"Cherenkov detectors in Cherenkov's laboratory." B.B. Govorkov Lebedev Physical Institute

1. "Cherenkov detectors in Cherenkov's laboratory."B.B. GovorkovLebedev Physical Institute Introduction The discovery of anisotropy of Cherenkov radiation as any great discovery went through 4 stages of its development: Prediction (O. Heaviside, 1888) [1] Discovery (P.A. Cherenkov, 1936) [2] Explanation (I. E. Tamm and I. M. Frank, 1937) [3] Application [4], [5] 1. Prediction In 1888 great Oliver Heaviside first predicted theoretically a "conical radiation" when a point charge (an electron) moves uniformly and rectilinearly both in the ether and in a medium with a speed higher than the speed of light in this medium.

2. Page 2 : In should be noted that until the development of the relativity theory by Einstein in 1905 nothing prevents scientists from considering particle movement both in vacuum (in the ether) and in a medium with speeds higher or equal to the speed of light. Heaviside did not distinguish between electron movement in the ether and in a substance. He studied these two problems simultaneously using values of the dielectric permeability ε and the magnetic permeability μ both for the ether and for a medium. Moreover, he noted that the permeabilities ε0 and μ0 he used for the ether were not equal to 1, as Hertz proposed. Heaviside emphasized that he considered the ether as a medium having certain electric and magnetic properties, and therefore he thought physically more correct to describe it with ε0 and μ0 as any other medium. It was not until the creation of the relativity theory when particle movement in vacuum and a medium became sharply differentiated. In vacuum a speed higher that the speed of light is absolutely prohibited while particle movement with a speed higher than the phase velocity in the medium is quite possible. Heaviside formulate his discovery in the following words:

3. Page 3 : "The displacement for a point charge is a conical sheet behind the charge, together with supplementary distribution inside the cone. The pulling back is obvious, and the energy is being masted at a steady rate by the constant growth of the cone at its apex, which is fully accounted for the activity of the applied moving force. This is as J suspected in 1888 (El. Papers vol. 2, p. 494) J later elaborated it by mathematical investigation. The solution is described in my "El. Papers", v. 2, p. 516." [1] In 19th century hypotheses became theories only after they were proved in experiment. Therefore after formulating the "subject of discovery" Heaviside made a comment concerning practical importance of his discovery. "February 26, 1898. – It may also beacome of some practical importance in connection with "Cathode Rays" and "X-Rays", for J. J. Thomson and others have lately concluded from experiment that immense speeds of the charged particles, comparable with the speed of light, are concerned. If this is fully confirmed, we may will believe that increased voltage will produce speeds exceeding that of light if they do not exist already and so bring in the conical theory".

4. Page 4 : In the third volume of his work "Electromagnetic theory" (v. III, p. 25–28) he informs about the results of his early works. He entitled this section "Theory of uniform and rectilinear movement of a point particle or an electron through the ether when u<v and when v<u." Where u is velocity of charge, v – velocity of light. He gives the formula for the half-angle of the light cone: u2/v2 – 1 = cosec2 Θ – 1 = cot2 Θ. (1) When dielectric permeability ε (Heaviside used a symbol c) is not equal to 1 the formula determinates a definite angle of radiation (in modern designation): cosΘ = 1/βn (2), where n – the refraction factor, β – the ration of the speed of charge and the speed of light in vacuum. A genius Heaviside was half a century in advance his time in his calculations and predictions, but unfortunately he was not understood by his contemporaries, his works was forgotten, and scientists had to start from the beginning, this time from the experiment.

5. Page 5 : 2. Discovery itself The discovery of directivity of Cherenkov-Vavilov radiation was preceded, naturally, by the discovery of the Cherenkov-Vavilov radiation itself. Cherenkov carried out his first experiments at the experimental facility shown in the Fig. 1 [6].

6. Page 6 : Fig.1 The design of one of employed modifications for quenching photometer. Here P is a silver mirror that reflects light coming from the liquid in the vessel A onto a circular aperture D ( 3 mm). Immediately behind the aperture there is a neutral grey wedge (groove K). The ocular lens L gives an enlarge image of the aperture on the retina. The groove D is designed for colour filters. N is a polarization prism. The photometric process consists in slow movement of the neutral grey wedge in front of the aperture through which the light from a constant source comes. The aim is to find such a position of the wedge where the light energy reaching the eye becomes insufficient to create a visual sensation (quenching moment).

