Hans Kraus

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.

Abstract:

The scintillation properties of a zinc tungstate crystal, shaped as a hexagonal prism (height 40 mm, diagonal 40 mm) were determined. An energy resolution of 10.7% for the 662 keV γ-line of ¹³⁷Cs was measured with the scintillator placed in a light collection setup similar to that used by the CRESST dark matter search. The light output and decay kinetics of ZnWO4 were examined over the temperature range 7-300 K and confirmed to be competitive with those of CaWO4. The radioactive contaminations of the ZnWO4 scintillator measured in the Solotvina Underground Laboratory do not exceed 0.1-10 mBq/kg (depending on radionuclide). Our study highlights the excellent feasibility of this ZnWO4 scintillator for a cryogenic dark matter experiment. © 2008 Elsevier B.V. All rights reserved.