Christopher Wrede

Several presolar grains have recently been discovered in primitive meteorites with non-solar 12C/13C, 14N/ 15N, 29Si/28Si and 30Si/ 28Si isotopic ratios that are qualitatively consistent with the isotopic ratios predicted to be ejected by massive oxygen-neon novae using hydrodynamic models of these thermonuclear astrophysical explosions. However, the models predict excesses of 30Si/28Si much larger than those measured in presolar nova grains. The same models (which are coupled to nuclear-reaction networks) predict that oxygen-neon nova nucleosynthesis in the Si-Ca mass region–including the 30Si/28 Si ratio–is highly sensitive to the thermonuclear rate of the unmeasured 30P(p, gamma)31S reaction in particular. The present work is an indirect experimental study of resonances in the 30P+p system using the 31P(3He,t)31S and 31P(3He,t)31S*( p)30P reactions to address this problem. Resonance energies, spins, parities and proton branching ratios have been measured and this information has been used together with existing experimental information on 31S to calculate the thermonuclear 30P( p, gamma)31S reaction rate at nova temperatures of T ≤ 0.35 GK. New resonances have been discovered at Ec.m. = 194.0(25) and 266.4(27) keV and the reaction rate is up to an order of magnitude faster than previous estimates based on nuclear energy-level schemes of 31S that were incomplete above the proton threshold of Ex = 6133.0(15) keV. The data acquired have also been used to refine the 31S level scheme up to Ex ≈ 9.5 MeV, far above the resonance energies relevant to novae. This information has been used to calculate the thermonuclear 30P(p, gamma)31S reaction rate up to T = 10 GK, which will be useful in astrophysical modeling involving other nucleosynthetic processes including the rp process and oxygen burning. We find our reaction rate to be up to a factor seven faster than a previous experimentally determined estimate for T > 1 GK. As a byproduct of the 31S work, the energies of the first two T = 3/2 levels in 31 S have been measured, and used together with existing experimental information on the T = 3/2, A = 31 isobaric multiplets and the isobaric multiplet mass equation to refine the level scheme of 31Cl and calculate the thermonuclear 30S( p, gamma)31Cl reaction rate up to T = 2 GK. This reaction and its inverse influence the rp/alphap-process waiting point at 30S which has been previously proposed to explain certain double-peaked type I x-ray burst profiles. The Q value for the 30S(p, gamma)31Cl reaction, which determines the equilibrium abundance ratio of 30S/ 31Cl in these environments, has also been constrained.