Pavel Alekseyevich Cherenkov undertook a detailed study of the properties of the faint blue light emitted by charged particles moving through a material faster than the speed of light in that material.
In the early twentieth century, many scientists noticed that transparent materials placed near intense radioactive sources emitted a very faint blue light. Pierre Curie, codiscoverer of the radioactive element radium, was said to have fascinated dinner guests by producing from his pocket a tube of radium salt that illuminated the dinner table with a mysterious blue glow visible in the fading evening light. At the time, however, scientists were interested in the isolation and identification of new chemical elements, and little or no effort went to understanding the origin of the bluish glow. The first systematic attempt to understand this emission of blue light was made by Lucien Mallet
Pavel Alekseyevich Cherenkov.
In 1934, Pavel Alekseyevich Cherenkov was working at the Institute of Physics of the Soviet Academy of Science in Moscow, where he undertook an exhaustive series of experiments to characterize the properties of the blue emission. He had been studying the problem of fluorescent emission from materials exposed to radiation. Apparently unaware of Mallet’s earlier efforts, Cherenkov noticed the very weak emission of visible light from liquids exposed to gamma rays, high-energy light invisible to the human eye. In his first experiment to determine the nature of this emission, Cherenkov inserted a vial containing about 100 milligrams of radium into a wooden block. The wood absorbed all the radiation from the decaying radium except the gamma rays. A container of liquid was then placed above the radioactive source and an optical microscope system was used to observe the intensity and color of the light emitted by the liquid. Sixteen different pure liquids including distilled water, paraffin, and various alcohols were examined. After studying the liquids, Cherenkov concluded that the intensity of the emission varied little when one liquid was substituted for another, that the emission was mainly in the blue and violet regions of the color spectrum, and that the color of the emission did not change significantly when a different liquid was substituted. These properties of the emission were quite different from what would be expected from the fluorescence emission process. In addition, when Cherenkov added compounds such as potassium iodide or silver nitrate (known to inhibit the fluorescence process) into the liquids, the intensity of the emitted light did not decrease.
By the early 1930’s, the mechanism for the fluorescent emission from materials bombarded with the rays from radioactive sources was well understood. Soviet scientists, who were the first to become aware of Cherenkov’s observations, took up the challenge of attempting to explain the mechanism for this new emission process. In 1934, Sergei Ivanovich Vavilov proposed the emission might be caused by the energy lost as electrons produced by the passage of the gamma rays through the liquid slowed down, a process known as bremsstrahlung. Cherenkov undertook a second series of experiments in 1936 to understand better the mechanism of the emission process. He investigated the influence of a magnetic field on the emitted light and concluded that the emission must be from electrons produced in the liquid by the gamma rays, not directly from the gamma rays themselves, but that the process was inconsistent with bremsstrahlung.
In 1937, two other Soviet physicists, Ilya Mikhailovich Frank and Igor Yevgenyevich Tamm, developed a theory to explain the emission process. Frank and Tamm recognized that the speed of light in solids, liquids, and gases is slower than in a vacuum. According to Albert Einstein’s theory of special relativity,
After applying their knowledge about electricity and magnetism, Frank and Tamm concluded that when a charged particle travels faster than the speed of light in the medium through which it is moving, it will give off light directed in a cone oriented along its direction of motion. This emission is analogous to the bow wave produced by a boat moving through water or the sonic boom produced by an airplane moving through air faster than the speed of sound in the medium. The theory predicted that the angle of the cone of emitted light would depend on the speed of the charged particle as well as the properties of the medium. Their theory also predicted the distribution of colors to be expected in the emitted light.
Cherenkov and two American physicists, George B. Collins
These early experiments all involved the examination of the Cherenkov radiation from an intense beam of particles. The emission from a single particle, however, was so faint that it could not be seen using the tools available in the 1930’s. The development of the photomultiplier tube, an extremely sensitive light detector, provided the hope that the Cherenkov radiation from a single particle passing through a liquid or solid could be detected. After several unsuccessful attempts by a number of researchers, in 1951 John V. Jelley
Although initially regarded as little more than a scientific curiosity, the distinctive Cherenkov effect has found widespread applications in the fields of particle physics, astronomy, and chemistry. In 1955, Emilio Gino Segrè and his colleagues set up an apparatus at the Bevatron particle accelerator in Berkeley, California, to search for the antiproton. They expected only a few antiprotons in a background of many other particles called pions, and their experiment was set up to allow the pions to move more rapidly than the antiprotons. To facilitate this movement, they built a Cherenkov detector filled with a particular organic liquid in which the speed of light was faster than the expected antiproton speed but slower than the pion speed. This detector provided a signal when a pion went through, but no signal was given when the antiproton passed. They also used a second detector sensitive to both types of particles, and by comparing the two outputs, they were able to identify the antiprotons. On September 21, 1955, Segrè and his colleagues obtained their first evidence for the antiproton using detectors based on Cherenkov radiation.
