Determininig the (n,γ) cross section of¹⁵³Gd using surrogate reactions

The astrophysical s-process is responsible for the synthesis of many of the nuclei heavier than iron through a series of low-energy (n,γ) reactions and β-decays. For nuclei for which the neutron capture and β-decay rates are comparable, the branching is crucial for tests of s-process models. Direct measurements of (n,γ) cross sections for these nuclei are extremely challenging due to the inherent difficulties associated with radioactive targets and the low intensity of available neutron beams. The surrogate reaction technique can be used to circumvent these difficulties by creating the same compound nucleus through light-ion reactions on a stable target. The cross section can be determined by combining optical model calculations for the formation of the compound nucleus with the measured exit channel probability for γ-ray emission. We have collected data to determine the low-energy (n,γ) cross section for the unstable nucleus ¹⁵³Gd by bombarding a stable ¹⁵⁴Gd target with protons from the 88-Inch Cyclotron at the Lawrence Berkeley National Laboratory to create the desired ¹⁵⁴Gd* compound nucleus. The STARS/LiBerACE silicon and clover germanium detector arrays were used to detect y-rays in coincidence with the scattered protons. Additional cross section measurements using ¹⁵⁶Gd and ¹⁵⁸Gd targets will be compared to direct measurements of the (n,γ) cross sections for ¹⁵⁵Gd and ¹⁵⁷Gd. The current status of the analysis is summarized.