Measurement of dielectric suppression of bremsstrahlung.

In 1953, Ter-Mikaelian predicted that the bremsstrahlung of low energy pho-tons in a medium is suppressed because of interactions between the produced photon and the electrons in the medium. This suppression occurs because the emission takes place over on a long distance scale, allowing for destructive interference between diierent instantaneous photon emission amplitudes. We present here measurements of bremsstrahlung cross sections of 200 keV to 20 MeV photons produced by 8 and 25 GeV electrons in carbon and gold targets. Our data shows that dielectric suppression occurs at the predicted level, reducing the cross section up to 75% in our data.