CHERENKOV, PAVEL ALEKSEYEVICH

(b. Novaya Chigla, Voronezh province, Russia, 28 July 1904;

d. Moscow, USSR, 6 January 1990), experimental physics, optics, nuclear and elementary particle physics, accelerators, cosmic rays.

The Soviet physicist Cherenkov (sometimes also spelled Čerenkov or Tscherenkow) is known primarily for the 1934 discovery of the Cherenkov effect, a distinctive type of electromagnetic radiation emitted when charged particles travel faster than light would travel through a particular medium. The effect serves as the basis for Cherenkov counters, commonly used detectors of high-energy particles in elementary particle accelerators. For his role in the discovery, Cherenkov shared the 1958 Nobel Prize in Physics. His scientific career can also be seen as a striking illustration of Communist “affirmative action” policies in the Soviet Union—reverse privileges in education that strongly encouraged the promotion of representatives of the lower classes into the ranks of the scientific profession. These educational opportunities, not ordinarily available to the son of a peasant, enabled Cherenkov to embark on what would become an illustrious career in science.

From Peasant Laborer to Scientist . Cherenkov’s parents, Aleksey and Mariya, were peasants in the village of Novaya Chigla in southern Russia. His mother died and his father remarried when Pavel was two years old. With eight siblings from his father’s two marriages, Cherenkov grew up in poverty and had to start work as a manual laborer at the early age of thirteen, having completed just two years of elementary schooling. After the Bolshevik Revolution and the civil war that followed, a new Soviet secondary school opened in the village in 1920, which allowed Cherenkov to resume his education while continuing to earn a living through occasional work at a grocery store. The door to further schooling beyond the incomplete secondary level was opened for him by the revolutionary Bolshevik government’s radical reform of the entire educational system.

In their attempts to democratize access to higher education, the Bolsheviks were not satisfied with simply removing formal barriers of gender, ethnicity, and religion. They also tried to compensate for economic disadvantages by preferential treatment of students from worker and peasant backgrounds. The latter measures included free tuition, stipends for low-income students, class quotas, preparatory courses (rabfaks, or workers’ faculties, for those who had not finished secondary school), and last but not least, softening of the formal criteria required for advancement from one educational level to another. It became possible, in principle, to enroll in a university without a formal high school diploma, start a graduate program without fully completing undergraduate education, and be hired as a professor without a PhD degree or equivalent. Many Soviet scientists of the Cherenkov generation skipped one or another of these formal steps while embarking on their academic careers. Cherenkov took advantage of promotional opportunities available for lower-class students and in 1924, apparently without completing secondary education, enrolled in the Pedagogical Department of Voronezh State University.

Graduation in 1928 enabled Cherenkov to become a teacher of physics and mathematics at an evening school for workers in Kozlov (now Michurinsk), a small town in Tambov province. But 1928 was also the year when a new, more radical cultural revolution broke out in the Soviet Union, and attempts intensified to draw more women, minorities, and lower-class students not only into colleges but also into the ranks of scientific researchers and to rapidly train a new and massive generation of scientists for the ongoing crash industrialization effort. “Affirmative action” measures were applied to the greatly expanded aspirantura, or graduate studies programs, at universities and institutes for research. Soviet educational policies at the time did not recognize academic degrees such as the PhD. The job of a graduate student, or aspirant, was not to write a thesis but instead to learn the trade of scientific research while working as a junior apprentice alongside established scientists. In 1930 Cherenkov was admitted as aspirant to one such program at the Physico- Mathematical Institute of the USSR Academy of Sciences in Leningrad.

Earlier that year Cherenkov married Marya Putintseva, daughter of a professor of Russian literature from Voronezh. Other events, typical of the revolutionary era, also affected his life. His father-in-law was dismissed as a “bourgeois professor” and sent to work in a labor camp, while Cherenkov’s own father was deprived of property and exiled as a kulak. Kulak literally means “a wealthy peasant,” but the label did not have to correspond to the true state of affairs—it received a very liberal and broad application and could also be applied to poorer peasants during the chaotic and violent collectivization of Soviet agriculture around 1930. In theory, associations with persecuted relatives could have undermined some of the advantages Cherenkov received due to his low class origin, but there is no evidence that he was affected in this way, at least not significantly.

