Measurement ofγ-emission branching ratios forGd154,156,158compound nuclei: Tests of surrogate nuclear reaction approximations for(n,γ) cross sections

Abstract

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 $(n,\ensuremath{\gamma})$ cross sections determined using the Weisskopf-Ewing and ratio approximations are explored for the $^{155,157}\mathrm{Gd}$$(n,\ensuremath{\gamma})$ reactions. Enriched gadolinium targets were bombarded with 22-MeV protons and $\ensuremath{\gamma}$ rays were detected in coincidence with scattered protons using the Silicon Telescope Array for Reaction Studies/Livermore-Berkeley Array for Collaborative Experiments (STARS/LiBerACE) silicon and germanium detector arrays. The $\ensuremath{\gamma}$-emission probabilities for the $^{154,156,158}\mathrm{Gd}$ compound nuclei were measured at excitation energies up to 12 MeV. It is found that the approximations yield results that deviate from directly measured $^{155,157}\mathrm{Gd}$$(n,\ensuremath{\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.