Monte Carlo simulation of neutron fluxes in the two irradiation sites of the Am-Be neutron source facility at NNRI

Monte Carlo (MCNP-5) simulations of the neutron fluxes were performed to determine radial and axial neutron fluxes of the two irradiation sites of 20 Ci 241Am-Be neutron irradiation facility at the National Nuclear Research Institute of the Ghana Atomic Energy Commission. The geometry of the 241Am-Be source as well as the irradiator design, constituted one cylindrical neutron source at the centre of a cylindrical barrel with water as moderator. In the further irradiation site and the closer irradiation site average thermal, epithermal and fast neutron fluxes axially increases exponentially from the bottom and peaks at the centre of the source 3.0 cm from the bottom of the source and decreases to very low value at end of the tube. The percentage of the average thermal flux increases as the distance from the source increases, while the percentage epithermal and fast fluxes decreases. The average radial thermal neutron flux decreases at rate of 307.02 n cm-2 s-1 and 961.54 n cm-2 s-1per cm along the diameter for the further and closer irradiation sites respectively. The average radial epithermal and fast neutron fluxes were fairly uniform along the diameter in the two irradiation sites. Large sample simulation shows that in the irradiation site (1) at maximum cylindrical sample radius of 0.5 cm and height 30.0 cm (volume 23.6 cm3) gold of density 19.3 g/cm3 can be irradiated at percentage thermal neutron flux depression of 70 %, at fairly stable average thermal neutron flux of 4.94x103 n cm-2 s-1. For site (2) the maximum cylindrical sample radius of 0.6 cm and height 30.0 cm (volume 33.9 cm3) gold can be irradiated at percentage thermal neutron flux depression of 75%, at fairly stable average thermal flux of 2.125x104 n cm-2 s-1.