Nuclear photon scattering byC12andO16

The elastic scattering cross sections for ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ and ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ have been measured in the energy ranges 23.5-39 and 25-39 MeV, respectively. These data have been compared with the Mainz total photonuclear absorption cross sections and interpreted in terms of a form factor applied to the gauge term of the Thomson amplitude as well as an E2 amplitude. For ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ the E2 strength found is ${1.9}_{-0.7\mathrm{}}^{+0.8\mathrm{}}$ total, isoscalar plus isovector, energy-weighted sums, and for ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ ${1.25}_{-0.9\mathrm{}}^{+1.3\mathrm{}}$. All of the ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ data taken at 23.5 MeV, the peak of the giant resonance, where the real part of the scattering amplitude vanishes, have been combined to determine the total photonuclear absorption cross section at that energy, 19.7±0.4 mb. The branching ratio $\frac{{\Gamma }_{4.4}}{{\Gamma }_0}$ at 23.5 MeV was found to be 0.23±0.07.