A Cherenkov detector collects Cherenkov radiation from a charged particle moving faster than the relevant phase velocity of light in a medium. For a transparent medium of refractive index and particle speed , emission requires and the cone angle satisfies .
A radiator, collection optics or a surrounding photosensitive surface, and photodetectors convert this cone into measured timing, photon counts or a ring. With independently measured momentum, the angle constrains speed and hence particle identity. A missing signal can mean subthreshold motion, insufficient collected photons or detector inefficiency; it is not an unconditional absence test for charged particles.
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