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 ε₁ to a medium of permittivity ε₂, 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 increases with the γ (γ = E / M = (1 - β ²)^-1/2 ) of the particle. The emission angle is close...