Vavilov – Cherenkov radiation: its discovery and application

This article has been downloaded from IOPscience. Please scroll down to see the full text article.

2009 Phys.-Usp. 52 1099

(http://iopscience.iop.org/1063-7869/52/11/R03)

Download details:

IP Address: 147.26.11.80

The article was downloaded on 08/04/2013 at 18:00

Please note that terms and conditions apply.

View the table of contents for this issue, or go to the journal homepage for more

Home Search Collections Journals About Contact us My IOPscience

Abstract. The history of the discovery of the Vavilov±Cheren-kov effect is outlined. Several important applications of theeffect are discussed.

In mid-1932, Sergei Ivanovich Vavilov, who had been electeda full member of the USSR Academy of Sciences only a shorttime previously, was appointed head of the Physics Depart-ment of the Institute for Physics andMathematics, the USSRAcademy of Sciences. At that time, the Institute for Physicsand Mathematics, was situated in Leningrad (then the nameof St. Petersburg). Before long, the Physics Department led byS I Vavilov was transformed into the Physical Institute of theAcademy of Sciences andmoved fromLeningrad toMoscow,and Vavilov became director of the Physical Institute. Thishappened later, in 1934. On Vavilov's initiative, the institutewas named after PetrNikolaevich Lebedev, a famousRussianscientist and the founder of the scientific school to whichVavilov himself belonged. That took place seventy five yearsago. Over the span of those years, the institute has gainedwide recognition by both the domestic and the internationalscientific communities. InRussia, the institute is referred to asFIAN, which is the acronym of its title in Russian: FizicheskiiInstitut Akademii Nauk (Physical Institute of theAcademy ofSciences). Abroad, it is customary to use the shortened nameLebedev Institute.

With Vavilov's accession to the institute, the scientificactivities of the Physics Department intensified. NikolaiAlekseevich Dobrotin, FIAN's elder staff member, who wasa postgraduate student of the Physics Department at thattime, recalls [1] that those changes concerned postgraduatesas well; not all postgraduate students were sufficientlygrounded in physics and mathematics. This applied primar-ily to those who had come from provincial institutes oruniversities to become postgraduate students. Several lecturecourses in mathematics and physics were organized for thepostgraduates. In his memoirs, N A Dobrotin gives animpressive list of the lecturers: S L Sobolev, A A Rukhadze,V D Kupradze, I N Vekua (in mathematics); V A Fock,Yu A Krutkov, G A Gamov (in physics). Furthermore, allpostgraduate students were given personal scientific super-visors. Vavilov assumed the scientific supervision of threepostgraduates: Nikolai AlekseevichDobrotin, Pavel Aleksee-

vich Cherenkov, and Anton Nikiforovich Sevchenko. Subse-quently, all three came to be famous physicists, and two ofthemÐCherenkov and DobrotinÐbecame FIAN staffmembers from the day of the Institute foundation.

Vavilov suggested subjects of investigation for his post-graduates. Dobrotin and Cherenkov were invited to choosefrom three subjects:

(i) the luminescence of uranyl salt solutions under thegamma-ray radiation of radium;

(ii) the investigation of the properties of neutrons;(iii) the study of isotopic effects.Dobrotin and Cherenkov selected their research subjects

by mutual consent: Cherenkov opted to take up the glow ofuranyl salts and Dobrotin neutron scattering by protons.

A notable fact is that Vavilov, an outstanding expert onoptics and luminescence, who made significant contributionsto these areas of physics, proposed three subjects to hisstudents related to the physics of the atomic nucleus, whichwas still in its infancy at that time. The neutron had beendiscovered only a year before, and the structure of the atomicnucleus was still under discussion. Few researchers foresawthe great future of nuclear physics, but Vavilov was amongthose few. Several years later, Vavilov, as Director of FIAN,ensured that investigations into the physics of the atomicnucleus would occupy a prominent place in the researchplans of the institute. As a consequence, when the SovietUnion was faced with the task of developing nuclearweapons at the end of the Second World War, FIAN playeda role in its fulfillment, and far from a minor one at that.

In 1934, the Lebedev Physical Institute moved fromLeningrad toMoscow. Vavilov's student IMFrank describedthe atmosphere at FIAN in those days in his memoirs [2]:

``In my youth, I had the good fortune of finding myself,even during the student years, in an environment in which theperception of scientific influence was especially intensive andversatile. I mean the scientific school of L I Mandel'shtam,which comprised my direct teachers and outstanding physi-cists S I Vavilov, G S Landsberg, and I E TammÐscientistsso unlike in their individuality. However, there was a featurecommon for all of them: permanent scientific communica-tion. Theoretical problems and experimental findings wereinvariably and constantly discussed, and no one consideredthese talks (which also occurred besides the scientificseminars) frequent and lengthy, to be a loss of time. At firstit seemed strange to me that these outstanding people werespending hours of their precious time, during which theycould have dove something remarkable, for talks, in whichmuch space was given to what produced no outcome orturned out to be rubbish. Nor did I understand then thatthese conversations quite often saw the emergence of newideas, long before their publication and, of course, withoutthe fear that they could be published by someone else. Inaddition, no one spared effort to promote the formation of

BM Bolotovskii P N Lebedev Physical Institute,Russian Academy of Sciences,Leninskii prosp. 53, 119991 Moscow, Russian FederationTel. (7-499) 135 85 33E-mail: bolot@td.lpi.ru

Received 6 July 2009

Uspekhi Fizicheskikh Nauk 179 (11) 1161 ± 1173 (2009)

DOI: 10.3367/UFNr.0179.200911c.1161

Translated by E N Ragozin; edited by A M Semikhatov

P N LEBEDEV PHYSICAL INSTITUTEÐ 75 YEARS PACS numbers: 01.30.Bb, 01.65.+g, 29.40.Ka, 41.60.Bq

Vavilov±Cherenkov radiation: its discovery and application

B M Bolotovskii

DOI: 10.3367/UFNe.0179.200911c.1161

Physics ±Uspekhi 52 (11) 1099 ± 1110 (2009) #2009 Uspekhi Fizicheskikh Nauk, Russian Academy of Sciences

new understanding, without a thought about coauthorship.In the moral atmosphere inherent in Mandel'shtam's school,that was only natural.''

This passage adequately depicts the remarkable creativeatmosphere that existed in FIAN in those years (to a certaindegree, it persisted in subsequent years as well) and which wasimprinted on Frank's memory. To a marked extent, thisatmosphere was to be credited to the Director of FIANS I Vavilov. I think the above passage is nevertheless inexactin one aspect. Frank wrote that Vavilov belonged toMandel'shtam's school of science. This is not so. Vavilovwas a student of P P Lazarev, one of the closest collaboratorsof P N Lebedev. When the student Vavilov was selecting thesubject of his research work, he wanted to work in Lebedev'slaboratory. At that time, Lebedev was already seriously illand the subjects for students research were given by his closestcollaborator, Lazarev, at that time a privatdocent andsubsequently academician. The subject of Vavilov's firstresearch was suggested by Lazarev, and Vavilov therebyjoined Lebedev's school of science to subsequently becomethe founder of a school of science. So, why did Frank rankVavilov with Mandel'shtam's school of science, which wasindeed an equally remarkable school of science but never-theless not the one which Vavilov had come from? The reasonmay lie with the following. Vavilov as director of the institutebelieved it was his duty to see that the conditions were highlyfavorable for the fruitful work of the staff. Science, institute,staff membersÐ these ranked highest for him, and heconsciously ranked himself second. Seeing Vavilov's highlyrespectful attitude to Mandel'shtam, the young Frank mighthave related Vavilov to Mandel'shtam's school of science.

However, ascribing members of the scientific communityto schools of science is ambiguous, especially so when twoschools are intimately interrelated. One way or the other,Frank had grounds to write what he wrote.

On obtaining the subject of investigation, Cherenkovstarted mastering his new range of effects and measurementtechniques.

