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

Abstract

The beta decay of ${}^{77}$Ge and ${}^{77\mathrm{m}}$Ge, both produced by neutron capture on ${}^{76}$Ge, 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 ${}^{77}$Ge decays in the full GERDA Phase II data set. A delayed coincidence method was employed to identify the decay of ${}^{77}$Ge via the isomeric state of ${}^{77}$As (9/2${}^{+}$, 475 keV, $T_{1/2}=114\hat{I}¼$s, ${}^{77\mathrm{m}}$As). 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 was set at $<0.216$ nuc/(kg$\cdot$yr) (90% CL). This corresponds to a total production rate of ${}^{77}$Ge and ${}^{77\mathrm{m}}$Ge of $<0.38$ nuc/(kg$\cdot$ yr) (90% CL), assuming equal production rates. A previous Monte Carlo study predicted a value for in-situ ${}^{77}$Ge and ${}^{77\mathrm{m}}$Ge production of (0.21$\pm$0.07) nuc/(kg$\cdot$yr), a prediction that is now further corroborated by our experimental limit. Moreover, tagging the isomeric state of ${}^{77\mathrm{m}}$As can be utilised to further suppress the ${}^{77}$Ge 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.

11 pages, 7 figures