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.

The Elementary Process of Bremsstrahlung

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The prediction of thick target electron bremsstrahlung spectra in the 0.25-50 keV energy range

The emission of x-rays following the electron bombardment of an atom is caused by three processes: electron-nucleus (e-n) bremsstrah-lung, electron-electron (e-e) bremsStrahlung and inner-shell ionisation. The bremsStrahlung processes have continuous spectra of photon energy (k) and outgoing electron energy (T), the ionisation is accompanied by emission of characteristic x-rays and by a continous electron spectrum (Auger electrons are not considered here).

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

In this chapter we discuss electron bremsstrahlung, the radiation of a photon in the scattering of an electron from an atom or ion. We shall focus our attention on incident electrons with kinetic energiesT i in the 100-eV to 10-MeV range (and especially for 1 keV to 1 MeV) and radiated photons in the vacuum ultraviolet, X-ray, and soft γ-ray regimes. Electron bremsstrahlung is only one particular consequence of the general coupling of the electromagnetic field and matter fields. The process is closely related, as we shall see, both to elastic electron scattering and to direct radiative recombination, in which the incident electron does not scatter but is captured by the atom. It is also closely related to pair production. Other charged particles also radiate in scattering, as follows from Maxwell’s equations, and we shall discuss the most closely related case of positron bremsstrahlung. More generally, the electromagnetic coupling to matter fields is responsible for radiative versions of all nonradiative processes, including “inverse bremsstrahlung” processes in which electromagnetic radiation is absorbed rather than emitted.

Electron—Atom Bremsstrahlung

We discuss new progress in bremsstrahlung, beyond the predictions previously available. A relativistic code for the calculation of bremsstrahlung in a screened central potential, utilizing an expansion in electron partial waves and photon multipoles, was developed some time ago by Tseng and Pratt [Phys. Rev. A 3 (1971) 100]. For electron bremsstrahlung, unlike for bremsstrahlung in ion-atom collisions, this potential bremsstrahlung contribution generally dominates the atomic or polarizational bremsstrahlung contribution associated with atomic structure, except for radiation of energies comparable to transition energies [M.Y. Amusia and R.H. Pratt, Commun. At. Mol. Phys. 28 (1992) 247]. Predictions from the potential bremsstrahlung calculations are in reasonably good agreement with experiment [R.H. Pratt and I.-J. Feng, in: Atomic Inner Shell Physics, ed. B. Crasemann (Plenum, New York, 1985) p.]. Tabulations are available for both the spectrum and the bremsstrahlung angular distribution [R.H. Pratt, Atom. Data Nucl. Data Tables 20 (1977) 175; L.D. Kissel et al., ibid. 28 (1983) 381]. A major shortcoming has been that the code only obtains the doubly differential cross section, summing over all states of the outgoing scattered electron. No adequate predictions have been available to compare with experimental results for the triply differential cross section, which show substantial disagreement with simpler theory [W. Nakel, in press, 1994]. This problem has recently been remedied [X.M. Tong, C.D. Shaffer and R.H. Pratt, to be published], directly summing numerically the partial wave contributions to the matrix element. In the first cases studied good agreement has been achieved with the experiments. A second shortcoming of the code was its inability to deal with incident electron energies below about 1 keV, needed to study the transition between classical and quantum behavior [L. Kim and R.H. Pratt, Phys. Rev. A 36 (1987) 45]. We discuss recent results in this classical domain. We also review new developments in polarizational bremsstrahlung and two photon bremsstrahlung.

New developments in the theory of bremsstrahlung

A coincidence measurement was made of the absolute cross section for electron-atomic-field bremsstrahlung, differential in photon energy, photon-emission angle, and electron scattering angle. The incident electron energy was 140 keV and the scattering materials were thin films of aluminum and gold. The data are compared to the theoretical calculations of Elwert and Haug and of Bethe and Heitler. Both theories give generally satisfactory agreement for aluminum. The Elwert-Haug theory is somewhat more accurate for gold.

J. D. Faulk

Papers

Coincidence measurement of the fully differential cross section for atomic-field bremsstrahlung