Gamma Rays from the Nuclear Photoeffect in Carbon, Oxygen, and Copper

A NaI crystal was used to study the spectra of gamma rays associated with the giant resonance cross section of the nuclear photoeffect in carbon, oxygen, and copper. The x-ray source was a bremsstrahlung beam whose energy was varied from 19 to 61 Mev. Cross sections for elastic, and mixed elastic and inelastic scattering were measured at 135° to the x-ray beam. The copper cross section has a magnitude which is well predicted by particle photoproduction cross sections coupled with a dipole dispersion theory. The elastic scattering cross section for carbon can be predicted in a similar fashion, but the oxygen cross section cannot. For oxygen, the observed cross section is much larger than the predicted one and this result is explained if narrow, isolated, resonances are an important part of the oxygen photonuclear cross section. The different behavior of carbon and oxygen which is found is consistent with other experiments. The angular dependence of the oxygen elastic scattering cross section near 22 Mev is predominantly dipole with a possible quadrupole admixture. For oxygen, an inelastic scattering cross section was observed which has a threshold at about 26 Mev, a peak at about 30 Mev, and is due to transitions to a state (or states) near 6 Mev. It is interpreted as the result of an overlap of two giant resonance cross sections—one for the ground state and one for the excited state. The consequences of this interpretation to the theory of the nuclear photoeffect is discussed. The yield of gamma rays from ${\mathrm{O}}^{15}$ and ${\mathrm{N}}^{15}$ which follow neutron or proton emission was also studied and several lines were observed, but none above 6.5 Mev. The photonuclear cross sections associated with these lines are estimated to be 45% of the whole photonuclear cross section at 23 Mev.