Presentation Speech by Professor K. Siegbahn, member of the Swedish Academy of Sciences

Your Majesties, Your Royal Highnesses, Ladies and Gentlemen.

The discovery of the phenomenon now known as the Cerenkov effect, for which the Nobel Prize is today being awarded, is an interesting example of how a relatively simple physical observation, if followed through in the right way, can lead to important findings and open up new paths to research. Here the scientist must be endowed with that unique intuitive experimental disposition which is the true incentive in scientific progress.

Among the students at the Lebedev Institute in Moscow in the early thirties was Pavel Cerenkov. The task assigned to him by his teacher, Professor Vavilov, for his thesis work, was to study what happens when the radiation from a radium source penetrates into and is absorbed in different fluids. The same problem had no doubt concerned many scientists before this young graduate student and, to be sure, many had also observed the weak bluish glow that emanated from the liquid as the radiation penetrated it. Special mention should be made of the important observation of the Frenchman Lucien Mallet. The bluish glow had – as it seemed on good grounds – always been considered a manifestation of the well-known fluorescence phenomenon. This phenomenon has for more than half a century been used, for instance, by radiologists in X-ray fluoroscopes, where the “invisible” X-radiation is allowed to strike a fluorescent screen, which then lights up.

Cerenkov, however, was not convinced that the light phenomenon he had observed was really of the fluorescence nature. Already his first experiments indicated that his suspicions were correct. He found, for instance, that the radiation was essentially independent of the composition of the liquid. This was in disagreement with the fluorescence explanation. By observing radiation even in doubly distilled water, he eliminated the possibility of minute impurities fluorescing in the liquids.

Cerenkov made the new, unknown radiation the subject of a systematic investigation. In his work he found that the radiation was ” polarized” along the direction of the incoming radium radiation and that it was the fast secondary electrons, produced by the latter, that were the primary cause of the visible radiation. This was verified by irradiating the liquids with only the electrons from a radium source.

The investigations that Cerenkov published in the Russian periodicals between 1934 and 1937 essentially established the general properties of the newly discovered radiation. However, a mathematical description of the effect was still lacking. Here two of Cerenkov’s colleagues in Moscow enter into the picture. How can a fast electron on passing through a liquid give rise to radiation with the properties observed by Cerenkov? In the beginning, the phenomenon seemed difficult to understand but in the work of Frank and Tamm (1937) an explanation was given that besides being both simple and clear, also satisfied the requirements for mathematical stringency.

The phenomenon can be compared to the bow wave of a vessel that moves through the water with a velocity exceeding that of the waves. This is, incidentally, a simple experiment that anybody can make. First one drops an object into a bowl of water and observes the propagation velocity of the circular wave front. Then one moves the object along the water surface very slowly to begin with but gradually increasing the velocity. When the latter exceeds the wave velocity previously observed, a bow wave is formed that extends obliquely backwards in the well-known way.

The wave velocity in the water surface is of course low and therefore it is easy to produce the bow wave in this case. In air, an analogous phenomenon occurs when a jet plane penetrates the so-called sound barrier at about 1,000 km/h, i.e. when the jet velocity exceeds the propagation velocity of the sound waves. This is accompanied by a bang.

The condition that is required to form the corresponding Cerenkov bow wave of ordinary light when a charged particle, e.g. an electron, traverses a medium is, analogously, that the particle moves with a velocity greater than that of light in the medium. At first, one might think this is impossible, for according to Einstein‘s famous theory of relativity the velocity of light is the highest possible velocity. This is in itself correct, but the velocity referred to in Einstein’s theory is the velocity of light in empty space or vacuum. In a medium, e.g. a liquid or a transparent solid, the velocity of light is lower than in vacuum and furthermore varies with the wavelength. This fact is well-known from school experiments on the refraction of light in a prism. In such a medium, it is thus entirely possible for an ultra-fast electron, emitted from a radioactive source, to move with a velocity greater than that of light in the medium. In that case, a Cerenkov bow wave is formed and the liquid glows with the bright blue magic shine from the hectic race of the electrons with the out-distanced light.

A beautiful sight is seen on looking down into a uranium reactor containing water; a so-called swimming-pool reactor. The whole core is aglow with the blue Cerenkov light and in this light one can even photograph the inside of the reactor.

In the successful studies of new elementary particles undertaken during the last few years, e.g. the discovery in 1955 of the antiproton – the negative hydrogen nucleus – the Cerenkov effect has played a decisive part. An instrument based on this effect has been designed that is capable of registering the passage of single particles. Only under the condition that the particle has a sufficiently high velocity will it be registered by the instrument which, at the same time, can measure the velocity. For the velocity determination, which can be made with considerable accuracy, one makes use of the fact that the angle of the bow wave depends on the particle velocity. The faster the particle moves, the smaller will be the angle between them. This is easily understood from the example with the vessel in the water. This new type of radiation detector has been named after Cerenkov and is now among the most important instruments at the big atomic laboratories, where elementary particles are accelerated to extremely high velocities.

The discovery of Cerenkov, Frank, and Tamm, about twenty years ago has thus, during the last few years, found an application of decisive importance in the study of the basic structure and nature of matter.

Professor Cerenkov, Professor Frank, Academician Tamm. The Swedish Royal Academy of Sciences has awarded to you the Nobel Prize for Physics for your discovery and explanation of the effect which now bears the name of one of you. This discovery not only throws light upon a hitherto-unknown physical phenomenon, but also provides a new and effective tool for the study of the atom. I congratulate you most heartily on behalf of the Academy, and ask you to accept the prize from the hands of His Majesty the King.