Optimization of an Am-Be neutron source shield design by advanced materials using MCNP code

Abstract

Nowadays, neutron sources are employing widely in many research as well as industrial and medical applications. A suitable neutron source with reasonable intensity, dimensions and weight is required in these applications. Am-Be is considered as an important and most famous neutron source,provides adequate neutron intensity for research and industrial applications. Typical shield materials of this source consist of some conventional and well known moderators as well as neutron and gamma absorbers. In many designs, a combined multi-layer configuration of these materials is required for better shielding efficiency which leads to a massive large shield design. Consequently, someone usually considers an Am-Be neutron source as a fixed one. However; some advantages are attributed to a portable neutron source in industriall applications. Some high-tech advanced materials have been proposed to reduce the weight and dimensions of shield design. These advance materials are much lighter and enhance shielding properties in a great extent. For example in neutron absorber advanced materials hydrogen and/or carbon content has been increased drastically leads to superior slowing down capability of these advanced materials in comparison to conventional competitors. The main objective of this work is dedicated to optimization of weight and dimensions in conventional 241Am-Be shield design using some state of the arts advanced materials. MCNP Monte Carlo code was employed for this purpose and equivalent neutron and gamma dose rates were calculated. The results showed that the use of advanced materials has a significant effect on total dose rate reduction. “Magnesium Borohydride” and “Kennertium” materials have the best performance in neutron and gamma dose reduction, respectively. The combination of these two materials can reduce the total dose rate in to one third of the reference value. Finally, two alternative shield designs were proposed. The first one is based on the combination of “Magnesium Borohydride” and “Kennertium”, for neutron and gamma absorption. The results showed that the volume and weight in the first case are reduced by 60% and 27%, respectively. In the second case the Kennertium was replaced by “Polyethylene-78.5% Bismuth” alloy to obtain much lighter shield design. The volume and weight in the second case were reduced by 49% and 67%, respectively.