The energy spectra and angular distributions of photoprotons from the ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}(\gamma ,p){}_{}{}^{11}{}_{}{}^{}\mathrm{B}$ reaction have been measured at seven angles simultaneously, using 30 MeV bremsstrahlung. The giant dipole resonance peaks at 22.5 MeV, and reaches an absolute cross section value of 13.1 ± 0.8 mb. The anisotropy parameter $a_2$ from the Legendre polynomial expansion of the angular distributions has an average value of about -0.55 but shows some structure especially around 25 MeV. On the other hand, the asymmetry coefficient $a_1$ is always positive and rises slowly with energy. Discussion of all coefficients leads to the conclusion that, although the photonuclear absorption mechanism in ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ leading to photoproton emission is dominated by the $E1$ component, a nonnegligible $E2\mathrm{}$ contribution (about 2%) might be present. A recent coupled-channel calculation by Birkholz satisfactorily describes our results.

NUCLEAR REACTIONS ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}(\gamma ,p)$, $E=16.0-30.0\mathrm{}$MeV, $\theta =37-143\mathrm{}°$; measured $\sigma (E,\theta )$; deduced $\sigma \mathrm{}\left(E\right)\mathrm{}$. Natural target.

• Received 2 February 1976

DOI:https://doi.org/10.1103/PhysRevC.14.456