Universal curve of epithermal neutron resonance self-shielding factors in foils, wires and spheres

The presence of a nuclide sample in an epithermal neutron field of a nuclear reactor creates a perturbation of the local neutron flux. This effect can be very important, especially if the nuclide cross-section exhibits a prominent resonance peak. To take into account the effect of the neutron flux perturbation in the sample activation, a resonance neutron self-shielding factor (Gres) must be considered. This factor depends on the geometry and dimension of the sample, as well as on the physical and nuclear properties of the nuclide. On the basis of a dimensionless variable which includes the relevant characteristics of the sample, an universal curve is proposed, which enables the determination of the factor Gres for isolated resonances of any nuclide and samples of various geometries (foils, wires and spheres). The proposed universal curve is in very good agreement with experimental and calculated values obtained from the literature.