COLLECTIVE E1 TRANSITIONS IN EVEN-A RA, TH, AND U-NUCLEI

Transitions between rotational states of the ground-state and lowest K(pi) = 0- bands in Ra-226 Th-224-232 and U-230,U-232 were studied by gamma-ray and conversion-electron spectroscopy following (d, pn) and (alpha, 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 Ra-226 and Th-228 no spin-dependence was found for spins up to I = 11 and 13, respectively. For the reflection-symmetric nucleus Th-230 the D0/Q0 ratios are consistent with the rotational predictions if the Coriolis coupling to the K(pi) = 1- band is taken into account.