An optimized pre-moderator improves uniformity of activation rate distribution in an ORNL phantom-IVNAA facility

Uniformity of activation rate distribution through the human body is extremely important for in vivo analysis of the body elements by neutron activation method. Achieving uniformity can be difficult because of the non-homogenous body shape and compositions. Pre-moderator is one of the most essential parts of the irradiation facility to provide uniform distribution over the sample. The aim of the present study was designation of an optimum pre-moderator, in terms of shape and material, which compensates the destructive effects of body shape and allows a satisfactory uniformity of activation rate in the sample. Our final calculations indicated that using two slabs of paraffin with a thickness of 1.8 cm as a pre-moderator in the presence of a reflector/moderator, achieve the most uniform distribution of activation rate in the body.

► We model IVNAA facility to provide an elemental profile of body composition. ► It is essential that the thermal neutron flux distribution is uniform. ► We study type and shape of pre-moderator to increase the uniformity of thermal neutron flux distribution. ► We examine the role of pre-moderator to assess uniform distribution. ► We conclude that 1.8 cm thickness of paraffin increase the uniformity of flux distribution.