum test set-up ..... 83 4.39 PSRD during the EMC test ...... 83 4.40 Emission curves measured in the EMC test 84 5.1 Time probability distributions for different thresholds 88 5.2 Time probability distributions for first-electron timing 89 5.3 Schematic set-up for time resolution measurements .. 90 5.4 Measured time distributions ...... 91 5.5 Absorption measurements for different reflector materials 93 5.6 X-ray transmission through 80 pm of Teflon . 94 5.7 Set-up for the detection efficiency measurements . 95 5.8 ADC spectra of the 511 keY photopeak in the CsI(TI) trigger crystal 96 5.9 Set-up for the afterpulse measurements . 98 5.10 Response of the R5900U photomultiplier to an LED pulse .. 99 5.11 Afterpulse spectra with varying counting threshold . 100 5.12 Afterpulse spectra with varying photomultiplier bias voltage. 101 6.1 Two raw signals from the small YAP array . 105 6.2 One event of a small YAP crystal . 105 6.3 Background rate spectrum as measured during the shuttle flight 106 LIST OF FIGURES 141 B.1 Plot of ri, R and r~ . 116 C.1 Spectra showing the accuracy of the energy calibration. 124 C.2 Background spectra taken with a CsI(TI) scintillator .. 125 Acknowledgements Several figures for this thesis were kindly provided by Hans Anderhub, Hanspeter von Gunten, Richard Kan, Vladimir Koutsenko, Michael Kraber, Silvia Waldmeier Wicki and Barbara Zimmermann. List of Ta bles 1.1 Supernova remnants within 1 kpc from earth and age below 4 . 105 years 19 2.1 Particle identification momentum ranges for AMS-02 22 3.1 Photon number distributions ...... 29 3.2 Required time resolutions for suppression against background 31 3.3 Required time resolutions for suppression against background 31 3.4 Properties of scintillators . 33 3.5 Properties of the YAP(Ce) scintillator . 35 4.1 Environmental requirements for PSRD components . 47 4.2 Weight of individual PSRD components ..... 48 4.3 Data downlinked through the thermistor lines .. 77 4.4 Drive letter assignment when one hard disk is on 79 5.1 Relative light yield of different YAP(Ce) crystals 91 5.2 Relative light yield of a YAP crystal with different reflectors. 93 5.3 Measured detection efficiencies for 511 keY photons . 96 6.1 Summary of PSRD data taking periods during STS-108 mission. 104 142 References [A1caOO] J. A1caraz et al., Helium in near earth orbit, Phys. Lett. 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A 419, 295 (1998) Contact Information AirBorn, 4321 Airborn Drive, Addison, TX 75001-0519, USA, tt +1- (1)972 - 931 3200, http: //www.airborn.com Alcan Airex AG, Speciality Foams, 5643 Sins, Switzerland, ft +41- (0)41-789 66 00, http: //www.alusuisse-airex.com Ampro Computers, Inc., 5215 Hellyer Avenue #110, San Jose, California 95138-1007, USA, ft +1- (1)408 - 360 0200, http://www.ampro.com Angst + Pfister AG, Thurgauerstrasse 66, Postfach, 8052 Zurich, Switzerland, ft +41- (0)1- 306 61 11, http://www.angst-pfister.com Anox AG, Lindenmoosstrasse 11, 8910 Affoltern a. A., Switzerland, tt +41- (0)1-762 16 80, http://www.anox-ag.ch Avocent, 4991 Corporate Drive, Huntsville, AL 35805, USA, ft +1- (1 )256 - 430 4000, http: //www.avocent.com Contraves Space AG, Schaffhauserstrasse 580, 8052 Zurich, Switzerland, ft +41- (0)1- 30622 11 Crismatec/Bicron Holland, p.a. 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'«'''.~._ '''~~~'~_'_"".'_·.n_·._.'''''''''''_''C~_'-~'''~'~'·~~' ,~-~ ...... -.,..~ ... ,._~-_ .... Personae Gratae The PSRD project allowed me for the first time to participate in the (almost) full life-cycle of a physics experiment, starting with the initial conception, going through construction and testing and arriving finally at data taking, with a glance at data analysis. The successful completion of this project owes much to the relaxed, yet professional atmosphere created by the head of the collaboration, Prof. Gert Viertel. I am much indebted to him also beyond this for sufficiently loose, still well-directed guidance throughout my thesis work and for a good personal relationship (not least thanks to sitting in the same office than him !). All at ETH Zurich contributed of course a lot towards this work. For any kind of ques tions regarding electronics (anything related to voltages and currents, in fact), Ulf Roser ("der Prazisionselektroniker") was a friendly information source, and he was also giving me a couple of joy-rides in the morning, sparing me the bus. Daning Ren was most helpful in getting me acquainted with all sorts of measurements and the art of tracking down unexpected effects he left behind a gap when he left the institute. Adrian Biland was indispensable not only for his expertise as Linux administrator, saving me hours and days by having the right tricks at hand, but also by his serious and critical view on the physics behind the experiments. Volker Commichau at RWTH Aachen was also a knowledgeable source of help on electronics, digital this time, and, apart from that, both professionally and personally a unique character. I believe that he has anytime a chain saw as well as a scalpel at his disposal, depending on the problem to solve. I profited a lot from my stays at Aachen with him and his colleagues. Alexei Lebedev put his finger relentlessly on even the slightest weak points in any argumen tation, and so drove the project forward at lot. His humour and sarcasm made working with him very enjoyable, helping also much in dealing with his non-documentation approach (which was nevertheless highly organised !). My former laboratory-course tutor from New Zealand, Phillip Webb, kindly took a thorough look at the language deficiencies, improving the "linguistic" level a lot using his Kiwi-English. The responsibility for remaining mistakes lies of course solely with the author. Solid and helpful criticism on this text came also from my neighbour at Waidspital, Safia Thaminy, who was co-discoverer of many Zurich Cafes as well. Support from further away, as always patient, discreet and kind, came from my parents, and from "moja cudna zaba", Karina ! 149 Lebenslauf Personliche Daten Oliver Grimm Steinbecker Str. 69 21244 Buchholz Deutschland 'It +49 - (0)4181- 33673 E-Mail: [email protected] Geboren am 24. Mai 1972 in Buchholz, ledig Deutscher Staatsangehoriger Ausbildung 1978 -1982 Grund- und Hauptschule Meckelfeld 1982 -1984 Orientierungsstufe 11, Buchholz 1984 -1991 Albert-Einstein-Gymnasium, Buchholz 5/1991 Erwerb der Allgemeinen Hochschulreife 1991-1995 Studium der Physik an der Universitat Hamburg, Unterbrochen durch ein jahrigen Wehrdienst 1995 Einjahriges Auslandsstudium an der Universitat Auckland, Neuseeland Abschlufi mit einem "Certificate of Proficiency" Verleihung eines "Senior Prize" 1996 -1998 Weiterfuhrung des Studiums an der Universitat Hamburg 8/1998 Abschlufi mit dem akademischen Grad "Diplom-Physiker" 11/1998 - 3/2002 Doktoratsstudium an der Eidgenossischen Technischen Hochschule, Zurich Wehrdienst 4/1992 - 3/1993 Grundausbildung in Coesfeld, dann als Funkmaterialmechaniker in der Este tal Kaserne, Buxtehude Sprachen Deutsch (Muttersprache), Englisch und Franzosisch flie.Bend