Study of the β-delayed multi-neutron emission process through γ-neutron spectroscopy of Gallium Isotopes.

Beta-delayed neutron emission occurs in neutron-rich nuclei where the decay energy window is large enough to populate states above the neutron separation energy in the daughter nucleus. Understanding the neutron emission process is crucial since one, or even multiple, neutron emissions serve as the dominant decay modes for the neutron-rich nuclei along the r-process path. A comprehensive β-delayed γ-neutron spectroscopy study on the decay of gallium isotopes A = 84 to 87 was conducted at the RIKEN Nishina Center's RI-beam Factory. The isotopes were examined using a high-efficiency array of ³He neutron counters (BRIKEN) and two clover-type HPGe detectors, enabling β-γ-2n coincidence measurements of the excited states of two-neutron daughter nuclei. Experimental results demonstrated that both neutron and γ branching ratios, as well as P_1n and P_2n ratios, are sensitive to the nuclear level densities of 1n daughter nuclei and the ground-state spin of the parent nucleus. Hauser-Feshbach statistical model calculations were optimized using experimental and shell-model levels in the daughter nuclei, thereby improving the reproduction of the P_2n/P_1n ratio. Given that neutron emissions significantly influence the final abundance pattern of r-process nucleosynthesis, these results underscore the importance of understanding level density and having detailed comprehension of the decay scheme, which includes neutron spectroscopy data for multi-neutron emitters.