F18States with Large Isospin Impurities

Abstract

We report extensive differential cross-section measurements for ${}_{}{}^{14}{}_{}{}^{}\mathrm{N}(\alpha , {\alpha }_i){}_{}{}^{14}{}_{}{}^{}\mathrm{N}$ over the energy range $10.2\le E_{\alpha }\le 18.24$ MeV. The ${\alpha }_0$ and ${\alpha }_2$ exit channels are isospin-allowed, while ${\alpha }_1$ is isospin-forbidden. Except for the Coulomb elastic scattering contribution to ${\alpha }_0$, compound-nucleus formation dominates all three channels. The ${\alpha }_1$ channel displays relatively isolated resonances which imply ${}_{}{}^{18}{}_{}{}^{}\mathrm{F}$ states with large isospin impurities. These ${\alpha }_1$ data, when expanded in partial waves, fix $J^{\pi }$ for many of the ${}_{}{}^{18}{}_{}{}^{}\mathrm{F}$ levels (∼20). Comparison of ${}_{}{}^{14}{}_{}{}^{}\mathrm{N}(\alpha , {\alpha }_1){}_{}{}^{14}{}_{}{}^{}\mathrm{N}$ with ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}(d, {\alpha }_1){}_{}{}^{14}{}_{}{}^{}\mathrm{N}$ data (some old and some new data) suggests that the same ${}_{}{}^{18}{}_{}{}^{}\mathrm{F}$ states are usually involved. The estimated isospin impurities are ∼10-25% and are still present at $E_x\left({}_{}{}^{18}{}_{}{}^{}\mathrm{F}\right)\sim 18$ MeV. Our data suggest that the isospin-impure levels in ${}_{}{}^{18}{}_{}{}^{}\mathrm{F}$ tend to occur as doublets, and that for low partial waves isospin may be rather well conserved at our higher excitation energies. Our data do not support Noble's suggestions concerning the importance of direct reactions in isospin-violating reactions.