Collective states fed by weak α-transitions in the 232U chain

The 232U decay chain has been investigated down to 212Pb using Ge(Li) spectrometers in singles and coincidence measurements. In the decay of 232U and 228Th some 10-5-10-6% α-branchings to collective states at > approximately 400 keV in 228Th and 224Ra, respectively, have been identified. Hindrance factors (HF) calculated for all α-transitions are discussed in terms of nuclear models. In particular, for the Ksup(π) = 0+ bands with the heads at 831.4 and 916.4 keV in 228Th and 224Ra, respectively, the low HF values are consistent with the quadrupole-pairing vibrational nature of these states indicated by previous (p,t) reaction studies. No evidence has been found (branching ratio < 2 x 10-6%, HF > 105) for an α-transition to a hypothetical two-phonon 0+ state in 224Ra which should be expected at approximately 430 keV, if the 215.98 keV 1- state were due to harmonic octupole vibrations. The alternative interpretation of this 1- state as a shape isomer is not supported by the analysis of the half-lives of the 3- and 5- rotational states. For the decay of 224Ra and its descendants, one new transition and improved energies for the levels are reported. (Auth.)