Foreword

The Čerenkov effect - also known as Vavilov-Čerenkov radiation - consists in the emission of electromagnetic radiation when a charged particle (suc as an electron) passes through a dielectric medium at a speed greater than the speed of light in the same medium.

As a charged particle travels, its electrical field polarizes the medium molecules. When going back to the initial state, provided that the charged particle has a velocity greater than a threshold value, the polarized molecules discharge emitting a short pulse of visible electromagnetic radiation.

More in general, Čerenkov radiation is the name assigned to this phenomenon when occurring in a medium that is not transparent to the visible radiation (light).

This phenomenon is named after the Soviet scientist Pavel Alekseevič Čerenkov who achieved in 1958 the Nobel prize for physics due to his research that led him to be the first in 1934 to prove experimentally the existence of such a radiation, theoretically predicted by the English polymath scientist Oliver Heaviside in a series of papers published in 1888-1889, based on Maxwell theory.

A complete theory of this effect was later developed, within the framework of Einstein's special relativity theory, by Igor Tamn and Ilya Frank, who also shared the 1958 Nobel prize.

Physical basis

When propagating in a dense medium, the speed of light, v, is less than the speed of light in a vacuum, c (that, according to the theory of relativity, cannot be exceeded and is a universal constant). This decrease is related to the refractive index of the medium, n, in such a way that it assumes the value v = c/n.

In a dense medium, a particle can occasionally exceed the speed of light in that medium.

When this particle is electrically charged the Čerenkov effect occurs, and γ rays are emitted.

An electrically charged particle along its path induces temporary dipole moments (polarization) on atoms or molecules of the medium through which it is passing.

When the speed of the particle is lower than the speed of light in that medium, the dipoles are symmetrically arranged around the particle path. Because of this symmetry, the radiation emitted when the excited molecules spring back to the inital configuration cancel each other out and no visible radiation rises.

When the speed of light in the medium is exceeded, this symmetry is broken and the resulting dipole moment is not zero anymore; in these conditions, there is emission of visble electromagnetic radiation.

A disturbance is left in the wake of particles and the energy contained in this disturbance radiates as a coherent shockwave.

It is a phenomenon similar to that occurring in the air when an object exceeds the speed of sound in the air (sound barrier): a conical shock front is generated (Mach cone) radiating from the point where the event occurred.

The electromagnetic radiation radiates from the disturbed point following a cone of semi-angle equal to the so-called Čerenkov angle, usually indicated with the Greek letter ϑ. It is given by the equation:

cos ϑ = (1/n)∙(c/n) = 1 /(n∙β)

where:

c is the speed of light in a vacuum,

v is the actual velocity of the charged particle,

n is the refractive index of the medium,

β = v/c is the ratio of the particle actual velocity and the speed of light in a vacuum (< 1).

The maximum traveling angle is achieved when the particle moves with a speed close to the speed of light in a vacuum, e.g. v ≈ c. In this case: cos ϑ_max ≈ (1/n).

Čerenkov radiation can be emitted only when β > 1/n, which is the threshold value for the occurrence of the effect.

Why does the Čerenkov light have the typical blue color?

The Čerenkov radiation is continuous, and has no typical spectral peaks as occurring, for example, with fluorescence. In the visible spectrum region, the relative intensity per unit frequency is approximately proportional to the frequency itself. This means that higher frequencies (shorter wavelength) are more intense in Čerenkov radiation. It follows that, being shifted towards higher visible frequencies, the radiation achieves the observed brilliant blue color. In fact, most Čerenkov radiation is in the ultraviolet region where the human sensitivity is very low.

Use of this effect

Čerenkov radiation is used in the gamma astronomy field and in experiments involving neutrinos. For instance, it allows to detect muons contained in cosmic rays that, traveling in water pools at a speed greater than the speed of light, generate the emission of Čerenkov radiation. When cosmic rays or high-energy (TeV) photons interact with the Earth's atmosphere, they can produce electron-positron pairs having very high velocity. The Čerenkov radiation generated by these charged particles allows to determine the source and intensity of the cosmic or gamma rays. The Čerenkov telescopes are used for these observations. This approach is used in the Imaging Atmospheric Cherenkov Technique (IACT), and in experiments such as VERITAS, H.E.S.S, MAGIC, and HAWC. As said above, the same principle is used in neutrino detectors, such as Super-Kamiokande, the Sudbury Neutrino Observatory (SNO) and IceCube.

