Hans Kraus

Abstract:

© 2017, Ivan Franko National University of Lviv. All rights reserved. The paper is devoted to the study of X-ray luminescence spectra, the scintillation light output and the decay time characterisation of undoped ZnTe at low temperatures down to 6 K. Also, the photoconductivity spectrum in a visible region has been investigated. Due to significant thermal quenching, the scintillations at α-particle excitation were detected in the sample only below T = 150 K. The emission of the crystal is attributed to the radioactive recombination of the holes trapped by Zn vacancies and electrons captured at the shallow levels of impurities or defects. The scintillation efficiency increased with further cooling. It has been found that at α-particle excitation undoped ZnTe exhibits a fairly competitive light output equal to 117 ± 20% of CaWO4reference scintillator. This finding underpins potential applications of ZnTe as a scintillation detector in the cryogenic experiments, particularly for the cryogenic search for neutrinoless double beta decay of130Te. It has been also found that ZnTe will be attractive as a conventional scintillation detector at the temperature of liquid nitrogen (T = 77 K). At this temperature, the scintillator exhibits a reasonably short decay time and a sufficient scintillation response to particle excitation. A practical implementation of this idea poses no real technical challenge since photomultipliers and Si-based photodetectors are proven to operate reliably and efficiently at this temperature.

Abstract:

It has been proven by several astronomical observations that dark matter exists, but no particle candidates have been observed yet. The CRESST experiment aims to directly detect dark matter particles elastically scattering off nuclei in CaWO4 crystals which are operated at mK temperatures. With nuclear recoil energy thresholds as low as 0.3 keV [2] and 0.6 keV [3], for the detector modules LISE and TUM40, respectively, CRESST is ideally suited for the detection of low-mass dark matter particles [5]. Additionally, the radiopurity of the crystals is another important factor for the detector performance. For a detailed understanding of the detector backgrounds, we simulate the radioactive contaminations of the TUM40 detector module with Geant4. The outcome of this simulation will be vital for the CRESST-III experiment. In this contribution, we discuss our results of the search for dark matter and dark photons achieved with the detector module Lise of CRESST-II. We will discuss the status of CRESST-III Phase 1 which started taking data in 2016.

Abstract:

CRESST is a multi-stage experiment directly searching for dark matter (DM) using cryogenic CaWO4 crystals. Previous stages established leading limits for the spin-independent DM-nucleon cross section down to DM-particle masses mDM below 1GeV/c2. Furthermore, CRESST performed a dedicated search for dark photons (DP) which excludes new parameter space between DP masses mDP of 300 eV/c2 to 700 eV/c2. In this contribution we will discuss the latest results based on the previous CRESST-II phase 2 and we will report on the status of the current CRESST-III phase 1: in this stage we have been operating 10 upgraded detectors with 24, g target mass each and enhanced detector performance since summer 2016. The improved detector design in terms of background suppression and reduction of the detection threshold will be discussed with respect to the previous stage. We will conclude with an outlook on the potential of the next stage, CRESST-III phase 2.

Abstract:

We report on a dark matter search for a Weakly Interacting Massive Particle (WIMP) in the mass range mx ∈ [4; 30] GeV/c^2 with the EDELWEISS-III experiment. A 2D profile likelihood analysis is performed on data from eight selected detectors with the lowest energy thresholds leading to a combined fiducial exposure of 496 kg-days. External backgrounds from γ-and β-radiation, recoils from 206Pb and neutrons as well as detector intrinsic backgrounds were modelled from data outside the region of interest and constrained in the analysis. The basic data selection and most of the background models are the same as those used in a previously published analysis based on Boosted Decision Trees (BDT) [1]. For the likelihood approach applied in the analysis presented here, a larger signal efficiency and a subtraction of the expected background lead to a higher sensitivity, especially for the lowest WIMP masses probed. No statistically significant signal was found and upper limits on the spin-independent WIMP-nucleon scattering cross section can be set with a hypothesis test based on the profile likelihood test statistics. The 90% C.L. exclusion limit set for WIMPs with mx = 4 GeV=c^2 is 1:6 X 10^-39 cm2, which is an improvement of a factor of seven with respect to the BDT-based analysis. For WIMP masses above 15 GeV/c^2 the exclusion limits found with both analyses are in good agreement.