When a charged particle passes through a dielectric medium the particle polarizes atoms along its path which quickly return to ground state via photon emission. If the particle's speed is faster than the speed of light in the medium, then this radiation is coherent due to constructive interference of wavelets from points along the track. A similar (though 2-dimensional) situation occurs when the wavelets which form along the path of a boat moving through the water interfere constructively to form a "wake". The Cherenkov photons form a cone-shaped wave front centered on the particle trajectory at an angle theta with respect to the particle direction. Using the boat analogy once more, as the velocity increases the angle theta increases, making a wider Vee shape (boat wake) or cone-shape (particle's Cherenkov). This effect is observed at nuclear reactor sites, as energetic charged particles emitted by radioactive material enter the water surrounding them. The light emitted appears blue because most photons produced by the Cherenkov Effect are in the ultraviolet part of the electro-magnetic spectrum. The Cherenkov Effect is often compared to the sonic boom created by aircraft flying faster than the speed of sound in air.
* Note: This relationship is defined by cos(thetac) = 1/nß where n is the refractive index of the material and ß= vp/c the velocity of the particle in units of c (speed of light in a vacuum). Therefore, as ß increases, the cosine of the angle decreases. Which implies that Cherenkov angle increases with the speed of the particle.
The Cherenkov angle thetac depends only on the index of refraction of the dielectric material and the particle velocity vp. Therefore, accurate measurement of thetac allows experimenters to calculate vp, the particle's velocity. This information is used together with the momentum measurements obtained elsewhere (ie. the drift chamber) to calculate the particle's mass, and ultimately identify the particle. Particle identification relies on making several different types of measurements. That's why the experimenters designed several different types of detectors to make up CLEO III.
Click Here for RICH Construction DetailsThis page was developed by five high school science teachers from the Detroit, Michigan metropolitan area and funded by the National Science Foundation RET Program. The teachers are: Ron Arscheene (Utica Community Schools), Gary Lalli (St. Florian), Robert Santavicca (Detroit Public Schools), Wilbert Sherrod (Detroit Public Schools), and Eric Wisniewski (Dearborn Public Schools).