That Eerie Blue Light from Nuclear Reactors? It's Cherenkov Radiation

Caused by particles traveling faster than light through a medium, Cherenkov Radiation is what gives nuclear reactors their eerie blue glow.

In the miniseries "Chernobyl" when the reactor first explodes, there's an eerie blue light emanating from it.

In scary movies, it's always a bad idea to enter a room that has a spooky blue light coming out of it.

As it turns out, that spooky blue light is a real phenomenon, and it's called Cherenkov Radiation.

Cherenkov radiation results when a charged particle, such as an electron, travels through a dielectric medium (an electrical insulator that can be polarized by an applied electric field) at a speed greater than the velocity of light in that medium.

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The speed of light (c) in a vacuum is a constant 299,792,458 meters per second, but light slows down to 225,000,000 meters per second in water, and to around 197,000,000 meters per second in glass. That means that when moving through the water, light travels at just 3/4 of the speed it reaches in a vacuum speed; and when traveling through glass, light moves at 2/3 of its speed in vacuum. 

During nuclear reactions and in particle accelerators, particles can be accelerated beyond those speeds, but still less than the speed of light.

Pavel Cherenkov

Cherenkov radiation is named after the Soviet physicist Pavel Alekseyevich Cherenkov who first noticed the effect in 1934 when a bottle of water that had been exposed to radiation glowed blue.

This type of radiation was first theorized in 1888 by the English polymath Oliver Heaviside, then again in 1904 by the German physicist Arnold Sommerfeld. In 1910, Marie Curie observed blue light in a concentrated solution of radium.

Marie Curie
Marie Curie. Source: Henri Manuel/Wikimedia Commons

An analogy to Cherenkov radiation is the sonic boom created when an aircraft flies faster than the speed of sound. The sound waves generated by the plane travel at the speed of sound. Because they travel slower than the speeding plane, they cannot propagate ahead of the plane and instead form a shock front.

In a similar way, since the particle emitting the radiation is in motion, it emits a cone of radiation that travels in the same direction of motion as the particle emitting it. This differs from the phenomenon of fluorescence, where electrons de-excite and emit visible radiation in all directions.

Two colleagues of Cherenkov's, Igor Tamm and Ilya Frank, described Cherenkov radiation in the context of electromagnetism and relativity, and for their work, all three were awarded the 1958 Nobel Prize in Physics.

Pavel Cherenkov
Pavel Cherenkov. Source: Nobel Foundation/Wikimedia Commons

"Seeing" the Invisible

If a charged particle strikes the vitreous humor of the human eye, flashes of Cherenkov radiation appear. This was used in the early days of particle physics experiments by scientists who would "look" into an invisible beam of electrons to "see" if it was on.

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If they saw the blue flashes of Cherenkov radiation, they knew it was on. As more was known about the dangers of radiation, this practice was stopped.

Cherenkov radiation in a nuclear reactor
Cherenkov radiation in a nuclear reactor. Source: Argonne National Laboratory/Wikimedia Commons

Today, nuclear power technicians use the amount of blue glow, or Cherenkov radiation, in a reactor to gauge the radioactivity of spent fuel rods.

Cherenkov radiation is also used in particle physics experiments to identify the particles being created, to detect neutrinos, and to study gamma-ray-emitting astronomical objects, such as the remnants of supernovas.

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