As long as the plane has not exceeded the speed of sound, the sound waves announce its arrival. When it reaches a supersonic speed, the plane passes over our heads before the sound wavefront shakes the windows and vibrates our eardrums. The Cherenkov effect derives from an analogous principle: the electron plays the role of the airplane, the waves of light that of the sound waves.
An electron must have an energy greater than 21 MeV 21 million electronvolts! This condition is never fulfilled for the electrons of radioactivity.
Cosmic radiation, on the other hand, contains electrons, positrons and also very high energy muons that can produce Cherenkov light flashes. This Cherenkov light is used to detect cosmic showers. The Cherenkov effect is used as a tool in many modern experiments: in nuclear physics for instance to detect solar neutrinos ; in high energy experiments such as the ones performed at CERN to identify the nature of particles; in the astrophysical experiments that study cosmic showers.
Access to page in french. EN FR. The number of photons from Cherenkov radiation at a given wavelength and the angle of radiation is given by. Unlike the fluorescence or emission spectra, the Cherenkov radiation spectrum given by the above formula, is continuous and its density is inversely related to the wavelength squared. Therefore, the number of photons increases as the wavelength decreases.
That explains why most of the Cherenkov radiation seems blue and mostly in UV range. Figure 1 shows the image of a neutron radiography reactor from National Laboratory of Idaho. In the realm of astronomy and cosmology, Cherenkov radiation is produced by charged particles traveling faster than the speed of the light in air. This emission is determined by the value of n, which is proportional to density of the atmosphere.
Integrates the above density within a wavelength range, the number of emitted photons in desired spectral range would be determined by the following equation. This photon number is proportional to the number measured by an intensity detector.
In particle physics, the Cherenkov radiation is used frequently in particle identification detectors PID. Particle identification plays a crucial role in particle physics to separate particles such as protons, electrons, muons, pions, etc at different velocities.
Cherenkov counters can be categorized to two main types: threshold and imaging counters. This can be set as a threshold measurement mechanism through which the number of particles at given velocity would be determined. By varying the pressure of the gas in a detector, the number of particles passing the threshold velocity changes. For example, the water surrounding nuclear reactors actually does glow bright blue! How does it work? It's due to the phenomenon called Cherenkov Radiation.
What is Cherenkov radiation? Essentially, it's like a sonic boom, except with light instead of sound. Cherenkov radiation is defined as the electromagnetic radiation emitted when a charged particle moves through a dielectric medium faster than the velocity of light in the medium. The effect is also called Vavilov-Cherenkov radiation or Cerenkov radiation. Cherenkov had first noticed the effect in , when a bottle of water exposed to radiation glowed with blue light. Although not observed until the 20th century and not explained until Einstein proposed his theory of special relativity, Cherenkov radiation had been predicted by English polymath Oliver Heaviside as theoretically possible in The speed of light in a vacuum in a constant c , yet the speed at which light travels through a medium is less than c, so it's possible for particles to travel through the medium faster than light, yet still slower than the speed of light.
Usually, the particle in question is an electron. When an energetic electron passes through a dielectric medium, the electromagnetic field is disrupted and electrically polarized. The medium can only react so quickly, though, so there is a disturbance or coherent shockwave left in the wake of the particle. One interesting feature of Cherenkov radiation is that it's mostly in the ultraviolet spectrum, not bright blue, yet it forms a continuous spectrum unlike emission spectra, which have spectral peaks.
As Cherenkov radiation passes through the water, the charged particles travel faster than light can through that medium. How can a charged body moving with constant velocity emit an electromagnetic radiation?
From the wiki article. The frequency spectrum of Cherenkov radiation by a particle is given by the Frank—Tamm formula. Unlike fluorescence or emission spectra that have characteristic spectral peaks, Cherenkov radiation is continuous. Around the visible spectrum, the relative intensity per unit frequency is approximately proportional to the frequency. That is, higher frequencies shorter wavelengths are more intense in Cherenkov radiation. This is why visible Cherenkov radiation is observed to be brilliant blue.
In fact, most Cherenkov radiation is in the ultraviolet spectrum—it is only with sufficiently accelerated charges that it even becomes visible; the sensitivity of the human eye peaks at green, and is very low in the violet portion of the spectrum.
From the wiki link again:. As a charged particle travels, it disrupts the local electromagnetic field in its medium. In particular, the medium becomes electrically polarized by the particle's electric field.
If the particle travels slowly then the disturbance elastically relaxes back to mechanical equilibrium as the particle passes. When the particle is traveling fast enough, however, the limited response speed of the medium means that a disturbance is left in the wake of the particle, and the energy contained in this disturbance radiates as a coherent shockwave. When high velocity particles travel faster than the speed of light in a medium they create a blue flash.
It should be noted that only electrically charged particles are capable of emitting Cerenkov radiation. The analogy we can use is that it is the speed of light equivalent to the speed of sound sonic boom. As a supersonic jet accelerates, the air piles up in front of the leading edges of the wing and tail of the aircraft.
The sonic boom is caused by a sudden pressure drop as the aircraft moves faster than the air molecules can get out of the way. The speed of sound is determined by how fast the air molecules can move.
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