Čerenkov radiation is commonly used in experimental particle physics for particle identification. The simplest type of particle identification device based on a Čerenkov radiation technique is the threshold counter, which gives an answer as to whether the velocity of a charged particle is lower or higher than a certain value by looking at whether this particle does or does not emit Čerenkov light in a certain medium. Knowing the particle momentum, one can separate particles lighter than a threshold from those heavier than the threshold. The most advanced type of such detectors is the RICH, or Ring-Imaging Čerenkov detector, developed in the 1980s. An example of a proximity gap RICH detector is the High Momentum Particle Identification Detector (HMPID), a detector currently under construction for ALICE experiment (A Large Ion Collider Experiment), one the six experiments at the LHC (Large Hadron Collider) at CERN.

In fission nuclear reactors, the intensity of the Čerenkov radiation provides an indication of the reactor activity since it is related to the frequency of the fission events. As the fission products decay, beta particles (high-speed electrons) are released producing the characteristic blue glow typical of water reactors.The blue glow continues after the chain reaction stops, dimming as the shorter-lived products decay. The Čerenkov effects is also used to evaluate the remaining radioactivity of the spent fuel rods stored in the decay pool.

Čerenkov radiation is also used in biological research to detect small quantities and low concentration of biomolecules. Enzymatic and synthetic methods are used to introduce radioactive atoms, as phosphorus 3. The Čerenkov effect allows researchers to detect these atoms even at low concentrations, study biological pathways and characterize the interaction of biological molecules, thus evaluating affinity constants and dissociation rates.

Curiosities

According to the Italian version of Wikipedia (https://it.wikipedia.org/wiki/Effetto_Čerenkov) the Čerenkov effect was used in science fiction novels and movies.

In the novel “Starship Troopers” by Robert A. Heinlein the Čerenkov drive is used to allow spacecraft to travel at the light speed. In the novel the “Čerenkov drive” is mentioned six times, among which:

<quote>

“…Even with Čerenkov drive, stars are far apart.”

“…you must return to base, even if the Čerenkov generators could still take you twice around the Galaxy.”

“…under Čerenkov drive she cranks Mike 400 or better—say Sol to Capella, forty six lightyears, in under six weeks.”

In Star Trek the starships make use of the imaginary "warp drive" and travel at super relativistic speed within a cone of Čerenkov blue radiation. The Čerenkov effect is mentioned at pages 81-83 of the "Spock’s World" book by Diane Douane, as follows:

<quote>

“No one paid much attention to the view out the windows while a ship was in warp. The other space in which Enterprise traveled at such times ...

... So they saw what not too many people have an opportunity to see—a starship decelerating hard from warp, the point of a silver spear piercing through from the far side of the darkness in a trailing storm-cone of rainbows, as Coromandel came out of warp in a splendor of Cherenkov radiation from the super relativistic particles she dragged into real-space with her...

... Coromandel accelerated away on impulse, then flung a cloak of spectrum-colored fire about herself, leaped away, and was gone from sight on the instant.”

A similar effect is shown in the Japanese anime "Cowboy Bebop". See “Unkown simettries in physics of 3rd millennium” by Jose Luis Armenta in Google Books.

In the "Mass Effect" gameplay the force field collapse, when the starship is traveling at super relativistic speed, can cause catastrophic effects because of the energy released in the form of lethal Čerenkov radiations (see also https://intelligentgames.wordpress.com/2010/05/29/hard-science-why-i-love-mass-effect/).

The English version of Wikipedia (https://en.wikipedia.org/wiki/Cherenkov_radiation) states that the blue color of "Doctor Manhattan" in "Watchmen" may be inspired by Čerenkov radiation (see also http://www.livescience.com/3356-watchmen-science-dr-manhattan.html).