Boric acid as converter material for semiconductor neutron detector- A GEANT4 simulation study

Abstract

Monte Carlo based GEANT4 simulations have been carried out to estimate the efficiency (η) of thermal neutron detection with boric acid (H3BO3) as a converter material. Different geometries of detector configurations — planar stack, sphere, cylinder and cuboid were considered and their efficiency were computed for relevant geometric parameters along with different 10B enrichment (BE) levels content in boric acid, with Low Level Discriminator (LLD) value fixed at 300 keV. The planar stack configuration detector was designed by replicating two dimensional detector units (DUs) – each consisting of boric acid as a converter material and Silicon (Si) as a detector material – in the third dimension. The simulation carried out for varied layer thicknesses (0.25μm to 10μm) of boric acid showed that efficiency (η) increased with increasing thickness, reached a maximum at a critical thickness (tsc) and thereafter found to decrease. Typically, at a critical thickness of ∼3.5μm, the η for 100% BE content in boric acid was found to be 15.96% for 30 DUs. For the case of sphere, cylinder and cuboid of boric acid embedded in Si, the simulations were performed for diameter and width both varying from 0.5μm to 9.5μm in steps of 0.5μm. In particular for cylinder and cuboid geometries, the depth was also varied from 25μm to 275μm with a step size of 25μm. In all these configurations η was found to increase initially and was maximum at a critical diameter and width of ∼8–9μm, and thereafter it was found to reduce. Apart from the simulated η estimation, histogram plots depicting the energy deposited in Si detector region by the generated charged particles (7Li = 0.839 MeV and α=1.47MeV) upon thermal neutron interaction in boric acid have also been studied for various geometrical parameters such as thickness, diameter and width of detector.