Photon spectra from radiative capture of stopped negative pions captured by ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ and ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$ nuclei have been measured with a high resolution pair spectrometer. In the bound region transitions have been observed to ${}_{}{}^{13}{}_{}{}^{}\mathrm{B}$(g.s.), ${}_{}{}^{13}{}_{}{}^{}\mathrm{B}$(3.5 MeV), ${}_{}{}^{19}{}_{}{}^{}\mathrm{O}$(g.s.), ${}_{}{}^{19}{}_{}{}^{}\mathrm{O}$(4.9 MeV), and ${}_{}{}^{19}{}_{}{}^{}\mathrm{O}$(6.3 MeV). In addition, strong transitions to narrow states above the neutron separation threshold were observed in both cases. The results for ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ are in agreement with detailed shell model calculations of this process. Major strength is observed for states formed by coupling extra nucleons or holes to the spin-quadrupole $d_{\frac{5}{2}}$ and $d_{\frac{3}{2}}$, $J^{\pi }=2^{-}$ states in ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$. The ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}({\pi }^{-}, \gamma ){}_{}{}^{13}{}_{}{}^{}\mathrm{B}_{\mathrm{g}.\mathrm{s}.\mathrm{}}^{}{}_{}{}^{}$ branching ratio is shown to be consistent with beta decay and electroexcitation data, through a combined phenomenological analysis. The observed transition strengths are accounted for by matrix elements of the spin-density operators ${[\sigma \times Y_0]}^1{t}^{+}$ and ${[\sigma \times Y_1]}^2{t}^{+}$.

NUCLEAR REACTIONS ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}({\pi }^{-}, \gamma )$, ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}({\pi }^{-}, \gamma )$; measured normalized $I\left(E_{\gamma }\right)$; deduced upper limit on $\delta \left(\frac{E2\mathrm{}}{M1\mathrm{}}\right)$ for ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$→${}_{}{}^{13}{}_{}{}^{}\mathrm{B}$(g.s.) at $q=125\mathrm{}$ MeV/c, systematics of SIGDR in $12\le A\le 20$. Stopped ${\pi }^{-}$; pair spectrometer; 850 keV at 129.4 MeV.

• Received 6 December 1982

DOI:https://doi.org/10.1103/PhysRevC.27.1621