A detector module with highly efficient surface-alpha event rejection operated in CRESST-II Phase 2

TL;DR

This paper introduces a new detector design for the CRESST-II experiment that significantly improves surface-alpha background rejection, enabling more sensitive searches for low-mass WIMPs in direct dark matter detection.

Contribution

The paper presents a novel detector design using CaWO$_4$ sticks and a fully-scintillating housing to enhance surface-alpha event rejection in cryogenic dark matter detectors.

Findings

Achieved a threshold of ~0.60 keV and resolution of ~0.090 keV at 2.60 keV.

Reduced background levels allow setting new limits on WIMP-nucleon cross sections.

Probed a new parameter space for WIMPs below 3 GeV/c$^2$.

Abstract

The cryogenic dark matter experiment CRESST-II aims at the direct detection of WIMPs via elastic scattering off nuclei in scintillating CaWO$_4$ crystals. We present a new, highly improved, detector design installed in the current run of CRESST-II Phase 2 with an efficient active rejection of surface-alpha backgrounds. Using CaWO$_4$ sticks instead of metal clamps to hold the target crystal, a detector housing with fully-scintillating inner surface could be realized. The presented detector (TUM40) provides an excellent threshold of ${\sim}\,0.60\,$keV and a resolution of $\sigma\,{\approx}\,0.090$ keV (at 2.60$\,$keV). With significantly reduced background levels, TUM40 sets stringent limits on the spin-independent WIMP-nucleon scattering cross section and probes a new region of parameter space for WIMP masses below 3$\,$GeV/c$^2$. In this paper, we discuss the novel detector design and the surface-alpha event rejection in detail.