Abstract
We report the first observation of a shoulder in the radiation spectrum from GeV electrons in a
structured target consisting of two thin and closely spaced foils. The position of the shoulder depends on
the target spacing and is directly connected to the finite formation length of a low-energy photon emitted
by an ultrarelativistic electron.With the present setup it is possible to control the separation of the foils on
a m scale and hence measure interference effects caused by the macroscopic dimensions of the
formation length. Several theoretical groups have predicted this effect using different methods. Our
observations have a preference for the modified theory by Blankenbecler but disagree with the results of
Baier and Katkov.