Speaker
Description
Neutron capture rates during the freeze-out phase of the astrophysical r process critically shape the final abundance distribution of heavy elements. For neutron-rich nuclei relevant to the r process, these rates remain largely unconstrained — direct measurements are experimentally inaccessible, and theoretical descriptions carry substantial uncertainties. In this contribution we present results from two experiments using state-of-the-art radioactive ion beam facilities and equipment.
At the CARIBU facility at Argonne National Laboratory, we applied the $\beta$-Oslo technique combined with the newly developed "Shape method" to constrain nuclear level densities of neutron-rich Cs isotopes northeast of $^{132}$Sn. These results allow us to critically assess the validity of the Hauser-Feshbach statistical model in this region. We find that along the N = 86 isotone chain, declining nuclear level densities shift the capture mechanism toward a regime dominated by individual resonances, with rates potentially exceeding Hauser-Feshbach predictions by up to two orders of magnitude for the most exotic species. Our measurements demonstrate that we are experimentally approaching the limits of statistical behaviour.
At the Facility for Rare Isotope Beams (FRIB), we targeted for the first time very neutron-rich nuclei southwest of $^{132}$Sn. near the N = 82 shell closure — a region identified as particularly sensitive for r-process freeze-out abundances, yet hitherto completely unexplored experimentally. Using the new SuN++ total absorption spectrometer, we obtained first spectra from these extremely neutron-rich nuclei and present a preliminary analysis of neutron capture rates below $^{132}$Sn.