Developing the S32(p,d)S*31(p)(γ) reaction to probe the P30(p,γ)S31 reaction rate in classical novae

Abstract

Background: The $^{30}\mathrm{P}(p,\ensuremath{\gamma})^{31}\mathrm{S}$ reaction rate is one of the largest remaining sources of uncertainty in the final abundances of nuclei created in a classical nova involving a ONe white dwarf. The reaction rate directly influences silicon isotopic ratios, which are used as identifiers of presolar grains with nova origins. In addition, the uncertainty in the $^{30}\mathrm{P}(p,\ensuremath{\gamma})^{31}\mathrm{S}$ reaction rate has been found to limit the use of nova nuclear thermometers based on observations of elemental ratios in nova ejecta.Purpose: Reduce uncertainties in the nuclear data for proton-unbound states in $^{31}\mathrm{S}$, which act as resonances for the $^{30}\mathrm{P}(p,\ensuremath{\gamma})^{31}\mathrm{S}$ reaction at classical nova temperatures, and develop a technique for high efficiency, high-resolution reaction-decay coincidence measurements.Methods: The $^{32}\mathrm{S}(p,d)^{31}\mathrm{S}$ reaction was used to populate the states of interest in $^{31}\mathrm{S}$. The experiment was performed at the Texas A Cyclotron Institute using the LLNL Hyperion array for the detection of charged particles and $\ensuremath{\gamma}$ rays. A downstream silicon telescope was used to select reaction deuterons, and a single upstream silicon detector was used to measure protons emitted in the decay of unbound $^{31}\mathrm{S}$ levels.Results: Several states in $^{31}\mathrm{S}$ above the proton separation energy were observed to have been populated. Decay protons from the resonant states in $^{31}\mathrm{S}$ were identified as events in the upstream silicon detectors that came in coincidence with deuterons in the downstream telescope. Protons emitted from these states were measured and branching ratios extracted.Conclusions: While no new reaction rate is derived, spin-parity assignments for several higher-lying proton unbound states have been confirmed. Measured ${p}_{0}$ branching ratios for these levels have been compared to previous measurements with good agreement, and in some cases provided a reduction in uncertainty. The previously identified $T=3/2$ state may have been incorrectly assigned a large ${p}_{0}$ branching ratio in a previous measurement. The technique of measuring reaction-decay coincidences with a particle-gamma setup appears promising.