Photoparticle spectra from the ${}_{}{}^{12}{}_{}{}^{}(\gamma$,$p_i$) and ${}_{}{}^{12}{}_{}{}^{}(\gamma$,${\alpha }_0$) reactions have been measured in an angular aperture (FWHM) of 60° centered around 90°, using 28 MeV quasimonochromatic photons from annihilation-in-flight of positrons. The branching ratio for the (γ,${\alpha }_0$) to (γ,$p_0$) transition amounts to (2.9±1.2)%, while the (γ,$p_i$) branching to excited states in ${}_{}{}^{11}{}_{}{}^{}\mathrm{B}$ contributes (24±2)% to the observed (γ,p) cross section.

• Received 6 July 1988

DOI:https://doi.org/10.1103/PhysRevC.39.253