Photon Spectra from Radiative Absorption of Pions in Nuclei

Abstract

The photon spectra following capture of stopped pions in ${}_{}{}^4{}_{}{}^{}\mathrm{He}$, ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$, ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$, ${}_{}{}^{24}{}_{}{}^{}\mathrm{Mg}$, and ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}$ have been measured for photon energies between 50 and 160 MeV. A pair spectrometer was used and a resolution of 2.0 MeV (full width at half maximum) was achieved. On the basis of several thousand events for each spectrum, we observe collective excitation in the residual nucleus for capture on ${}_{}{}^4{}_{}{}^{}\mathrm{He}$, ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$, and ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$. Such excitation is predicted theoretically, and in some cases the detailed comparison with the data is good. For ${}_{}{}^{24}{}_{}{}^{}\mathrm{Mg}$ and ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}$, no significant structure is seen. In addition, the transition rates to particle-stable states in ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ and ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ have been measured. A continuum background consistent with a direct-reaction mechanism is also observed for all the elements studied except for ${}_{}{}^4{}_{}{}^{}\mathrm{He}$. Results for the pion-capture rates in C${\mathrm{H}}_2$ and ${\mathrm{H}}_2$O are given.