Dead layer and degradation effects in cryogenic germanium detectors for dark matter search

We investigate degradation effects in cryogenic Ge detectors subjected to penetrating gamma radiation and to alpha and electron surface bombardment. Based on an analysis of the collection efficiencies and on time-resolved measurements of the ionization signals, we show that the space-charge following gamma irradiation is strongly inhomogeneous, with a maximum density underneath the collection electrodes. Degradation effects induced by alpha and by electron bombardment strongly suggest a dependence of dead layer phenomena on carrier trapping at defect states in the vicinity of the electrodes. Mechanisms of space-charge formation are discussed, including the role of impact ionization of impurities as compared to the more usually considered effects of trapping.