Direct Photodisintegration Processes in Nuclei

The unexpectedly large ($\ensuremath{\gamma},p$) cross sections found in many moderately heavy nuclei are interpreted in terms of a direct photodisintegration process. The gamma-ray quantum is supposed to be absorbed by one proton in the nucleus, which is then emitted without the formation of an intermediate compound nucleus. The cross section for this process is small compared with that for the formation of a compound nucleus (which would almost always lead to the eventual emission of a neutron), but it has to be only a few millibarns to account for the experimental results. It is shown in this paper that if the nucleus is represented by a square well potential, the calculated cross sections turn out to be smaller than the experimental ones, but much larger than those obtained from the statistical theory.The angular distribution of the emitted protons should be of the form $A+B{sin}^{2}\ensuremath{\theta}$, where $\ensuremath{\theta}$ is the angle between the incoming photon and the proton, and the ratio $\frac{B}{A}$ depends on the angular momentum of the proton in the nucleus. This is confirmed by experimental results; from the experimental values of $\frac{B}{A}$ it appears that protons with $l=0$ in the nucleus contribute considerably to the effect in Rh and Ag. Feenberg's "wine bottle" potential can account for this more easily than the square well.The theory is applicable to fast photoneutron emission as well, and recent experiments by Poss show that fast photoneutrons are indeed emitted anisotropically.It is suggested that similar direct interaction mechanisms may account for the deviations of ($n,p$) cross sections from those predicted by the theory, as found recently by W\"affler.