Hans Kraus

Abstract:

The structure of calcium molybdate was studied by means of synchrotron based high-resolution powder diffraction methods in the temperature range 12-300 K. The scheelite structure type was confirmed for CaMo O4 in the temperature region investigated and no structural anomalies were observed. Thermal expansion coefficients extracted from the thermal dependencies of the cell sizes are found to be in good agreement with the predictions from our lattice dynamics calculations that form the background for microscopic interpretation of the experimental data. From the analyses of experimental results and the calculated thermal expansion coefficients, elastic constants, phonon density of states, heat capacities, entropy, and Grüneisen parameters it is concluded that a quasiharmonic lattice dynamics approach provides a good description of these properties for CaMo O4 at temperatures below 800 K. © 2006 The American Physical Society.

Abstract:

A new method for measurements of the scintillation characteristics of materials has been developed. This method, called multiple photon counting coincidence (MPCC) technique, is based on the recording of a sequence of individual photon pulses resulting from a scintillation event. The distribution of the arrival times of these individual photon pulses provides information about the decay characteristic of the scintillation process and the number of photons recorded per scintillation event is proportional to the scintillation light yield. The ability to reject spurious events through off-line analysis is an important advantage of the MPCC method since it allows cleaning of the data set from pile-up events. It is shown that the MPCC technique is particularly well suited for the analysis of slow scintillation processes in the investigation of temperature-dependant scintillator properties. It is now used extensively by our group for the identification and optimisation of scintillating targets for cryogenic low-background rare event searches, such as Dark Matter and 0-v double beta decay experiments.

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.