β-delayed Fission in r-process Nucleosynthesis

Abstract

We present β -delayed neutron emission and β -delayed fission ( β df) calculations for heavy, neutron-rich nuclei using the coupled Quasi-Particle Random Phase Approximation plus Hauser-Feshbach (QRPA+HF) approach. From the initial population of a compound nucleus after β -decay, we follow the statistical decay, taking into account competition between neutrons, γ -rays, and fission. We find a region of the chart of nuclides where the probability of β df is ∼100%, which likely prevents the production of superheavy elements in nature. For a subset of nuclei near the neutron dripline, neutron multiplicity and the probability of fission are both large, leading to the intriguing possibility of multi-chance β df, a decay mode for extremely neutron-rich heavy nuclei. In this decay mode, β -decay can be followed by multiple neutron emission, leading to subsequent daughter generations that each have a probability to fission. We explore the impact of β df in rapid neutron-capture process ( r -process) nucleosynthesis in the tidal ejecta of a neutron star–neutron star merger and show that it is a key fission channel that shapes the final abundances near the second r -process peak.