Cherenkov Radiation . Living the life of a young up-and-coming physicist in the revolutionary Soviet society meant also to be particularly receptive to the on-going radical revolution in fundamental laws of physics. The news of the 1919 triumphant confirmation of Einstein’s relativity theory was among the first to arrive in the country at the end of the Civil War and created a wave of public astonishment and obsession, made perhaps even stronger than in the rest of Europe by metaphorical associations with the concurrent political revolution. The year 1923 brought yet another radical crack in the established foundation of physics: the Compton Effect—electrons kicked out of their atoms by incoming x-rays—confirmed an even more daring prediction by Einstein that electromagnetic radiation behaves like an assembly of particle-like light quanta. In 1925 quantum mechanics revealed the new strange laws of microscopic particles at the atomic levels that contradicted not only the classical Newtonian mechanics but also seemingly the laws of reason and causality. Quantum mechanics’ relativistic generalization in the 1928 Dirac equation explained the Compton Effect and the electron spin, but also led to prediction of the yet unknown anti-electron and antimatter in general, which were seen as wild speculations even by the majority of otherwise radically inclined theorists. Shockingly, in 1932 anti-electron turned out in cosmic rays, and the discovery of the neutron—another previously unknown elementary particle—that same year shifted physicist’s interests to the emerging field of nuclear physics.

Although the news about the most recent breakthroughs often came to them with a certain delay, the young Soviet physicists jumped at every opportunity to contribute to the great developments in physics, often ahead of their more senior peers. In 1922 Alexander Friedmann revealed for the first time that Einstein’s relativity, when applied to cosmology, means that the Universe is not stable, but expands in what came to be called the Big Bang explosion. Upon receiving the first news about quantum mechanics, theorists Yakov Frenkel, Vladimir Fock, Igor Tamm, and Lev Landau immediately joined the new revolutionary theory and made important contributions to it. In 1928 Georgiy (George) Gamow pioneered the application of quantum mechanics to atomic nucleus with his theory of alpha-decay.

Despite its elite affiliation, the Physico-Mathematical Institute of the USSR Academy of Sciences, where Cherenkov began his aspirant studies, had been a very small and almost nominal institution. The main events in Soviet physics occurred elsewhere, in much larger and better funded universities and research institutes associated with industry. In 1932, however, the situation was about to change with the appointment of Sergei Ivanovich Vavilov (1891–1951) as the director of the institute’s Department of Physics.

A physics professor from Moscow State University and a newly elected member of the USSR Academy of Sciences, Vavilov was an expert in experimental research on luminescence—a kind of quantum interaction between light and matter typically observed as a property of some substances to glow long after being exposed to light or some other excitation (the phenomenon is used in luminescent screens). In the course of his research, Vavilov became a firm believer in Einstein’s light quanta and made their study his lifelong commitment in experiment. As director, he harbored broader ambitions and wanted to develop in his institute the most exciting and cutting-edge fields, in particular the study of the atomic nucleus. His appointment at the Physico-Mathematical Institute was connected with the plans by the academy to greatly expand and separate the institute into two independent ones, for physics and mathematics respectively.

The institute’s only specialist in nuclear physics, Gamow (1904–1968), decided to stay in the West and did not return after attending the 1933 Solvay conference in Brussels. In a bold administrative move, and also in the spirit of revolutionary times, Vavilov entrusted several aspirants to start nuclear research on their own, while appointing himself the pro forma head of the nuclear laboratory to ensure administrative protection. Cherenkov received a more precise assignment, a fusion between Vavilov’s own research program in luminescence and the new interest in nuclear physics. He was asked to study what happens to luminescent solutions of uranium salts excited not by ordinary light, as in the common method, but by much more energetic gamma rays from a radioactive source.

