THE ELEMENTARY PROCESS OF BREMSSTRAHLUNG

In this report we review the experimental and theoretical developments concerning the elementary process of electron bremsstrahlung. The ter m ''elementary process'' means that not only the emitted bremsstrahlun g photons are considered but also the pertinent decelerated electrons. Experimentally the elementary process is observed by detecting the ph otons in coincidence with outgoing electrons scattered into a particul ar direction. The results of such electron-photon coincidence experime nts yield stringent tests for theoretical predictions of the fully dif ferential cross section. Much deeper insight into the bremsstrahlung p rocess is achieved by also taking into account polarization variables such as the spin orientation of the incoming electron or the polarizat ion of the emitted photon. Thus conspicuous effects of the weak spin-o rbit interaction can be observed which are usually masked in non-coinc idence experiments. Bremsstrahlung is produced not only in the Coulomb field of an atomic nucleus but also in a collision between an incomin g electron and an atomic electron which then is ejected. By using the electron-photon coincidence technique it is possible to differentiate exactly between electron-electron bremsstrahlung and electron-nucleus bremsstrahlung. All coincidence experiments done for studying the elem entary bremsstrahlung process have been performed in the energy region of a few hundred keV. In the MeV to GeV region the coincidence method is mainly used to test radiative corrections from quantum electrodyna mics or to obtain ''monochromatic'' tagged photon beams for further ap plications.