Elastic Scattering of Gamma Rays

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Abstract
Gamma rays of 0.411, 0.662, and 1.33 Mev were scattered from tin and lead targets and the absolute differential cross sections for elastic scattering were measured at six angles between 15 and 90 degrees. At 0.411 and 0.662 Mev, the scattering is expected to be almost entirely Rayleigh scattering from the tightly bound electrons, and these data should therefore provide a reasonably stringent test of the theory of that process. However, the only calculations of Rayleigh scattering presently available are form-factor calculations which are expected to be inadequate for several reasons—in particular, for neglect of electron binding in the intermediate state of the scattering process, and for certain nonrelativistic approximations—and consequently comparisons of experiment with the form-factor results are inconclusive. Nevertheless, for the scattering from tin at both 0.411 and 0.662 Mev, there is fair but possibly fortuitous agreement between experiment and form-factor calculations using Dirac wave functions for the electrons; for lead at the same energies there are appreciable discrepancies particularly at the larger angles between the experimental results and similar calculations. In view of the latter discrepancies, it appears that unambiguous identification of the elastic scattering of gamma rays by the electric field of a nucleus, i.e., potential scattering, cannot be obtained from measurements of the total elastic scattering by lead at 1.33 Mev without improvements in the theory of Rayleigh scattering.

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