Virtual depth by active background suppression: Revisiting the cosmic muon induced background of GERDA Phase II

TL;DR

This study re-evaluates cosmogenic backgrounds in the GERDA experiment, demonstrating that active background suppression techniques can significantly reduce muon-induced isotopic backgrounds, effectively increasing the experimental depth.

Contribution

It introduces a novel Monte Carlo simulation analysis showing that active background suppression can mitigate muon-induced backgrounds, enabling deeper underground searches without additional depth.

Findings

Active background suppression reduces muon-induced backgrounds by over an order of magnitude.

The $^{77(m)}$Ge production rate is quantified as 0.21 nuclei/(kg·yr).

Suppression techniques achieve a background contribution of 2.7×10^{-6} cts/(keV·kg·yr).

Abstract

In-situ production of long-lived isotopes 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$\pm$0.01) nuclei/(kg$\cdot$yr). After application of state-of-the-art active background suppression techniques and simple delayed coincidence cuts this corresponds to a background contribution of (2.7$\pm$0.3)$\cdot10^{-6}$ cts/(keV$\cdot$kg$\cdot$yr). 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.