Luminescence is the cold glow of a substance. Whenexhibiting luminescence, excited molecules of the substancetransit to the ground state and emit visible light. The methodsof excitation may be quite different: ultrasound, chemicalreactions, pre-irradiation by visible light, or gamma rays. Theessential feature that defines the luminescence phenomenon isthat an excited molecule radiates not immediately but residesin an excited state for some time. The lifetimes of excitedstates differ widely for various luminescent media: from daysto hundred millionths of a second. It is significant that themolecule residence time in the excited state is much longerthan the period of the light wave emitted in the luminescence.This distinguishes luminescence from other phenomenainvolving light emissionÐreflection, refraction, diffraction,and other kinds of radiation. In these phenomena, unlike inluminescence, secondary radiation terminates upon comple-tion of excitation, in a time comparable to the period of thelight wave.

Vavilov made a decisive contribution to the theory ofluminescence. In particular, the above definition of lumines-cence in terms of the emission time is credited to him.Together with his collaborators, he developed experimentaltechniques that enabled determining the main characteristicsof luminescent media, including the emission time.

The glow that Cherenkov was to observe (the lumines-cence of uranyl salt solutions under the gamma-ray radiation

of radium) was quite weak, although a relatively large (forthose times) amount of radium, tenths of a gram, was used.The glow intensity was close to the visibility threshold.Sensitive light detectors had not been made by those days,and the human eye was selected for themeasuring instrument.Shortly before Cherenkov took up his postgraduate studies,Vavilov and Brumberg developed a photometry methodbased on the visibility threshold [3], the so-called quenchingmethod. This method turned out to be well suited for theinvestigation of weak radiation, and Cherenkov used it in hismeasurements.

The apparatus on which Cherenkov carried out hismeasurements is depicted in Fig. 1. Vessel 1 contains the liquidwhose glow characteristics are measured. Located under thevessel is an ampoule with the radioactive radium sample,which excites the glow. In some measurements, the ampoulewas placed on the side of the vessel (where it was required todetermine the dependence of radiation polarization on thegamma-ray propagation direction). The light emanating fromthe liquid is reflected by silver mirror 2 onto round aperture3 with a diameter of 3 mm. Optical wedge 4 is located behindthe aperture. The optical wedge is a strip of glass that istransparent on one side, i.e., transmits all incident light. Thetransmittance of the optical wedge gradually decreases towardthe other end according to a certain law to make the wedgeopaque at the other end. The wedgemigrates along the slots inthe direction perpendicular to the drawing plane. Eyepiece 7produces a magnified aperture image on the retina.

This facility was used to measure not only the brightnessbut also the spectral composition and polarization of theglow. To determine the spectral composition, color lightfilters were used, which were slid into an additional slot, 5.A Glan prism, 6, was used to measure the polarizationdegree. The prism was removed in the measurements of thebrightness and spectral composition of the glow.

To protect the observer from radioactive emission, theinstrument was mounted on a massive lead block, 8, whichshielded the observer from the source. This shield was alsorequired because the radiation of radium excited the glow notonly of the liquid under investigation but also of thetransparent substance that fills observer's eyeball.

6

7

8

R0

3

2

1

45

Figure 1. Cherenkov's apparatus for measuring the brightness of liquid

glow by the quenching method.

1100 B M Bolotovskii Physics ±Uspekhi 52 (11)

The measurements were performed as follows. Prior toevery measurement session, the observer had to stay an houror anhour andahalf in complete darkness for his eyes to adaptto the dark. As a consequence, the eye sensitivity increased bya factor of tens of thousands. Then the measurements started.The optical wedge or, to be more precise, its transparent endwas placed between the light source and the eye. The wedgewas moved to the position whereby the eye ceased to see theglow of the liquid. In this case, the optical wedge absorbed theexcess of source brightness over the perceptibility threshold.Of course, the brightness of the light source had to exceed theperceptibility threshold (tens of visible light photons persecond). From the position of the optical wedge, it waspossible to assess the source brightness; the darker is theportion of the wedge that effects quenching, the brighter thesource.This photometry technique is basedon the fact that thevisibility threshold of a given observer is constant.

All measurements were performed in total darkness. Theobserver could not even measure the position of the opticalwedge: doing this required illuminating the wedge, butunwanted light immediately upset retinal adaptation todarkness. That is why the wedge readings were taken by anassistant. Prior to that, the observer covered his head with athick opaque cloth; then the assistant switched on the lightand took the optical wedge readings. After that, the light wasturned off and measurements were resumed. To avoid visualfatigue, individual measurements were separated by 3±5 minbreaks. The overall duration of the measurements did notexceed 2±2.5 h per day; otherwise, eye strain and consequen-tial errors emerged.

Cherenkov quickly mastered the procedure of measure-ments and performed them thoroughly and with the max-imum accuracy attainable under the arduous conditionsdescribed above. None of his findings were later proved to befallacious. In his experiments, he was assisted at one time byN L GrigorovÐ then a laboratory technician and subse-quently a professor atMoscow State University and a famouslecturer in high-energy physics; by M N Alentsev, who wasVavilov's collaborator for many years and a remarkableperson in many respects, both as a physicist and as a person;lastly, byFrank, Vavilov's student and (despite his young age)a mature scientist by that time. Frank's participation in thecapacity of assistant in Cherenkov's experiments meant thathe knew the state of affairs quite well. Also noteworthy is thefact that in those years, Cherenkov and Frank shared anapartment and hence could discuss the scientific problems ofinterest during their off hours as well. The communalapartment they lived in was unique as regards the set of itsdwellers. There were four rooms in the apartment. One roomwas occupied by the family of the future Nobel LaureateCherenkov and another room by the family of the futureNobel Laureate Frank. The third room was occupied by thefuture honorary member of the Royal Danish Academy ofSciences L V Groshev. The fourth room was occupied by aperson who was not on the staff of FIAN. In a way, theapartment was no less remarkable than the apartment onSadovaya Street described by M Bulgakov in his novel TheMaster andMargarita.

Being Cherenkov's scientific supervisor, Vavilov partici-pated in these measurements from the very beginning. Duringthe first days of the work, he familiarized his postgraduatestudent with the technique of measurements performed withlow-intensity light sources. Later on, as the results were beingaccumulated, he would cross-check them. Once or twice a

week, Vavilov performed, so to say, a check experiment. Priorto the measurement, he would, as required, sit an hour and ahalf in complete darkness. His students and collaboratorsused this time to discuss the daily state of affairs, to listen tothe opinion of their leader, and to lay down the program oftheir further work. After that, the `outsiders' were drivenaway from the room and Vavilov started his measurements.

Cherenkov investigated the glow of uranyl salt solutionsunder gamma-ray irradiation. Once, in the autumn of 1933, itso happened that glass 1 (see Fig. 1) in his apparatus was filledwith a pure solvent, sulfuric acid [1]. And it turned out thatsulfuric acid glowed under gamma-ray irradiation, the glowintensity being of the same order of magnitude as the glow ofthe uranyl salt solution in the same sulfuric acid.

Cherenkov was confused by this result, because theexistence of a backgroundÐ the glow of pure solventsÐsubstantially hinderedmeasuring the principal effect, the glowof solutions, which was the subject of his thesis research [4].

However, on learning that sulfuric acid glowed undergamma-ray irradiation, Vavilov became interested in this factand suggested that Cherenkov investigate other solvents. Itturned out that all other pure solvents, water in particular,glowed when irradiated by gamma rays, and the glow of puresolvents was not negligible in comparison with the glow ofsolutions. The glow brightness turned out to be about thesame for liquids of quite different chemical compositions. Itwas believed at that time that pure solvents should not glowunder gamma-ray irradiation and that the glow, if never-theless observed, was due to impurities, contaminants. But itturned out that it was precisely the pure solvents that glowed.Cherenkov distilled ordinary tap water sequentially thriceand measured the gamma-ray-induced glow intensity eachtime after the next distillation. The glow intensity remainedalmost invariable.

Then Vavilov proposed that the glow of pure liquids bemeasured using standard techniques elaborated in hislaboratory for the investigation of luminescence. These wereexperiments for luminescence quenching.

