Precision Elastic Scattering Measurement Using the St. George Recoil Mass Separator*

Helium radiative capture plays a key role in most stellar environments beyond their hydrogen burning phase. The cross section of these reactions, for the most part, are dominated by resonances. The strength of these resonances are in turn partially dominated by the alpha partial width. Recoil mass separators, such as St. George at the University of Notre Dame, are designed to study these low energy radiative capture reactions in inverse kinematics. 

Here the use of a recoil separator to measure elastic scattering will be reported, with the goal of deducing the alpha partial width of two resonances contributing to the stellar reaction rate of ¹⁵N(α,γ)¹⁹F. While the recoil separator approach constrains the measurement to 180 degrees in the center of mass frame, this angle is where the interference between the Coulomb and nuclear force is the strongest. In addition the HIPPO gas jet target can be made as thin as reasonably possible to minimize the impact of target effects on the measured yield.