Mean Lives of Excited Rotational States of Heavy Even-Even Nuclei

Abstract

The lifetimes of the 2+ and 4+ excited states of the ground-state rotational band of three even-even nuclides in the rotational region beyond Pb, and the 2+ first excited state of a nearby vibrational nuclide, were measured by means of delayed-coincidence techniques. The alphas to the excited state were detected with a semiconductor charged-particle detector, and the conversion electrons from the de-exciting state were detected with a plastic scintillator. The full width at half maximum of the resulting prompt spectrum was 0.5 nsec, with a limiting equivalent half-life of 60 psec. A system of multiple Peierls analyses was carried out on the MIT 7094 computer to obtain the best lifetime values, which are presented below, along with the ratio of the reduced transition probabilities in the case of rotational nuclides. The uncertainties quoted for the $B(E2)$ ratios do not include estimates of absolute uncertainties in the theoretical internal-conversion coefficients employed. The ratios of the reduced transition probabilities are seen to be quite close to the value of 1.43 predicted by the collective model, with ${\mathrm{Ra}}^{224}$ being 4% low, ${\mathrm{Th}}^{228}$ 2% high, and ${\mathrm{Th}}^{230}$ 2% low.