Experimental study of Cl-35 excited states via S-32(alpha, 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 (S-33/S-32 and S-34/S-32) are compared with the theoretical ones. The accuracy of such a comparison depends on reliable S-33(p, gamma)Cl-34 and S-34(p, gamma)Cl-35 reaction rates at the nova temperature regime. The latter rate has recently been computed based on experimental input, and many new excited states in Cl-35 were discovered above the proton threshold. As a result, the experimental S-34(p, gamma)Cl-35 rate was found to be less uncertain and 2-5 times smaller than the theoretical one. Consequently, the simulated S-34/S-32 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 S-34(p, gamma)Cl-35 reaction rate. Methods: Energies and spin-parities of the Cl-35 excited levels were investigated via high-resolution charged-particle spectroscopy with an Enge split-pole spectrograph using the S-32(alpha, p)Cl-35 reaction. Differential cross sections of the outgoing protons were measured at E-alpha = 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, gamma)Cl-35 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 Cl-35 states were assigned tentatively for the first time. Conclusions: The present S-34(p, gamma)Cl-35 experimental thermonuclear reaction rate at 0.1-0.4 GK is consistent within 1 sigma 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 Cl-35: 6643, 6761, 6780, and 6800 keV.