Spin flip transitions inC13andF19probed with the (π−, γ) reaction

Abstract

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 ${d}_{\frac{3}{2}}$, ${J}^{\ensuremath{\pi}}={2}^{\ensuremath{-}}$ states in $^{16}\mathrm{O}$. The $^{13}\mathrm{C}({\ensuremath{\pi}}^{\ensuremath{-}}, \ensuremath{\gamma})^{13}\mathrm{B}_{\mathrm{g}.\mathrm{s}.}$ 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 ${[\ensuremath{\sigma}\ifmmode\times\else\texttimes\fi{}{Y}_{0}]}^{1}{t}^{+}$ and ${[\ensuremath{\sigma}\ifmmode\times\else\texttimes\fi{}{Y}_{1}]}^{2}{t}^{+}$.NUCLEAR REACTIONS $^{13}\mathrm{C}({\ensuremath{\pi}}^{\ensuremath{-}}, \ensuremath{\gamma})$, $^{19}\mathrm{F}({\ensuremath{\pi}}^{\ensuremath{-}}, \ensuremath{\gamma})$; measured normalized $I({E}_{\ensuremath{\gamma}})$; deduced upper limit on $\ensuremath{\delta}(\frac{E2}{M1})$ for $^{13}\mathrm{C}$\ensuremath{\rightarrow}$^{13}\mathrm{B}$(g.s.) at $q=125$ MeV/c, systematics of SIGDR in $12\ensuremath{\le}A\ensuremath{\le}20$. Stopped ${\ensuremath{\pi}}^{\ensuremath{-}}$; pair spectrometer; 850 keV at 129.4 MeV.