Investigating the Phosphorus-30 Proton Capture Reaction in Classical Novae

Accurate modeling of classical nova nucleosynthesis is fundamentally dependent on the thermonuclear reaction rates of the nuclei involved. In particular, it has been shown that the ³⁰P(p,γ)³¹S reaction rate is the largest remaining source of uncertainty in the final abundances of nuclei created in a classical nova involving an ONe white dwarf. Currently, a direct measurement of the ³⁰P(p,γ)³¹S reaction is not feasible. Previous attempts at constraining the ³⁰P(p,γ)³¹S reaction rate have been made using the Hauser-Feshbach statistical approach to calculate the cross section. However, the level density in ³¹S may not be sufficiently high for this approach. A more accurate calculation can be made by assuming the reaction proceeds via narrow and isolated resonances. Considering the contribution from individual resonance states, the calculation of the ³⁰P(p,γ)³¹S reaction rate at nova temperatures requires knowledge of the spin and parity assignments and resonance strengths of the levels in ³¹S just above the proton threshold. To obtain the relevant nuclear data, a measurement of the ³²S(p,d)³¹S* reaction has been performed at the Texas A&M Cyclotron Institute using a proton beam from the K150 cyclotron and a target consisting of ZnS deposited on a thin carbon backing. The particle-gamma array, Hyperion, was used in a configuration with a dE-E telescope of silicon detectors downstream of the target position for the detection of direct reaction products, and a single silicon detector placed upstream of the target for the detection of decay protons. In this experiment states above the proton separation energy in ³¹S were populated via the ³²(p,d)³¹S* reaction. A single proton unbound state was observed to decay via γ-ray emission, and several states were observed to decay by proton emission. New analysis techniques for measuring angular correlations between reaction products and decay protons were developed. Branching ratios and constraints on the angular momentum of decays for several proton emitting states are presented. The impact of the new data on the ³⁰P(p,γ)³¹S reaction rate is assessed. Finally, a discussion of the challenges encountered in this experiment and a proposal for a follow-up experiment to mitigate these challenges is presented.