Enhanced ballistic phonon production for surface events in cryogenic silicon detector

We present evidence of an enhanced ballistic phonon component resulting from surface events in a 100 g silicon cryogenic dark matter detector. Surface events, calibrated using a 14C electron source, were found to have faster rise times (∼5 μs) than bulk gamma and neutron events (∼7 μs). Using this effect, we were able to discriminate bulk nuclear recoil events from a surface electron background at better than the 97% level above 25 keV recoil energy. The phonon risetime for bulk gamma events was dependent on the applied voltage, confirming that phonons produced from electron-hole emission are ballistic.