7. Page 7 : The main feature of all Cherenkov's experiments which he carried out with the assistance and under the direction of S. I. Vavilov was their simplicity and elegancy, their clearness and consistency. In most experiments the method of quantitative measurements of luminous flux using the human eye visual threshold which was developed by Vavilov and his disciples was used [7]. The methods required long eye adaptation in absolute darkness, careful preparation of all components of the experimental facility that Cherenkov always perform himself. It should be noted that the first scientist who suggested using the human eye as a reliable instrument for measuring luminous flux was A. Lavoisier. In 1763 very young Lavoisier who was 21 years old took part in a competition for the best project of street lighting of Paris announced by the Royal Academy of Science. He compared illumination of various lamps (oil lamps and candle lamps, with and without reflectors) to find the optimum variant of street lighting. Lavoisier carried out all his measurements in a dark room upholstered with black cloth. From his own experience he made certain that the best instrument for assessing illumination is the human eye. Lavoisier propositions was set out in a fundamental work and was awarded with the gold medal of Royal Academy of Science in 1766 [8].

8. Page 8 : I am lucky enough working in Cherenkov's laboratory all my life. So I know many details about the discovery of the Cherenkov effect from Pavel Alexeevich himself. Thus when I asked how he succeeded in seeing a new extremely weak radiation he answered that he first observed this new radiation during background experiments. Vavilov gave him, then a post-graduate, a task to study luminescence of uranyl salts while exposing them to γ-quants from radium radioactive source. While measuring luminescence of these solutions Cherenkov decided to examine whether the walls of the glass vessel or the pure solvent, sulphuric acid or water, influenced the luminescence. Pavel Alexeevich told me that he was very surprised when found the radiation of the vessel with the pure solvent. Then he went to the storehouse of Lebedev Physical Institute and took all clear liquids from there. Returning to the laboratory he repeated his experiments with other substances. All liquids gave radiation! Moreover the radiation has approximately equal intensity for all liquids ( 15%). Attempts to quench the luminescence using methods developed by Vavilov and his disciple (quenching additives, heating of liquids etc) proved unsuccessful – all liquids glows stubbornly! Next time when Pavel Alexeevich met with his scientific supervisor he told him in detail about the unexpected results of background measurements. The following discussion led to new plans and ideas for experiments to prove that the observed radiation has nothing to do with luminescence.

9. Page 9 : It is how Pavel Alexeevich told me about his first observation of the radiation that subsequently was named as the Cherenkov-effect. About the prominent role of Sergey Ivanovich Vavilov in the Cherenkov-effect discovery one can to read in the last CERN Courier article [B. Bolotovsky, Y. Vavilov, A. Shmeleva. CERN Courier V. 44, № 9, (2004), p.37]. In the following experiments in 1936 P.A. Cherenkov studied the influence of magnetic fields on polarization of the discovered radiation and seen unexpectedly (he was first!) that the new radiation has directivity. In the next experiment designed for investigation of this directivity discovered by him P. Cherenkov used a very simple facility shown in Fig. 2 where the main components were a clear vessel with a liquid, a source of γ-radiation and a cone mirror.

10. Page 10 : Fig. 2 Diagram of apparatus for photographing the angular distribution of radiation: 1 – vessel with liquid; 2 – mirror; 3 – direction of glow towards the photographic lens.

11. Page 11 : Fig 3a and 3b shows a photography of the intensity distribution in the radiation cone with the axis in the direction of charged particles and with the half angle at the axis (2) again: cos Θ = 1/βn (2′)

12. Page 12 : Fig. 3a Photos of angular distribution of the intensity of radiation produced in liquids by fast electron: a – fluorescence of an aqueous solution of esculin – radiation of water caused by fast electron; b – radiation of ethyl cinnamate, n = 1,58043; c – radiation of benzene, n = 1,5133; d – radiation of cyclohexane, n = 1,4367; e – radiation of water, n = 1,3371. (In all cases the γ ray beam is directed from the lower part of the figure to the upper one.)

