Use of spherical targets to minimize effects of neutron scattering by hydrogen in neutron capture prompt .gamma.-ray activation analysis

For hydrogenous targets that are thinner than they are wide, element sensitivities (counts.s-1.mg-1) for determining concentrations of elements by neutron capture prompt gamma-ray activation analysis (PGAA) are enhanced relative to sensitivities obtained from measurements on nonhydrogenous materials. These enhancements are caused mainly by elastic neutron scattering by H, which changes the average neutron fluence rate within the matrix. The magnitude of the effect depends on the macroscopic scattering and absorption cross sections and on the size, shape, and orientation of the target with respect to the neutron beam. Sensitivities increase linearly with H density for thin targets of constant size and shape and also vary with target shape. Theoretical work was shown that element sensitivities for hydrogenous targets in the form of spheres are least affected by neutron scattering. Methods were devised for creating solid spheres and for containing liquids in spherical shapes. Element sensitivities were determined for spheres and disks of several hydrogenous materials. For H, B, Cl, K, Br, and Cd, sensitivities for spheres were found to be less affected by neutron scattering. Exceptions were Sm and Gd sensitivities measured in liquids contained in quartz globes.