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Petricca, Federica (2005): Dark Matter Search with Cryogenic Phonon-Light Detectors. Dissertation, LMU München: Faculty of Physics
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Abstract

The CRESST (Cryogenic Rare Event Search with Superconducting Thermometers) Dark Matter search is aiming to directly detect Weakly Interacting Massive Particles (WIMPs) via their elastic scattering off nuclei in a target. Due to the low event rate expected for WIMP-nucleus scattering the suppression of background which would hide or mimic the WIMP signal is of crucial importance. Moreover, since the energy transferred to a nucleus in a WIMP-nucleus elastic scattering is extremely low (a few tens of keV), the energy threshold and the sensitivity of detectors are additional fundamental issues in a Dark Matter search. CRESST detectors consist of a 300g CaWO4 scintillating crystal operated as a cryogenic calorimeter in close proximity to a second much smaller cryogenic calorimeter used to detect the scintillation light produced in the target crystal. The lower light yield of nuclear recoils, caused by neutrons and WIMPs, with respect to electron recoils resulting from alpha, beta and gamma interactions is used to identify the event in the scintillating absorber. In the second phase of the CRESST experiment, active background suppression is achieved by the simultaneous measurement of a phonon and a light signal from a scintillating cryogenic calorimeter. Passive background suppression is achieved by operating CRESST detectors in a low background facility located in a deep underground site. About 1% of the energy deposited in CaWO4 by beta or gamma interactions can be detected as scintillation light; therefore the sensitivity of light detectors is a fundamental issue for the discrimination of electron recoils from nuclear recoils at energies relevant for WIMP searches. This work reports on the development of extremely sensitive cryogenic calorimeters for the detection of the scintillation light. These detectors applied a new thermometer geometry characterized by phonon collectors and a thin film thermal coupling to the heat sink. This concept allows a high sensitivity by decoupling the area required for the collection of non-thermal phonons and the heat capacity of the sensor and permits to make the thermal relaxation time of the thermometer long enough to allow for the integration of the scintillation light, despite the slow light emission of CaWO4 at low temperature. Results on new materials to be used as absorbers for a new generation of light detectors are presented. First competitive limits on WIMP Dark Matter established by the CRESST experiment running scintillating CaWO4 cryogenic detectors in association with the light detectors developed in the course of this work are reported together with main results of the detector prototyping phase.