Measurement of γ-emission branching ratios for Gd154,156,158 compound nuclei: Tests of surrogate nuclear reaction approximations for (n,γ) cross sections

The surrogate nuclear reaction method can be used to determine neutron-induced reaction cross sections from measured decay properties of a compound nucleus created using a different reaction and calculated formation cross sections. The reliability of (n,γ) cross sections determined using the Weisskopf-Ewing and ratio approximations are explored for the Gd155,157(n,γ) reactions. Enriched gadolinium targets were bombarded with 22-MeV protons and γ rays were detected in coincidence with scattered protons using the Silicon Telescope Array for Reaction Studies/Livermore- Berkeley Array for Collaborative Experiments (STARS/LiBerACE) silicon and germanium detector arrays. The γ-emission probabilities for the Gd154,156,158 compound nuclei were measured at excitation energies up to 12 MeV. It is found that the approximations yield results that deviate from directly measured Gd155,157(n,γ) cross sections at low energies. To extract reliable cross sections, a more sophisticated analysis should be developed that takes into account angular-momentum differences between the neutron-induced and surrogate reactions.