Interpretation of Experiments on the Photonuclear Effect in Heavy Elements

An analysis of the experiments to date on photonuclear reactions in heavy nuclei leads to the following results. There is a correlation between the energy at which the ($\gamma ,n$) cross section is a maximum and the ($\gamma ,2n$) threshold. The shape of the total photon absorption cross sections in Sb and Ta can be estimated up to 22 Mev, and there is an indication that the cross section drops off strongly above that energy. Using the estimate of the shape of the total cross section in Ta, one can calculate the neutron yield to be expected in experiments with 330-Mev bremsstrahlung. The calculated value is only about 60 percent of the experimental value. Evidence is presented that the discrepancy is due to neutrons produced by high energy photons, presumably due to mesonic effects. The integrated cross sections for ${\mathrm{Zn}}^{64}$, Sb, and ${\mathrm{Ta}}^{181}$ can be evaluated and by comparison with the Levinger-Bethe formula lead to values of 0.56, >0.44, and 0.50, respectively, for the fraction of exchange force in the neutron-proton interaction, assuming that all of the photoeffect is due to electric dipole transition.