Introduction

Electrons impinging on solid materials are slowed down principally through ‘inelastic’ interactions with outer atomic electrons, while ‘elastic’ deflections by atomic nuclei determine their spatial distribution. Some leave the target again, having been deflected through an angle of more than 90°. Both these ‘backscattered’ electrons and ‘secondary’ electrons dislodged from the surface of the sample are used for image formation. In addition, interactions between bombarding electrons and atomic nuclei give rise to the emission of X-ray photons with any energy up to E0, the energy of the incident electrons, resulting in a ‘continuous X-ray spectrum’ (or ‘continuum’). ‘Characteristic’ X-rays (used for chemical analysis) are produced by electron transitions between inner atomic energy levels, following the creation of a vacancy by the ejection of an inner-shell electron.

Inelastic scattering

In the SEM or EMP the electrons with which the specimen is bombarded have an energy typically in the range 5–30 keV, which is dissipated in interactions of various types with bound electrons and the lattice, known collectively as ‘inelastic scattering’. Individual energy losses are mostly small; hence it is a reasonable approximation to assume that the electron decelerates smoothly as a function of distance travelled. The rate of energy loss is dependent on the property of the target material known as the ‘stopping power’, defined as − dE / d(ρs), where ρ is the density of the target and s the distance travelled.