PAMELA's TRD:
When a charged particle crosses the boundary between two media with different dielectric properties (that is to mean, with different index of refraction), it can emit an electromagnetic radiation, known as transition radiation, which was first observed by Ginzburg and Frank in 1946. The nice feature of this radiation is that its intensity is approximately linear with the Lorentz γ factor (=E/mc²) of the particle.
Practically, this radiation only becomes useful for detectors in the case of ultrarelativistic particles (γ > 1000), when the emission is mainly along the particle direction(*), and in the energy range of X-rays. Thus, a Transition Radiation Detector (TRD) can be employed as a mean of identifying particles measuring their γ, or as a threshold detector, to distinguish between particles which emit or not transition radiation.
(...)
Since soft X-rays, in the useful range between 2 and 20 KeV, are radiated with about 1% probability per boundary crossing, practical detectors must use radiators with several hundreds interfaces.

(*) Angle of emission typically around 1/γ.

The photons from the front and back faces have opposite phases, so if the two faces are very close, the two waves cancel out.