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

TL;DR

This study analyzes GERDA Phase I data to set upper limits on uranium and thorium contaminations in germanium detectors, crucial for reducing background in neutrinoless double beta decay experiments.

Contribution

It provides the first stringent upper limits on bulk uranium and thorium contamination levels in high purity germanium detectors used in GERDA Phase I.

Findings

No full decay chain events were observed.

Upper activity limits are in the range of a few nanobecquerels per kilogram.

Background levels from these contaminants are sufficiently low for future large-scale experiments.

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.