Hans Kraus

Abstract:

EURECA (European Underground Rare Event Calorimeter Array) is a new project, searching for dark matter, with largely the present groups of the CRESST and EDELWEISS experiments and already a few new groups. The aim is to explore scalar cross sections in the 10(-9) - 10(-10) pico-barn region with a target mass of up to one tonne. A major advantage of EURECA is our planned use of more that just one target material (multi target experiment for WIMP identification). In preparation for this large-scale experiment, R&D for EURECA is provided through the current phases of CRESST and EDELWEISS.

Abstract:

Spectroscopic properties of CaMoO4 and MgMoO4 crystals were studied in view of their application to cryogenic scintillation detectors. Luminescence spectra and the luminescence decay kinetics were measured over a wide range of temperatures (8-300 K). For the first time we measured time-resolved luminescence spectra of CaMoO4. In addition to the green emission arising from the triplet state of self-trapped excitons (STEs), a new band at around 430 nm with a decay time constant 10 ± 3 ns was observed at T ≤ 8 K. This emission is assigned to the radiative decay of a singlet STE. The relaxation of electronic excitations in the crystals under study is discussed on the basis of our current understanding of their electronic structures and a configuration coordinate model for the radiative decay of STEs. The model includes adiabatic potential energy surfaces (APESs) associated with singlet and triplet states and explains the variation of the luminescence kinetics with temperature as a result of a re-distribution in the population of these states. Thus, judging from the change of the singlet STE emission due to temperature variation, we infer the existence of an energy barrier between the singlet and triplet APESs. The multi-exponential character of the decay of the triplet emission can be understood assuming that the relevant radiative transitions originate from different minima of the triplet APES. Non-radiative energy transfer processes control the population of these states, resulting in thermal variation of the intensities of the different emission components. © 2005 IOP Publishing Ltd.

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.