Effects of target shape and neutron scattering on element sensitivities for neutron-capture prompt gamma-ray...

Results are presented for a study of the effects of neutron scattering by hydrogen on element sensitivities for in-beam neutron capture prompt gamma-ray activation analysis. In a scattering matrix, sensitivities vary as a function of both the scattering density, i.e., the number of scatters per unit volume and the magnitude of scattering cross sections, and the target geometry, i.e., the target shape and orientation with respect to the neutron beam. Element sensitivities (counts.s-1.mg-1) increased linearly with H density (g.mL-1) for H, B, Na, Cl, K, Mn, Br, Ag, Cd, I, Sm, and Gd, measured for liquids packaged in Teflon bags. Nine of the 12 elements studied had 1.69 +/- 0.18% sensitivity enhancement per percent increase in H density. Samarium sensitivity was enhanced by only 0.54 +/- 0.07%/% H, which may indicate that neutron scattering by H causes a shift in the energy distribution of the neutrons. Manganese sensitivity was enhanced by 2.44 +/- 0.26%/% H. The enhancement of the sensitivity for H itself varied with matrix composition. For several series of disk-shaped, solid, hydrogenous targets, element sensitivities increased with decreasing target thickness until, at some limiting thickness, this trend was reversed. Consistent with theory, sensitivities measured for spherical hydrogenous targets showed no enhancement.