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

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.