Because a molecule takes some time to transit from anexcited state to the normal state with the emission of light, it ispossible to act on excited molecules during this time such thatthey transit to the normal state without radiating light. In thiscase, the stored energy is released not in the form of radiationbut in a different way, for instance, it is transferred from theexcited molecule to a molecule of a different sort, which doesnot emit light. Luminescence quenching occurs. The quench-ing may be achieved in several ways. For instance, specialsubstancesÐquenchersÐmay be added to the solution ofluminescent substances. Silver nitrate, potassium iodide, andnitrobenzene are active luminescence quenchers. In thecollision with a quencher molecule, the excited molecule ofsubstance transfers its energy to the quencher and transits tothe normal state without radiating light. Luminescence maybe weakened in a different way, for instance, by heating theglowing solution. On heating, the kinetic energy of themolecules increases, their mobility increases, and theyexperience more frequent collisions with each other; in thecollision, the molecule that resides in the excited statetransfers the excitation energy to a `foreign' molecule.

Cherenkov commenced experiments involving the appli-cation of the two above techniques (the addition of quenchersand heating) to glowing pure liquids.

Both of these quenching techniques are based on the factthat the excited state of the molecules of a glowing substance

November 2009 VavilovëCherenkov radiation: its discovery and application 1101

has a finite lifetime. For luminescence quenching to occur,this time must allow an excited molecule to collide once orseveral times with quencher molecules or with `foreign'molecules. By varying the temperature of the solution of theglowing substance or the density of quenchers, it is possible todetermine the most important characteristic of the glowingsubstance, the lifetime of the excited state. Measurements ofthe glow brightness in relation to the quencher densityshowed that the glow brightness hardly changed when thequencher density changed by a factor of several hundred. Wenote for comparison that increasing the quencher density by afactor of 10±30 in luminescence suffices to decrease the glowbrightness several fold. It turned out that heating the liquidsalso left the glow brightness practically unaffected.

Cherenkov also determined the polarization of the glowand its spectrum. Within the limits of the capabilities of theexperimental apparatus, it was determined that the electricvector was primarily directed parallel to the gamma-raybeam. Measurements of the spectral composition showedthat the glow brightness peaked in the blue part of thespectrum. Were the observer able to discern the brightnesscolor, it would have appeared bluish. But the human eye doesnot discern colors near the visibility threshold (``when thecandles are out all cats are grey''). The blue color of the novelglow was determined with the aid of color filters.

The first reports on the discovery were sent to the journalDoklady Akademii Nauk (DAN) SSSR in late May 1934 andwere published several months later. One of the papers waswritten by Cherenkov [5] and the other by his supervisorVavilov [6]. These were two brief papers: DAN publishespapers no longer than four pages in journal format. Thesetwo papers were actually two parts of one investigation,which resulted in the discovery of a new, previouslyunknown effect, a special kind of radiation later named forits discoverers, the Vavilov±Cherenkov radiation. Outlinedin the paper by Cherenkov were the results of experimentsinvolving the addition of luminescence quenchers, theheating of glowing liquids, and the findings of experimentsstaged to measure the properties of the new glow: thebrightness, the polarization, and the spectral composition.Vavilov's paper, ``On the possible causes of the blue gamma-glow in liquids,'' followed immediately after Cherenkov'spaper. Proceeding from the experiments conducted, Vavilovmade an assertion that the observed blue glow ``cannot beany kind of luminescence, for which a finite lifetime ofexcitation is an immanent feature.''

In that paper, Vavilov next expressed his view on thenature of the blue glow. He took into account that hardgamma rays knock electrons out of the atoms of liquids.When moving through the liquid, these electrons produce theradiation that was observed in the measurements. At thattime, only one kind of radiation by electronsÐbremsstrah-lungÐwas well known. That is why Vavilov conjectured thatthe observed blue glow was the bremsstrahlung of electronsthat were knocked out of atoms by the gamma rays of radium.

As it became clear later, the blue glow was not electronbremsstrahlung: it had a different nature. As for Vavilov'sassertion that the radiation was caused by electrons, it provedto be quite correct and determined the course of furtherinvestigations. Vavilov himself, although he had madeassumptions about the nature of the glow, did not regardthese considerations as being final. He would discuss the stateof affairs with his colleagues, these discussions taking placeboth at seminars and in daily conversations. These discus-

sions served the purpose of outlining future experimentsaimed at elucidating the nature of the glow.

The following expression by Vavilov is well known:``There are observations and there are experiments.'' Anobservation describes the face of a phenomenon withoutrevealing its nature. An experiment is staged with preciselythe aim of understanding the nature of the regularitiesobserved.

That the source of radiation was precisely due to electronswas confirmed as follows: the source of gamma-ray radiationwas replaced with a source of beta particles (electrons), aradioactive radium source in a thin-wall glass ampoule. Thissource excited a glowwith the same characteristics as the glowexcited by gamma rays.

During the discussions of the new phenomenon, anexperiment was proposed that played a crucial role in theexplanation of the new glow. Specifically, it was suggestedthat the vessel with the liquid under irradiation should beplaced in a magnetic field. Now it is hard to tell who was thefirst to propose this experiment. Different people ascribe thissuggestion to different physicists. Mentioned in this connec-tion areVVAntonov-Romanovskii [7],MALeontovich, andI M Frank. It is likely that several people at about the sametime came up with the idea of placing the apparatus in amagnetic field. The primary purpose of this experiment was toelucidate the relationship between the radiation polarizationand the direction of electron motion through the liquid. Themagnetic field deflects moving electrons. If the observedradiation is indeed emitted by electrons, variations of thedirection of motion should be attended with changes in theradiation polarization. It turned out that the radiationpolarization did change in a magnetic field. This resultbecame another confirmation that the electrons movingthrough the liquid are the source of the radiation.

The experiments with the magnetic field yielded one moresignificant new result. It turned out that the magnetic fieldchanged not only the radiation polarization but also the glowbrightness. Hence, it followed that the radiation was aniso-tropic. The angular radiation distribution rotated uponrotation of the electron beam induced by the magnetic field.The glow brightness measured by the observer then eitherincreased or decreased, depending on the direction of themagnetic field. As a rule, the intensity changes induced by themagnetic field were significant, which was evidence of apronounced radiation directivity.

The radiation directivity turned out to be the key factorthat enabled constructing the theory of the phenomenon.Frank reminisced [2]: when he told I E Tamm that theradiation was directional, Tamm immediately made aremark: ``This signifies that there occurs coherent emissionover the electron path comparable to the wavelength of thelight wave.'' The directivity of radiation is related to theradiator size. When the size of the radiating domain is smallcompared with the radiated wavelength, any radiationdirectivity is ruled out. When the size of the radiating domainis large compared with the wavelength, the radiation becomesdirectional; the angular spread (the width of the angulardistribution) Dj is by the order of magnitude given by

Dj � lL; �1�

where l is the radiation wavelength and L is the linear size ofthe radiating domain. The above magnitude of Dj essen-tially defines the diffraction-limited radiation divergence.

1102 B M Bolotovskii Physics ±Uspekhi 52 (11)

The greater the dimensions of the radiating domain (incomparison with the wavelength), the smaller the angulardivergence.

Tamm's remark implied yet another significant feature ofthe radiation observed. The radiation could not stem from theinteraction of a moving electron with an individual atom ofthe medium. The radiation directivity implies that theradiating domain size exceeds the wavelength, and thewavelength is such that a large number of atoms (from ahundred to a thousand) fit into the corresponding distance.

Tamm's remark received the full attention of Frank, whoused a simple model to study the composition of wavesemitted by a moving source from each point in its path. Heused the illustrative formulation of the Huygens principle inthe form it appeared in numerous courses of physical optics(i.e., in almost the same form as in C Huygens' book Treatiseon Light, which had appeared two and a half centuries prior tothe events under description). The simple picture obtained byFrank immediately supplied a qualitative explanation of theangular directivity of the new glow.

