CHARGE COLLECTION AND TRAPPING IN LOW-TEMPERATURE SILICON DETECTORS

Charge collection efficiency measurements in silicon detectors at low temperature (T<0.5 K) and low applied electric field (E=0.1-100 V/cm) were performed using a variety of high-purity, p-type silicon samples with room-temperature resistivity in the range 2-40 k Ohm cm. Good cha rge collection under these conditions of low temperature and low elect ric field is necessary for background suppression, through the simulta neous measurement of phonons and ionization, in a very low event rate dark matter search or neutrino physics experiment. Charge loss due to trapping during drift is present in some samples, but the data suggest that another charge-loss mechanism is also important. We present resu lts which indicate that, for 60 keV energy depositions, a significant fraction of the total charge loss by trapping occurs in the initial el ectron-hole cloud near the event location which may briefly act as a s hielded, field-free region. In addition, measurements of the lateral s ize, transverse to the applied electric field, of the initial electron -hole cloud indicate large transverse diffusion lengths. At the lowest fields a lateral diameter on the order of 1 mm is found in a detector similar to 5 mm thick. (C) 1996 American Institute of Physics.