Strong one-neutron emission from two-neutron unbound states in β decays of the r -process nuclei Ga86,87

Le résumé fourni par la source

$\ensuremath{\beta}$-delayed one-neutron and two-neutron branching ratios $({P}_{1n}$ and ${P}_{2n})$ have been measured in the decay of $A=84$ to 87 Ga isotopes at the Radioactive-Isotope Beam Factory (RIBF) at the RIKEN Nishina Center using a high-efficiency array of $^{3}\mathrm{He}$ neutron counters (BRIKEN). Two-neutron emission was observed in the decay of $^{84,85,87}\mathrm{Ga}$ for the first time and the branching ratios were measured to be ${P}_{2n}=1.6(2)%,1.3(2)%$, and $10.2{(28)}_{\mathrm{stat}}{(5)}_{\mathrm{sys}}%$, respectively. One-neutron branching ratio of $^{87}\mathrm{Ga}\phantom{\rule{4pt}{0ex}}({P}_{1n}=81{(9)}_{\mathrm{stat}}{(8)}_{\mathrm{sys}}%)$ and half-life of 29(4) ms were measured for the first time. The branching ratios of $^{86}\mathrm{Ga}$ were also measured to be ${P}_{1n}=74{(2)}_{\mathrm{stat}}{(8)}_{\mathrm{sys}}%$ and $16.2{(9)}_{\mathrm{stat}}{(6)}_{\mathrm{sys}}%$ with better precision than a previous study. The observation that ${P}_{1n}>{P}_{2n}$ for both $^{86,87}\mathrm{Ga}$ was unexpected and is interpreted as a signature of dominating one-neutron emission from the two-neutron unbound excited states in $^{86,87}\mathrm{Ge}$. In order to interpret the experimental results, shell-model and Hauser-Feshbach statistical model calculations of delayed particle and $\ensuremath{\gamma}$-ray emission probabilities were performed. This model framework reproduces the experimental results. The shell model alone predicts ${P}_{2n}$ significantly larger than ${P}_{1n}$ for the $^{87}\mathrm{Ga}$ decay, and it is necessary to invoke a statistical description to successfully explain the observation that ${P}_{1n}>{P}_{2n}$. Our new results demonstrate the relevance and importance of a statistical description of neutron emission for the prediction of the decay properties of multineutron emitters and that it must be included in the $r$-process modeling.

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