Neutron and gamma-ray radiation fields characterisation in an 241Am-Be irradiator in view of its use as reserach tool

Abstract

Introduction A neutron irradiation facility based on four 241Am–Be sources, activity of 1.11·1011 Bq each (yield ~7.0 ·106 n s-1), consisting in four source-channels and an irradiation channel placed in a tank filled with light water both as moderator and biological shielding, was realized at DEIM Department. The aim of this work is to evaluate neutron and gamma fluxes in several points inside the irradiation channel, in view of its use as a testing tool in various research activities. For this goal, an experimental measurement campaign was started also to validate Monte Carlo (MCNP5 code) simulation results. Methods Experimental measurements of neutron radiation fields (fast and thermal) were realized both by neutron activation with bare and cadmium covered gold foils and with TLD600 and TLD700 dosimeters compared with the values obtained by a MCNP5 simulation performed adopting a previously validated model. As regards gamma-ray fluxes, the contribution at 60 keV energy, associated with the decay of 241Am, and the one at 2.2 MeV, due to the radiative capture of neutrons in the biological shield (water) was determined (for difference) with use of bare and lead shielded TLD700 dosimeters. Results Both gamma and neutron fields behaviours along irradiation channel axis were determined, besides thermal and fast contributions of neutron flux were evaluated. For gamma-ray fluxes, from the lying plane of the sources up to about 8 cm, the main contribution in the gamma field comes from 60 keV of 241Am, while, above 8 cm, the contribution of 2.2 MeV prevails. Experimental values confirmed with a good correspondence (within 5%) MCNP5 evaluation for each examined position inside irradiation channel. Conclusions The comparison between the experimental data and those obtained performing a Monte Carlo simulation allowed the determination of both neutron (fast and thermal) and gamma field in several points inside the irradiation channel. So, the main parameters for any irradiation plans can be easily established.