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High flux reactor at oak ridge national laboratory. (Credit: Genevieve Martin, ORNL) In nuclear reactors, charged particles sometimes travel faster than the speed of light in water, producing a blue glow. This blue light is not useless, it has important applications in physics, biology and medicine.

Wen| Chen Qiang

If you've ever seen a photograph of a nuclear reactor, you might notice a blue glow around the core. This phenomenon is called Cherenkov radiation and is essentially a shock wave of light.

Cerenkov radiation was first discovered in 1934 by Soviet physicist Pavel Cerenkov. Later, Soviet physicists Ilya Frank and Igor Tamm succeeded in explaining the cause of Cherenkov radiation, and the three won the Nobel Prize in Physics in 1958.

How is Cerenkov radiation generated? Special relativity holds that objects cannot move faster than the speed of light in vacuum c. However, the propagation speed of light in the medium is less than c, for example, the propagation speed in water is about 0.75c. Thus, objects can be accelerated beyond the speed of light in the medium, either by nuclear reactions or particle accelerators. When a charged particle travels faster than the speed of light in a medium, it emits Cerenkov radiation.

Specifically, when a charged particle moves through a medium, it interacts with molecules in the surrounding medium, causing them to excite to higher energy levels. When molecules return to their ground state, they emit photons that form electromagnetic waves.

If the charged particles move slowly, these electromagnetic waves converge slightly in the direction of motion, but do not interfere. But if charged particles move faster than the speed of light in the medium, these electromagnetic waves "stack" in front of the particles and interfere with each other, resulting in coherent radiation in a direction at an angle to the direction of particle motion, known as Cherenkov radiation.

Cherenkov radiation can be compared to supersonic aircraft produced sonic boom phenomenon. As the supersonic plane flies, sound waves generated in front of it "stack" to form a shock wave and a sonic boom. Cerenkov radiation can therefore be regarded as a shock wave of light.

The faster and more charged particles move, the stronger the Cerenkov radiation. The spectrum of Cerenkov radiation is continuous, and the relative intensity at a frequency is proportional to that frequency. Therefore, Cerenkov radiation in the visible part of the spectrum appears blue because blue has shorter wavelengths and higher intensities. In fact, most Cerenkov radiation is in the ultraviolet.

What are the applications of Cerenkov radiation? In nuclear power plants, Cerenkov radiation can be used not only to detect the presence of energetic charged particles, but also to examine the residual radioactivity of spent nuclear fuel rods. Cerenkov radiation also has important applications in astrophysics and particle physics.

Cerenkov radiation, for example, is often used in particle physics to identify particles. By measuring the properties of the Cerenkov radiation emitted by a charged particle in a medium, the particle's velocity can be determined. If a particle's momentum can be measured in other ways, it can be identified by calculating its mass from its momentum and velocity.

On the other hand, cosmic rays or gamma ray bursts from space interacting with the Earth's atmosphere may produce electron-positron pairs with extremely high velocities. Cerenkov radiation emitted by these charged particles in the atmosphere can be used to determine the direction and energy of cosmic rays or gamma rays.

In addition, some large detectors that detect neutrinos also use Cerenkov radiation to detect whether neutrinos interact with matter in the detector.

Cerenkov radiation also has applications in biology and medicine. Cerenkov radiation, for example, can be used to monitor the activity of biomolecules. Scientists can introduce radioactive atoms, such as phosphorus-32, into biomolecules and monitor them using Cherenkov radiation produced by the molecules. In this way, scientists can analyze the role and changes of these molecules in organisms even if the amount of labeled biomolecules is small.

As for medicine, whether it's injecting radiopharmaceuticals into the body or using radiation to treat cancer, these produce high-speed electrons in specific parts of the body that emit detectable Cerenkov radiation. In recent years, some scientists have begun to use Cerenkov radiation for medical imaging in order to diagnose diseases more accurately.

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