Strong one-neutron emission from two-neutron inbound states in beta decays of $r$-process nuclei, $^{86, 87}$Ga*

Beta-delayed one- and two-neutron branching ratios ($P_{1n}$ and $P_{2n}$) are measured in the decay of $^{86}$Ga and $^{87}$Ga at the RI-beam Factory at RIKEN Nishina Center using a high-efficiency array of $^{3}$He neutron counters (BRIKEN). Large $P_{1n}$ values are observed in $^{87,86}$Ga compared to $P_{2n}$ which was not predicted by theoretical calculations [PRL117,092502(2016), PRC67,055802(2003), PRC93,025805(2016)]. Our shell model calculation explains the observed $P_{1n} > P_{2n}$ successfully only when combined with Hauser-Feshbach statistical model [Nucl.Phys.A913,51(2013)]. This result is the first experimental demonstration that competition between multi-particle emission channels has to be considered to predict the decay properties of multi-neutron emitters which must be included in the astrophysical $r$-process modeling.