First observation of excited states in the 138I nucleus

Excited states in the I-138 nucleus, including T-1/2=1.3 mu s isomer decaying by a stretched E2 transition of 68 keV, were observed for the first time. The I-138 nucleus was populated in the spontaneous fission of Cm-248 and studied by means of prompt gamma-ray spectroscopy using the EUROGAM 2 array. The microsecond isomer was populated in the neutron-induced fission of U-235 and observed at the LOHENGRIN separator. Excitation scheme consists of a low-spin part and a medium-spin, Delta I=1, band based on the 7(-) state with the (pi g(7/2)nu f(7/2))(7)(-) dominating configuration, as predicted by the shell model. The shell-model calculations of I-138 provide the optimum reproduction of the experimental scheme when the pi d(5/2) orbital is lowered by 600 keV relative to its position in Sb-133. In the calculation the isomeric level has spin and parity 3(-) and deexcites by an E2 isomeric transition to the 1(-) level, located only 9 keV above the predicted 0(-) ground state. Considering additional information on the ground-state spin from the literature, we propose that in I-138 the 1(-) level corresponds to the ground state and the 0(-) is located above. We note, however, that additional measurements are required to resolve this problem.