Modeling and design of a buried explosive detector based on neutron scattering

Main goal of this thesis was to design a buried explosive detector based on the neutron scattering method. Detection of land mines and massive ammonium nitrate based explosives were targeted. In this work, the trace element is the hydrogen present in buried explosives. Detection method aims to detect the scattered thermal neutrons from the explosive. Design of the detector includes an array of Si-PIN photodiodes, a 14 MeV neutron generator and a shielding material to protect detector electronics. Monte Carlo simulations have been made to determine the optimal design parameters and the detection limits. Geant4 physics simulation package has been used for this purpose. Si-PIN photodiode is the detection element of the detector. On its own, Si-PIN photodiodes are not sensitive to neutrons. They are rather sensitive to charged particles. To be able to detect neutrons, a neutron to charged particle converter should be applied to Si-PIN photodiode surface. Possible converters were investigated and 6 LiF, 10 B and polyethylene were selected as the potential candidates. Simulations have been made to determine optimal coating thicknesses for these materials. Neutrons path through the soil is recorded via tracking implementation in Geant4. Neutron thermalization in the explosive and scattered neutron spectrum have been analysed. A significant difference has been found in the thermal neutron spectrum between the case with only soil and the case where an explosive was buried in soil. Further simulations have been prepared on this basis for landmines and ammonium nitrate based explosives. Simulations were made for different converters, explosive masses, bury depths, explosives and soil types. Signals detected at Si-PIN detector have been compared to each other. Detection efficiency were calculated in terms of separation quality z, which describes the difference between explosive and soil signal in terms of number of standard deviations. Additional studies have been made to investigate the performance of the detector when it is mounted on a vehicle. To understand the effect of humidity in the soil and the effect of asphalt, additional simulations were performed and detection efficiencies in each case were determined. Between the three candidate converter materials, 6 LiF outperformed other converters. Sensitivity of 6 LiF coated Si-PIN photodiode was high enough to detect a 500 kg ammonium nitrate based explosive down to 30 cm depth and a 15 kg anti tank mine down to 20 cm bury depth. For the same explosives, 10 B coating managed to detect ammonium nitrate based explosives down to 10 cm and anti tank mines to 15 cm. Polyethylene converter did not manage to offer a reliable detection. Possible upgrades like large area silicon pad detectors or gamma detectors have been discussed.