Discrimination between Nuclear Recoils and Electron Recoils by Simultaneous Detection of Phonons and Scintillation Light

We have developed a detector, consisting of a cryogenic calorimeter with a scintillating crystal as absorber, and a second calorimeter for the detection of the scintillation light, both operated at 12 mK. Using a CaWO4 crystal with a mass of 6 g as scintillating absorber, we have achieved a discrimination of nuclear recoils against electron recoils with a suppression factor of 99.7% at energies above 15 keV. This novel method will be applied for background rejection in the CRESST (Cryogenic Rare Event Search with Superconducting Thermometers) experiment looking for dark matter Weakly Interacting Massive Particles (WIMPs). c © American Institute of Physics, 1999. Direct dark matter WIMPs (Weakly Interacting Massive Particles) searches are looking for nuclear recoil events caused by WIMPs scattering off a nucleus in a detector. The kinetic energy of a recoiling nucleus due to a WIMP interaction is expected to be of order of a few tens of keV. Such recoils have a low ionization and scintillation efficiency and are therefore hard to detect in conventional detectors. In contrast, cryogenic calorimeters are fully sensitive to nuclear recoils and can achieve much lower energy thresholds1–3 making them the most promising detectors for a dark matter search. Since these dark matter events are expected to be very rare, such experiments have to be carefully shielded against cosmic radiation and ambient radioactivity. The sensitivity of