Measurement of dielectric suppression of bremsstrahlung

In 1953, Ter-Mikaelian predicted that low energy bremsstrahlung from relativistic electrons moving through a medium is suppressed, because of interactions between the emitted photon and the electrons in the medium. This suppression occurs because the emission takes place on a long distance scale, allowing for destructive interference between different instantaneous photon emission amplitudes. We present 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 show-that dielectric suppression occurs at the predicted level, reducing the measured cross section up to 75%.