Computational Study of a Joint Neutron-Gamma Detection System for Large-Volume Ammonium Nitrate-Based Landmine Identification

A joint neutron-gamma detection system was designed using a two-stage Monte Carlo simulation approach to identify large-volume ammonium nitrate from buried landmines. To investigate possible spectral anomalies associated with the presence of landmines, the phase space shell concept was used to study the response of a LaBr3 scintillator gamma-ray detector to photons from neutron activation of specific chemical elements in the explosive and soil, as well as the response of a He-3 thermal neutron counter to backscattered thermal neutrons. At a neutron emission energy of 5 MeV, the soil-specific gamma-ray peak energies of 4.94 MeV (Al), 7.65 MeV (Al), and 8.45 MeV (Si) showed a decrease in peak area, which increased as the detection system approached the landmine. The ammonium nitrate-specific peak at 10.82 MeV (N) occurred only in the presence of a landmine. The difference in count rate recorded by the He-3 counter between soil without explosive and soil with explosive increased as the detector system approached the landmine. The combined information from the gamma and neutron detectors provides a good fingerprint for the presence of a suspected landmine. The source shielding, featuring a spherical multi-layered graphite, polyethylene, and lead shielding, improved the useful gamma ray intensity and the registered difference in thermal neutron count rate and reduced the background gamma radiation and direct neutrons reaching the same detectors.