Cherenkov detectors have been employed in the investigation of the stability of the proton. Although the proton was once believed to be a stable particle, certain theories predict that eventually the proton will decay. Its lifetime must be very long; therefore, to see a decay, scientists would have to watch a single proton for billions and billions of years or watch a large number of protons for a shorter time. Cherenkov radiation provides the tool to undertake such an experiment. Physicists from the University of California at Irvine, the University of Michigan, and the Brookhaven National Laboratory costructed a huge pool of eight thousand tons of pure water at a Morton Thiokol salt mine in Ohio. If any single proton in one of the water molecules were to decay, the resulting fragments traveling through the water would produce a pulse of Cherenkov light to be detected by one or more of the 2,048 individual Cherenkov detectors surrounding the pool.
Another large, water-filled Cherenkov detector, the Kamiokande II, operated by Japanese physicists at a site 300 kilometers west of Tokyo (originally designed to search for proton decays) has been upgraded to allow detection of neutrinos
Cherenkov detectors are also employed by radiochemists to identify and count decaying nuclei. Because Cherenkov radiation occurs only if the charged particle is traveling faster than the speed of light in the detection material, Cherenkov detectors can be used to count rare decays in which a high energy, or fast-moving, particle is emitted in a background of many more low-energy events below the detection threshold. Cherenkov detectors have been used to determine the amount and type of radioactive material present in plant and animal tissue, environmental materials, nuclear reactor effluents, and biomedical fluids.
Astronomers have used Cherenkov detectors to understand the properties of the cosmic rays. In 1956, Frank McDonald
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Close, Frank, Michael Marten, and Christine Sutton. The Particle Explosion. New York: Oxford University Press, 1987. This well-illustrated book makes extensive use of color photographs in explaining the world of subatomic particles to general audiences. It describes how Cherenkov detectors were used in Segrè’s discovery of the antiproton, the search for proton decay, neutrino experiments, and efforts to reveal other elusive subatomic particles. -
Collins, George B., and Victor G. Reiling. “Cherenkov Radiation.” Physical Review 54 (October 1, 1938): 499-503. This article reviews the discovery of Cherenkov radiation, describes the theory of its emission, and reports the authors’ results in determining the color spectrum of the emitted light. Although a technical article, it should be understandable to students in a high school-level physical science course. -
Cropper, William H. Great Physicists: The Life and Times of Leading Physicists from Galileo to Hawking. New York: Oxford University Press, 2001. Presents portraits of the lives and accomplishments of important physicists and shows how they influenced one another with their work. Includes glossary and index. -
Jelley, John V. Cherenkov Radiation and Its Applications. Elmsford, N.Y.: Pergamon Press, 1958. This book is the definitive scientific description of Cherenkov radiation. Although intended for specialists, the first chapter, which presents an extensive historical account of Cherenkov’s contribution and the observations of this phenomenon that predate his work, is appropriate for general audiences. The extensive citations and reference list will assist readers interested in locating original papers on the topic. -
Jordan, W. H. “Radiation from a Reactor.” Scientific American (October, 1951): 54-55. Provides a nonmathematical description of how Cherenkov light is emitted and describes the early experiments in the United States to follow up on Cherenkov’s discovery. The blue glow of Cherenkov radiation surrounding an operating nuclear reactor is shown on the cover. -
Koshiba, Masa-Toshi. “Observational Neutrino Astrophysics.” Physics Today 40 (December, 1987): 38-42. This well-illustrated, nontechnical article describes the construction, operation, and results from the Japanese Kamiokande II proton decay and neutrino detector. How Cherenkov radiation from these particles is produced and detected is described, and the results for solar neutrino observations and the 1987 supernova are discussed. -
Piel, Gerard. The Age of Science: What Scientists Learned in the Twentieth Century. New York: Basic Books, 2001. An overview of the scientific achievements of the twentieth century. Includes many illustrations and index. -
Ross, H. H., and G. T. Rasmussen. “Modern Techniques and Applications in Cherenkov Counting.” In Liquid Scintillation Counting: Recent Developments, edited by Philip E. Stantey and Bruce A. Scoggins. New York: Academic Press, 1974. Although written for specialists, this chapter provides a clear description of many of the applications of Cherenkov detectors to problems in chemistry, biology, and environmental science. The authors explain how the unique characteristics of Cherenkov detectors make them the instrument of choice for particular scientific experiments. -
Segrè, Emilio. From X-Rays to Quarks: Modern Physicists and Their Discoveries. San Francisco: W. H. Freeman, 1980. Segrè was one of the few physicists who both participated directly in nuclear physics (for which he received a Nobel Prize) and wrote a number of popular accounts on the history of physics. The earlier sections of this volume cover the discoveries and theories of those who produced a coherent picture of the atom.
Elster and Geitel Study Radioactivity
Becquerel Wins the Nobel Prize for Discovering Natural Radioactivity
Einstein States His Theory of Special Relativity
Thomson Wins the Nobel Prize for Discovering the Electron
Rutherford Discovers the Proton
De Broglie Explains the Wave-Particle Duality of Light
Discovery of the Compton Effect
Heisenberg Articulates the Uncertainty Principle
Yukawa Proposes the Existence of Mesons