Comparison of flux density distributions from massive or ring-shaped isotopic neutron sources

The comparison of flux distributions obtained for both massive and ring-shaped cylindrical isotopic neutron sources shows that the latter source geometry has superior qualities that can justify the inevitably higher production costs. Indeed a considerable gain in direct fast neutron flux is obtained, as well as a neat separation of fast and thermal flux density maxima creating irradiation positions with widely varying fast to thermal flux ratios very conveniently within a single irradiation rabbit. As a result the use of Cd shields, causing a loss in thermal neutron density as well as sharp thermal flux gradients, can be avoided in most cases requiring the double irradiation technique and leads to an improved sensitivity and precision. However, it should be pointed out that the distribution of activity over the volume of the source can be expected a priori to be less homogeneous for annular sources compared to massive sources, making the requirements on homogeneity of the samples more stringent. (T.G.)