Virtual depth by active background suppression: revisiting the cosmic muon induced background of Gerda Phase II

Abstract

In-situ production of radioisotopes by cosmic muon interactions may generate a non-negligible background for deep underground rare event searches. Previous Monte Carlo studies for the Gerda experiment at Lngs identified the delayed decays of 77 Ge and its metastable state 77m Ge as dominant cosmogenic background in the search for neutrinoless double beta decay of 76 Ge. This might limit the sensitivity of next generation experiments aiming for increased 76 Ge mass at background-free conditions and thereby define a minimum depth requirement. A re-evaluation of the 77(m) Ge background for the Gerda experiment has been carried out by a set of Monte Carlo simulations. The obtained 77(m) Ge production rate is (0.21±0.01) nuclei/(kg·year). After application of state-of-the-art active background suppression techniques and simple delayed coincidence cuts this corresponds to a background contribution of (2.7 ± 0.3) × 10 −6 cts/(keV·kg·year). The suppression achieved by this strategy equals an effective muon flux reduction of more than one order of magnitude. This virtual depth increase opens the way for next generation rare event searches.

Cosmogenic Activation in Double Beta Decay Experiments

“…For the GERDA experiment, production rates of induced nuclides in detectors and materials of the set-up like the cyogenic liquid were firstly estimated from a MC simulation of muons in Gran Sasso using GEANT4 [109]; the delayed decays of 77 Ge and its metastable state 77m Ge following neutron captures in 76 Ge were identified as the most relevant background. A re-evaluation of this cosmic muon induced background has been carried out for GERDA Phase II, obtaining a production rate of 77 Ge/ 77m Ge of (0.21 ± 0.01) nuclei per kg and year [110]. The estimated background contribution from this is well below the background level of GERDA Phase II and even below the one estimated for LEGEND thanks to the use of active background suppression techniques and the applications of delayed coincidence cuts.…”

Present and Future of 0ν2β Searches with Germanium

“…The two-order of magnitude gap at LNGS can be filled by adding a "virtual depth" effect looking at events with a certain time pattern and topology. The idea was first proposed in the context of GERDA [33], and it has now been updated for LEGEND-1000 [24]. The strategy is being optimized using simulations based on LEGEND-1000 detector geometries.…”

Neutron Background Simulations for LEGEND-1000 in a Geant4-based Framework

“…The radiogenic neutron flux and energy spectra were calculated using material assay measurements [3] performed for the GERDA experiment as well as the NeuCBOT software package [4]. Cosmogenic muon flux and spectra were based on a parametrization from [5] and detector signal estimates from neutron capture are from [6].…”