Development and Implementation of a New Technique to Study (p,α) Reactions Relevant to Nucleosynthesis in Binary Systems

The accurate determination of the strengths and energies of resonances in (p,alpha) reactions is important for understanding the influence of reaction cycles to element synthesis in many astrophysical environments. Thus far, (p,alpha) studies in inverse kinematics have employed solid polypropylene targets. These are not always advantageous, especially when the energy loss of the incoming beam in the solid target is significantly larger than the resonance width. At the Holifield Radioactive Ion Beam Facility, a new technique has been developed for measuring the strengths and energies of (p,alpha) reactions. In this technique, a large differentially-pumped scattering chamber is filled with hydrogen gas at pressures up to 4 Torr. No windows or foils obstruct the incoming beam and reaction products are detected in coincidence by two silicon strip arrays. The vertex of the (p,alpha) reaction is determined from the known kinematics of the alpha particle and heavy recoil.

This new technique was applied to study the strength of the 183 keV resonance in ¹⁷O(p,alpha)¹⁴N reaction that was previously reported to significantly increase the reaction rate at nova temperatures and decrease ¹⁸F production by as much as a factor of 10 in low-mass ONeMg novae. This larger strength was confirmed using the new technique and nova simulations showed a substantial decrease in ¹⁸F production in lower-mass novae though a much smaller effect was seen in higher-mass novae.

Low-energy resonances in ³¹P(p,alpha)²⁸Si and ³⁵Cl(p,alpha)³²S were also studied using the same technique in order to gain a better understanding of reaction cycling in the Si-Ar region. Resonance strengths at E_CM = 599 and 622 keV in ³¹P(p,alpha)²⁸Si were measured as well as the E_CM = 611 keV resonance in ³⁵Cl(p,alpha)³²S, the lowest energy that any resonance in this reaction has been observed. The strengths of these resonances were found to be lower than previously determined through indirect methods, resulting in weak cycling in the Si-Ar region.