Werner Nakel

The elementary process of bremsstrahlung

Abstract In this report we review the experimental and theoretical developments concerning the elementary process of electron bremsstrahlung. The term “elementary process” means that not only the emitted bremsstrahlung photons are considered but also the partinent decelerated electrons. Experimentally the elementary process is observed by detecting the photons in coincidence with outgoing electrons scattered into a particular direction. The results of such electron-photon coincidence experiments yield stringent tests for theoretical predictions of the fully differential cross section. Much deeper insight into the bremsstrahlung process is achieved by also taking into account polarization variables such as the spin orientation of the incoming electron or the polarization of the emitted photon. Thus conspicuous effects of the weak spin-orbit interaction can be observed which are usually masked in non-coincidence experiments. Bremsstrahlung is produced not only in the Coulomb field of an atomic nucleus but also in a collision between an incoming electron and an atomic electron which then is ejected. By using the electron-phonon coincidence technique it is possible to differentiate exactly between electron-electron bremsstrahlung and electron-nucleus bremsstrahlung. All coincidence experiments done for studying the elementary 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 electrodynamics or to obtain “monochromatic” tagged photon beams for further applications.

Koinzidenzexperimente zum Elementarprozeß der Bremsstrahlungserzeugung

By using a coincidence method, the angular distribution of bremsstrahlung has been measured for definite energy loss and definite angle of the outgoing electron. The incident electron energy was 300 keV. With a magnetic spectrometer, outgoing electrons with an energy of 170 keV and scattering angles of 0°, 5° and 10° were selected. The electrons and photons were both detected in scintillation counters. A gold foil of 250 Å thickness served as the target.The experimental results are compared with theoretical calculations.

Relativistic (e,2e) processes on atomic inner shells

The investigation of electron impact ionization by the electron-electron (e,2e) coincidence technique has been restricted to the nonrelativistic energy region (typically 10 eV to 10 keV). The authors report on an (e,2e) experiment using a 500 keV electron beam impinging upon thin-foil targets. Absolute triply differential cross sections and angular distributions for K-shell ionization of silver and gold were measured, showing appreciable discrepancies compared with theoretical predictions.

Absolute triply differential cross section for K-shell ionisation by relativistic electron impact for high atomic number

The absolute triply differential cross section for K-shell ionisation by 500 keV electron impact for high atomic number (Z=73) was measured as a function of the energy sharing between outgoing electrons. By using a two-parameter set-up, electron-electron coincidences in a wide energy sharing range could be registered in the same run. Scattering angle pairs of 40 degrees /-20 degrees and 30 degrees /-30 degrees , respectively, were used in a coplanar geometry. The results show the inadequacy of the theory of Das and Konar (1974) for the present parameters.

Two-Parameter Coincidence Measurements of Bremsstrahlung, Electron-Electron Bremsstrahlung, and K-Shell Ionisation for 300-keV Electron Impact

An electron beam of 300 keV was directed on to thin target foils (C, Cu, Ag, and Au) to investigate the processes of electron-nucleus bremsstrahlung, electron-electron bremsstrahlung and K-shell ionisation. Using a two-parameter arrangement the outgoing electrons from all these processes were detected in coincidence with the emitted bremsstrahlung and K X-ray photons, respectively, as a function of electron and photon energies for fixed detector angles. So it was possible to measure the triply differential cross sections of the processes of electron-nucleus bremsstrahlung and electron-electron bremsstrahlung and the doubly differential cross section of K-shell ionisation simultaneously. A comparison with theoretical calculations is given.

Relativistic (e, 2e) collisions on atomic inner shells in symmetric geometry

We report here on an (e, 2e) experiment at relativistic electron energies (E0=300 keV and 500 keV) in coplanar symmetric geometry. Absolute triple differential cross section measurements forK-shell ionisation of gold, silver and copper are compared with a number of simple first order approximations. Appreciable discrepancies between theory and experiment are found, which reduce with decreasingZ and increasing primary energy. The theoretical calculations show that spin flip effects are important in symmetric geometry, in earlier works these had been neglected.

Messung der Winkelverteilung und des Wirkungsquerschnittes beim Elementarprozeß der Elektron-Elektron-Bremsstrahlung

By using an electron-photon coincidence technique, we have measured the absolute cross section of electron-electron bremsstrahlung in dependence of the photon emission angle for a definite energy and a definite angle of the outgoing electron. An electron beam of 300 keV was used, incident on a carbon target. Within the experimental errors we find agreement with theoretical calculations.Simultaneously with the electron-electron bremsstrahlung the elementary process of electron-nucleus bremsstrahlung of carbon was measured.

Eine Koinzidenzmessung zum Elementarprozeß der Elektron-Elektron-Bremsstrahlung bei 300 keV

The absolute cross section of electron-electron bremsstrahlung which is differential in all the free kinematical variables was measured by using an electron-photon coincidence technique. An electron beam of 300 keV was used, incident on a carbon target. Within the experimental errors we found agreement with recent theoretical calculations. Simultaneously with the electron-electron bremsstrahlung the absolute cross section of electron-nucleus bremsstrahlung of carbon was measured.

Measurement of the asymmetry in the emission of bremsstrahlung by transversely polarized electrons

Bremsstrahlung emitted by transversely polarized electrons shows a “right-left” asymmetry in spatial distribution. Measurements and remeasurements of this asymmetry are presented using electrons of 300 and 128 keV, respectively, impinging on a gold target. Within the accuracy of the experiment there are no significant discrepancies to the partial wave calculations of Tseng and Pratt.

Relativistic (e,2e) processes on inner shells of heavy atoms

The investigation of electron impact ionization by the electron‐electron (e,2e) coincidence technique has been until recently restricted to the nonrelativistic energy region (typically: 10 eV to 10 keV). We report here on the continuation of an (e,2e) experiment using a 500 keV electron beam impinging upon thin‐foil targets. Absolute triply differential cross sections and angular distributions for K‐shell ionization of silver and gold were measured and are compared with theoretical predictions.