13. Page 13 : Fig. 3b Angular distribution of the radiation intensity of water, cyclogexane (C6H12), benzene (C6H6) and ethyl cinnamate (C11H12O2), measured a cording to the data of Fig. 3a. The upper curve for every liquid was obtained by means of exciting the Cherenkov radiation by Compton electrons from γ rays of ThC´´ (βeff =0.896); the lower from γ rays of Ra (βeff =0.847).

14. Page 14 : The detector shown in Fig. 2 is practically the first RICH detector! We expect to know about the development and future prospects of this extremely useful detector from reports at this conference. It is interesting how Cherenkov himself regarded his discoveries. At one of the meetings during International Conference on Instruments in High Energy Physics that took place in Dubna (JINR) in 1970, where his name was mentioned in each report – Cherenkov detectors, Cherenkov spectrometers, Cherenkov radiation etc. – Pavel Alexeevich bent to me and whispered in my ear: "It always seems to me, Boris Borisovich, that all this has no relation to me. As if formerly somewhere another Cherenkov lived, and everybody talk about him." Wonderful modesty and decency!

15. Page 15 : 3. Explanation The correct theory of the Cherenkov effect that explained completely all properties of the new radiation was developed on the basis of electrodynamics by two outstanding scientists working in Levedev Institute I.E. Tamm and I. M. Frank in 1937 [3]. They demonstrated that the radiation discovered by Cherenkov is a radiation of a charged particle (electron) moving uniformly and rectilinearly with a speed higher than the speed of light in the material medium, and they obtained the main formula for the directivity of the radiation and energy losses – the famous Tamm-Frank formula. All calculation took into account the dispersion i.e. the dependence of the refraction factor upon the frequency of the emitted light. The number of photons produced per unit path length of a particle with the charge Ze and per unit energy interval of the photons is

16. Page 16 : The most clear explanation of directivity if the Vavilov-Cherenkov radiation was given by I.M. Frank on the basis of the Huygens principle [9] in fig 4:

17. Page 17 : Fig. 4 Movement of a charge particle in a medium with a velocity v>u. v – particle velocity; u – the speed of light in the medium having the refraction factor n.

18. Page 18 : Each point of the trajectory of the charged particles (A, B, C etc) should be considered a source of a wave arising when a particle goes through the point. If the medium is optically isotropic such partial waves have the spherical form and propagates with the speed c/n. Suppose that a particle moving uniformly and rectilinearly with the speed v is in the point E in the moment of observation. In the moment t before that the particle was in the point A (AE = vt – coherent length or the length of formation). The wave emitted from A becomes a sphere with the radius R=ut=ct/n in the moment t (in Fig. 3 circle 1 corresponds to this wave, while circles 2, 3, 4 correspond to the waves from B, C, D). According to the Huygens principle interfering partial waves quench each other everywhere except for their common envelope which is corresponded to the wave surface of light propagating in a medium; this surface is a cone with the apex in the point E and it coincides with the instantaneous position of the particle, while normal lines to this envelope determine the wave vectors – the direction of light propagation. The angle Θ between the wave vector and the direction of the particle movement is described by the formula (2). Quantum mechanical examination executed by V. L. Ginsburg gave the same results [10].

19. Page 19 : 4. Application After the mechanism of the Cherenkov-Vavilov radiation became fully clear the final and still continuing stage began – the stage of application. Again Pavel Alexeevich Cherenkov was the first who proposed to use the Cherenkov effect for measuring particle velocities. In the work of 1937 he wrote [2], "The Frank-Tamm theory indicates another kind of experiments where the dependence of the effect on the refraction factor should manifest itself and that could give new quantitative confirmation of the theory – experiments to determine lower limits of electron velocities when the glow just begins to appear. The condition nβ > 1 shows this limit for liquids with different n should be different. The inverse problem is also possible: to find a liquid for electrons with specified velocity (for this the initial beam should be uniform) where the effect just begins to appear. After the development of appropriate methods this second type of experiments could be used for determining electron velocities."

