Collective E1 transitions in even-A Ra, Th, and U nuclei

Transitions between rotational states of the ground-state and lowest Kπ=0- bands in 226Ra, 224-232Th and 230,232U were studied by gamma-ray and conversion-electron spectroscopy following (d, pn) and (α, xn) reactions in beam, and following the (p, xn) production of radioactive Pa isotopes out of beam. Coincidence measurements allowed to establish the level schemes and to determine E1/E2 branching ratios which can be interpreted in terms of the ratios of intrinsic electric dipole to quadrupole moments, D0/Q0, of a rigid rotor. This ratio was found to be independent of spin from I=8 to 19 for 226Th, whereas there is an indication that it increases for 224Th in the same spin region by almost a factor of two. For the transitional nuclei 226Ra and 228Th no spin-dependence was found for spins up to I=11 and 13, respectively. For the refection-symmetric nucleus 230Th the D0/Q0 ratios are consistent with the rotational predictions if the Coriolis coupling to the Kπ=1- band is taken into account. (orig.)