Constraining the ^77(m)Ge Production with GERDA Data and Implications for LEGEND-1000

The delayed decay of ^77(m)Ge, produced by neutron capture on ⁷⁶Ge, is a potential background for Germanium based neutrinoless double beta decay search experiments such as Gerda or the future LEGEND-1000 experiment. In this work we present a search for ⁷⁷Ge in the full Gerda Phase II data. We employ a delayed coincidence method to identify the decay of ⁷⁷Ge via the isomeric state of ⁷⁷As (9/2+, 475.5 keV, t₁/₂ = 114 μs). New digital signal processing methods were employed to select and analyze pile-up signal. We found no signal and are able to set an upper limit on the production rate of ⁷⁷Ge at < 0.235 nuc/(kg · yr) (90% CL), which translates into < 0.40 nuc/(kg · yr) (90% CL) for the total ^77(m)Ge production assuming equal production of ⁷⁷Ge and ^77mGe. Given the very similar configuration - bare germanium detectors in liquid argon - this limit benchmarks our LEGEND-1000 predictions. Scaled with the production rate obtained from simulations and using a new delayed coincidence cut condition on ⁷⁷Ge, we estimate a ^77(m)Ge background index contribution in LEGEND-1000 at Lngs (Laboratori Nazionali del Gran Sasso) of 4.0^+3.0_-2.9 × 10^-7 cts/(keV · kg · yr), which is about 4 % of the target background budget.