Although some authors have claimed that the effect is not detectable, we show experimentally for the first time that as the quantum parameter $\chi$ grows beyond 1, an increasingly large part of the hard radiation emitted arises from the spin of the electron. Results for the energy loss of electrons in the energy range 35–243 GeV incident on a W single crystal are presented. Close to the axial direction the strong electromagnetic fields induce a radiative energy loss which is significantly enhanced compared to incidence on an amorphous target. In such continuously strong fields, the radiation process is highly nonperturbative for ultrarelativistic particles and a full quantum description is needed. The remarkable effect of spin flips and the energy loss is connected to the presence of a field comparable in magnitude to the Schwinger critical field, $E_0{=m}^2{c}^3/eħ$, in the rest frame of the emitting electron.

• Received 17 April 2001

DOI:https://doi.org/10.1103/PhysRevLett.87.054801