Intrinsic states and collective structures in 181Ir

Abstract

One- and three-quasiparticle states, and associated rotational bands have been identified in the odd-proton nucleus 181Ir. They were populated using the 169Tm( 16O, 4n) 181Ir reaction and established using a variety of time-correlated γ-ray and electron techniques. The 9 2 −[514] and 5 2 +[402] intrinsic states (from the h 11 2 and d 5 2 parents) were found to be metastable with mean of 193 and 430 ns, respectively. A third isomer, with K π = 23 2 + and a meanlife of 42 ns was identified and attributed to a one-quasiproton, two-quasineutron configuration, as were two other high- K band heads. The B( M1) B( E2) and g K − g R values extracted from the in-band decay properties of the observed rotational bands, based on these and other intrinsic states, were used to identify and test the proposed configurations. Interference effects were observed between states of the band assigned to the α = + 1 2 signature sequence from the i 13 2 configuration, with those from a second α = + 1 2 sequence, and the states from one of the high- K bands (leading to complicated branchings in the decay scheme). The α = + 1 2 sequence of the h 9 2 proton band was extended to higher spins than known previously and its (unflavoured) α = − 1 2 sequence identified. Both show alignments and alignment gains which are essentially well-behaved and consistent with a stable quadrupole deformation and small γ-asymmetry. In contrast, the h 11 2 and d 5 2 bands show complex alignment gains at low frequencies. These, and the in-band decay properties can be reproduced in a three-band model. However, essentially all bands, except for the h 9 2 one-quasiparticle cases, exhibit an underlying alignment increase with frequency if reference parameters appropriate to the h 9 2 band are used. Comparisons between the one-, two- and three-quasiparticle bands in which different orbitals are blocked, suggest the presence of alignment gains due to the h 9 2 protons but conflicting evidence remains. Some of the alignment effects may be artefacts of larger deformations, or alternatively a reduction in pairing.