Strong one-neutron emission from two-neutron unbound states in decays of neutron-rich Ga isotopes

Beta-delayed one-neutron and two-neutron branching ratios (P1n and P2n) have been measured in the decay of neutron-rich Ga isotopes A = 84 to 87 at the RI-beam Factory 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, and 87Ga for the first time. P1n values are also obtained with more precision than previous measurements. The observation of the large P1n values compared to the P2n values in the Ga isotopes are interpreted as a signature of one neutron emission from the two-neutron unbound excited states in Ge daughters. We demonstrated that inclusion of Hauser Feshbach statistical model reproduces our experimental results better both on QRPA[P. Moller et al.] and Shell Model[M. Madurga et al.] calculations. This shows the relevance and importance of a statistical description of neutron emission for the prediction of the decay properties of multi-neutron emitters and that it must be included in the r-process modeling.