Charge-collection and trapping effects in cryogenic silicon detectors

The physics of charge measurement in silicon at low temperature (T < 0.5 K) and low applied electric field (E = 0.1-100 V/cm) has been examined in a variety of high purity, p-type silicon samples with room temperature resistivity in the range 2-40k Omega cm. Charge collection under these conditions is necessary for background suppression, through the simultaneous measurement of phonons and ionization, in a very low event rate dark matter search. Recent improvement in the data analysis of our phonons and ionization experiment is given. Charge loss at low electric field due to trapping during charge drift is present but the data suggest that another charge-loss mechanism is also important. We present results which indicate that a significant fraction of the total charge loss (compared to full collection) occurs in the initial charge cloud near the event location. In addition, measurements of the lateral size, 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.