Figure 2 shows the field pattern for a moving source,obtained by directly applying the Huygens principle.

A charged particle moves from left to right with a constantspeed v along straight line O1O4. At the moment depicted inthe drawing, the particle is at point O4. Spherical wavesemanate from the points the particle had passed. Thepropagation speed of these waves is equal to the phase speedof light c=n, where c is the speed of light in the vacuum and n isthe refractive index of the medium. We assume that thecharged particle travels the distance O1O4 in a unit time, i.e.,this distance is numerically equal to the charged particlespeed v. When the particle is at O4, the wave emanatingfrom O1 has traveled the distance c=n from that point. Thewaves radiated from subsequent points of the path havetraveled through shorter distances, because their propaga-tion time is shorter: it is proportional to the time the particletakes to travel from the corresponding point to O4. It is easilyverified that all these spherical waves have a commonenvelopeÐa conical surface with an apex at O4 and an axisthat coincides with the particle trajectory. In accordance withthe Huygens principle, this conical surface is the front of thewave radiated in the motion of the charged particle. Thepropagation direction of this wave coincides with the normalto the conical surface, and the angle y between the normal andthe particle velocity is defined by

cos y � c

nv; �2�

as can be easily seen from Fig. 2.

Because cos y does not exceed unity for real angles y, itfollows from formula (2) that a conical wave is producedwhen

v >c

n; �3�

i.e., when the particle speed exceeds the phase speed of light.This is also clear from Fig. 2: the particle leaves behind all thewaves it produces.

Not only did the simple consideration given above explainthe radiation directivity but it also gave a value for theradiation angle y defined by formula (2).

Frank discussed his simple treatment based on theHuygens principle with several colleagues. Among his firstaudience were N A Dobrotin, M A Markov, andM A Leontovich. Frank's treatment did not provokeobjections, but his audience did not express keen interest inhis explanation (true, many years later Leontovich would halfseriously say when the conversation turned to Frank:``[Frank] is a serious man, he should be listened attentively.At one time I did not, and missed a Nobel Prize.'') Bycontrast, Vavilov listened to Frank's considerations withinterest and approval, and anticipated their further develop-ment. Lastly, Tamm viewed Frank's ideas quite seriously andwith enthusiasm that was so characteristic of him. We recallthat it was precisely Tamm's primary remark (that theradiation is collected from a long path compared tointeratomic distances) that came to be the starting point forthe treatment used by Frank.

Tamm told Mandel'shtam about the results of hisdiscussions with Frank, and Mandel'shtam made a remarkthat cast doubt on the qualitative picture of the emissionoutlined above. He pointed out the well-known fact that acharged particle that executes a uniform rectilinear motionthrough empty space does not radiate. This followed from theMaxwell equations. In this connection, he raised the question:will the result change if the speed of light in the medium c=nreplaces the speed of light in the vacuum in the waveequation? To state it in different terms: while a chargedparticle that executes a uniform motion in the vacuum doesnot radiate, will the charged particle emit radiation in itsuniform motion through a medium? The answer to thisquestion was not evident at that time.

In the autumn of 1936, after several discussions withFrank, Tamm wrote the system of Maxwell equations forthe field of a point-like charged particle executing a uniformmotion through a medium with dispersion and obtained asolution of this system. Tamm immediately called up Frankand asked him to come to his home. Later on, Frankreminisced [2]:

``I found Tamm sitting at his desk, deep at work, withmany sheets of paper already covered with formulas. Straightaway he started telling me of what he had done prior to myarrival. Today I can no longer recall what precisely wediscussed during that night. I believe we discussed both thecourse of the solution proposed by Tamm and the validity ofcalculations, as well as the physical foundations of the theory,in which much was still unclear. I only remember that we satfor a long time.

I returned home by foot at daybreak, because the urbantransport had finished (or had not yet resumed) working.''

A joint paper by Tamm and Frank [8] was submitted tothe journal DAN SSSR in the first days of 1937. This paperwas an outline of the complete theory of the radiation

O1

r1

O3 O4

r3 y\yO2

r2

Figure 2.

November 2009 VavilovëCherenkov radiation: its discovery and application 1103

discovered by Cherenkov. The authors considered the field ofa point-like charge moving uniformly and rectilinearlythrough an infinite uniform medium with dispersion, i.e., amedium whose refractive index n�o� depends on the fre-quencyo. It was shown that when the charged particle speed vexceeds the phase speed of light c=n�o� in the medium, adirectional radiation at the frequency o occurs. The radiatedwaves propagate in the direction that makes an acute angle ywith the particle motion direction, such that

cos y � c

n�o�v : �4�

Therefore, the angle y depends not only on the speed v of thecharged particle but also on the frequency of the wave(because the refractive index n�o� depends on the frequencyo of the wave).

Much later, in themid-1950s inDubna, V PZrelovmade abeautiful photograph illustrating formula (4). It follows from(4) that the greater the refractive index n, the larger the angle yat which the wave is radiated. Normally (in the case of normaldispersion), the refractive index for blue rays is greater thanfor red ones. That is why the angle y at which blue waves areemitted is larger than the angle of red-wave emission. Ingeneral, as the frequencyo increases, so does the angle y. Theemission is spectrally decomposed.

The setup of the experiment performed by Zrelov isdepicted in Fig. 3. A transparent plate is placed in the pathof 660 MeV protons, which produce radiation in passingthrough the plate.Waves whose phase speed c=n is lower thanthe proton speed are radiated. All radiation angles areconfined between two conical surfaces. The cone angle ofthe inner conical surface is equal to the radiation angle for thelowest-frequency wave (the red part of the spectrum), and theouter cone angle is equal to the radiation angle for the highest-frequency wave (the blue part of the spectrum). It is worthnoting that in reality, radiation also occurs in other parts ofthe spectrum, at radio frequencies in particular. However,

photography is involved in this case, and therefore only thevisible part of the spectrum is considered. If the photographicplate is placed perpendicular to the radiating proton beam, acolored annular domain is recorded on it, a particular colorcorresponding to each value of the radius. Figure 3b shows apart of the annular domain photographed on a color film.

Tamm and Frank calculated the emission intensity I�o�,i.e., the energy radiated by an electron at a frequency o perunit time per frequency interval do,

I�o� do � v e 2

c 2

�1ÿ c 2

v 2n 2�o��o do ; �5�

where e is the charge and n�o� is the refractive index at thefrequencyo. Expression (5) is physicallymeaningful when thedifference in the square brackets is positive. This conditionimplies the inequality

v >c

n�o� ; �6�

i.e., the speed of a charged particle should exceed the phasespeed of the wave at the frequency o. This inequality wasobtained above proceeding from other considerations [seeinequality (3)].

The total radiation energy losses for a charge movinguniformly through a refractive medium are obtained byintegrating expression (5) over all frequencies that satisfyinequality (6):

dW

dx� e 2

c 2

��nv=c>1�

�1ÿ c 2

v 2n 2�o��o do ; �7�

the inequality in parentheses under the integral defines theintegration domain. Formula (7) gives the energy loss per unitpath length. As regards the spectral composition of theradiation, it follows from the above relations that the higherthe radiation frequency, the higher the energy radiated at thisfrequency. In the normal-dispersion spectral region (wherethe refractive index increases with frequency), the radiationintensity also increases with frequency. When the refractiveindex varies only slightly as the frequency varies, we canassume in the first approximation that the glow intensity isproportional to the frequency. The blue component in thevisible spectrum is therefore the highest in brightness, whichwas indeed observed in the measurements. Tamm and Frankalso determined the polarization of the radiated waves. Theyshowed that the electric vector E of the radiated wave isperpendicular to the wave vector k and lies in the plane thatcontains the wave vector and the charged particle velocity. Inother words, when a plane wave of the form

E � E0 exp�i�krÿ ot��

is radiated, the vector E0 is aligned with the vector

vÿ k�kv�k 2

:

Not only did the theory constructed by Tamm and Frankprovide an explanation for all observations published byCherenkov but it also contained additional informationabout the properties of the new glow.

