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

Internal contaminations of U-238, U-235 and Th-232 in the bulk of high purity germanium detectors are potential backgrounds for experiments searching for neutrinoless double beta decay of Ge-76. 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 Ra-226, Ac-227 and Th-228, the long-lived daughter nuclides of U-238, U-235 and Th-232, respectively, have been derived. With these upper limits a background index in the energy region of interest from Ra-226 and Th-228 contamination is estimated which satisfies the prerequisites of a future ton scale germanium double beta decay experiment.