Using sources of "Na, differential cross sections for elastic scattering of 2.754 MeV photons have been measured for angles between 60 and 120' and charge numbers of 30, 42, 50, and 58 with an accuracy of 2.8% on the average. For the cases covered by experiment, Delbriick scattering amplitudes have been calculated on the basis of the lowest-order Feynman graphs with an average numerical accuracy of 1%. For the same cases Rayleigh scattering amplitudes have been calculated on the basis of the second-order S matrix using Dirac-Hartree-Fock-Slater wave functions. These investigations confirm the lowest-order Delbruck theory with an accuracy of 5%. This corresponds to a new test of vacuum polarization of the order a(Za)'.NUCLEAR REACTIONS Photon scattering Zn, Mo, Sn, Ce(y, y), E=2.754 MeV: measured do/dQ(8), 8= 60'-120', deduced test of Delbriick scattering.
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