It is pertinent to note that the explanation provided byTamm and Frank did not gain unreserved recognitionimmediately. According to their theory, the glow was due toelectrons that were traveling through a refractive medium at aspeed exceeding the speed of light. In this connection, two

a

b

Figure 3. (a) Schematic of the experiment carried out by Zrelov.

(b) Vavilov±Cherenkov radiation photographed on a color film.

1104 B M Bolotovskii Physics ±Uspekhi 52 (11)

objections were most frequently raised against Tamm andFrank's theory in the discussion of the theoretical findings.

The first objection was as follows: according to therelativity theory, no material body can move at a speedexceeding the speed of light. But the relativity theory assertsthat the limiting speed for material bodies is the speed of lightin the vacuum, c � 300;000 km sÿ1. It is precisely this valuethat cannot be exceeded by particles. The phase speed of lightin a medium with a refractive index n is c=n, i.e., n times lowerthan the speed of light in the vacuum. For instance, for atransparent plastic with the refractive index n � 1:5, the speedof light is equal to 200,000 km sÿ1. An electron having amoderate energy 700,000 eV moves with a speed exceeding200,000 km sÿ1. For a higher electron energy, the electronspeed becomes even higher, but it never exceeds the speed oflight in the vacuum, in complete agreement with the relativitytheory. Therefore, the speed of a particle can exceed the in-medium phase speed of light but remain below the speed oflight in the vacuum.

The second objection against the explanation provided byTamm and Frank proceeded from the well-known fact that auniformly moving charged particle does not radiate electro-magnetic waves. The answer to this objection is essentiallycontained in the plot shown in Fig. 2. As is evident from theplot, the charged particle emits waves at each point in its path.The resultant field is the composition of waves emitted fromall portions of the trajectory. This picture applies for any lawof the charged particle motion, including uniform motion.For a charge moving uniformly in the vacuum, it can beshown that the radiation from different portions of thetrajectory cancels, with the effect that a uniformly movingcharged particle does not radiate. The same occurs when acharged particle propagates at a constant speed through arefractive medium when this speed is lower than the phasespeed of light in the medium. But if the charged particle speedexceeds the phase speed of light, then, as is evident fromFig. 2,the radiation from different portions of the trajectory nolonger cancels but is coherently combined.

Interestingly, Vavilov was among the first to accept thetheory constructed by Tamm and Frank. He even demon-strated a beautiful experiment to his collaboratorsÐahydrodynamic analogy for the blue glow. M N Alentsev toldme about this. Vavilov took a plane glass cuvette, pouredsome water into it, and placed a lamp below the cuvette.

When the lamp was turned on, a magnified image of thecuvette appeared on the ceiling. Vavilov took a sharp penciland passed its point over the water surface in the cuvette. Onthe ceiling, two waves were seen to diverge, making an anglewith the pencil point at its vertex.

After the emergence of the theory, Cherenkov performedseveral experiments to verify its theoretical predictions. Hismeasurements turned out to agree nicely with the theory.Following Vavilov's advice, Cherenkov summarized themainfacts about the new effectÐboth experimental data and sometheoretical resultsÐ in a brief paper in English and sent thispaper to the well-known London natural science journalNature. The paper was entitled ``Visible Radiation Producedby Electrons Moving in a Medium with Velocities Exceedingthat of Light.'' But the editors of the journal did not acceptthe paper for publication. The reason undoubtedly lay with adistrust created by the content of the paper, beginning with itstitle. For the sake of fairness it should be admitted thatdistrust in the entire complex of problems related to the blueglow was at that time expressed by some well-known andrespected physicists not only abroad but also in the USSR.Doubt was aroused both by the measurement techniques andthe data obtained at the visibility threshold, as well as by thetheoretical explanations.

Vavilov advised Cherenkov to send the rejected paper tothe American physical journal Physical Review. The paperwas published there in 1937 [9]. Next year, the same journalpublished paper [10], which was motivated by Cherenkov'spaper [9] and concerned with experimental verification ofrelation (2). American physicists Collins and Reiling [10]measured the angular distribution of the glow excited in thinplatelets of substance by the beam of electrons accelerated tothe energy about 2 MeV. The beam current was equal to10 mA. The glow in the experiments staged by Collins andReiling was much brighter than in Cherenkov's experiments.Photographing this glow required the exposure time 10 s(Cherenkov needed an exposure time of three days when hewas photographing the glow). The accelerator turned out tobe a considerably more intense source of fast electrons thanthe small amounts of radium with which the measurements atFIAN were performed. The data of Collins and Reiling werein perfect agreement with the theory constructed by Tammand Frank. However, the authors did not understand (or didnot accept) the foundations of the theory by Tamm andFrank. This is what they wrote in their paper: ``Electronsconstantly lose energy as they pass through a medium. Theresultant acceleration is responsible for Cherenkov's radia-tion.'' Collins and Reiling believed that the cause of radiationwas precisely the acceleration (to be more precise, decelera-tion) of particles in the medium. Meanwhile, according to thetheory constructed by Tamm and Frank, the radiationemerges in the uniform motion of a charged particle.

The quotation from the paper by Collins and Reilingcomes as no surprise considering how novel and unconven-tional the conclusion was that a uniformly moving chargebecomes a radiation sourcewhen its speed exceeds the speed oflight.We emphasize, by theway, thatCollins andReilingwerethe first to use the term ``Cherenkov radiation'' in their work.Presently, the names ``Cherenkov radiation'' and ``Cherenkoveffect'' are commonly accepted in the West. As for Russia,even when Vavilov was alive, his students, collaborators, andin general those physicists who witnessed the discoveryproposed that the blue glow should be termed ``Vavilov±Cherenkov radiation'' or the ``Vavilov±Cherenkov effect'' in

j y y

yPhotomultiplier

a

b

Figure 4.

November 2009 VavilovëCherenkov radiation: its discovery and application 1105

order to emphasize the decisive role played by Vavilov in itsdiscovery. Vavilov would invariably object to such proposalsand he himself resorted to the name ``Cherenkov glow.''However, several years after Vavilov's death, several Sovietphysicists would nevertheless use the term ``Vavilov±Cher-enkov effect.'' I believe that this name does not diminish therole played by Cherenkov and reflects both the history of thediscovery and the role of Vavilov much better. However, wedo well to bear in mind that the name ``Cherenkov effect'' isrooted in modern physics. The future will show how the glowdiscovered by Vavilov and Cherenkov will finally be referredto. In the long run, it is not amatter of the name: it is importantto knowhow the discoverywasmade.As forme, I use the term``Vavilov±Cherenkov radiation.''

In 1940, Cherenkov defended his doctoral thesis entitled``Radiation of Electrons in Their Motion at SuperluminalVelocity in Substance.'' During the defense procedure,Mandel'shtam raised the question of what regions of themediumÐthose close to the path of a superluminal electronor distant onesÐwere responsible for the Vavilov±Cheren-kov radiation. The answer to this question was provided in apaper by Ginzburg and Frank [11] seven years later. Theyconsidered the motion of an electron along the axis of acylindrical channel made in a medium with a permittivity e1and filled with a medium with a permittivity e2. Ginzburg andFrank solved the problem exactly; for brevity, we here giveonly the qualitative result of their treatment in the case of anempty channel (i.e., e2 � 1). Let the electron speed exceed thephase speed of light outside the channel. Then outside thechannel, the Vavilov±Cherenkov radiation exists, with theradiation intensity depending on the channel radius R. Forradiation of a frequencyo, the dependence of the intensity onthe radius is qualitatively as follows.When the channel radiusR satisfies the inequality

R <2pv

o��������������1ÿ b 2

q �8�

(where b � v=c), the radiation is little different from whatwould be the case for a channel-free continuous medium.When the opposite inequality holds, the intensity of theVavilov±Cherenkov radiation decreases rapidly with anincrease in the channel radius. It is assumed that the speed vof an electron that travels along the channel axis exceeds thephase speed of light in themedium. For a qualitative estimate,the electron speed v in the numerator in (8) may be replacedwith the speed of light in the vacuum c. The quantityl � �2pc=o� is the wavelength in the vacuum that corre-sponds to the radiation of the frequency o. The quantityg � �1ÿ b 2�ÿ1=2 is the so-called Lorentz factor. This quantityis proportional to the particle energy. Therefore, the quantitylg may be regarded as the critical parameter for the channelradius. For high particle energies, lg becomes much greaterthan the wavelength. This signifies that the main role in theformation of the Vavilov±Cherenkov radiation is played bythe regions of the medium that are remote from the particletrajectory, this remoteness increasing proportionally to theparticle energy.

