Investigation of CaWO4 Crystals for Simultaneous Phonon-Light Detection in the CRESST Dark Matter Search

The goal of the CRESST Dark Matter Search is the direct detection of Weakly Interacting Massive Particles (WIMPs) via elastic scattering off the nuclei in a target absorber. Due to the low event rates expected for WIMP-nucleus scattering 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 underground 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, produce 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 CaWO4 single crystals of different origin were studied. Special attention was attributed to the radiopurity and the optimization of the light output of large cylindrical crystals (m = 300 g). A low level of radioactive contamination (~100 microBq/kg) by isotopes of the natural decay chains was obtained. The influence of different parameters and conditioning procedures on the light yield of CaWO4 crystals was studied resulting in a significantly improved reproducibility of the scintillation properties. The effect of different mechanical and thermal treatments on the energy resolution and the light output of the crystals was investigated and an energy resolution of ~8% at E = 662 keV was obtained. In particular, a new technique for the measurement of nucleus dependent quenching factors was developed and the quenching factor of tungsten in CaWO4 was measured for the first 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