Pavel A. Cherenkov was a physicist whose work helped build the foundation for modern nuclear physics. He is best known for his discovery in 1934 of Cherenkov radiation, which he first noticed as a faint blue light in water that was absorbing radiation. In 1937, Ilya Frank and Igor Tamm established that this light was caused by radioactive particles that move faster through water than the speed of light does, and for their work the three men shared the 1958 Nobel Prize in physics. Cherenkov radiation has been used to design and build a sensitive tool for measuring high-energy particles, known as the Cherenkov counter, which has important uses in experimental physics.
Cherenkov was born to peasant parents in the Voronezh region of Russia on July 28, 1904. He went to Voronezh University, where he graduated in 1928 with a degree in physics and mathematics. From there he taught in a high school, then moved to Leningrad in 1930, where he began graduate studies in physics. He studied at the Institute of Physics and Mathematics under Sergei I. Vavilov ; soon after he began his affiliation there, the institute was transferred to Moscow and renamed the P. N. Lebedev Institute of Physics. Cherenkov would remain there for the rest of his life.
It had long been observed that irradiated liquids produced certain kinds of luminescence, and Cherenkov was still a graduate student when he began researching the causes of this phenomenon. He was quickly able to establish that in most cases this light was the result of atomic and molecular activity in the fluids themselves, a phenomenon known as fluorescence. But he discovered that high-energy gamma rays, when passed through liquid, produced a light that was not so easily explained. It was Cherenkov's assumption that this light was caused by the radiation itself and not the liquid, and he set out to prove this theory in a series of taxing experiments. He used little more than a weak source of gamma radiation and a variety of liquids and measured the light with only his eyes. In an obituary for Cherenkov in Physics Today, Alexander E. Chudakov writes: "I imagine a young and enthusiastic fellow who for several years started his working day by spending an hour in a totally dark room to prepare his eyes to observe faint light, and who scrupulously repeated the observations again and again, varying the liquids and the geometry of the experiment, trying to find the clue to the nature of the puzzling radiation that now bears his name."
Cherenkov was first able to show that the gamma rays produced the same light in a number of different liquids, thus establishing that the light was not affected by the medium through which the high-energy particles passed. After verifying that the effect was not the result of the medium, he proved that this light lacked a characteristic which fluorescence had: The light was not dimmed or "quenched" by the introduction of additives. The light produced by gamma rays was also not polarized in the same way fluorescent light was, and the results of this last experiment established definitively that what Cherenkov had been observing was a new and distinct phenomenon.
Cherenkov was only able to explain the radiation he had discovered with the theoretical assistance of Ilya Frank and Igor Tamm. Frank and Tamm developed a mathematical theory that explained the phenomenon as the result of high-energy particles moving faster than the speed of light within the liquid. Light travels more slowly in a transparent medium than it does in a vacuum, and gamma rays have enough energy to pass it, creating a visible cone or wave of light, analogous to a supersonic boom when an airplane exceeds the speed of sound. Cherenkov, Frank, and Tamm were able to prove these theories after an additional series of experiments. Though the majority of their scientific colleagues initially showed little interest in their work, the growth of nuclear physics during and after World War II made the importance of their discovery increasingly apparent, and they were awarded the State Prize from the Soviet Government in 1946 and the Nobel Prize in 1958.
Cherenkov was always modest about his role in the identification and explanation of Cherenkov radiation. As Chudakov notes in his obituary: "Limiting his own contribution to the period of the 1930s, Cherenkov always emphasized the crucial role of Sergei Vavilov, Frank and Tamm in the discovery." Tamm went on to work with Andrei Sakharov and others on the development of the first Soviet hydrogen bomb in 1953; he also helped formulate the theoretical basis for controlled thermonuclear fusion--the use of thermonuclear power for generating electricity. The Cherenkov effect has been used by others to develop counters to identify the velocity of high-energy particles. Cherenkov detectors are used in instruments sent up in satellites or balloons to study primary cosmic rays, exploring the effect of some particles that reach Earth's atmosphere from further out in the galaxy. The detectors have also been used in studies of Cherenkov radiation under the Antarctic ice cap.
Cherenkov earned his doctorate in 1940. He became a professor of experimental physics in 1959, then a senior scientific officer at the institute. Cherenkov was elected a member of the U.S.S.R. Academy of Sciences in 1970, and he headed the department of high-energy physics of the Lebedev Institute until he died January 6, 1990.
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