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Someone told me that Cherenkov radiation is analogous to breaking the sound barrier. In the latter, an object travels faster than the speed of sound and in the former, a particle travels faster than the speed of light. Can you comment on the accuracy of this?

The analogy is correct. Just as a sonic boom is what results when a physical object exceeds the speed of sound, Cherenkov radiation is what comes out when a subatomic particle exceeds the speed of light! "What!" you say "Exceeds the speed of light? Einstein said that was impossible!" Well it is and it isn't.

The speed of light in a vacuum - 186,000 miles per second - is an absolute speed limit. Only light can ever go this fast. However, when light goes through glass, air, crystal, plastic or water, the light slows down. The speed of light in a refractive media is always lower than the speed of light in vacuum. Sometimes, a lot lower. In general, the bigger the refractive index, the slower the light travels. That means that there can be a situation when physical objects exceed the speed that light is allowed to travel. A subatomic particle such as an electron, proton or one of the mesons can exceed the reduced speed that light travels at when going through glass, gases, water etc... What results is a shock wave effect. The particle gives off light that is emitted at an angle to the particle's direction of travel. This angle can be used to determine the particle's velocity. The frequency of the light is also sharply peaked around a characteristic frequency. At Jefferson Lab we use devices that measure particle velocity using this effect. They are called Cherenkov detectors. They are used to discriminate between slower particles like protons and mesons from the faster particles such as electrons. These are called threshold detectors. The velocity is set so that a particle must exceed a certain minimum speed to give off the Cherenkov light. Slower particles then won't give a signal. Sometimes several Cherenkov detectors with different thresholds are stacked up one after another. You can then use them to differentiate between electrons, protons and pions. Cherenkov detectors are also used to measure velocity accurately. A popular type of detector that actually measures the emitted ring of light is the Ring Imaging Cherenkov Detector.

You have probably seen photos of swimming pool Nuclear Reactors in science books. The blue glow is the Cherenkov light that is emitted by the electrons from beta decay going on in the nuclear fuel. It is a beautifully eerie sight.


Paul Brindza, Experimental Hall A Design Leader (Other answers by Paul Brindza)

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