In the years that saw the discovery of Vavilov±Cherenkovradiation and the finding of its explanation, its possibleapplications were not yet considered. The radiation was soweak, indeed, that the very observation of this radiation raninto serious problems. The situation changed when photo-multipliersÐdevices that enabled reliable detection of so

weak a radiation as Vavilov±Cherenkov radiation producedby a single charged particleÐemerged. In 1947, ten yearsafter TammandFrank constructed the complete theory of theeffect, Physical Review published Getting's suggestion [12]that photomultipliers be used to record Vavilov±Cherenkovradiation. The use of Cherenkov counters goes back toprecisely this suggestion. The simplest devices of thoseproposed by Getting are depicted in Fig. 4.

A fast charged particle that travels along the axis of a conemade of a transparent plastic produces Vavilov±Cherenkovradiation (Fig. 4a). The cone angle is selected such that theradiation experiences the total internal reflection from theconic surface, is normally incident on the plane base of thecone, and emanates in the form of a parallel beam of rays. Alens focuses this beam onto the photocathode of a photo-multiplier. The part of the Cherenkov counter in which theradiation is generated is called a radiator. Figure 4a shows aconic radiator.

Figure 4b depicts a radiator consisting of a cylindricalpart and an adjacent conical part. This radiator also producesa parallel beam of rays; in this case, the charged-particleenergy losses due to the Vavilov±Cherenkov radiation in thisradiator may far exceed those in the conical one, because thepath of the particle in the cylindrical part of the radiator israther long. Accordingly, the energy that reaches thephotomultiplier is also higher than the conical radiator flashenergy, which facilitates detection.

We emphasize that because we are considering the historyof the discovery, we here discuss the first simplest designs ofCherenkov counters. Nevertheless, even these simplest ver-sions offer several advantages over, say, aGeiger counter. Forexample, once a Geiger counter operates, an observer knowsthat a charged particle has traversed the working volume ofthe counter. But the observer cannot determine the directionthe particle has passed: from right to left or from left to right,or downwards, or in some other direction. By contrast, theCherenkov counter not only detects the passage of a chargedparticle but also determines the direction of its motion owingto the directivity of Vavilov±Cherenkov radiation. When aparallel beam emanates, e.g., from the base of the radiatordepicted in Fig. 4a, this signifies that the charged particle hastraversed the radiator in the direction from the apex to thebase of the cone.

The second advantage of a Cherenkov counter consists inits fast response, which is faster than that of a Geiger counterby many orders of magnitude. In passing through theworking volume of the Geiger counter, a charged particleexcites a gas discharge, which permits recording the particle.The charge duration is of the order of 10ÿ4 s. If anotherparticle passes through the counter during the discharge, thecounter would record the two particles as one. In theCherenkov counter, a charged particle produces a radiationflash that is several orders of magnitude shorter in durationthan the Geiger counter discharge.

When comparing a Cherenkov counter with a Geigercounter, perhaps it would be appropriate to speak abouttheir differences rather than about the advantages of one ofthem over the other. Each counter has a field of application ofits own, and is widely used within this field. For instance,Cherenkov counters cannot record particles whose speeds arelower than the phase speed of light in the radiator material,while the Geiger counter does detect such particles.

After the publication of Getting's proposal, a rapiddevelopment of the Cherenkov counter technology started.

1106 B M Bolotovskii Physics ±Uspekhi 52 (11)

Counters designed to determine the speed, charge, totalenergy, and other characteristics of a charged particleemerged. Cherenkov counters were rapidly incorporatedinto the arsenal of high-energy physics.

Tamm, Frank, and Cherenkov were awarded the 1958Nobel Prize in Physics ``for the discovery and the interpreta-tion of the Cherenkov effect.'' In the Soviet Union, theimportance of the discovery had been recognized muchearlier: in 1946, Vavilov, Tamm, Frank, and Cherenkovwere awarded a Stalin Prize of First DegreeÐat that time,the highest official sign of scientific recognition.

� � �Interestingly, the properties of Vavilov±Cherenkov radia-

tion give researchers a unique chance: it is possible to make aCherenkov counter such that it is simultaneously the site of anuclear reaction and the means for its recording.

In the early 1960s, a large Cherenkov counter wasmade atthe Lebedev Physical Institute of the Russian Academy ofSciences. It was used to investigate the production of muonsunder the action of high-energy protons, which are aconstituent of cosmic rays. The counter was made in theLaboratory of Cosmic Rays by a team led by A E Chudakov.The function of the radiator in the counter was fulfilled bywater, which filled a huge tank welded of steel sheets. Thetank was a truncated cone in shape (Fig. 5). The base of thetruncated cone measured 6.5 m in diameter, the upper conebase measured 2.5 m in diameter, and the height was 5 m. Torecord light, 16 photomultipliers accommodated symmetri-cally about the cone axis on its surface were used. The innertank surface was painted white to prevent the wall fromabsorbing the radiation that did not immediately find its wayinto the photomultipliers. The water filling the facility waspurified for the same purpose, to minimize absorption of theVavilov±Cherenkov radiation.

On entering the volume of such a counter, a high-energyparticle produced an electron±photon or electron±nucleusshower. From the integral Vavilov±Cherenkov glow pro-duced by reaction products, it was possible to assess theprimary particle energy.

This counter was operated for several years. It was used,in particular, to investigate multiple production of muons inhigh-energy nuclear interactions [13]. In the mid-1960s, thecounter was disassembled. At that time, this was the world'slargest Cherenkov counter.

� � �In 1996, a gigantic Cherenkov detector was put into

service in Japan. It is accommodated in a mine one kilometerbeneath the surface of the Earth in theKamioka region, about300 km north of Tokyo. The detector is a cylindrical tank ofwelded stainless steel (Fig. 6). The tank measures 41.4 m inheight and 39.3m in diameter, and holds 50,000 t of water, thewater being carefully purified to minimize the absorption andscattering of light in it. Located on the walls as well as on thelower and upper bases of the tank are 11,146 photomulti-pliers.

This detector received the name Super-Kamiokande. Thethree last letters stand for Nuclear Decay Experiment. Thisdetector was intended for use in the quest for proton decay.This quest is underway but so far has not yielded definiteresults. However, the Super-Kamiokande detector hasenabled obtaining significant results (making importantdiscoveries) in neutrino physics. The neutrino is still a

mysterious particle in many respects, exhibiting an extremelyweak interaction with other elementary particles. The gigan-tic size of the counter permits recording individual andinfrequent events of the interaction of neutrinos with protonsand neutrons in the atomic nuclei of the elements that makeup water (oxygen and hydrogen). Fast charged particles areproduced in the collision of energetic neutrinoswith nucleons.The fast particles produce Vavilov±Cherenkov radiation intheir passage through the water column of the counter. The

Section through AB

Water

� 6.5 m

5m

� 2.5 m

5

15

64

16 14

113

1 13

9

10

7

8

122A B

Figure 5. Large total-absorption Cherenkov counter.

Electronic controls

PM mounts

Rock

Concrete wall

Hoisting crane

PMs

Figure 6. Water Cherenkov detector, 50 thousand tons of water,

11200 photomultipliers (PMs).

