CROSS-SECTIONS FOR K-SHELL IONIZATION BY ELECTRON-IMPACT

A semi-phenomenological method to compute the cross section for bound shell ionisation of atoms by impact of relativistic electrons is proposed. This method involves a simple schematisation of the Bethe surface, which is obtained from the experimental optical oscillator strength distribution. The authors' approach may be used to describe the ionisation from any bound shell. They consider in detail the case of K-shell ionisation and derive an analytical formula for the differential cross section on the basis of a hydrogenic optical oscillator strength density. Close collisions are described by the Moller differential cross section, thus incorporating exchange effects. Empirical corrections to the Born approximation for energies near the ionisation threshold are introduced. The relationship of their approximation and the Weizsacker-Williams method of virtual quanta is also discussed.