Measurement of rates of fast E1 transitions in AcRa nuclei

Abstract

The half-lives of the 40.0 keV level in 225Ac, 31.6 keV level in 225Ra, and 27.4 keV level in 227Ac have been measured to be 0.72 ± 0.03 ns, 2.1 ± 0.2 ns, and 38.3 ± 0.3 ns, respectively. The three levels decay only by E1 transitions. The lifetimes were measured by delayed coincidence techniques using pure samples. The reduced transition probabilities in 225Ac and 225Ra are about two orders of magnitude larger than the values encountered in the mid-actinide nuclei. On the other hand, the transition rate in 227Ac is similar to the values measured in heavier actinides. Previous studies suggest the presence of octupole deformation in all three nuclei. Our results demonstrate that large enhancements in E1 transition rates can occur for nuclei with octupole deformation. However, the results also indicate that there is no simple relationship between the E1 rate and the octupole deformation. The half-life of the 25.4 keV state in 225Ra, which decays by a pure rotational E2 transition, has been measured to be 0.88 ± 0.04 ns. This lifetime yields a quadrupole moment of 6.47 ± 0.15 b and a β 2 deformation of 0.17 + 0.004.