Experimental study of Cl 35 excited states via S 32 (α,p)

Background: The presolar grains originating in oxygen-neon novae may be identified more easily than those of other stellar sources if their sulfur isotopic ratios (S33/S32 and S34/S32) are compared with the theoretical ones. The accuracy of such a comparison depends on reliable S33(p,γ)Cl34 and S34(p,γ)Cl35 reaction rates at the nova temperature regime. The latter rate has recently been computed based on experimental input, and many new excited states in Cl35 were discovered above the proton threshold. As a result, the experimental S34(p,γ)Cl35 rate was found to be less uncertain and 2-5 times smaller than the theoretical one. Consequently, the simulated S34/S32 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. Purpose: The present study was performed to confirm the existence of these new resonances, and to improve the remaining uncertainties in the S34(p,γ)Cl35 reaction rate. Methods: 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 S32(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 S34(p,γ)Cl35 reaction rate over the nova temperature regime. Results: 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. Conclusions: The present S34(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 states of Cl35: 6643, 6761, 6780, and 6800 keV.

Duke Scholars

Author John H Kelley Physics