Development of a Prompt Gamma Neutron Activation Analysis (PGNAA) system at the Institute of Nuclear Science and Technology (INST)

The purpose of this research was to conduct a computational investigation into developing a PGNAA irradiation system for elemental analysis using the Am-Be neutron source stationed at INST. The research focused on the parametric study of the variables that influence the design of the PGNAA system through Monte Carlo simulation using the MCNP5 particle transport code. The MCNP5 was used to model the PGNAA system and to generate the flux profile of fast, epithermal and thermal neutrons along the irradiation sites and the prompt gamma flux at the detector. The geometry of the facility was modeled as one cylinder with the neutron source at the center of the cylinder and paraffin wax as the moderator. The original irradiation site was modeled directly above the source while sites 2, 3 and 4 were modeled at radial distances of 5 cm, 10 cm and 15 cm from the source respectively. The optimum thermal neutron fluxes are obtainable within the range of 0 cm to 6 cm for site 1 and a range of -6 cm to 6 cm for sites 2, 3 and 4 (measurement carried out with reference to the source upper surface). The axial neutron fluxes peak near the center of the source and then reduce exponentially to very low values at the end of the channel. The percentage of the average radial thermal flux increases with the distance from the source while those of epithermal and fast neutrons decrease.
Site 2, which was simulated at 5 cm from the center of the Am-Be neutron source was selected as the ideal sample location for PGNAA application. The thermal neutron flux of 45,152 ±388−2s−1 and the neutron ratios ΦT/ΦE and (ΦT+ΦE)/ ΦF of 3 and 2.8 respectively were achieved at this channel. Since the simulated thermal neutron flux is very low, the sensitivity obtained for the elements are very low. Apart from hydrogen which showed discrepancy due to the interference from the high background peak, boron had the best sensitivity. The boron sensitivity of 7.887(cps)/mg with a lower detection limit of 10 mg could be applied in analysis of boron samples.

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