Measurement of γ -emission Branching Ratios for 154,156,158 Gd 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,gamma) cross sections determined using the Weisskopf-Ewing and ratio approximations are explored for the {sup 155,157}Gd(n,gamma) reactions. Enriched gadolinium targets were bombarded with 22-MeV protons and gamma 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 gamma-emission probabilities for the {sup 154,156,158}Gd compound nuclei were measured at excitation energies up to 12 MeV. It is found that the approximations yield results that deviate from directly measured {sup 155,157}Gd(n,gamma) 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.