Isospin structure of transitions inO17from inelastic pion scattering at 164 MeV

Differential cross sections for inelastic scattering of ${\pi }^{+}$ and ${\pi }^{-}$ from a cooled gas target, enriched in ${}_{}{}^{17}{}_{}{}^{}\mathrm{O}$ (49.9%), were measured at an incident energy $T_{\pi }=164\mathrm{}$ MeV between ${\theta }_{\mathrm{lab}}=18\mathrm{}° \mathrm{and} 74°$. Data were also taken on a natural oxygen and an enriched ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$ target (94.9%) to allow a substraction of the ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ and ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$ yields from the data taken with the ${}_{}{}^{17}{}_{}{}^{}\mathrm{O}$ target. A microscopic distorted wave impulse approximation analysis of five collectively enhanced transitions to the lowest $J^{\pi }={\frac{3}{2}}^{-}, {\frac{5}{2}}^{-}, {\frac{7}{2}}^{-}, {\frac{9}{2}}^{-}, \mathrm{and} {\frac{11}{2}}^{-}$ states in ${}_{}{}^{17}{}_{}{}^{}\mathrm{O}$ was performed using transition densities from three different models. The data were fitted best with transition densities derived from a large-space shell-model calculation; provided that their $\mathrm{}\left[\mathrm{LSJ}\right]=\left[303\right]\mathrm{}$, isoscalar parts were enhanced by a factor ($1+2\delta$). Both the ${\pi }^{+}$ and ${\pi }^{-}$ data for the five transitions in ${}_{}{}^{17}{}_{}{}^{}\mathrm{O}$ and the transition to the $3^{-}$ state in ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ (6.13 MeV) were fitted using a single polarization charge, $\delta$. Evidence was found for $E2\mathrm{}$ strength near 8 MeV and for $M4\mathrm{}$ strength to two states at 15.7 and 17.1 MeV.