Transition counter

We report a new type of counter based on the transition radiation from ultrarelativistic charged particles. The efficiency of the "transition counter" depends almost linearly on the Lorentz factor of the particle in the energy region covered and is over 80% for 2-GeV electrons. Particles heavier than electrons in a 2-GeV unseparated beam are effectively rejected. A possible application of the counter at multihundred-GeV accelerators is discussed.