The external electron beam of the University of Illinois 25-Mev betatron and a post-bremsstrahlung electron spectrometer are used to provide a time coincidence which selects monochromatic photons with an energy resolution of 0.67%. For this resolution and an electron beam current of 3×${10}^{-9\mathrm{}}$ amp, the maximum photon intensity is about 1.5×${10}^5$/sec for each electron detector placed in the spectrometer.

The elastic scattering of photons from Au was measured using the background limited intensity of ${10}^4$ photons/sec for each of the three electron detectors used simultaneously. The peak 135° differential scattering cross section was 0.71±0.05 mb/sr at 14.5 Mev. Other measured values at 135° were 0.11 (11.7 Mev), 0.32 (12.6 Mev), 0.67 (13.5 Mev), 0.54 (15.4 Mev), 0.38 (16.7 Mev), and 0.35 (17.6 Mev).

These scattering results would be in better agreement with photon absorption measurements if the absolute absorption cross sections were reduced by 9%. The results are consistent with dipole scattering, a cross section which varies smoothly with energy, and no high-energy inelastic scattering.

• Received 16 August 1961

DOI:https://doi.org/10.1103/PhysRev.126.228