Measurement of cosmic muon-induced neutron background with ISMRAN detector in a non-reactor environment

Abstract

The Indian Scintillator Matrix for Reactor Anti-Neutrinos (ISMRAN) is an above-ground, very short baseline reactor anti-neutrino (${\overline{\nu }}_e$) experiment, located inside the Dhruva research reactor facility, Mumbai, India. The primary goal of the ISMRAN experiment is the indirect detection of reactor ${\overline{\nu }}_e$ through an inverse beta decay (IBD) process, using a cluster of 90 optically segmented plastic scintillator detectors, weighing $\sim$1 ton. However, the most difficult to distinguish correlated background for the ISMRAN experiment is from fast neutrons, which cannot be actively rejected and as a consequence mimics the IBD process through proton recoil inside the detector’s volume. In this work, we present the neutron capture time response and energy deposition of neutron capture signals generated by cosmic muons in the ISMRAN geometry, and we compare these experimental results with Geant4-based Monte Carlo (MC) simulations. The obtained mean capture time of fast neutrons is 74.46 $\pm$ 5.98 $\mu$s and is comparable with the MC simulation results. The efficiency-corrected rate of muon-induced neutron background inside the ISMRAN geometry, due to the presence of a passive shielding structure of 10 cm lead followed by 10 cm borated polyethylene with a surface area of 600 cm², deployed on top of the ISMRAN setup, is reported to be 1334 ± 64 (stat.) $\pm$ 70 (sys.) per day. This result shows good agreement with the expected background rate from MC simulations using Geant4. Additionally, we also estimate the muon-induced fast-neutron rate in the ISMRAN geometry for the actual shielding configuration of 9000 cm² surface area to be 3335 ± 160 (stat.) $\pm$ 175 (sys.) neutrons day^-1 through an extrapolation, after incorporating the model dependent acceptance correction factor from the Geant4 MC simulation. Finally, using these results, we evaluate the neutron production yield due to the composite shielding in the ISMRAN geometry, which is 2.81$\times$ 10^-5 neutrons per $\mu$ per (g/cm²) at sea level. These results will be significant in the context of differentiating correlated background from true ${\overline{\nu }}_e$ events at the actual measurement site inside the reactor facility.