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Analytic expressions for bremsstrahlung spectra from neutral atoms and ions, including the polarizational bremsstrahlung contribution in a stripped atom approximation, are developed for electron scattering at energies of 10-2000 keV. A modified Elwert factor and a simple higher Born correction are used for the Coulomb spectrum, with ordinary bremsstrahlung screening effects in ions and atoms adequately characterized in the non-relativistic Born approximation. In parallel with the development of this analytic description, new numerical results are obtained for ordinary bremsstrahlung from ions and from bare nuclei, appreciably extending the available data set which can be used to study dependences on element, ionicity, energy and the fraction of incident energy radiated. The accuracy of predictions with the analytic expressions is then determined by comparison with the full numerical relativistic partial-wave results for ordinary bremsstrahlung and with non-relativistic numerical results in the Born approximation or in partial waves for the polarizational amplitude.

A simple analytical expression for calculating the generalized oscillator strengths for optically allowed atomic transitions in the forward direction is obtained. We found the correction to the first Born approximation by exploiting the analytical properties of the generalized oscillator strength function at small momentum transfers. A new procedure for experimental measurement of the differential cross sections for optically allowed transitions at small angles is proposed.

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The bremsstrahlung from solid material caused by runaway electrons may blur severely the bremsstrahlung that is recorded from gases or plasmas in order to determine the energy distribution of their hot electrons. The expressions for these emissions are presented in order to compare their characteristics. Explanations are then given for an irregular feature observed at the tip of the spectrum for solid targets. More importantly, a type of diagnostics of energy distribution functions is proposed, based on analysis of the bremsstrahlung emissivity from thick targets. It can be implemented on experimental setups by purposely inserting a well-defined solid target into the electron trajectories, and it is applied as an example to the electron beam of microwave tubes. These diagnostics could also be taken advantage of to study runaway electrons hitting vessel walls.

Expressions are obtained for the energy distribution of fast non-relativistic outgoing electrons resulting from double photoionization of helium by highenergy photons, taking into account both final state interaction and quasi-free mechanisms. In these expressions the behaviour of the energy spectrum is sensitive to the choice of the initial state wavefunction. The magnitude of the distribution in the centre of the spectrum is much reduced by correlation effects. We also analyse the corresponding results which have been obtained by other authors.