Spins and parities of astrophysically important S30 states from Si28(He3, nγ)S30

The structure of proton-unbound S30 states strongly determines the thermonuclear P29(p,γ)S30 reaction rate at temperatures characteristic of explosive hydrogen burning in classical novae and type I x-ray bursts. Specifically, the rate had been previously predicted to be dominated by two low-lying, unobserved, levels in the Ex= 4.7–4.8 MeV region, with spin and parity assignments of 3+ and 2+. In recent experimental work, two candidate levels were observed with energies of 4.699 MeV and 4.814 MeV, but no experimental information on their spins and parities was obtained. We have performed an in-beam γ-ray spectroscopy study of S30 with the Si28(He3, nγ)S30 reaction. By constructing the decay schemes of proton-unbound states with a γ-γ coincidence analysis of their decay γ rays, their Jπ values were inferred from a comparison to the known decay schemes of the corresponding mirror states in Si30. For the two aforementioned states, our results strongly corroborate the spin-parity assignments assumed in recent evaluations of the P29(p,γ)S30 reaction rate.