Measurement of cosmic muon-induced events in an HPGe detector using time-coincidence technique

TL;DR

This study measures muon-induced background events in an HPGe detector using time-coincidence, providing data crucial for background suppression in neutrino experiments with lead shielding.

Contribution

It presents the first detailed measurement of muon-induced secondary particle yield in lead shielding using a high-purity germanium detector and time-coincidence technique.

Findings

Measured muon-induced event rate: 34 ± 1 (stat.) ± 3 (sys.) per day per kg.

Characteristic time of residual background events: 11 ± 4 μs, matching simulations.

Yield of secondary events in 10 cm Pb shielding: (11 ± 1 stat. ± 1 sys.) per kg per m² per muon.

Abstract

Detailed understanding and suppression of backgrounds are among the key challenges faced by Coherent Elastic Neutrino-Nucleus Scattering (CE\ensuremath{\nu}NS) experiments. The sensitivity of these experiments is largely determined by the background levels arising from various sources. Above-ground and shallow-overburden neutrino experiments typically employ passive shielding, primarily composed of lead (Pb), to suppress environmental $\gamma$ background. However, such shielding can introduce additional backgrounds that are particularly challenging for CE\ensuremath{\nu}NS experiments. These backgrounds arise mainly from $\gamma$ and neutrons produced by cosmic muon interactions in the shielding, and their contribution can become significant depending on the amount of Pb shielding used. In the current work, we measure the yield of secondary particles originating from Pb as a result of high-energy cosmic muon interaction, using a high-purity germanium (HPGe) detector and plastic scintillators. A time-coincidence technique is used to identify and reject these secondary background events from the experimental data. The obtained mean characteristic time of these residual background events is 11 $\pm$ 4 $\mu$s, which is consistent with the Geant4-based MC simulation result of 11 $\pm$ 1 $\mu$s. The measured efficiency-corrected rate of muon-induced events in the HPGe detector is 34 $\pm$ 1 (stat.) $\pm$ 3 (sys.) day$^{-1}$kg$^{-1}$ within the energy range of 30 keV to 2000 keV. The yield of muon-induced secondary backgrounds in 10 cm thick Pb shielding is evaluated to be $(11 \pm 1 (\text{stat.}) \pm 1 (\text{sys.}))$ secondary events$\thinspace\text{kg}^{-1}\thinspace\mathrm{m^{-2}}\thinspace\text{muon}^{-1}$ at sea level.