The CDMS II Z-Sensitive Phonon and Ionization Detectors

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The last year saw major progress in the development of the nuclear-recoil dark matter detector for the Cryogenic Dark Matter Search (CDMS II) experiment. The basic γ-ray background discrimination is based on ionization yield. However, as shown in separate experiments, surface events and especially electrons also results in low ionization yield due to incomplete charge collection. The two-fold strategy to reduce these unwanted backgrounds was to improve charge collection at the detector surface and to employ fast phonon sensors on Ge. A 250 g Ge detector with an Al Schottky contact on an amorphous Si blocking layer and a Quasiparticle-trap-assisted Electrothermal-feedback Transition-edge (QET) phonon sensor has been characterized. The phonon collection efficiency of the new detector is similar to that of the latest 100 g Si Fast Large Ionization and Phonon (FLIP) detector, which has the same phonon sensor design. The fall times of the phonon pulses are longer and consistent with simulations. The charge collection at the surface is high and very similar to that of smaller test devices and the last revision of Berkeley Large Ionization and Phonon (BLIP) detectors. The four fast non-thermal phonon sensors yield position resolution similar to the Si FLIP detector. A rise time surface effect was demonstrated on both sides of the detector. The effect results in a much more effective rejection of surface events than the rejection based on charge yield alone and therefore improves the sensitivity for a dark matter search.