Nuclear levels in228Th populated in the decay of228Pa

Abstract

The electron-capture decay of228Pa to levels in228Th has been studied using mass-separated sources and high-resolutionγ-ray and conversion-electron spectroscopy. A level at 979.5 keV is assigned as 2+ member of a second excited Kπ=0+ band, with the 0+ band head at 938.6 keV. The 2+ and 3+ members of a second excited Kπ=2+ band at 1153.5 and 1200.5 keV, which decay by strongE0 transitions to the 969 keVγ-vibrational band, are confirmed. In addition we tentatively propose a Kπ=1+ band at 944 keV. The Kπ=0−, 1− and 2− members of the octupole quadruplet are confirmed, and theγ decay of these levels is analysed in an approach, in which the mixing of the octupole bands by the Coriolis interaction is taken into account. It is suggested that octupole correlations might be important for theE1 transition moments. A total of 29 levels is observed between ∼1.4 and ∼2.0 MeV, for which the nuclear structure, and the possible assignment to rotational bands, is unclear.