Influence of the effective mass on the relative neutron density distribution inside a large sample in prompt-gamma neutron activation analysis

A Monte Carlo study was carried out to determine the influence of the effective scattering mass (M _e) of the atoms on the neutron density profile inside and outside the sample illuminated by a thermal neutron beam as in large-sample prompt-gamma neutron activation analysis (LS-PGNAA). From theory it is known that the spatial neutron density distribution (n(r)) inside a large sample is not the same for atoms with the same macroscopic scattering and absorption cross-section (Σ _s and Σ _a) but different M _e, due to anisotropic scattering at low M _e. The probability of neutron absorption in the sample was found to be the same for materials with equal Σ _s and Σ _a but different M _e, even though the neutron density distribution in the sample was found to change slightly. In view of typical sample, collimator and detector dimensions, it is concluded that M _e does not need to be taken into account in a correction method for neutron self-shielding in LS-PGNAA.