Hans Kraus

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.

Abstract:

Large volume zinc tungstate crystal scintillators with improved scintillation characteristics were produced. Energy resolution and relative light output of samples with various sizes and shapes (cylinders, rectangular, and hexagonal prisms) were investigated. The energy resolutions of ZnWO 4 scintillator modules for the 662 keVγ line of 137Cs were 8.5% for a 1 cm3 sample and 10.7% for the large hexagonal module of dimensions ◇ 40 × 40 mm. A variation of the light output by 7%, depending on direction of observation was found for the 1 cm3 cubic crystal. Afterglow,measured 20 ms after termination of irradiation, was 0.002%. The relative light output of the scintillator and the decay kinetics were studied over the temperature range 7-300 K. The level of radioactive contamination of the ZnWO4 scintillator was determined in the Solotvina Underground Laboratory at a sensitivity of ≈0.1-10 mBq/kg. © 2006 IEEE.

Abstract:

The CRESST cryogenic direct dark matter search at Gran Sasso, searching for WIMPs via nuclear recoil, has been upgraded to CRESST-II by several changes and improvements. The upgrade includes a new detector support structure capable of accommodating 33 modules, the associated multichannel readout with 66 SQUID channels, a neutron shield, a calibration source lift, and the installation of a muon veto. We present the results of a commissioning run carried out in 2007. The basic element of CRESST-II is a detector module consisting of a large (∼ 300 g) CaWO4 crystal and a very sensitive smaller (∼ 2 g) light detector to detect the scintillation light from the CaWO4. The large crystal gives an accurate total energy measurement. The light detector permits a determination of the light yield for an event, allowing an effective separation of nuclear recoils from electron-photon backgrounds. Furthermore, information from light-quenching factor studies allows the definition of a region of the energy-light yield plane which corresponds to tungsten recoils. A neutron test is reported which supports the principle of using the light yield to identify the recoiling nucleus. Data obtained with two detector modules for a total exposure of 48 kg-days are presented. Judging by the rate of events in the "all nuclear recoils" acceptance region the apparatus shows a factor ∼10 improvement with respect to previous results, which we attribute principally to the presence of the neutron shield. In the "tungsten recoils" acceptance region three events are found, corresponding to a rate of 0.063 per kg-day. Standard assumptions on the dark matter flux, coherent or spin independent interactions, then yield a limit for WIMP-nucleon scattering of 4.8 × 10- 7 pb, at MWIMP ∼ 50 GeV. © 2009 Elsevier B.V. All rights reserved.