Study of $\gamma$-ray background from cosmic muon induced neutrons

Abstract

The contribution to $\gamma$-ray background from secondary neutrons, originating from cosmic muon interactions in Pb-Cu composite shield, has been measured via $(\mathrm{n},\mathrm{n}^{\prime} \gamma)$ reactions in Cu and Ge. The minimization of background plays a key role in improving the sensitivity of rare event experimental searches like neutrinoless double beta decay and neutron background is often a major concern. It is important to understand secondary neutron production from cosmic muons, especially in materials like Cu and Pb, which often form a part of the detector or shields. The direct contribution of fast neutrons generated from cosmic muon interactions to the $\gamma$-ray background via $(\mathrm{n},\mathrm{n}^{\prime} \gamma)$ reactions is investigated for the first time. Measurements are carried out in the low background HPGe detector setup, TiLES, using Pb-Cu composite shield as target material for muon interactions. Simulations have been carried out with GEANT4.10.00 and GEANT4.10.05, each with two different Physics lists. The observed intensities of $(\mathrm{n},\mathrm{n}^{\prime} \gamma)$ for Cu are significantly under-predicted by GEANT4.10.00, while GEANT4.10.05 shows good agreement with the experimental data.