20. Page 20 : It is impossible to cover thousands bright works that develop methods using Cherenkov detectors in a single report, therefore it would be reasonable to refer to previous reviews of J. Seguinot and T. Ypsilantis [4], V.P. Zrelov [5] etc. In this report I restrict myself with several experiments conducted under the direction of P. A. Cherenkov at the time when Cherenkov detectors was already used. It was mainly Cherenkov spectrometers for full absorption of γ-quants and electrons. It should be noted the basis for such spectrometers is another property determined by P. A. Cherenkov, "The glow intensity reduced to one electron is proportional to the length that an electron goes in the liquid." In accordance with this proportionality we obtain a linear dependence between the photomultiplier signal and total path length of the electromagnetic shower and therefore the energy of the incident particle E0. This dependence follows directly from the Tamm-Frank formula.

21. Page 21 : 5. Cherenkov in the laboratory Some words about the work of P.A. Cherenkov after the World War II. In 1949 Pavel Alexeevich together with his friend and colleague Vladimir Iosifovich Veksler started up the first in Europe electron synchrotrone with the energy of electrons 250 MeV. After the accelerator was put into operation P.A. became the head investigation on particle physics at this accelerator. Compton effect on protons, threshold pion photoproduction, photodisintegration of systems with small number of nucleons (D2, 4He …) etc.

22. Page 22 : The staff of the laboratory was about 150 physicists, engineers and technicians. This laboratory had 3 main scientific directions: • The investigation of photonuclear processes with the 250 MeV synchrotron. • Accelerator physics and different kinds of radiation (ondulator radiation, synchrotron radiation etc) • Work in other laboratories at home and abroad. Experiments at the largest accelerators and colliders: Fermilab, DESY, Dubna, Protvino etc. Let's point at some experiments where Cherenkov detectors were used to obtain new physical results.

23. Page 23 : In the 1970s one of the most interesting work was first generation of high energy γ and e-beams at the proton accelerator in Serpukhov. This work was made at the 76 GeV proton accelerator in collaboration with Institute of High Energy Physics (IHEP) (Serpukhov, Protvino) and Yerevan Physical Institute. According an idea of M. A. Markov [13] interaction of a high energy proton with a nucleus results in production of many pseudoscalar mesons (π0, η, η'…) that move in the forward direction in a narrow cone and then decay π0→2γ etc. As a result a γ-beam is generated in the forward direction at high energies of tens and hundreds GeV, and this beam can be transformed with a converter in an e+, e–-beam and inversely. At first one had to make sure that electrons always present in any hadron magneto-optical channel. A differential Cherenkov counter [12] and a total absorption Cherenkov spectrometer were placed on one of the channels. The TACS consists of two detectors – KQS (11 cm) and TF1 (30 cm), the total length of the spectrometer was 23X0. Fig. 5 shows design of the composite spectrometer. In Fig. 6 we can see the pulse distribution for the combined TACS in the electron beam extracted from the Serpukhov accelerator with the momentum 45 GeV/c.

24. Page 24 : Fig 5

25. Page 25 : Fig 6

26. Page 26 : The experiment shows that the proposed method can be used for electron beam generation in existing magnetic channels. The intensity of the electron beam was about 105 e/impulse with the energy spread  2%. Having obtained an electron beam with the highest energy at the moment the team under the direction of P.A. Cherenkov conducted an experiment at this beam for measuring of the total hadronic cross section at the energies 20–40 GeV. Fig. 7 shows what facilities were used in this experiment.

27. Page 27 : Fig 6

28. Page 28 : A distinctive feature of this experiment was an original tagging system of bremsstrahlung photons using the fact that electromagnetic processes (e+e–-pair production and bremsstrahlung) and photoprocesses (production of high energy hadrons) have different transverse dimensions [14]. The electron beam passed through a 2 meter long liquid hydrogen (or deuterium) target. Hadrons created in this target were detected with two hadrons detectors placed behind the target and having a central hole to let electromagnetic events through to the combined TACS. The value of the TACS signal was recorded when there was a simultaneous signal from one of the hadron detectors. Since the target was simultaneously the radiator of bremstrahlung radiaton and the target of the photohardronic process the energy of the interacted photon was determined as the difference between the energy of the incident electron and the electron energy measured with the TACS. Fig. 8 and 9 show experimentally measured cross sections σt(γp) and σt(γd) and calculated from these cross sections the neutron cross section σt(γn).

