Level structure of S31 via S32(p,d)S31

Background: Properties of proton-unbound S31 states determine the P30(p,γ)S31 reaction rate, which has a significant impact on explosive hydrogen burning in classical novae and type-I x-ray bursts. Despite several previous studies, uncertainties still remain with respect to the nuclear structure of S31 near the proton threshold. Purpose: The level structure of S31 has been presently investigated via a charged-particle spectroscopy experiment using the S32(p,d)S31 reaction. Method: Deuterons corresponding to S31 excited states with 3.285≤Ex≤10.8 MeV were momentum analyzed via an Enge split-pole spectrograph at six laboratory angles between 10∘ and 62∘. Differential cross sections of the S32(p,d)S31 reaction were measured at Ep=34.5 MeV. Distorted-wave Born approximation calculations were performed to constrain the spin-parity assignments of several of the observed levels. Results: We have detected 72 excited states of S31, out of which 17 are within the astrophysical region of interest corresponding to the temperature range of 0.1–1.5 GK. We have resolved the discrepancy in the spin and parity of an excited state with Ex=6542 keV, showing that is it not Jπ=3/2−, and therefore the contribution of this state to the P30(p,γ) reaction rate is likely much less significant than previously thought owing to the larger angular-momentum transfer required to populate this excited state. Moreover, our measurement results help consolidate the spin-parity assignments for the 6377 and 6636 keV states in S31. Conclusions: This work presents the most comprehensive spin-parity assignments to date from a single-neutron transfer reaction on S32 to S31 excited states in the region between 6 to 7 MeV excitation energy. This region is significant for the determination of the P30(p,γ)S30 reaction rate over the temperatures characteristic of explosive hydrogen burning in novae.