Measurements of gamma ray, cosmic muon and residual neutron background fluxes for rare event search experiments at an underground laboratory

TL;DR

This study measures gamma ray, cosmic muon, and neutron background fluxes at an underground lab to inform rare event search experiments, providing data and simulations crucial for background reduction strategies.

Contribution

The paper provides comprehensive measurements and simulations of background radiation levels at an underground laboratory, aiding the design of sensitive rare event detection experiments.

Findings

Gamma ray background dominated by rock radioactivity below 3 MeV.

Cosmic muon flux measured at 2.051e-7 cm^-2 s^-1, aligning with simulations.

Neutron flux from radiogenic sources measured at 1.61e-4 cm^-2 s^-1, with simulation validation.

Abstract

Ambient radiation background contributed by the penetrating cosmic ray particles and the radionuclides present in the rock materials have been measured at an underground laboratory located inside a mine at 555 m depth. The laboratory is being set up to explore rare event search processes, such as direct dark matter search, neutrinoless double beta decay, axion search, supernova neutrino detection, etc., that require specific knowledge of the nature and extent of the radiation environment in order to assess the sensitivity reach and also to plan for its reduction for the targeted experiment. The gamma ray background, which is mostly contributed by the primordial radionuclides and their decay chain products, have been measured inside the laboratory and found to be dominated by rock radioactivity for $E_\gamma \lesssim 3 \,{\rm MeV}$. Shielding of these residual gamma rays for the experiment was also evaluated. The cosmic muon flux, measured inside the laboratory using large area plastic scintillator telescope, was found to be: $(2.051 \pm 0.142 \pm 0.009) \times 10^{-7}\, {\rm cm}^{-2}.{\rm sec}^{-1}$, which agrees reasonably well with simulation results. The neutron background flux has been measured for the radiogenic neutrons and found to be: $(1.61 \pm 0.03) \times 10^{-4} \, {\rm cm}^{-2}.{\rm sec}^{-1}$ for no threshold cut. Detailed GEANT4 simulation for the radiogenic neutrons and the cosmogenic neutrons have been carried out. Effects of multiple scattering of both the types of neutrons within the surrounding rock and the cavern walls were studied and the results for the radiogenic neutrons are found to be in reasonable agreement with experimental results. Neutron fluxes contributed by those neutrons of cosmogenic origin have been reported as function of the energy threshold.