Quenched E1 transition rates in Th-231

The fast timing beta gamma gamma(t) method has been used to measure lifetim es of the low-lying levels in Th-231 populated in the beta(-) decay of Ac-2 31. The half-life of the K-pi = 5/2(-) band-head at 185.7-keV was measured as T-1/2 = 1073(79) ps, yielding D-0 = 1.76(8) x 10(-2) e fm and 1.65(6) x 10(-2) e.fm for the El transitions to the 7/2(+) and 5/2(+) members respect ively of the K-pi = 5/2(+) ground state band. Furthermore, T-1/2 less than or equal to 74 ps and less than or equal to 74 ps were measured for the K-p i = 3/2(+) and 1/2(+) bandheads at 221.4- and 247.6-keV, and T-1/2 = 503(12 ) ps and 98(36) ps for the 1/2(-) and 3/2(-) states at 554.7- and 593.6-keV , members of the K-pi = 1/2(-) band. The latter results consistently yield D(0)values of the order of 1.1 x 10-2 e.fm for three transitions connecting the K-pi = 1/2(+/-) bands. The observed transition rates have been interpr eted within the quasiparticle-phonon model with the inclusion of Coriolis c oupling. (C) 1999 Elsevier Science B.V. All rights reserved.