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.