Limits on uranium and thorium bulk content in GERDA Phase I detectors

Abstract Internal contaminations of 238 U, 235 U and 232 Th in the bulk of high purity germanium detectors are potential backgrounds for experiments searching for neutrinoless double beta decay of 76 Ge. The data from Gerda Phase I have been analyzed for alpha events from the decay chain of these contaminations by looking for full decay chains and for time correlations between successive decays in the same detector. No candidate events for a full chain have been found. Upper limits on the activities in the range of a few nBq/kg for 226 Ra, 227 Ac and 228 Th, the long-lived daughter nuclides of 238 U, 235 U and 232 Th, respectively, have been derived. With these upper limits a background index in the energy region of interest from 226 Ra and 228 Th contamination is estimated which satisfies the prerequisites of a future ton scale germanium double beta decay experiment.