We have demonstrated a new type of phonon sensor for cryogenic particle detectors with high-bandwidth SQUID readout. Our Quasiparticle-trap assisted Electrothermal feedback Transition edge sensor (QET) utilizes aluminum quasiparticle traps attached to a tungsten superconducting transition edge sensor patterned on a silicon substrate. The tungsten lines are voltage biased and self-regulate in the transition region. Phonons from particle interactions in the silicon deposit energy into the creating quasiparticles. The quasiparticles are trapped into the tungsten and cause its electrical resistance to increase. The resulting decrease in current through the sensor is measured with a DC SQUID array. We have been able to demonstrate xy-position resolution of /spl sim/0.3 mm for 6 keV X-rays from an /sup 55/Fe source in a 2 cm/spl times/2 cm/spl times/4 mm (4 g) detector. We describe our optimizations of the sensor design for improved energy and position. By simultaneously measuring the ionization yield we have demonstrated discrimination between electron and nuclear recoil events in a 100 g Si detector for the CDMS (Cryogenic Dark Matter Search) experiment.
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