Measuring cross-sections of unbound mirror states: a systematic study of one-proton and one-neutron reactions adding on N=Z targets*

One-neutron adding reactions are often used to infer proton-capture rates in mirror systems. While isospin is approximately symmetric, the degree to which it's still symmetric in systems with weak binding is unknown. Isobaric analog states should exhibit the same behavior, but weakly bound states experience Thomas-Erhman shifts and it is unknown how this affects the cross section. A series of experiments at Florida State University has been completed with the aim to be able to quantify asymmetries in spectroscopic factors due to weakly bound or unbound protons for calculating astrophysical reaction rates.

Targets of N=Z nuclei ²⁴Mg and ²⁸Si were used to populated mirror states via one-neutron and one-proton adding reactions. Beams of deuterium and ³He were impinged onto these targets, and four reactions were measured in the Super-Enge Split-Pole Spectrograph: ²⁴Mg(d,p)²⁵Mg, ²⁴Mg(³He,d)²⁵Al, ²⁸Si(d,p)²⁹Si and ²⁸Si(³He,d)²⁹P. Using the same targets, experimental setup and beam energy per nucleon, these systematic measurements populated states up to and beyond the respective neutron and proton separation energies (from 0 to at least 8 MeV in all cases), which in each mirror pair differs by ~5 MeV. Preliminary cross sections will be presented for these bound and unbound mirror states.