Search for the Splitting of Gamma-Ray Photons in the Electric Field of the Nucleus

A search has been made for an effect predicted by quantum electrodynamics closely connected with vacuum polarization, namely, the splitting of gamma-ray photons in the electric field of the nucleus. The scattering of 1.1-MeV gamma rays from a 2-Ci source of ${\mathrm{Zn}}^{65}$ in various target elements has been investigated. The spectrum of the sum of the two photon pulses appearing simultaneously in two NaI(Tl) scintillating crystals, situated at backward angles with respect to the target, was recorded over long periods of time. Evidence for the production of photon pairs, whose energy sum is equal to that of the original incident photon, has been obtained for copper and cobalt nuclei. For photon pairs produced in these nuclei, at average angles of 105° with respect to the incident photon direction, and of 130° with respect to each other, we estimate the differential cross section to be $(3\mathrm{}\pm 1)Z^2\left(\frac{\Delta w}{m{c}^2}\right){10}^{-35\mathrm{}}$ ${\mathrm{cm}}^2$/${\mathrm{}\left(\mathrm{sr}\right)\mathrm{}}^2$, where $\Delta w$ is the energy interval corresponding to one component of the pair. This is about six times larger than the value predicted theoretically by Shima. However, double Rayleigh scattering from bound electrons may also contribute to the splitting process, and no exact quantitative calculations of this have yet been made.