Characterization of neutron flux spectra in the irradiation sites of a 37 GBq 241Am-Be isotopic source

Abstract

For the applicability of instrumental neutron activation analysis (NAA) technique, an irradiation unit with a 37GBq 241Am-Be neutron source was installed at Institute of Nuclear Sciences of Ankara University. Design and configuration properties of the irradiation unit are described. It has two different sample irradiation positions, one is called site #1 having a pneumatic sample transfer system and the other is site #2 having a location for manual use. In order to characterize neutron flux spectra in the irradiation sites, the measurement results were obtained for thermal (Фth) and epithermal neutron fluxes (Фepi), thermal to epithermal flux ratio (f) and epithermal spectrum shaping factors (α) by employing cadmium ratios of gold (Au) and molybdenum (Mo) monitors. The activities produced in these foils were measured by using a p-type, 44.8% relative efficiency HPGe well detector. For the measured γ-rays, self-absorption and true coincidence summing effects were taken into account. Additionally, thermal neutron self-shielding and resonance neutron self-shielding effects were taken into account in the measured results. For characterization of site #1, the required parameters were found to be Фth=(2.11±0.05)×103ncm−2s−1, Фepi=(3.32±0.17)×101ncm−2s−1, f=63.6±1.5, α=0.045±0.009, respectively. Similarly, those parameters were measured in site #2 as Фth=(1.49±0.04)×103ncm−2s−1, Фepi=(2.93±0.15)×101ncm−2s−1, f=50.9±1.3 and α=0.038±0.008. The results for f-values indicate that good thermalization of fast neutrons on the order of 98% was achieved in both sample irradiation sites. This is because an optimum combination of water and paraffin moderator is used in the present configuration. In addition, the shielding requirements are met by using natural boron oxide powder (5.5cm) and boron loaded paraffin layers against neutrons, and a 15cm thick lead bricks against gamma-rays from source and its surrounding materials.