Formation length effects in very thin targets

Experimental results for the radiative energy loss of 178GeV positrons in Cu, Au, and W targets are presented. It is shown that for a few micron thick target, effects related to the formation zone disappear, in particular, the suppression due to the Landau-Pomeranchuk-Migdal and Ternovskii-Shul'ga-Fomin mechanisms. This disappearance may restrict the region of applicability of thin foils as a target for energy-selective production of high energy photons. Furthermore, transition radiation dominated by multiple scattering and structured target interference effects are shown to be likely ingredients for an accurate description of the data obtained at low photon energies. © 2005 The American Physical Society.