The $\beta$-delayed one- and two-neutron emission probabilities ($P_{1n}$ and $P_{2n}$) of 20 neutron-rich nuclei with $N\ge 82$ have been measured at the RIBF facility of the RIKEN Nishina Center. $P_{1n}$ of ${}^{130,131}\mathrm{Ag}$, ${}^{133,134}\mathrm{Cd}$, ${}^{135,136}\mathrm{In}$, and ${}^{138,139}\mathrm{Sn}$ were determined for the first time, and stringent upper limits were placed on $P_{2n}$ for nearly all cases. $\beta$-delayed two-neutron emission ($\beta 2n$) was unambiguously identified in ${}^{133}\mathrm{Cd}$ and ${}^{135,136}\mathrm{In}$, and their $P_{2n}$ were measured. Weak $\beta 2n$ was also detected from ${}^{137,138}\mathrm{Sn}$. Our results highlight the effect of the $N=82$ and $Z=50$ shell closures on $\beta$-delayed neutron emission probability and provide stringent benchmarks for newly developed macroscopic-microscopic and self-consistent global models with the inclusion of a statistical treatment of neutron and $\gamma$ emission. The impact of our measurements on $r$-process nucleosynthesis was studied in a neutron star merger scenario. Our $P_{1n}$ and $P_{2n}$ have a direct impact on the odd-even staggering of the final abundance, improving the agreement between calculated and observed Solar System abundances. The odd isotope fraction of Ba in $r$-process-enhanced ($r\text{−}\mathrm{II}$) stars is also better reproduced using our new data.

• Received 4 June 2022
• Revised 30 July 2022
• Accepted 25 August 2022
• Corrected 15 August 2023

DOI:https://doi.org/10.1103/PhysRevLett.129.172701