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Transition radiation is a radiation in the X-ray region, that arises
when ultrarelativistic particles cross a boundary between 2 media with
different dielectric constants. It depends strongly on the relativistic
factor of the particle which makes it usable for particle
identification at energies where time of flight methods or detection
of Cherenkov radiation no longer work. As an example, the large
mass difference between electrons and pions makes it possible to separate
electrons from pions in an energy interval from 0.5 GeV to
To build a detector based on transition radiation is far from easy
though since the number of X-ray photons from a single boundary are of
the order , the fine structure constant.
The theory here is mainly based on the review articles 
and  and the section on applications to detectors
on  and .