Hans Kraus

Abstract:

Magnesium tungstate (MgWO4) crystals of ~1 cm³ volume were obtained for the first time using a flux growth technique. The crystal was subjected to comprehensive characterisation that included room-temperature measurements of the transmittance, X-ray luminescence spectra, afterglow under X-ray excitation, relative photoelectron output, energy resolution, non-proportionality of scintillation response to γ-quanta, response to α-particles, and pulse shape for γ-quanta and α-particles. The light output and decay kinetics of MgWO4 were studied over the temperature range 7-305 K. Under X-ray excitation the crystal exhibits an intense luminescence band peaking at a wavelength of 470 nm; the intensity of afterglow after 20 ms is 0.035%. An energy resolution of 9.1% for 662 keV γ-quanta of ¹³⁷Cs was measured with a small (≈0.9 g) sample of the MgWO4 crystal. The photoelectron output of the MgWO4 crystal scintillator is 35% that of CdWO4 and 27% that of NaI(Tl). The detector showed pulse-shape discrimination ability in measurements with α-particles and γ-quanta, which enabled us to assess the radioactive contamination of the scintillator. The results of these studies demonstrate the prospect of this material for a variety of scintillation applications, including rare event searches. © 2008 Elsevier B.V.

Abstract:

The luminescence properties of Ba3Tb0.9Eu0.1(PO4)3 and Ba3Gd0.9Eu0.1(PO4)3 phosphors were studied for excitation over the 120-300 nm wavelength range. It is found that Tb³⁺, which exhibits a strong vacuum-ultraviolet (VUV) absorption band, provides sensitisation of Eu³⁺ emission in this host. This effect can be used to develop phosphors with enhanced conversion efficiency of the VUV radiation into visible light. © 2009 Elsevier B.V. All rights reserved.

Abstract:

The CRESST-II direct Dark Matter search is located in the Gran Sasso underground laboratories, Italy. CaWO4 crystals are used as scintillating targets for WIMP (weakly interacting massive particle) interactions. They are operated as cryogenic calorimeters in combination with a second cryogenic detector used to measure the scintillation light produced in the target crystal. For each particle interaction, the combination of phonon and light signals provides an event by event discrimination which allows to distinguish known particles (alphas, betas, gammas, neutrons) from the expected signal of WIMPs. A major upgrade of the setup comprises modifications of the shielding, installation of a muon-veto, and new read out electronics, as well as a new detector-support structure to accommodate up to 33 detector modules, i.e. 10 kg of target mass. The experiment was thereafter successfully commissioned in 2007. Data obtained during this commissioning phase from 2 detector modules are presented here. Combining the data collected with these two detector modules with data from one single module obtained during the CRESST-I phase, the experiment could already place a limit of ∼6 × 10^-7 pb for the spin independent WIMP-nucleon scattering cross section at a WIMP mass of ∼60 GeV/c². © BAS, BDP Sciences 2009.

Abstract:

Results are presented on our latest research, aimed at the development and study of oxide scintillation crystals (ZnWO4, ZnMoO4, PbWO4, PbMoO4, and MgWO4) with high scintillation yield and low intrinsic radioactivity. We report on the improvement of these properties for conventional scintillators, as well as on new promising crystals based on metal tungstates and molybdates. The results are discussed in view of applying these materials in cryogenic experiments searching for dark matter and/or neutrinoless double beta decay. © 2006 IEEE.

Abstract:

In order to optimize sapphire as a cryogenic scintillation-phonon detector for dark matter, Al2 O3 : Ti crystals with different concentrations of doping have been studied using continuous X-ray excitation in the 30-300 K temperature range. Light yields vary by 20% for Ti concentrations between 10 and 1000 ppm at room temperature; they roughly double as the crystals are cooled from room temperature to 45 K. From the analysis of the change in the X-ray luminescence spectra of Al2 O3 with the concentration of Ti, it is concluded that the well-known blue emission of Ti-doped Al2 O3 is due to the radiative decay of F-centers. Recommendations are given for improving the performance of Al2 O3 scintillators. © 2009 Elsevier B.V. All rights reserved.