Orbital Recoupling Dominance in theA=20Isovector GiantM1Transition

Abstract

The photon spectrum from $^{20}\mathrm{Ne}$(${\ensuremath{\pi}}^{\ensuremath{-}}$,$\ensuremath{\gamma}$)$^{20}\mathrm{F}$ has been measured. The branching ratio to $^{20}\mathrm{F}$(${1}^{+}$, 1.06 MeV) is compared to the analogous $M1$ decay width $^{20}\mathrm{Ne}$(${1}^{+}$, 11.24 MeV) \ensuremath{\rightarrow}$^{20}\mathrm{Ne}$(g.s.). This determines the relative importance of spin and orbital magnetization in the transition amplitude. The $M1$ amplitude is found to contain only (47 \ifmmode\pm\else\textpm\fi{} 16)% spin flip, in agreement with shell-model calculations. Excitation of ${2}^{\ensuremath{-}}$ states by $^{20}\mathrm{Ne}$(${\ensuremath{\pi}}^{\ensuremath{-}}$,$\ensuremath{\gamma}$) is also observed.