Electric dipole moments in 230, 232U and implications for tetrahedral shapes

The nuclei U-230 and U-232 were populated in the compound nucleus reactions Th-232(alpha,6n) and Th-232(alpha,4n), respectively. Gamma rays from these nuclei were observed in coincidence with a recoil detector. A comprehensive set of in-band E2 transitions were observed in the lowest lying negative-parity band of U-232 while one E2 transition was also observed for U-230. These allowed B(E1; I- -> I+ - 1)/B( E2; I- -> I- - 2) ratios to be extracted and compared with systematics. The values are similar to those of their Th and Ra isotones. The possibility of a tetrahedral shape for the negative-parity U bands appears difficult to reconcile with the measured Q(2) values for the isotone Ra-226.