Eur. Phys. J. C (2025) 85: 809
https://doi.org/10.1140/epjc/s10052-025-14445-x

Regular Article - Experimental Physics

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

¹ INFN Laboratori Nazionali del Gran Sasso, Assergi, Italy
² INFN Laboratori Nazionali del Gran Sasso and Gran Sasso Science Institute, Assergi, Italy
³ INFN Laboratori Nazionali del Gran Sasso and Università degli Studi dell’Aquila, L’Aquila, Italy
⁴ INFN Laboratori Nazionali del Sud, Catania, Italy
⁵ Institute of Physics, Jagiellonian University, Cracow, Poland
⁶ Institut für Kern- und Teilchenphysik, Technische Universität Dresden, Dresden, Germany
⁷ Joint Institute for Nuclear Research, Dubna, Russia
⁸ European Commission, JRC-Geel, Geel, Belgium
⁹ Max-Planck-Institut für Kernphysik, Heidelberg, Germany
¹⁰ Department of Physics and Astronomy, University College London, London, UK
¹¹ INFN Milano Bicocca, Milan, Italy
¹² Dipartimento di Fisica, Università degli Studi di Milano, INFN Milano, Milan, Italy
¹³ Institute for Nuclear Research, Russian Academy of Sciences, Moscow, Russia
¹⁴ Institute for Theoretical and Experimental Physics, NRC “Kurchatov Institute”, Moscow, Russia
¹⁵ National Research Centre “Kurchatov Institute”, Moscow, Russia
¹⁶ Max-Planck-Institut für Physik, Munich, Germany
¹⁷ Physik Department, Technische Universität München, Munich, Germany
¹⁸ Dipartimento di Fisica e Astronomia, Università degli Studi di Padova, Padua, Italy
¹⁹ INFN Padova, Padua, Italy
²⁰ Physikalisches Institut, Eberhard Karls Universität Tübingen, Tübingen, Germany
²¹ Physik-Institut, Universität Zürich, Zurich, Switzerland
²² NRNU MEPhI, Moscow, Russia
²³ Duke University, Durham, NC, USA
²⁴ Moscow Inst. of Physics and Technology, Moscow, Russia
²⁵ Semilab Zrt, Budapest, Hungary
²⁶ Dubna State University, Dubna, Russia
²⁷ Nuclear Science Division, Berkeley, USA
²⁸ Department of Physics and Astronomy, University College London, London, UK

^a gerda-eb@mpi-hd.mpg.de
^b gerda-eb@mpi-hd.mpg.de

Received: 22 February 2025
Accepted: 20 June 2025
Published online: 26 July 2025

Abstract

The beta decay of $_{}^{77}$Ge and $_{}^{77\text{m}}$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 ($9/2^{+}$, $475\text{keV}$, $T_{1/2}=114\mathrm{\mu }\text{s}$, $_{}^{77\text{m}}$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 $_{}^{77}$Ge was set at $<0.216$ nuc/(kg$·$ yr) (90% CL). This corresponds to a total production rate of $_{}^{77}$Ge and $_{}^{77\text{m}}$Ge of $<0.38$ nuc/(kg$·$ yr) (90% CL), assuming equal production rates. A previous Monte Carlo study predicted a value for in-situ $_{}^{77}$Ge and $_{}^{77\text{m}}$Ge 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 $_{}^{77\text{m}}$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.