Experimental study of Cl35 excited states via S 32(α,p)

The presolar grains originating in oxygen-neon novae may be identified more easily than those of other stellar sources if their sulfur isotopic ratios ($^{33}\mathrm{S}/^{32}\mathrm{S}$ and $^{34}\mathrm{S}/^{32}\mathrm{S}$) are compared with the theoretical ones. The accuracy of such a comparison depends on reliable $^{33}\mathrm{S}(p,{\gamma})^{34}\mathrm{Cl}$ and $^{34}\mathrm{S}(p,{\gamma})^{35}\mathrm{Cl}$ reaction rates at the nova temperature regime. The latter rate has recently been computed based on experimental input, and many new excited states in $^{35}\mathrm{Cl}$ were discovered above the proton threshold. As a result, the experimental $^{34}\mathrm{S}(p,{\gamma})^{35}\mathrm{Cl}$ rate was found to be less uncertain and 2–5 times smaller than the theoretical one. Consequently, the simulated $^{34}\mathrm{S}/^{32}\mathrm{S}$ isotopic ratio for nova presolar grains was predicted to be smaller than that of type II supernova grains by a factor of 1.5 to 3.7. The present study was performed to confirm the existence of these new resonances, and to improve the remaining uncertainties in the S 34(p,γ)Cl35 reaction rate. Energies and spin-parities of the Cl35 excited levels were investigated via high-resolution charged-particle spectroscopy with an Enge split-pole spectrograph using the S 32(α,p)Cl35 reaction. Differential cross sections of the outgoing protons were measured at Eα=21 MeV. Distorted-wave Born approximation calculations were carried out to constrain the spin-parity assignments of observed levels, with special attention to those significant in determination of the S 34(p,γ)Cl35 reaction rate over the nova temperature regime. The existence of these newly discovered states are largely confirmed, although a few states were not observed in this study. The spins and parities of a few Cl35 states were assigned tentatively for the first time. The present S 34(p,γ)Cl35 experimental thermonuclear reaction rate at 0.1–0.4 GK is consistent within 1σ with the previous evaluation. However, our rate uncertainty is larger than before due to a more realistic treatment of the uncertainties in the rate input. In comparison with the previous rate evaluation, where the high and low rates differed by less than a factor of 2 over the nova temperature regime, the ratio of the present limit rates is at most a factor of 3.5 at 0.12 GK. At temperatures above 0.2 GK, we recommend the future work to focus on determination of the unknown properties of four excited s