Measurement of Near-Threshold Proton Branching Ratios in 31S Important for Novae

Classical novae are stellar explosions that contribute to the nucleosynthesis of isotopes on the proton-rich side of the valley of stability up to 40Ca. In ONe novae, the incompletely understood reaction rate of 30P(p,γ)31S is known to strongly influence the production rate of several stable isotopes such as 30Si, 31P, and 32,33,34S. A precise measurement of this reaction rate has several potential implications towards matching astrophysical observables to the physical composition of the nova site -- the observed elemental abundance ratios of O/S and S/Al have been suggested as useful `thermometers\u27 to gauge the temperature of nova explosions, while the Si/H ratio observed is predicted to be correlated with the degree of mixing between accreted hydrogenic matter and the heavier elements in the white-dwarf surface. In addition, the abundance of 30Si relative to 28Si has been suggested as a potential tracer to distinguish between pre-solar meteoritic grains with ONe nova origins and grains with supernova/solar origins. We used the Super Enge Split-pole spectrograph (SE-SPS) at Florida State University to study levels close to the proton threshold of 31S using the 28Si(6Li, t)31S transfer reaction. The Silicon Array for Branching Ratio Experiments (SABRE) was used to detect protons emitted from unbound states in 31S, in coincidence with tritons detected at the focal plane of the SE-SPS. Decay protons from the Ex = 6.636 MeV resonance in 31S were observed for the first time. The measured 31S level structure, proton-branching ratios and spin-parities were used to determine an updated 30P(p,γ)31S reaction rate