Modeling of tagged neutron method for explosive detection using GEANT4

Abstract The study has been focused on modeling the capabilities of tagged neutron method (TNM) for detection and identification of explosive materials using an approximated experimental configuration. The underlying principle is to use D-T (14.1 MeV) neutrons tagged by its associated charged particle and incorporating neutron time-of-flight technique to identify explosives by estimating carbon, nitrogen and oxygen contents and their ratios. This research was carried out to facilitate the design and construction of a prototype laboratory based TNM system and a simulation model was developed in the GEANT4 framework. The model has established an environment for (a) simulating production of D-T tagged neutrons, their transport and interactions with an object to induce emission, detection of characteristic gamma-rays and (b) image reconstruction (3D) of the interrogated object using time-of-flight information of neutron and gamma-ray. The geometrical configuration includes 14.1 MeV tagged neutrons, a position sensitive alpha detector and an array of BGO (3” x 3”) gamma detectors arranged in a square geometry around the sample to collect the neutron induced gamma-ray spectra. With this configuration of tagged neutron system, a reference data set of pure elements’ (C, O, N, Al and Fe) neutron induced gamma energy spectra was generated. Simulated spectra of benign (urea, melamine) and explosive (RDX, TNT, ammonium nitrate) samples were analyzed and their elemental compositions of C, N and O were extracted with linear combination of reference data set. The resulting compositions were found to be close to the actual values. Further, the effects of the matrix materials on energy spectra of sample were studied with explosive (1 kg RDX) hidden inside a small container filled with (1) metallic matrix composed of iron ( 56 Fe) or (2) organic matrix ( C 2.59 H 5.05 N 0.55 O 1.6 ) with an average densities of 0.1, 0.2 g/cc. Simulation results have shown the system’s capability in detecting and imaging of hidden explosives (1 kg RDX) within a metallic matrix is better than that in a wood matrix.