November 2009 VavilovëCherenkov radiation: its discovery and application 1107

radiation is detected by the photomultipliers mounted on theinner surface of the tank and is minutely analyzed. Thisanalysis enables sufficiently accurately determining the kindof neutrino that has caused the reaction, as well as the energyand the momentum direction. As is generally known, thereare three types of neutrinosÐelectron, mu-meson (muon),and tau-meson (taon), in accordance with the three differentkinds of nuclear reactions in which these particles areproduced or absorbed.

Measurements performed with the Super-Kamiokandedetector [14] have given firm evidence in favor of neutrinooscillations. The effect consists in the neutrino experiencing`in-flight' changes: a neutrino of one type transforms into aneutrino of another type. Neutrino oscillations were pre-dicted by B Pontecorvo in 1957.

The Super-Kamiokande detector is simultaneously thesite of nuclear reactions and the means for their detection.

Furthermore, using large Cherenkov detectors like Super-Kamiokande and the previously constructed Kamiokandedetector, it has been possible to record neutrinos coming fromdistant cosmic objects. In 1987, neutrinos produced in thesupernova outburst in the Large Magellanic Cloud weredetected [15]. Today, it is valid to say that neutrinoastronomy has come into being.

� � �The glow of different liquids under gamma-ray irradiation

had been observed prior to Cherenkov's experiments. Inparticular, in 1926±1929, the French physicist M Malletobserved such a glowand even photographed its spectrum [16].But Mallet believed that the glow he had observed wasluminescence and did not undertake any further investiga-tions. It took Vavilov's knowledge and experience to deter-mine that the glow discovered by Cherenkov was different innature from luminescence.

As regards Tamm and Frank's theory, which interpretsthe Vavilov±Cherenkov radiation, here, too, it is possible toindicate earlier papers containing some quite close physicalideas but, of course, not so complete an explanation. Forinstance, back in 1904, the famous German theorist ArnoldSommerfeld calculated the field of a charged particle thatmoved in the vacuum at a constant speed exceeding the speedof light [17]. Sommerfeld showed that radiation of directionalelectromagnetic waves occurred. However, the special theoryof relativity was formulated shortly thereafter, the motion ofmaterial bodies at a superluminal velocity turned out to beimpossible, and Sommerfeld's work was forgotten. WhenTamm and Frank were discussing their work with A F Ioffe,he remembered Sommerfeld's work and informed Tamm andFrank about it. A reprint of the paper by Tamm and Frankwas sent to Sommerfeld. The authors received a letter ofthanks from him. This exchange of letters had taken placebefore the Second World War broke out. Later, Sommerfeldincluded an article entitled ``Cherenkov radiation'' in histextbook Optics.

Tamm and Frank had another precursor who hadapproached their theory rather closely, even more closelythan Sommerfeld. However, his work was consigned tooblivion for a longer time than Sommerfeld's work. It wasnot recalled until the mid-1970s. The author of this work wasthe famous English physicist, mathematician, and engineerOliver Heaviside. In 1889, proceeding from the Maxwellequations, Heaviside calculated the field of a charged particlemoving at a constant velocity through a medium with a given

dielectric constant (or through a medium with a givenrefractive index, which is the same) [18]. It followed from hiscalculations that the particle radiated electromagnetic waveswhen its speed exceeded the speed of light in the medium.Heaviside determined the radiation angle, and it turned outthat the angle was defined by relation (2). He also determinedthe radiation spectrum. Heaviside's theory was not ascomplete as the theory constructed by Frank and Tamm. Inparticular, Heaviside did not take dispersion into account,and therefore his expression for the total radiation lossdiverged. Furthermore, Heaviside did not regard the speedof light in the vacuum as the speed limit for all materialbodies. In his theory, for instance, the charged particle couldtravel through the refractive medium at a speed exceeding thespeed of light in the vacuum. But in many respects, his theorywas close to the theory by Frank and Tamm.

Heaviside's work was forgotten even more so thanSommerfeld's. This was attributable to the fact that Heavi-side had been ahead of his time. In particular, as indicatedabove, he considered the field of a point-like charged particlemoving at a constant speed in a refractive medium. But nocharged particles had been discovered by that time, not eventhe electron, to say nothing of the other particles carryingelectric charge. Furthermore, Heaviside considered the casewhere the charged-particle speed exceeded the speed of lightin the medium. Such speeds seemed to be inconceivable formaterial bodies at that time. The first fast particles wereobtained in the decay of radioactive elements, but theradioactivity itself was discovered only ten years later. Thatis why Heaviside's work appeared to be distant from realityand was promptly forgotten. It was remembered many yearslater, in the mid-1970s.

By contrast, when Tamm and Frank set themselves thetask of interpreting the Vavilov±Cherenkov glow, they wereto explain a physical effect observed in reality.

When these old studies by Heaviside became known, Irecall, Frank was recovering in the Uzkoe academicsanatorium. On his request I took the third volume ofHeaviside's book Electromagnetic TheoryÐa part of thevolume was dedicated to the radiation of superluminalsourcesÐand brought it to him. After reading the book,he returned it to me saying: ``It is a great honor to have sucha predecessor.''

� � �During the past years, Vavilov±Cherenkov radiation has

found numerous applications. The technology of Cherenkovcounters has made rapid strides. Nowadays, they arecomplicated devices, combining the achievements of optics,electronics, and radiophysics. Using the Vavilov±Cherenkovradiation, they ensure high-efficiency detection of fastcharged particles that pass through the radiator. Moreover,it has become possible to make detectors that need not beplaced in the path of fast particles: these detectors are capableof detecting particle fluxes at a distance. The idea of makingthese detectors was conceived by Askar'yan [19, 20]. Let ahigh-energy electron enter the terrestrial atmosphere fromspace. In its path, it interacts with the nuclei of atoms thatconstitute the atmosphere. The electron produces brems-strahlung in the field of a nucleus, the emitted photon energybeing of the same order of magnitude as the electron energy.This photon next interacts with the nucleus or electron ofanother atom to produce an electron±positron pair. Each ofthe particles that make up the pair also emits bremsstrahlung

1108 B M Bolotovskii Physics ±Uspekhi 52 (11)

photons (second-generation photons, so to say). Thesesecond-generation photons in turn produce electron±posi-tron pairs, and so on: a so-called electromagnetic showerdevelops.

It was assumed that an electromagnetic shower did notproduce electromagnetic radiation because a shower waselectrically neutral, the electrons and positrons in the showerbeing equal in number. In Ref. [19] and in the subsequentpaper [20], Askar'yan showed that the shower actuallycontains an excess of electrons. This is because the electronsand photons that make up the shower ionize the atoms of theair in their path (ionize the atoms of gases that compose theair) and knock electrons out of them, and these electronsbecome part of the shower. That is why the shower turns outto be negatively charged, the excess of electrons amounting toseveral dozen percent of the total number of particles in theshower. The shower may therefore be a source of Vavilov±Cherenkov radiation in the atmosphere. The emission occursat those frequencies for which the phase speed of electro-magnetic waves is lower than the speed of shower particles.Askar'yan estimated the intensity of the Vavilov±Cherenkovradiation produced by the shower at radio frequencies (e.g., atthe wavelength 10 cm). Its intensity was found to substan-tially exceed the intrinsic noise of radio receivers, whichopened up the possibility of recording cosmic showers bytheir radiation (including the Vavilov±Cherenkov radiation)at radio frequencies. This signifies that the recording unitneed not be placed in the shower shaft, it may be located awayfrom the shower. In recent years, recording showers throughVavilov±Cherenkov radiation at radio frequencies has beentaken up by many radio astronomical stations.

Speaking of applications that involve Vavilov±Cherenkovradiation, we have so far considered only high-energy physics.But the ideas underlying the interpretation of this effect areactually widely used in many fields of physics. The principalideaÐ the idea of wave±particle synchronismÐ is at theheart of several radiophysical applications; this idea is basicto the explanation of wave damping in collisionless plasmas,as well as of several acoustic and hydrodynamic effects.Owing to space limitations, we cannot discuss the subject atgreater length in this paper.

