Hans Kraus

Abstract:

Luminescence of a CaWO4 scintillator was studied in the temperature range 77-500 K using intense laser excitation in the 450-600 nm spectral region. Characteristics of the observed blue emission are similar to those of the intrinsic luminescence of calcium tungstate. The emission is concluded to be caused by cascade excitation of CaWO4 that results in a two-photon excited (TPE) luminescence. The features of TPE luminescence of CaWO4 are analysed in comparison to those obtained with UV and X-ray excitation. (C) 2004 Elsevier Ltd. All rights reserved.

Abstract:

Luminescence properties of CaWO4 have been investigated using complementary one- and two-photon excitation techniques. Analysis of the thermal changes in the luminescence spectra indicates that the observed high-energy shift of the CaWO4 emission maximum is mainly caused by a change of position of the intrinsic blue band. The thermal broadening and shift of this band can be interpreted satisfactorily in terms of a model of the luminescence center interacting with the vibrating crystalline environment. The characteristic parameters of the phonon system obtained from the experiment agree with those from earlier independent studies. Intense laser stimulation of CaWO4 in the spectral region <505 nm (>2.45 eV) results in emission with spectral and kinetics features that are characteristic of the radiative decay of a WO42- oxyanion complex in this crystal. The kinetics of luminescence decay under two-photon excitation changes with increasing excitation density due to exchange interaction of the elementary excitations. From a comparative analysis of the optical properties of CaWO 4 obtained in the course of two-photon and one-photon spectroscopic studies (absorption, reflection, one- and two-photon excitation spectra) it is concluded that the energy gap of the crystal is 5.2 ± 0.3 eV.

Abstract:

In the CRESST-II experiment, scintillating CaWO4 crystals are used as absorbers for direct weakly interacting massive particles (WIMP) detection. Nuclear recoils can be discriminated against electron recoils by measuring phonons and scintillation light simultaneously. The absorber crystal and the silicon light detector are read out by tungsten superconducting phase transition thermometers. Results on the sensitivity of the phonon and the light channel, radiopurity, the scintillation properties of CaWO4, and on the WIMP sensitivity are presented. (C) 2003 Elsevier B.V. All rights reserved.

Abstract:

We have developed a 66-channel SQUID system to read out the detectors for the second phase of the CRESST dark matter search using SQUID sensors, electronics and cryocables supplied by different manufacturers. The system has been extensively tested at Oxford to characterise the sensitivity, bandwidth, slew rate, noise, thermal drift, and crosstalk. By using the SQUID sensors as magnetometers we can confirm the input circuit for the detectors is superconducting, thus ensuring there will be no noise from parasitic resistance. Multichannel SQUID systems have further applications in particle physics experiments, such as precision magnetometry in the search for the neutron electric dipole moment. (C) 2003 Elsevier B.V. All rights reserved.