THE STRUCTURE OF THE PARITY-DOUBLET BANDS IN PA-231

The level structure of Pa-231 was investigated by means of the Pa-231( d,d'), Pa-231(p,p') and Th-232(p,2n gamma) reactions and Coulomb excit ation and by means of the electron-capture decay of U-231 to Pa-231. T wo low-lying opposite-parity bands based on the K-pi = 1/2(+/-) states were observed up to spins 31/2(-) and 21/2(+), respectively. From the E1/E2 branching ratios the intrinsic electric dipole moment was deter mined to be \D-0\ approximate to 0.04 e . fm. Weak Coulomb excitation of other bands was observed. The structure of Pa-231 is compared to th at of Pa-229, in particular with regard to octupole effects. The level scheme and transition intensities in Coulomb excitation are analyzed in the framework of the quasiparticle + phonon model.