Search for the in-situ production of 77Ge in the GERDA neutrinoless double-beta decay experiment

The beta decay of 77Ge and 77mGe, both produced by neutron capture on 76Ge, is a potential background for Germanium based neutrinoless double-beta decay search experiments such as GERDA or the LEGEND experiment. In this work we present a search for 77Ge decays in the full GERDA Phase II data set. A delayed coincidence method was employed to identify the decay of 77Ge via the isomeric state of 77As (9/2+, 475keV, T1/2=114μs, 77mAs). New digital signal processing methods were employed to select and analyze pile-up signals. No signal was observed, and an upper limit on the production rate of 77Ge was set at <0.216 nuc/(kg· yr) (90% CL). This corresponds to a total production rate of 77Ge and 77mGe of <0.38 nuc/(kg· yr) (90% CL), assuming equal production rates. A previous Monte Carlo study predicted a value for in-situ 77Ge and 77mGe production of (0.21 ± 0.07) nuc/(kg.yr), a prediction that is now further corroborated by our experimental limit. Moreover, tagging the isomeric state of 77mAs can be utilised to further suppress the 77Ge background. Considering the similar experimental configurations of LEGEND-1000 and GERDA, the cosmogenic background in LEGEND-1000 at LNGS is estimated to remain at a sub-dominant level.