Cherenkov Radiation . Earlier in the 1920s, when economic conditions were extremely poor, Vavilov with coworkers developed a clever observational method for registering extremely faint luminescence—almost at the threshold of single light quantum—by using an experimenter’s naked eye adapted to greater sensitivity by several hours spent in complete darkness. Cherenkov mastered this technique and applied it in his study of luminescence induced by gamma rays in uranium salt solutions. An extremely diligent and meticulous observer, he noticed that gamma rays also produced a faint background blue glow in ordinarily nonluminiscent pure solvents, such as sulfuric acid or water. Vavilov’s expertise helped him to recognize the effect as a new phenomenon, different from luminescence. The institute’s unpublished annual report for 1933 first mentioned the discovery of a new radiation in pure liquids by aspirant Cherenkov working under Vavilov’s supervision. The publication came in 1934 as two back-to-back papers in Doklady (Reports) of the USSR Academy of Sciences: one by Cherenkov on the experimental discovery of the “blue light,” and another by Vavilov with theoretical discussion of the observed results. Vavilov attributed the radiation to Bremsstrahlung, or “stopping radiation,” emitted by rapidly decelerating Compton electrons that had been dislodged from their atoms by incident gamma rays.

Also in 1934 the Soviet government enacted a major buildup of the academy’s research infrastructure, including the reorganization of its Physico-Mathematical Institute into separate institutes for physics and mathematics and their relocation, along with a number of other institutes, from Leningrad to Moscow. Vavilov’s numerous

administrative chores, including the task of organizing the Physical Institute of the Academy of Sciences (FIAN)— eventually the nation’s main research center in fundamental physics—no longer allowed him to engage actively in hands-on research. Cherenkov, who also moved to Moscow, continued his studies of the new effect despite the strong skepticism he encountered. In the preceding decades, physicists had witnessed several embarrassing cases of widely publicized “discoveries” of spurious rays. Some influential Soviet colleagues and foreign visitors also mocked FIAN’s search for an almost non-visible radiation in complete darkness as “spiritism” and “ghost-hunting.” With Vavilov’s backing, Cherenkov defiantly persevered. His English-language article reporting the chief results of the investigation was rejected by Nature in 1937 but published later that year by the Physical Review.

Soviet educational policies started returning to more traditional forms around that time, with diminishing attention to class criteria and a renewed emphasis on regularity and quality of training. In particular, from 1934 on, graduate students were once again required to defend a thesis for the kandidat nauk, the Russian equivalent of the PhD degree. After receiving his degree in 1935 Cherenkov remained at FIAN as a research associate and in 1940 defended his second thesis for a higher academic degree, the doktor nauk. In the course of these studies he discovered further characteristics of the new radiation, including its specific anisotropy—the radiation propagated at a very particular angle relative to the direction of the incident gamma ray. This feature helped the FIAN theoreticians Igor Yevgenyevich Tamm (1895–1971) and Ilya Mikhailovich Frank (1908–1991) explain the true cause of the phenomenon in 1937.

Cherenkov (or Vavilov-Cherenkov) radiation is produced when Compton electrons, kicked by high-energy gamma quanta, move through a substance relatively uniformly (rather than decelerating rapidly, as Vavilov had thought) and faster than light would travel in the substance. It is the optical analog of shock waves in acoustics, the sound produced in the air by an ultrasonic projectile or jet, which also propagates at a certain angle relative to the direction of the projectile’s movement (see Figure 1).

