Investigation of CaWO_1tn4 crystals for simultaneous phonon-light detection in the CRESST dark matter search [Elektronische Ressource] / Jelena Ninković

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• Astronomie

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• Publié par: technische_universitat_munchen
• Publié le: 01 janvier 2005
• Nombre de lectures: 9
• Langue: English
• Poids de l'ouvrage: 11 Mo

Extrait

Investigation of CaWO Crystals for4
Simultaneous Phonon-Light Detection
in the CRESST Dark Matter Search
Jelena Ninkovi¶cTechnische Universit˜at Munc˜ hen
Max-Planck-Institut fur˜ Physik
(Werner-Heisenberg-Institut)
Investigation of CaWO Crystals for4
Simultaneous Phonon-Light Detection
in the CRESST Dark Matter Search
Jelena Ninkovi¶c
Vollst˜andiger Abdruck der von der Fakult˜at fur˜ Physik der Technischen Univer-
sit˜at Munc˜ hen zur Erlangung des akademischen Grades eines
Doktors der Naturwissenschaften
genehmigten Dissertation.
Vorsitzender: Univ.-Prof. Dr. Manfred Lindner
Prufer˜ der Dissertation: 1. Priv.-Doz. Dr. Rainer Kotthaus
2. Univ.-Prof. Dr. Lothar Oberauer
Die Dissertation wurde am 13.01.2005 bei der Technischen Universit˜at Munc˜ hen
eingereicht und durch die Fakult˜at fur˜ Physik am 16.02.2005 angenommen.Abstract
The goal of the CRESST Dark Matter Search is the direct detection of Weakly
Interacting Massive Particles (WIMPs) via elastic scattering ofi the nuclei in a
target absorber. Due to the low event rates expected for WIMP-nucleus scat-
tering it is of crucial importance to suppress background signals which might
mimic WIMP events as much as possible. Passive suppression is achieved by
the use of radiopure materials and by operating the detectors in a deep un-
derground location. In the second phase of the CRESST experiment an active
signal-to-background discrimination is achieved with the use of low temperature
scintillating calorimeters. Nuclear recoils, caused by neutrons and WIMPs, pro-
duce considerably less light than electron recoils of the same energy. Therefore,
a simultaneous measurement of scintillation light and temperature rise can be
used to identify the type of interaction.
In this work the properties and quality of CaWO single crystals of difierent4
origin were studied. Special attention was attributed to the radiopurity and the
optimization of the light output of large cylindrical crystals (` = 40mm; h =
40mm; m = 300g). A low level of radioactive contamination (»100„Bq=kg)
by isotopes of the natural decay chains was obtained. The in uence of difierent
parameters and conditioning procedures on the light yield of CaWO crystals4
was studied resulting in a signiflcantly improved reproducibility of the scintil-
lation properties. The efiect of difierent mechanical and thermal treatments on
the energy resolution and the light output of the crystals was investigated and
an¢E=E » 8%atE = 662keV wasobtained. In particular,
a new technique for the measurement of nucleus dependent quenching factors
was developed and the quenching factor of tungsten in CaWO was measured4
for the flrst time. Based on the obtained results, restrictive upper limit could be
obtained for the spin-independent WIMP-nucleus scattering cross section using
CRESST Phase II prototype detectors.
iContents
Abstract i
1 The quest for Dark Matter 1
1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 Motivation for Dark Matter search . . . . . . . . . . . . . . . . . 1
1.2.1 The evidence for the existence of Dark Matter . . . . . . 3
1.3 Dark Matter candidates . . . . . . . . . . . . . . . . . . . . . . . 6
1.3.1 Baryonic . . . . . . . . . . . . . . . . . . . . . 7
1.3.2 Non-baryonic candidates . . . . . . . . . . . . . . . . . . . 7
1.4 The direct detection of Dark Matter. . . . . . . . . . . . . . . . . 10
1.4.1 Nuclear form factor . . . . . . . . . . . . . . . . . . . . . . 11
1.4.2 Energy spectrum . . . . . . . . . . . . . . . . . . . . . . . 12
1.4.3 WIMP exclusion limits . . . . . . . . . . . . . . . . . . . . 13
1.4.4 Experimental sensitivities . . . . . . . . . . . . . . . . . . 14
1.4.5 Sources of particle background . . . . . . . . . . . . . . . . 15
1.4.6 Description of leading direct detection experiments . . . . 17
