Surface Production of Charged Mesons by Photons on Nuclei

An estimate has been made of the photoproduction cross section of charged mesons from nuclei, and in particular from "surface-nucleons," i.e., the weakly interacting nucleons which make up the less dense nucleon atmosphere surrounding the main body of a nucleus. Comparison with experiment indicates that the production of mesons from the core of a nucleus is appreciably suppressed, apart from the effects of the initial momentum distribution of the nucleus, and of meson absorption. It is found, for example, that apart from having the correct $A^{\frac{2}{3}}$ dependence for the ${\pi }^{+}+{\pi }^{-}$ cross section, the surface production alone can account for large fractions of the observed yields, and because of differences in average binding between neutrons and protons in nuclei, gives $\frac{{\pi }^{-}}{{\pi }^{+}}$ ratios which have the same trends as a function of $A$ as the observed ratios. A possible explanation of these results is that there occurs a large competing photodisintegration process as a result of meson exchange effects between strongly coupled nucleons in the interior of a nucleus.