Hans Kraus

Abstract:

The future application of ZnWO 4 scintillator in a cryogenic search for rare events is the motivation for optimization of this material. We present results on the effect of Ca doping on the structure and scintillation properties of ZnWO 4. X-ray diffraction analysis revealed that there is no mixing in the CaWO 4-ZnWO 4 pseudobinary system due to a significant mismatch of the crystal structures of CaWO 4 and ZnWO 4. The lattice parameters of Ca-doped ZnWO 4 samples obtained from X-ray powder diffraction data confirmed this finding. It is also shown that ZnWO 4 retains the monoclinic wolframite structure when cooling, at 12 K exhibiting the following lattice parameters: a = 4.6826(2) Å, b = 5.7088(2) Å, c = 4.9230(2) Å and β = 90.541(2)°. The scintillation light yield of the Zn 1-x,Ca xWO 4 was measured using the multi-photon counting technique and it is found that small concentrations of Ca (x = 0.001 - 0.02) cause no deterioration of this parameter. Ca doping of ZnWO 4 is expected to facilitate production of a single-crystalline scintillator. © 2007 WILEY-VCH Verlag GmbH & Co. KGaA.

Abstract:

The Monte-Carlo refractive index matching (MCRIM) technique was developed to determine the physical properties of heavy inorganic scintillators (HIS) which are difficult to measure experimentally. It was designed as a method for obtaining input parameters for Monte-Carlo (MC) simulations of experimental arrangements incorporating HIS in their setups. The MCRIM technique is used to estimate the intrinsic light yield, the scattering coefficient and the absorption coefficient, herein referred to as indirect measurement properties. The MCRIM technique uses an experiment/MC combination to determine these indirect measurement properties. The MCRIM experimental setup comprises a crystal placed on a photomultiplier tube window with the possibility of introducing materials of different refractive indices in a small gap between the crystal and photomultiplier tube (PMT) window. The dependence of the measured light yield on the refractive index of the material in the gap can only be reproduced by simulations if the correct values of scattering, absorption and intrinsic light yield are used. The experimental setup is designed to minimise the presence of optical components such as unpolished surfaces and non-ideal reflectors, which are difficult to simulate. The MCRIM technique is tested on a 1.03×1.00×0.82 cm³ crystal of CaWO4 which is found to have a scattering coefficient of 0.061±0.005 cm^-1, an absorption coefficient of 0.065±0.005 cm^-1, and an intrinsic light yield of 22700±1700 photons/MeV. © 2006 Elsevier B.V. All rights reserved.

Abstract:

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

Abstract:

The temperature dependence of the decay time and scintillation light yield of pure CaF2 crystal was measured over the temperature range 8-305 K using the multiphoton coincidence counting technique. Pure CaF2 exhibits emission of triplet self-trapped excitons at 280 nm with a slow decay, the time constant of which changes significantly with temperature. The main decay time constant increases by three orders of magnitude when cooled, from 0.96±0.06 μs at 295 K to 930±40 μs at 8 K. The results obtained demonstrate that the scintillation light yield of pure CaF2 increases with decreasing temperature down to 20 K below which it is roughly constant. At low temperatures the light yield of CaF2 is estimated to be 60% relative to that of pure CaWO4. It is concluded that undoped calcium fluoride is a very attractive target material for experimental searches for rare events based on the detection of phonon and scintillation signals. © 2006 Elsevier B.V. All rights reserved.