Persistence of octupole correlations in Ra-231

The structure of Ra-231, the heaviest odd Ra nucleus currently accessible f or detailed spectroscopic investigation, has been studied in the beta (-) d ecay of Fr-231. The main purpose has been to verify whether fast B(E1) rate s and significant octupole correlations recently established in Ra-229 pers ist in Ra-231. The excited stares in Ra-231 have been determined for the fi rst time. Spins and parities have been deduced from conversion electron mea surements, while level half-lives in the ps and ns ranges have been measure d with the Advanced Time Delayed beta gamma gamma (t) method. The moderatel y fast B(E1) rates found For transitions connecting the lowest-lying K-pi = 5/2(+/-) and K-pi = 1/2(+/-) hands, reveal the persistence of octupole col lective effects in Ra-231, although the B(E1) rates are lower than in Ra-22 9. These findings reinforce the differences in the B(E1) systematics betwee n isotonic Ra and Th nuclei. (C) 2001 Elsevier Science B.V. All rights rese rved.