29. Page 29 : Fig8

30. Page 30 : Fig 9

31. Page 31 : Later experiments conducted at the electron beam of the Serpukhov accelerator involved ρ-mesons photoproduction at high energies. An original feature of the experimental facility consisted in simultaneous measurement of photoprocesses in two targets (hydrogen and beryllium) and using a tagging system for bremsstrahlung photons. A TACS separated muons from electrons and so allowed the muon background to be suppressed [15]. Later electron beams with the energy of hundreds GeV was generated and used in Fermilab, CERN etc. The staff of the Cherenkov's laboratory took part in some of these experiments. For example, they took part in the first experiment on J/ψ-photoproduction [16]. The swan song of P.A.Cherenkov the arrangement of collaboration between DESY and the HEP department of LPI on the H1 experimental facility. October 19, 1991 in 6.54 p.m. the H1 [17] luminosity detector recorded first e-p-collisions on the HERA collider [17]. This detector (Fig. 10) was made in the LPI and consisted of 2 TACC hodoscopes and one water counter. The bremsstrahlung process e + p → p + e' + γ was used as a monitoring process.

32. Page 32 : Fig 10

33. Page 33 : Unfortunately Pavel Alexeevich could not see this memorable day of fisrt e-p-collisions. January 6, 1990 the life of this brilliant figure in science and the pure nice person in life came to the end. But life goes on, and every day new areas of application for Cherenkov detectors are found. And the better we became familiar with high energy physics, the wider, the fuller Cherenkov detectors are used. Without doubt the main detectors at LHC will be RICH-detectros, total absorption Cherenkov detectors and perhaps Cherenkov detectors of some new type. For example, total absorption Cherenkov detectors to register particles in the maximum of the shower, combined scintillating-Cherenkov detectors etc. As an illustration. Fig. 11 shows a scintillating crystal in which a through-passing electron generates 3 types of radiation: • scintillation radiation (isotropic) • Cherenkov radiation (conical) • bremsstrahlung (practically at 0°)

34. Page 34 : Fig 11

35. Page35 : Bremsstrahlung and the Cherenkov radiation occur during the time approximately equal to the time of the passing of a particle (3·10-10 s), while the scintillation radiation does during the time more than 10-9 s. Using all three types of radiation can open up new possibilities. In my opinion, new possibilities may also appear when rectilinear spaces of magnet-optical channels and linear accelerators are considered as Cherenkov detectors, in a sense as RICH-detectors, working on the rarefied residual gas. This might make possible to create new extremely long Cherenkov detectors for solving such problems as charged particles beam alignment, monitoring and other problems at colliders and super high energy accelerators. In conclusion I would like to express my acknowledgment to A.S.Belousov, V. Baskov, V, Polyansky and A.Verdi for the assistance in preparing this report.

36. Page 36 : REFERENCES • O. Heaviside Electromagnetic Theory. L. 1922. • P.A. Cherenkov. Phys. Rev., 52, 378 (1937). • I.E. Tamm, I.M. Frank. ДАН СССР. 14, 109 (1937). • I. Seguinet, T. Ypsilantis CERN-LAA/91-04, 3/3/9. • V.P. Zrelov, Radiation of Vavilov-Cherenkov and its application in high energy physics, M. 1968. • P.A. Cherenkov. ДАНСССР, 372, (1954), 451. • E.M. Brumberg, S.I. Vavilov. Изв. АН СССР (1933) серия VII, 919. • Apployard R. “Oliver Heaviside. In pioneers of electrical communication” 1930. • I.M. Frank. Cherenkov-Vavilov radiation. Phys. Encyclopaedic Dictionary, M., Sov. Encyclopaedia, (1983), 850. • V.L. Ginsburg. ЖЭТФ 10, (1940) 589; 10 (1940) 608. • P.A. Cherenkov. Изв. АН СССР, серия физ. ОМЕН (1937) 455. • Y.B. Bushnin et al. ПТЭ, 1971, № 1, 65. • M.A. Markov. Preprint JINR, D-577, Dubna, 1960, 12. • B.B. Govorkov. Preprint FIAN, № 87, Moscow, (1972). • Preprint FIAN, № 31, Moscow, (1979); №52 Moscow, (1980). • T. Nash et al. Phys. Rev. Letter 36 (1976) 1233. • H1-Collaboration. Phys. Lett., B299 (1993) 374.