Quenched E1 transition rates in 231Th

Abstract The fast timing βγγ ( t ) method has been used to measure lifetimes of the low-lying levels in 231 Th populated in the β − decay of 231 Ac. The half-life of the K π = 5 2 − band-head at 185.7-keV was measured as T 1 2 = 1073(79) ps , yielding D 0 = 1,76(8) × 10 −2 e ·fm and 1.65(6) × 10 −2 e ·fm for the E1 transitions to the 7 2 + and 5 2 + members respectively of the K π = 5 2 + ground state band. Furthermore, T 1 2 ⩽ 74 ps and ⩽ 74 ps were measured for the K π = 3 2 + and 1 2 + band-heads 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 π = 1 2 − band. The latter results consistently yield D 0 values of the order of 1.1 × 10 −2 e ·fm for three transitions connecting the K π = 1 2 ± bands. The observed transition rates have been interpreted within the quasiparticle-phonon model with the inclusion of Coriolis coupling.