Developing the 32S(p, d)31S*(p) reaction to probe the 30P(p, γ)31S reaction rate in classical novae

Background: The 30 P(p, γ ) 31 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 P(p, γ ) 31 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 S, which act as resonances
for the 30 P(p, γ ) 31 S reaction at classical nova temperatures, and develop a technique for high efficiency, high-
resolution reaction-decay coincidence measurements.
Methods: The 32 S(p, d ) 31 S reaction was used to populate the states of interest in 31 S. The experiment was
performed at the Texas A&M Cyclotron Institute using the LLNL Hyperion array for the detection of charged
particles and γ 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 S levels.
Results: Several states in 31 S above the proton separation energy were observed to have been populated. Decay
protons from the resonant states in 31 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 p0 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 p0 branching ratio in a previous
measurement. The technique of measuring reaction-decay coincidences with a particle-gamma setup appears
promising.