Gamma Decay of Unbound Neutron-Hole States in 133Sn

Excited states in the nucleus Sn-133, with one neutron outside the double magic Sn-132 core, were populated following one-neutron knockout from a Sn-134 beam on a carbon target at relativistic energies at the Radioactive Isotope Beam Factory at RIKEN. Besides the gamma rays emitted in the decay of the known neutron single-particle states in Sn-133 additional gamma strength in the energy range 3.5-5.5 MeV was observed for the first time. Since the neutron-separation energy of Sn-133 is low, S-n = 2.402(4) 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 Sn-132 nuclear structure effects may play a significant role in the neutron versus 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 beta-decay properties for astrophysical simulations may have to be reconsidered.