Hans Kraus

Abstract:

The properties of CaWO4 (CaWO) crystals in γ-spectrometry were studied at room and liquid nitrogen(LN2) temperatures. Two small samples of 10×10×4 mm3 and 10×10×8 mm 3 size were tested, coupled to a Photonis XP3212 photomultiplier at room temperature and a large area avalanche photodiode at LN2 temperature. Light pulse shape and light output at room and LN2 temperatures were measured. Energy resolution and non-proportionality of the CaWO response versus γ-ray energy were studied and compared with those of small BGO and CdWO4 crystals to discuss further the origin of the intrinsic resolution of undoped scintillating crystals. A high light output of 4800±200 phe/MeV and a good energy resolution of 6.6±0.2% for 662 keV γ-rays from a 137Cs source were measured for the small samples coupled to the XP3212 photomultiplier. © 2005 Elsevier B.V. All rights reserved.

Abstract:

The luminescence properties of Ti-doped Al2O3 crystals have been studied using monochromatic VUV and X-ray radiation in the temperature range 9-300 K. In addition to the emission band of Ti3+ in the near IR region and blue emission at 420 nm that are commonly observed in this material, we detected a UV band at 290 nm which contributes approximately one-third to the integrated emission under X-ray excitation. Based on the results of the study this band was assigned to the radiative decay of excitons localised at activator ions. Particular attention has been given to the assessment of the feasibility of the material as cryogenic scintillation detector. The low-temperature scintillation light yield of Al2O 3-Ti (0.20 wt%) is found to be 2300±200 ph/MeV at 32 keV and an estimate shows that it is two times larger for a sample with 0.07 wt% of activator. Given this, we concluded that Ti-doped Al2O3 appears to be very suitable material for cryogenic Dark Matter search experiments that rely on the simultaneous detection of phonon and scintillation signals. © 2005 Elsevier B.V. All rights reserved.

Abstract:

The CRESST Phase II experiment at Gran Sasso is using 300 g scintillating CaWO4 crystals as absorbers for direct WIMP (weakly interactive massive particles) detection. The phonon signal in the CaWO4 crystal is registered in coincidence with the light signal, which is measured with a separate cryogenic light detector. The absorber crystal and the silicon light detector are read out by tungsten superconducting phase transition thermometers (W-SPTs). As a result an active discrimination of the electron recoils against nuclear recoils is achieved. Results on the properties of the detector modules and on the WIMP sensitivity are presented. (C) 2005 Published by Elsevier B.V.

Abstract:

The luminescence spectra of CaW O4, CaMo O4, and ZnW O4 scintillating crystals were investigated in the temperature range 8-400 K. The excitation photon energy was varied from the ultraviolet (4.5 eV) to the hard x-ray region (35 keV). It is found that as the excitation energy decreases the relative intensity of the low-energy luminescence band, attributed to the extrinsic emission of defect centers in CaW O4 and CaMo O4 crystals, increases. This observation is interpreted in terms of the total absorption of incident radiation, i.e., the variation of the mean penetration depth of the photons with their energy. It indicates that the centers responsible for the extrinsic emission in the crystals with scheelite structure are mainly localized in a thin (∼100 nm) surface layer. On the other hand no noticeable changes with the excitation energy were found in the emission spectra of ZnW O4 crystals with wolframite structure. The possible implication of this finding is discussed. The light yield of the crystals is compared at low temperature using monochromatic x-ray excitation and it is shown that ZnW O4 has ∼10% higher light yield than CaW O4, while this parameter has a factor of 4 lower in CaMo O4. © 2005 American Institute of Physics.