A measurement of the LPM effect

We have performed an experiment to measure accurately the Landau‐Pomeranchuk‐Migdal (LPM) effect in the production of 5 to 500 MeV photons due to bremsstrahlung of 8 and 25 Gev electron beams traversing thin (2 to 6% X0) targets of varying densities. Our measurements confirm that the LPM effect exists and that the Migdal calculations are accurate. We see that, for thin targets, LPM suppression disappears leaving a Bethe‐Heitler spectrum, as predicted by theory. For intermediate target thicknesses, we lack an acceptable theory, but have measured energy spectra for targets of differing thickness.We have also measured the production rate of 500 keV to 5 MeV photons at the same electron energies, to study dielectric suppression. We see qualitative agreement with the theory of Ter‐Mikaelian; more work is needed before we can make quantitative comparisons.