Estimation of low energy neutron flux (Eₙ≤15 MeV) in India-based Neutrino Observatory cavern using Monte Carlo techniques

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The neutron flux at low energy (Eₙ≤15 MeV) resulting from the radioactivity of the rock in the underground cavern of the India-based Neutrino Observatory is estimated using Geant4-based Monte Carlo simulations. The neutron production rate due to the spontaneous fission of ²³⁵, ²³⁸U, ²³²Th and (α, n) interactions in the rock is determined employing the actual rock composition. It is been shown that the total flux is equivalent to a finite size cylindrical rock (D=L=140 cm) element. The energy integrated neutron flux thus obtained at the centre of the underground tunnel is 2.76 (0.47) × 10⁻⁶ n cm⁻² s⁻¹. The estimated neutron flux is of the same order (10⁻⁶ n cm⁻² s⁻¹) as measured in other underground laboratories.