Simulation of a collimator and sapphire filter for PGAA facility of the Moroccan TRIGA MARK II research reactor

The Prompt Gamma Neutron Activation Analysis (PGAA) facility will be commissioned at the NB1 beam tube of the Moroccan TRIGA MARK II research reactor. PGAA is a non-destructive analytical technique used for multi-elemental analysis and especially for light element analysis. For better performances, PGAA requires small and focused beam, and high Phi(th)/Phi(ep) and Phi(th)/Phi(fast) ratios, and low background gamma radiation. For this purpose, fast neutron filters are commonly used to reduce fast and epithermal components of neutron flux. In this paper, the impact of sapphire filter, introduced, within the collimator hole, at the beam port on neutron spectra was evaluated. Monte Carlo, MCNP6.2, code has been used with ENDF VII.1 neutron cross section library to perform this study. An approximate TRIGA MARK II reactor MCNP model has been used to generate neutron spectra at the entrance of the collimator. Then an independent model for both collimator and sapphire filter has been brought into calculation to fulfil our purpose. Results showed an increase of about 8 times for both Phi(th)/Phi(ep), and Phi(th)/Phi(fast) ratios using a 10 cm length of sapphire.