As was realized post-factum, the phenomenon is rather general and must have been seen by many physicists, starting with Pierre and Marie Curie, who had worked with gamma radiation prior to Cherenkov but either did not attribute special importance to the glow or, like Lucien Mallet (1885–1981) in 1926, considered it luminescence. Also, as early as 1904 Arnold Sommerfeld (1868–1951) had calculated waves emitted by an electron traveling faster than the speed of light, according to the turn-of-the-century electron theory, and arrived at the formulas almost identical to those derived by Tamm and Frank. Sommerfeld’s calculation seemed meaningless and was forgotten almost immediately because of Einstein’s relativity theory of 1905, which prohibits any physical object from moving faster than light. The limitation, however, refers to light in a vacuum, whereas, as explained by Tamm and Frank, objects can travel faster than the phase speed of light in a medium (always less than the speed of propagation in a vacuum) without entering into any conflict with Einstein’s postulate. So although Cherenkov radiation seemed to contradict relativity theory, it was in fact fully compatible with it.

Cherenkov radiation can be produced by any electrically charged particles, not just electrons, if they propagate through a medium with sufficient velocity. It is often seen, for example, in photos of nuclear reactors as bright blue light emanating from water. The effect proved especially useful for registering high-energy elementary particles encountered in cosmic rays or produced in particle accelerators. Detectors using this principle, Cherenkov counters, were designed and became widely used soon after World War II, and they helped physicists find new elementary particles, such as the antiproton discovered in 1955. Cherenkov also pointed out that the radiation could be used to measure the velocities of particles, an idea later realized in the so-called RICH detectors. The 1958 Nobel Prize in Physics “for the discovery and the interpretation of the Cherenkov effect” was shared between the experimentalist Cherenkov and the theoreticians Tamm and Frank. Vavilov had died in 1951 and thus could not be nominated for the Nobel Prize. In the Soviet Union the discovery of the new radiation was recognized by a Stalin Prize awarded

to Vavilov, Cherenkov, Tamm, and Frank in 1946. Generally Russian-language sources attribute partial credit for the experimental discovery to Vavilov and often prefer the term Vavilov-Cherenkov radiation.

Later Years . Cherenkov worked at FIAN for the rest of his life. He participated in FIAN’s Elbrus expedition in 1934 that established the first Soviet high-altitude cosmic ray station in the Caucasus Mountains and studied the new phenomenon of cosmic ray showers in the atmosphere. In the 1940s he constructed Wilson chamber detectors for the Pamir Mountains cosmic rays expedition and station. During World War II, as the German armies approached Moscow, FIAN was evacuated to Kazan on the Volga River, where Cherenkov worked on acoustical systems for air defense. During the patriotic upsurge of the war, many scientists joined the Communist Party, which also became less exclusive. Cherenkov became a Communist in 1944 and remained a loyal party member for the rest of his life.

From 1946 to 1958 Cherenkov assisted Vladimir Iosifovich Veksler (1907–1966) in designing new types of particle accelerators, the synchrotron (1947) and the first Soviet betatron (1948). A larger 250 MeV synchrotron started operating in FIAN in 1951, for which the main contributors, including Veksler and Cherenkov, received that year’s Stalin Prize. Most of the research done by Cherenkov during those years was classified—not necessarily for good reasons but simply because of its relation to nuclear physics at a time when the development of atomic weapons constituted one of the country’s top security priorities. Starting in 1959 Cherenkov directed his own laboratory at FIAN, which studied how photons interacted with mesons and nucleons (a series of investigations recognized by the USSR State Prize in 1977). He also helped organize and design a new acceleration laboratory with a 1.2 Gev synchrotron in Troitsk, near Moscow, in the 1970s. Between 1951 and 1977 he taught as professor at the Moscow Institute of Physical Engineers (MIFI), which educated specialists for the field of nuclear energy.

After World War II, as the use of Cherenkov’s 1934 discovery and his fame grew in the West, his reputation and standing in his own country remained very modest, overshadowed by more illustrious colleagues. Perhaps, owing to his peasant background, he was still perceived as an outsider by elitist representatives of the Soviet intelligentsia. His lifestyle was also rather unprivileged: At the time of his 1958 Nobel Prize the Cherenkov family with two children still lived in a so-called communal apartment (a flat with rooms occupied by several different families and with shared facilities), which was a typical arrangement for common urban dwellers in the overcrowded Soviet cities from the 1920s to the 1960s. They moved into a separate flat—the ultimate sign of a middle class standing in the Soviet society—only in 1962.

