Surrogate ratio methodology for the indirect determination of neutron capture cross sections

The relative γ -decay probabilities of the 162Dy to 161Dy and 162Dy to 164Dy residual nuclei, produced using light-ion-induced direct reactions, were measured as a function of excitation energy using the CACTUS array at the Oslo Cyclotron Laboratory. The external surrogate ratio method (SRM) was used to convert these relative γ -decay probabilities into the 161Dy(n,γ ) cross section in an equivalent neutron energy range of 130–560 keV. The directly measured 161Dy(n,γ ) cross section, obtained from the Evaluated Nuclear Data Files (ENDF/B-VII.0), was compared to the experimentally determined surrogate 161Dy(n,γ ) cross section obtained using compound-nucleus pairs with both similar (162Dy to 164Dy) and dissimilar (162Dy to 161Dy) nuclear structures. A γ -ray energy threshold was identified, based upon pairing gap parameters, that provides a first-order correction to the statistical γ -ray tagging approach and improves the agreement between the surrogate cross-section data and the evaluated result.