Design of an in-tank thermal neutron beam for PGNAA application at Isfahan MNSR

Calculations have been done in order to achieve a thermal neutron beam for Prompt Gamma Neutron Activation Analysis (PGNAA) applications using Miniature Neutron Source Reactor (MNSR) as neutron source. The MNSR is a 30 kW tank-in-pool reactor and there are ten small occupied in-tank irradiation positions. Also, there are four positions in order to install or remove aluminum Regulating Rod Tubes (RRTs) that are bigger than that ten irradiation positions. In this study, the eastern RRT has been selected for more simulations of thermal neutron beam. Thermal neutron flux (& phi;th), the ratio of thermal neutron flux to gamma dose rate (& phi;th D & gamma; ), and the ratio of thermal neutron flux to total neutron flux (& phi;th & phi;tot ) have been considered in calculations as important neutron beam parameters. Some designs have been done and simulated using MCNP code. The effect of the height of beam inlet, the effect of the different materials and thickness of moderators, and the effect of different thickness and height of gamma filters on the beam parameters at the beam outlet have been investigated. The results of different designs showed that using appropriate heights and thicknesses, by using bismuth as gamma filter and heavy water as moderator, & phi;th, & phi;th & phi;tot and & phi;th D & gamma; , are 1.64E + 06n.cm- 2.s- 1, 0.789, and 4.36E + 05n.cm- 2.& mu;Sv- 1 respectively. Also, by using bismuth as gamma filter and graphite as moderator, these parameters are 1.57E + 06n.cm- 2.s- 1, 0.761, and 4.69E + 05E + 05n.cm- 2.& mu;Sv- 1, respectively.