Levels in 128Te

Abstract

The half-life of the 6 + level at 1811 keV in 128Te has been studied by the β−γ delayed coincidence method using a mass separated source of 128Sb ( T 1 2 = 11 min ) in equilibrium with 128Sn ( T 1 2 = 59 min ). A discrepancy between the results obtained by two different methods of measurement led to the postulate that there was no significant direct β-feeding to the 4 + level at 1497 keV in 128Te from 11 min 128Sb, and this was confirmed by a study of the chemically separated antimony activity using a Ge(Li) detector. A revised level scheme was constructed for 128Te incorporating four new states above 2.4 MeV, and a consistent value of 0.42±0.03 ns was deduced for the half-life of the 1811 keV level. This implies a B(E2) of 0.05 e 2 · b 2 for the 314 keV transition which is therefore enhanced by a factor of 13 with respect to the Weisskopf estimate. This result is related to theoretical work, in particular to that dealing with ground state quasirotational bands. It is shown that the properties of neutron-rich tellurium nuclei are of special interest for an understanding of recent predictions based on the variable-moment-of-inertia model.