Gamma Decay of Unbound Neutron-Hole States in Sn133

Excited states in the nucleus $^{133}\mathrm{Sn}$, with one neutron outside the double magic $^{132}\mathrm{Sn}$ core, were populated following one-neutron knockout from a $^{134}\mathrm{Sn}$ beam on a carbon target at relativistic energies at the Radioactive Isotope Beam Factory at RIKEN. Besides the $\ensuremath{\gamma}$ rays emitted in the decay of the known neutron single-particle states in $^{133}\mathrm{Sn}$ additional $\ensuremath{\gamma}$ strength in the energy range 3.5--5.5 MeV was observed for the first time. Since the neutron-separation energy of $^{133}\mathrm{Sn}$ is low, ${S}_{n}=2.402(4)\text{ }\text{ }\mathrm{MeV}$, this observation provides direct evidence for the radiative decay of neutron-unbound states in this nucleus. The ability of electromagnetic decay to compete successfully with neutron emission at energies as high as 3 MeV above threshold is attributed to a mismatch between the wave functions of the initial and final states in the latter case. These findings suggest that in the region southeast of $^{132}\mathrm{Sn}$ nuclear structure effects may play a significant role in the neutron versus $\ensuremath{\gamma}$ competition in the decay of unbound states. As a consequence, the common neglect of such effects in the evaluation of the neutron-emission probabilities in calculations of global $\ensuremath{\beta}$-decay properties for astrophysical simulations may have to be reconsidered.