Relatively recently, Vavilov±Cherenkov radiation hap-pened to bear on the interpretation of an interestingphenomenon. All deep-sea fish have eyes, but it was unclearuntil recently why they have retained the organ of sight. Seawater exhibits a high electric conductivity, and hence itfollows that the daylight is strongly attenuated with depth insea water and complete darkness should reign at a depth ofseveral hundredmeters. Eyes are not needed in total darkness,and the organ of sight should have died out according to thelaws of evolution. It turns out, however, that there is nocomplete darkness at great depths. A radioactive isotope ofcalcium dissolved in seawater emits fast electrons. Theseelectrons are responsible for Vavilov±Cherenkov glow inseawater, and therefore twilight reigns at great depthsÐ``the Vavilov±Cherenkov twilight.'' The illumination at greatdepths turns out to be such that vision may well provebeneficial to fish [21].

In the late 1960sÐearly 1970s, several papers werepublished concerned with the action of fast particles that areconstituents of cosmic radiation on human vision. Thesequestions arose in connection with the development ofastronautics, specifically in connection with the Apollo-11,Apollo-12, and Apollo-13 space missions aimed at achieving

a lunar landing of a human being. The Apollo-11 missiontook place in July 1969. The astronauts Neil Armstrong andMichael Collins landed on the Moon, and Edwin Aldrinremained in the command module, which orbited the Moon.The astronauts stayed on the Moon for about a day, andAldrin stayed in the cabin the entire time. For a significantportion of this time, the cabin was isolated from externallight. On returning to Earth, Aldrin reported that he observedpoint-like short-duration flashes in white color. Similarflashes were seen by Armstrong as well. The astronauts whoparticipated in the following missions also observed short-time flashes when they were in the darkness or closed theireyes [22]. The possible causes of these flashes were discussed inRef. [23]. The authors considered two possible causes: theionization produced by primary or secondary cosmic radia-tion particles in or near the retina of the eye, and (or) Vavilov±Cherenkov radiation induced by fast particles traversing thetransparent substance in the eyeball. In this connection, wenote that the glow of the transparent substance in the eyeballcaused by radioactive radiation was known long before theonset of the age of spaceflight.

References

1. Dobrotin P A ``V `doistoricheskom' FIANe'' (``In `prehistoric'

FIAN'', in Pavel Alekseevich Cherenkov. Chelovek i Otkrytie

(Pavel Alekseevich Cherenkov. Man and Discovery) (Exec. Ed.,

Comp. A N Gorbunov, E P Cherenkova) (Moscow: Nauka, 1999)

p. 171

2. Frank I M ``O kogerentnom izluchenii bystrogo elektrona v srede''

(``On the coherent radiation of a fast electron in a medium''), in

Problemy Teoreticheskoi Fiziki. Pamyati Igorya Evgen'evicha Tam-

ma (Problems of Theoretical Physics. In Commemoration of Igor'

Evgen'evich Tamm) (Exec. Ed. V I Ritus) (Moscow: Nauka, 1972)

p. 350

3. Brumberg E M, Vavilov S I ``Fotometriya slabykh istochnikov

izlucheniya'' (``Photometry of weak radiation sources''). Izv. Akad.

Nauk SSSR, OMEN, Ser. 7 919 (1933)

4. Dobrotin N A, Feinberg E L, Fock M V Priroda (11) 58 (1991)

5. Cherenkov P A ``Vidimoe svechenie chistykh zhidkostei pod

deistviem g-radiatsii'' (``Visible glow of pure liquids under g-irradia-tion''). Dokl. Akad. Nauk SSSR 2 (8) 451 (1934)

6. Vavilov S I ``O vozmozhnykh prichinakh sinego g-svecheniyazhidkostei'' (``On the possible causes of blue g-glow of liquids'')

Dokl. Akad. Nauk SSSR 2 (8) 457 (1934)

7. Grigorov N L ``Kak ya pomogal P.A. Cherenkovu v laboratorii''

(``How I assisted P A Cherenkov in laboratory''), in Pavel Aleksee-

vich Cherenkov. Chelovek i Otkrytie (Pavel Alekseevich Cherenkov.

Man and Discovery) (Exec. Ed., Comp. A N Gorbunov, E P Cher-

enkova) (Moscow: Nauka, 1999) p. 174

8. Tamm I E, Frank I M ``Kogerentnoe izluchenie bystrogo elektrona

v srede'' (``The coherent radiation of a fast electron in a medium'')

Dokl. Akad. Nauk SSSR 14 107 (1937); see also a reprint of this

paper: Usp. Fiz. Nauk 93 388 (1967)

9. �Cerenkov P A ``Visible radiation produced by electrons moving in a

medium with velocities exceeding that of light'' Phys. Rev. 52 378

(1937)

10. Collins G B, Reiling V G ``�Cerenkov radiation'' Phys. Rev. 54 499(1938)

11. Ginzburg V L, Frank I M ``Izluchenie elektrona i atoma, dvizhu-

shchikhsya po osi kanala v plotnoi srede'' (``The radiation of

electrons and atoms moving along the axis of a channel in a dense

medium'') Dokl. Akad. Nauk SSSR 56 699 (1947)

12. Getting I A ``A proposed detector for high energy electrons and

mesons'' Phys. Rev. 71 123 (1947)

13. Vavilov Yu N, Pugacheva G I, Fedorov V M ``O myu-mezonnykh

gruppakh vblizi ot osi shirokikh atmosfernykh livnei'' Zh. Eksp.

Teor. Fiz. 44 487 (1963) [`` m-meson groups near the axis of board

atmospheric showers'' Sov. Phys. JETP 17 333 (1963)]

November 2009 VavilovëCherenkov radiation: its discovery and application 1109

14. The complete list of publications on neutrino research pursued on

the Kamiokande facility is found in the Internet, at http://www-

sk.icrr.u-tokyo.ac.jp/sk/pub/index.html

15. Koshiba M ``Rozhdenie neitrinnoi astrofiziki'' Usp. Fiz. Nauk 174

418 (2004); ``Nobel Lecture: Birth of neutrino astrophysics'' Rev.

Mod. Phys. 75 1011 (2003)

16. Mallet L C.R. Acad. Sci. Paris 183 274 (1926); 187 222 (1928); 188

445 (1929)

17. Sommerfeld A ``Simplified deduction of the field and the forces of an

electron, moving in a given way'' Proc. Amsterdam Acad. 7 346

(1904)

18. Heaviside O ``The electro-magnetic effects of a moving charge''

Electrician 83 (1888); reproduced in in: Heaviside O Electrical

Papers Vol. 2 (London: Macmillan, 1892) p. 492

19. Askar'yan G A ``Izbytochnyi elektronnyi zaryad elektronno-foton-

nogo livnya i kogerentnoe radioizluchenie ot nego'' Zh. Eksp. Teor.

Fiz. 41 616 (1961) [``Excess negative charge of an electron-photon

shower and its coherent radio emission'' Sov. Phys. JETP 14 441

(1962)]

20. Askar'yan G A ``Kogerentnoe izluchenie ot kosmicheskikh livnei v

vozdukhe i plotnykh sredakh'' Zh. Eksp. Teor. Fiz. 48 988 (1965)

[``Coherent radio emission from cosmic showers in air and dense

media'' Sov. Phys. JETP 21 658 (1965)]

21. Baboshin Yu B, Lopatnikov S L, Popov N I ``Informatsionnaya

znachimost' sobstvennogo cherenkovskogo svecheniya morskoi

vody dlya glubokovodnykh zhivotnykh'' (``Information signifi-

cance of the intrinsic Cherenkov glow of sea water for deep-sea

animals'') Dokl. Akad. Nauk SSSR 290 991 (1986)

22. Fazio G G, Jelley J V, Charman W N ``Generation of Cherenkov

light flashes by cosmic radiation within the eyes of the Apollo

astronauts'' Nature 228 260 (1970)23. Tobias C A, Budinger T F, Lyman J T ``Radiation-induced light

flashes observed by human subjects in fast neutron, X-ray and

positive pion beams'' Nature 230 596 (1971)

1110 B M Bolotovskii Physics ±Uspekhi 52 (11)