The rapid (r) neutron capture process is responsible for the synthesis of about half of the nuclei heavier than iron. The astrophysical site(s) of this process continue to be an open question. In particular, the synthesis of the elements with atomic mass 80 (near the neutron N=50 shell closure) likely require more than one scenario, often described as a “weak” r process. In 2014 Surman and colleagues [1] analyzed the impact of unknown neutron capture rates in reproducing observed “weak” r-process abundances. Near shell closures, neutron capture could proceed via two processes: direct (semi-direct) (DSD) and via the population and decay of a compound nucleus (CN). While the nuclei important for weak r-process nucleosynthesis are unstable, they can be accelerated to enable reaction studies to inform neutron capture.<br/><br/>Direct neutron capture (and the semi-direct process that proceeds via the giant dipole resonance) has a sensitive dependence on specific low-spin neutron single-particle excitation energies and spectroscopic strengths that can be deduced from neutron transfer reactions such as (d,p). The Oak Ridge Rutgers University Barrel Array (ORRUBA) collaboration has informed the DSD capture on several key A≈80 nuclei. More recently, ORRUBA has been coupled to large arrays of gamma-ray detectors, including GRETINA, to inform CN neutron capture via (d,pγ) reactions.<br/><br/>The present talk will summarize the efforts to inform neutron capture on A≈80 nuclei important to understand weak r-process nucleosynthesis and prospects for informing the competition between DSD and CN capture including with the Facility for Rare Isotope Beams.

Synthesizing elements with the weak r process

ACS Fall 2021

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