A Novel Approach to Direct (p,n) Reaction Measurements of Astrophysical Interest with SECAR.*

Low energy (p,n) and (n,p) reactions on unstable nuclei affect the synthesis of heavy elements in core-collapse supernovae, especially during explosive silicon burning and the $\nu$p-process. The SECAR recoil separator was recently constructed at FRIB to directly measure astrophysical reactions with unstable nuclei in inverse kinematics when the reaction product and the beam differ in mass. However, using a machine-learning-based approach, combining a multi-objective evolutionary algorithm with ion-optical simulations, a sufficient beam rejection rate is achieved, enabling the measurement of (p,n) reactions, despite the nearly equivalent masses involved. This novel technique was verified via the stable-beam $^{58}$Fe(p,n)$^{58}$Co reaction measurement, and the new cross-section resulted at 20.3 ± 6.3 mb at the energy of 3.66 ± 0.12 MeV/u. Statistical model predictions yielded overall higher cross-section values, though exhibit a strong dependence on the optical model potential used. The new technique is expected to be used with radioactive beams, allowing direct low-energy (p,n) reaction measurements on short-lived nuclei of astrophysical importance.