Nuclear structure of 229,231Th studied with the 230,232Th(d,t) reactions

Detailed angular distributions for the 230,232Th(d,t)229,231Th reactions were measured with 17 MeV deuterons, using a magnetic spectrograph to analyze the reaction products. Comparisons with distorted wave Born approximation predictions were used to determine ℓ-values and spectroscopic strengths for many levels in both nuclides. These results reconfirm previous assignments for the 3/2+[631] band in 229Th, with its bandhead unresolved from the 5/2+[633] ground state, providing support for the existence of the much-sought “3.5 eV” isomer (for which the latest reported energy is 7.6±0.5 eV). Although 231Th was previously well studied, many new spin-parity assignments have been made and some changes to adopted model assignments are proposed. There is evidence in both isotopes for appreciable mixing of single-particle orbitals with other configurations, as predicted by the Quasiparticle Phonon Model. Single-nucleon transfer strengths are described much better with earlier values for the Nilsson model parameters κ and μ than later values that have been widely used. An earlier 232Th(d,t) study concluded that serious errors could result in the commonly-used method of predicting cross sections by combining distorted wave Born approximation calculations using spherical form factors with Nilsson wave functions for a deformed potential, but the present work shows this conclusion is not valid because it was based on inadequate knowledge of the 231Th nuclear structure.