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

The beta decay of Ge-77 and (77)mGe, both produced by neutron capture on Ge-76, 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 Ge-77 decays in the full GERDA Phase II data set. A delayed coincidence method was employed to identify the decay of Ge-77 via the isomeric state of As-77 (9/2(+), 475 keV, T-1/2 = 114 mu s, As-77m). 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 Ge-77 was set at < 0.216 nuc/(kg yr) (90% CL). This corresponds to a total production rate of Ge-77 and Ge-77m of < 0.38 nuc/(kg yr) (90% CL), assuming equal production rates. A previous Monte Carlo study predicted a value for in-situ Ge-77 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 As-77m can be utilised to further suppress the Ge-77 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.