Transition write-off
Particle-matter interaction - Theory

When a charged particle (of charge ze ) with velocity β = v /c passes from a medium of permittivity ε ₁ to a medium of permittivity ε ₂ , the continuity of the electromagnetic field as the two media pass forces the emission of radiation, called transition radiation . This emission takes place backwards (relative to the direction of the particle), in the visible range, and forwards in the X-ray range (0-100 keV). It is this latter emission that is used to detect relativistic particles.

The amount of radiation...