A transition radiation detector (TRD) is a particle detector using the γ γ -dependent threshold of transition radiation in a stratified material. It contains many layers of materials with different indices of refraction. At each interface between materials, the probability of transition radiation increases with the relativistic gamma factor. Thus particles with large γ γ give off many photons, and small γ γ give off few. For a given energy, this allows a discrimination between a lighter particle (which has a high γ γ and therefore radiates) and a heavier particle (which has a low γ γ and radiates much less).