Level structure of S 31 via S 32 (p,d) S 31
Abstract
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.