A study of ${}^{35}$Cl excited states via ${}^{32}$S($\alpha ,p$)

Abstract

Presolar grains originating in oxygen-neon novae may be identified by their sulfur isotopic ratios compared with theoretical estimates. These ratios depend on reliable ${}^{33}$S($p,\gamma$)${}^{34}$Cl and ${}^{34}$S($p,\gamma$)${}^{35}$Cl reaction rates. The latter rate has recently been computed based on experimental input, and many new excited states in ${}^{35}$Cl were discovered above the proton threshold. The experimental ${}^{34}$S($p,\gamma$)${}^{35}$Cl rate was found to be 2 - 5 times smaller than the theoretical one, and the simulated ${}^{34}$S/${}^{32}$S isotopic ratio for nova presolar grains was thus predicted to be smaller than that of type II supernova grains by up to a factor of 3.7. The present study was performed to confirm the existence of these new resonances, and to improve the remaining uncertainties in the ${}^{34}$S($p,\gamma$)${}^{35}$Cl reaction rate. Energies and spin-parities of the ${}^{35}$Cl excited levels were investigated with an Enge split-pole spectrograph using the ${}^{32}$S($\alpha ,p$)${}^{35}$Cl reaction. Differential cross sections of the outgoing protons were measured at $E_{\alpha }$ = 21 MeV. The existence of the newly discovered states are largely confirmed, although a few states were not observed in this study. The spins and parities of several ${}^{35}$Cl states were assigned tentatively for the first time. The present ${}^{34}$S($p,\gamma$)${}^{35}$Cl experimental thermonuclear reaction rate is consistent within 1$\sigma$ with the previous evaluation. However, our rate uncertainty is larger due to a more realistic treatment of the experimental uncertainties. The uncertainty in the present rate is up to a factor of 3.5 at nova temperatures. We recommend future work to focus on the unknown properties of four excited states of ${}^{35}$Cl at 6643 keV, 6761 keV, 6780 keV, and 6800 keV.

arXiv comment17 pages, 9 figures, accepted to Phys. Rev. C

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