2 The CRESST Dark Matter search 23
2.1 The CRESST experimental set-up . . . . . . . . . . . . . . . . . . 23
2.1.1 Gran Sasso laboratory . . . . . . . . . . . . . . . . . . . . 23
2.1.2 Cryostat . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
2.1.3 The passive shielding . . . . . . . . . . . . . . . . . . . . . 27
2.2 The CRESST detectors . . . . . . . . . . . . . . . . . . . . . . . . 28
2.2.1 Basic principles of cryogenic detectors. . . . . . . . . . . . 29
2.2.2 The absorber . . . . . . . . . . . . . . . . . . . . . . . . . 30
2.2.3 The thermometer . . . . . . . . . . . . . . . . . . . . . . . 30
2.3 The readout system . . . . . . . . . . . . . . . . . . . . . . . . . . 31
2.4 Detector operation . . . . . . . . . . . . . . . . . . . . . . . . . . 34
2.5 Basic elements of data analysis . . . . . . . . . . . . . . . . . . . 34
2.6 CRESST Phase I . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
2.6.1 Limitations of the CRESST Phase I . . . . . . . . . . . . . 35
2.7 CRESST Phase II: Idea . . . . . . . . . . . . . . . . . . . . . . . 37
iii2.7.1 The proof-of-principle experiment . . . . . . . . . . . . . . 38
2.7.2 Detector module . . . . . . . . . . . . . . . . . . . . . . . 39
2.7.3 Projected sensitivity of CRESST Phase II . . . . . . . . . 41
3 The Scintillating CaWO Crystals 434
3.1 The basics processes . . . . . . . . . . . . . . . . . . . . . . . . . 43
3.2 Creation of the electron-hole pairs . . . . . . . . . . . . . . . . . . 44
3.2.1 Absorption of the incident radiation . . . . . . . . . . . . . 44
3.2.2 Relaxation of the primary electrons and holes . . . . . . . 46
3.2.3 Thermalization of the low-energy secondaries . . . . . . . . 47
3.3 Excitation and emission of luminescence centers . . . . . . . . . . 47
3.4 The scintillation process in CaWO . . . . . . . . . . . . . . . . . 484
3.5 Quenching mechanisms . . . . . . . . . . . . . . . . . . . . . . . . 48
3.6 Characteristics of inorganic scintillators . . . . . . . . . . . . . . . 51
3.6.1 Light yield . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
3.6.2 Duration of a scintillation pulse . . . . . . . . . . . . . . . 52
3.6.3 Afterglow . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
3.6.4 Temperature dependence of the response . . . . . . . . . . 53
3.6.5 Optical properties . . . . . . . . . . . . . . . . . . . . . . . 53
4 Measurement techniques 55
4.1t of scintillator light output at room temperature . . 55
4.2 Transmission measurement . . . . . . . . . . . . . . . . . . . . . . 56
4.3 Luminescencets . . . . . . . . . . . . . . . . . . . . . 57
4.4 Quenching factor measurements . . . . . . . . . . . . . . . . . . . 58
4.4.1 Linear time-of- ight mass spectrometer - TOF-MS . . . . 59
4.4.2 Mass spectrometer with a LDI ion source . . . . . . . . . . 59
4.4.3 MCP measurements . . . . . . . . . . . . . . . . . . . . . 61
4.4.4 Room temperature set-up . . . . . . . . . . . . . . . . . . 62
4.4.5 Low temp set-up . . . . . . . . . . . . . . . . . . . 63
5 Results 67
5.1 Properties of the crystals . . . . . . . . . . . . . . . . . . . . . . . 68
5.1.1 Purity of the crystals . . . . . . . . . . . . . . . . . . . . . 71
5.1.2 Transmission of the crystals . . . . . . . . . . . . . . . . . 73
5.1.3 Luminescence measurements . . . . . . . . . . . . . . . . . 77
5.1.4 Scintillation properties . . . . . . . . . . . . . . . . . . . . 86
5.1.5 Annealing of the CaWO crystals . . . . . . . . . . . . . . 934
5.2 Light collection and detector module optimization . . . . . . . . . 94
5.2.1 Light trapping. . . . . . . . . . . . . . . . . . . . . . . . . 95
5.2.2 Optimization of detector module . . . . . . . . . . . . . . 97
5.3 Quenching factor measurements . . . . . . . . . . . . . . . . . . . 105
5.3.1 MCPts . . . . . . . . . . . . . . . . . . . . . 105
iv5.3.2 Room temperature measurements . . . . . . . . . . . . . . 105
5.3.3 Low tempts . . . . . . . . . . . . . . . 121
5.3.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
6 Conclusions and Perspectives 129
A High Purity Germanium ?-spectroscopy measurements - HPGe133
B Inductively Coupled Plasma Mass Spectroscopy measurements
- ICP-MS 136
C De ection plate parameters 142
D SRIM2003 Simulation 143
Bibliography 148
v