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 kΩ cm. Good charge collection under these conditions of low temperature and low electric field is necessary for background suppression, through the simultaneous 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 results which indicate that, for 60 keV energy depositions, a significant fraction of the total charge loss by trapping occurs in the initial electron‐hole cloud near the event location which may briefly act as a shielded, field‐free region. 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 in a detector ∼5 mm thick. © 1996 American Institute of Physics.