M'SONE MSc Abstract - 2011

Supervisor : Dr P Maleka : iThemba and Prof SH Connell : UJ

A study of neutron to gamma-ray transport for interpreting neutron activation analysis using the GEANT4 simulation package.

Introduction

The high penetrating ability of neutrons offers an effective way of probing elemental content of interrogated materials [1, 2], and leads to methods that are non-destructive [3] and needs no sample preparation [4] even in bulk materials. The reaction products reveal information not only about the elemental content of the material but also about its isotopic composition. The irradiation of materials with neutrons and studying the induced γ-rays for element identification and concentrations [5, 6] for geological formations [7-11] has developed fast in recent years. The energies of the emitted γ-rays are characteristic of the isotopic composition of the elements present in the matrix and their intensities are proportional to their concentrations [5, 7, 12, 13].

The neutron-induced γ-ray techniques have developed further in studies ranging from mineral exploration [8-11] to cargo inspections [14] for explosives and contraband detection [15-18]. Moreover, coupled with the advances in -ray detection instruments and the introduction of Monte Carlo methods, these techniques have become highly sophisticated. Their application is based either on, fast-neutron induced reactions (n,n'γ), (n,p), (n,γ) or thermal-neutron induced (n,γ) reactions.

This study investigates the possibility of using GEANT4 [19] simulation package to reproduce and interpret gamma-ray spectra for the coupled neutron gamma-ray field. The results will be benchmarked against the measured spectra and other simulation packages like MCNPX [20] and FLUKA [21].

Experimental set-up

The experiments were conducted inside a 10 cm thick, 2 m diameter and 3 m high cylindrical concrete tank [22]. The tank is covered on top with a wooden platform that has a 12 cm diameter hole at the centre. A PVC tube is mounted along the central axis of the tank, and extends to almost the bottom of the tank. The instrument is lowered into the tank through the PVC tube by means of a pulley that hangs from the roof of the building above the tank. The PVC tube serves as a positioning structure for the instrument at the centre of the tank and also for balancing. The tank is filled with water up to a height of 2.1 m, thereby maintaining, except for the soil below it, at least 1 m of water around the neutron source target area. The instrument refers to the neutron generator (~14.2 MeV neutrons with 108 neutrons/s output) and the gamma-ray detector (based on BGO scintillator) systems.

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