His election to the USSR Academy of Sciences—the most prestigious body in Soviet science—also came rather late. Cherenkov became a corresponding member of the academy in 1964 and a full member in 1970. In 1985 he was elected to the U.S. National Academy of Sciences as a foreign member. Until the end of his life he tried to avoid using the phrases “Cherenkov effect” and “Cherenkov detectors,” which had become standard terminology in physics. He was also very reluctant to allow the use of his name and fame for public relations purposes and for the promotion of scientific projects. He did, however, represent the Soviet Union internationally—in the Soviet Peace Committee, in the Soviet OSCE (Organization for Security and Co-operation in Europe) Committee, and in the Pugwash Conferences on Science and World Affairs—in a manner compatible with his membership in the Communist Party. Cherenkov remained occupied with scientific research at FIAN almost until his death at age eighty-five. His two children, a son and a daughter, also became scientists.

BIBLIOGRAPHY

WORKS BY CHERENKOV

“Vidimoe svechenie chistykh zhidkostei pod deistviem gamma-radiatsii” [Visible glow of pure liquids under gamma irradiation]. Doklady AN SSSR, t. 2 (1934): 451–457.

“Visible Radiation Produced by Electrons Moving in a Medium with Velocities Exceeding That of Light.” Physical Review 52 (1937): 378–379.

“Spatial Distribution of Visible Radiation Produced by Fast Electrons.” Comptes Rendus de l’Académie des Sciences de l’URSS 21 (1938): 319–321.

With Igor E. Tamm and Ilya. M. Frank. “Svechenie chistykh zhidkostei pod deistviem bystrykh elektronov” [The glowing of pure liquids under the influence of fast electrons]. Izvestiia AN SSSR, Seriia Fizicheskaia 1–2 (1938): 29–31.

“Radiation of Particles Moving at a Velocity Exceeding that of Light, and Some of the Possibilities for Their Use in Experimental Physics.” Nobel Lecture, 11 December 1958. Available from http://nobelprize.org

OTHER WORKS

Afanasiev, Georgy N. Vavilov-Cherenkov and Synchrotron Radiation: Foundations and Applications. Dordrecht and Boston: Kluwer, 2004.

Bolotovsky, Boris M. Svechenie Vavilova-Cherenkova. Moscow: Nauka, 1964.

Cherenkov, Pavel Alekseevich (1904–1990): Materialy k bibliografii uchenykh. Moscow: Nauka, 1997. Contains a full bibliography plus a biographical essay and a list of secondary literature.

Frank, Ilya M. “A Conceptual History of the Vavilov-Cherenkov Radiation.” Soviet Physics Uspekhi 27 (1984): 385–395.

Gorbunov, Andrei N., and E. P. Cherenkova, eds. Pavel Alekseevich Cherenkov: Chelovek i otkrytie. Moscow: Nauka, 1999. Recollections about Cherenkov.

“Izluchenie Vavilova-Cherenkova: 50 let otkrytiia” [Recollections and materials published on the occasion of the discovery’s 50th anniversary]. Priroda 10 (1984): 74–86.

Jelley, John V. Čerenkov Radiation and Its Applications. New York: Pergamon Press, 1958.

Nobel Lectures, Including Presentation Speeches and Laureates’ Biographies: Physics: 1942–1962. Amsterdam: Elsevier, 1964.

Tamm, Igor E., and Ilya M. Frank. “Coherent Visible Radiation of Fast Electrons Passing through Matter.” Comptes Rendus de l’Académie des Sciences de l’URSS 14 (1937): 109–114.

Vavilov, Sergi I. “O vozmozhnykh prichinakh sinego gamma-svecheniia zhidkostei” [On the possible causes of blue gamma-glow of liquids]. Doklady AN SSSR, t. 2 (1934): 457–461.

Alexei Kojevnikov