Strong one-neutron emission from two-neutron unbound states in Beta decays of the r-process nuclei 86,87 Ga

Abstract

Beta-delayed one-neutron and two-neutron branching ratios (P1n and P2n) 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 3He neutron counters (BRIKEN). Two-neutron emission was observed in the decay of 84,85,87Ga for the first time and the branching ratios were measured to be P2n = 1.6(2)%, 1.3(2)%, and 10.2(28)stat(5)sys%, respectively. One-neutron branching ratio of 87Ga (P1n = 81(9)stat(8)sys%) and half-life of 29(4) ms were measured for the first time. The branching ratios of 86Ga were also measured to be P1n = 74(2)stat(8)sys% and 16.2(9)stat(6)sys% with better precision than a previous study. The observation that P1n > P2n for both 86,87Ga was unexpected and is interpreted as a signature of dominating one-neutron emission from the two-neutron unbound excited states in 86,87Ge. In order to interpret the experimental results, shell-model and Hauser-Feshbach statistical model calculations of delayed particle and ¿ -ray emission probabilities were performed. This model framework reproduces the experimental results. The shell model alone predicts P2n significantly larger than P1n for the 87Ga decay, and it is necessary to invoke a statistical description to successfully explain the observation that P1n > P2n. 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.