Detailed study of the decay Cl31(βγ)S31

Background: Cl31 is a neutron-deficient isotope with a half-life of T1/2=190(1) ms. The nuclear structure of its daughter, S31, is important for the determination of the thermonuclear P30(p,γ)S31 reaction rate, which affects the final isotopic abundances of the ejecta from classical oxygen-neon novae. Purpose: We aim to determine the β feedings, γ-decay branchings, and excitation energies of states populated in S31 and create a comprehensive decay scheme for comparison with predictions based on the shell model. Methods: Using a Cl31 rare istope beam implanted into a plastic scintillator and an array of high-purity Ge detectors, γ rays from the Cl31(βγ)S31 decay sequence were measured. Shell-model calculations using the USDB and the recently-developed USDE interactions were performed for comparison. Results: A Cl31 β-decay scheme was constructed from the experimental data and compared to the USDB and USDE shell-model calculations based on the β feeding and γ-decay branches of each observed state. 33 new γ-ray transitions and ten new β decay branches were observed. The β feeding and γ-decay branches of each observed state were compared to those from the USDB and USDE shell-model calculations. For every allowed transition predicted by the USD calculations up to an excitation energy of 6.4 MeV in S31, an analogous transition was found in the experimental data, enabling a one-to-one comparison with the shell model. Using these identifications, spin and parity arguments were made for observed states. Conclusions: The new Cl31 γ-decay scheme presented in this work is the most complete and precise one for this nucleus constructed to date, incorporating statistics over an order of magnitude higher than previous work on Cl31 β-delayed γ decay. Of particular interest is the discovery of a 6390-keV state that mixes with the isobaric analog state and affects the P30(p,γ)S31 reaction rate. Other states observed in the decay are not expected to strongly affect the P30(p,γ)S31 reaction rate, but the comprehensive comparison to the shell model helps to clarify spin and parity assignments of resonances that might affect the rate.