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Pavel Alekseyevich Cherenkov
(28th July 1904-6th January 1990)
Twice received State Prizes
Discoverer of Cherenkov radiation
1958 Nobel Laureate for Physics

Early life

In the summer of 1904, Mariya Cherenkov gave birth to a son - he was called Pavel. It was the only the beginning of the twentieth century when the young Cherenkov, a son to poor peasants, came into the world in Novaya Chigla, a small Russian village in the Voronezh province. Only two years after he was born, his mother tragically died and Cherenkov was left to be brought up with his sister by his stepmother. Still, this did nothing to curb his curiosity and, as a child, he was very inquisitive and enjoyed working things out for himself. He constantly marvelled at the remarkable things his youthful eyes saw and struggled to explain them for himself. He also enjoyed working problems out without resorting to adults.

Education

Cherenkov was a voracious reader as a teenager and greatly benefited from his village having the only library in the region. Through books he developed his knowledge of the world and was able to explore many aspects of science that would later cause him to be grasped by the study of physics. Despite coming into a country overcome by war and revolution, he received glowing reports from his teachers and went on to further education. In 1928, Cherenkov graduated from the Physics and Mathematics Faculty of Voronezh State University and, in 1930, went onto a postgraduate course at the Academy of Science in St. Petersburg. He continued working in physics and was soon promoted to a senior scientific officer as a section leader at the P.N. Lebedev Institute, which is the physics branch of the Academy in Moscow. In the same year, he married his wife, Marya, and they later parented a boy and girl and also had two grandchildren. In 1932 he took up research and, supervised by the prominent optical physicist Sergei Vavilov, began to study the visible electromagnetic radiation shown to be given off when gamma rays passed through specific liquids.

Research

These new observations took over his work and he worked for six years studying the phenomenon which gave rise to the blue light being emitted. In 1936, Cherenkov's father, Aleksei, was shot as a kulak (a prosperous peasant berated by the Communists for supposedly being exploitative). A.M. Putintsev, his father-in-law and a professor, was also oppressed and sent to a labour camp for two years. Meanwhile, Cherenkov's work was often dismissed by his colleagues as being trivial and useless. Despite all the disadvantages he encountered, he continued his research and, through various experiments and working with other esteemed Soviet physicists, he discovered a new effect which now bears his name: Cherenkov radiation.

Discovery

Despite functioning in a different way and having subtle differences, Cherenkov radiation can be easily explained with an analogy to sonic booms. When a hypersonic aircraft approach the sound barrier, wavefronts begin to "bunch up" in front of it as the sound waves are barely moving faster than the aeroplane itself. As a result, a high pressure area is formed in front of the 'plane and, when it is moving supersonically, because of the sudden pressure change, an intense shock wave is propagated away from the aircraft. The waves superpose and nothing is heard until the high-amplitude sound waves simultaneously reach the observer. When an aircraft is moving much faster than the speed of sound, it actually begins to overtake its emanated sound waves. This means that, instead of the constructive superposition occurring directly in front of the aircraft in an intense shock wave, a cone of high pressure is produced with shock waves at the edges, and the sound travels away from the 'plane in this conical shape.

A similar effect occurs when high-energy charged particles travel through insulating media with high refractive indices. As the particle traverses the medium, the local electromagnetic field is disrupted by the particle's electrostatic charge and the electrons in the atoms that make up the medium become polarised. As the particle's electromagnetic field passes, the atoms quickly go back to the ground state and the displaced electrons return to their original positions to retain equilibrium. This causes radiation to be emitted in the form of photons. Under usual conditions, these photons would destructively superimpose, but if the particle has sufficient velocity, the electron's energy loss after being disrupted is high enough for a coherent wavefront to be propagated. In a similar process to the sonic booms, when the particle is moving faster than the speed of light, cascades of coherent electrons are emitted and the observed radiation is intensified in a conical shape. Thus, for Cherenkov radiation to be readily observable, the charged particles must have a minimum threshold velocity which is greater than the speed of light.

But how can particles travel faster than light? Well, light does not always travel at 299,792,458 ms-1 - that is only in a vacuum. In a medium with refractive index n, the speed of light in that medium will be c/n and thus, in materials with common refractive indices, fast moving electrons can often reach the minimum supraluminal velocity required for the Cherenkov effect to take place with relatively low energies (even below 20MeV). Cherenkov radiation is continuous and the relative intensities of light frequencies are inversely proportional to their wavelength. Ergo, for lower wavelengths (the blue end of the electromagnetic spectrum), the radiation is observed to be more intense. This explains why Cherenkov radiation appears bright blue. The cone of light produced is centred on the particle's velocity vector - that is the Cherenkov radiation is propagated along the direction in which the particle is moving. Similarly to sonic booms, the angle of the radiation cone is inversely proportional to the particle's velocity. This is because, when the particle is travelling faster, the cone of light from photon emitting electrons will be "stretched" more in the plane of its direction due to a greater distance being covered in a given time. This light can be detected by very sensitive photomultipliers capable of detecting individual photons. By analysing the pattern of radiation, several properties of the particle can be learned including its mass, energy and direction.

Achievements

Cherenkov was awarded a doctorate for his work at the Academy of Science in 1940. Together with Ill'ja Frank and Igor Tamm the distinguished theorists, he was also awarded the Russian State Prize. He was awarded it again in 1951. By 1953, his academic performance was acknowledged again when he was initiated as a Professor of Experimental Physics. Also, in the same year, Ascoli recorded the first detection of Cherenkov radiation in a gas. In 1958, he was made a Nobel Laureate when he was awarded the Nobel Prize in Physics shared with Frank and Tamm for the discovery and interpretation of the Cherenkov effect. That same year, the Cherenkov counter which, thanks to him, was now standard equipment, was installed as part of Sputnik III's payload when it was launched in the beginning stages of the Soviet Space Programme. Cherenkov continued his research in the Lebedev Institute and controlled the photo-meson processes lab since 1959.

The man

Cherenkov taught in higher education for fourteen years and worked at the Lebedev Institute in Moscow for sixty years of his life. Cherenkov was a very modest and reserved man in spite of his fantastic achievements. He was fastidious and required accuracy and precision in his work and that of others. As well, he enjoyed playing tennis. He had a passion for less scientific and logical pursuits too: he adored photography, was a keen historian and took good care of his pets. Cherenkov lived to be eighty-five and passed away early in 1990.

But still the legacy of his work lives on. The Cherenkov effect is invaluable in today's modern nuclear and particle physics - it is used in the study of subatomic particles and cosmic rays as well as being very important in managing nuclear power stations. Because of him, we are able to explain many aspects of nuclear physics and our knowledge of Cherenkov radiation has recently been put to great use in the Super-Kamiokande facility in Kamioka, Japan. Because it can be used to detect individual particles, Cherenkov radiation is an ideal method to study weakly interacting neutrinos which usually pass though most matter. Even though it takes a light year of lead to stop half the neutrinos from the sun, this large detector, equipped with an acre of photocathode, is able to provide a lot of information about the mysterious leptons.

Nowadays, Cherenkov is little known and few scientists are aware of the man behind the name which is now so common in physics. Without doubt, Cherenkov was one of the greatest Russian physicists of the twentieth century.


Sources:
  • http://www.nobel.se/physics/laureates/1958/cerenkov-bio.html
  • http://www.britannica.com/nobel/micro/120_32.html
  • http://www.bartleby.com/65/ch/CherenkPA.html
  • http://www.norskfysikk.no/nfs/epsbiografer/CHEREN~1.PDF
  • Microsoft Encarta '99

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