Nuclear Structure and Decay Data for A=231 Isobars

The present evaluation of currently known nine nuclides of A=231: (²³¹Rn, ²³³Fr, ²³¹Ra, ²³¹Ac, ²³¹Th, ²³¹Pa, ²³¹U, ²³¹Np and ²³¹Pu) represents an update and revision of previous A=231 evaluation by 2013Br04. All the known decay and reaction data are evaluated, with recommended properties given in the Adopted Levels, Gammas datasets, for gamma-ray energies, photon branching ratios, level half-lives, spins, parities, transition probabilities, and configuration assignments. For ²³¹Rn, only an identification of the nuclide has been made without an experimental determination of its half-life. For ²³¹Fr, no excited states are known. About 30 levels in ²³¹Ra up to 1774 keV excitation are known only from β⁻ decay of ²³¹Fr with level half-lives determined for five excited states. For ²³¹Ac, excited states up to 3122 keV are known from β⁻ decay, (t,α), and fragmentary information for high-spin levels from two heavy-ion reaction studies, with lifetimes of excited states measured for five levels. Extensive structure and rotational band information for ²³¹Th is available for a large number of levels up to 1714 keV from β⁻ and α decays, (n,γ) thermal and resonance, single-particle transfer reactions (d,p), (d,t) and (³He,α), with limited data for high-spin levels from one secondary reference, and half-lives available for five excited states. Very detailed structure data are available for ²³¹Pa up to 2139 keV from β⁻, ε and α decays, (α,t), (d,d′) and high-spin data from heavy-ion Coulomb excitation and (p,2nγ) reaction, with level half-lives known for 34 excited states, mostly deduced by evaluators from transition probabilities determined in Coulomb excitation. Only eight excited states are known in ²³¹U up to 1268 keV from ε and α decays, with no data for half-lives of excited states. For ²³¹Np and ²³¹Pu, only the ground-state is known from α decays, with a tentative level at 324 keV in ²³¹Pu. Measurements of nuclear rms charge radius and isotope shifts for ²³¹Ra have been made by 2018Ly01, and that for rms charge radius for ²³¹Fr by 2014Bu06.

²³⁵U is a well known nuclide for nuclear structure and reactor applications. There have been a large number of studies of its alpha decay to levels in ²³¹Th, including some recent ones such as 2018Ma03 and 2017Le03, yet evaluators' analysis of the decay scheme suggests that, while α-particle transitions are well established, but several issues still remain about intricacies of gamma-ray and conversion electron spectroscopy which need to be resolved through dedicated experiments for a complete understanding of the decay scheme characteristics, although, this sort of research project would seem a challenging spectroscopic problem. As commented in individual datasets for other decays, important spectral information is lacking for most of the decay schemes, with almost no information available for some of these, resulting in many incomplete decay schemes in A=231 mass chain.