The Discoveryof CherenkovRadiation

Roger Huang

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The Discovery of Cherenkov Radiation

Roger Huang

April 22, 2020

The Discoveryof CherenkovRadiation

Roger Huang

2/18

Cherenkov’s Scientific Beginnings

Pavel Alekseevich Cherenkov began postgraduate studiesat the Institute for Physics and Mathematics in Leningradin 1930, and began working under direct supervision ofSergei Ivanovich Vavilov in 1932

This institute was transformed into the Lebedev Institutein Moscow in 1934, with Vavilov as director

Cherenkov chose to study the luminescence of uranyl saltsolutions under the gamma-ray radiation of radium

The Discoveryof CherenkovRadiation

Roger Huang

3/18

Investigating Luminescence

Lumniscence is a weak glow caused when molecules areexcited by some external source and then release lightwhen decaying back to their ground state after some finitetime

This glow is weak, close to the human visibility threshold,and photomultipliers had not yet been developed

Vavilov and Brumberg had developed the optical wedgemethod to quantify this glow, featuring the human eye hasthe measuring instrument

The Discoveryof CherenkovRadiation

Roger Huang

4/18

The Experimental Setup

Vessel 1 contains the saltsolutions under study, withslots for radium samples tobe inserted below and tothe side

A Glan prism at 6 is usedto measure polarization

An optical wedge (opaqueon one side andtransparent on the other)is inserted into 4, movingperpendicular to thisfigure’s plane, to absorbsome portion of the light

The Discoveryof CherenkovRadiation

Roger Huang

5/18

The Experimental Procedure

The observer (Cherenkov) prepares by sitting in thecompletely dark room for 1-1.5 hours to attain maximumeye sensitivity

The intensity of the glow is measured by moving theoptical wedge until the light is no longer visible

The position of the wedge (corresponding to the amountof light it absorbs) is used as the measurementThe observer must be very consistent with their eye’ssensitivity!

An assistant maneuvers the wedge to avoid experimenterbias

The Discoveryof CherenkovRadiation

Roger Huang

6/18

A Strange Finding

In 1933, Cherenkov happened to do a test with a puresolvent in the vessel, sulfuric acid

He observed a glow of comparable intensity to the uranylsalt solution in the same sulfuric acid

Vavilov suggested investigating other solvents as well, tosee if this background was due to some contamination

Cherenkov observed the same glow intensity for everysolvent he tried, including thrice-distilled water

The Discoveryof CherenkovRadiation

Roger Huang

7/18

Investigating More

Vavilov suggested using some standard techniques forluminescence investigation he had helped develop

Adding quenchers and varying the temperature of asolution can suppress luminescence, as the excited stateenergy can be lost through collisions before it decays backto ground state

The Discoveryof CherenkovRadiation

Roger Huang

8/18

Investigating More

Temperature variation and addition of large amounts ofquenchers both had no notable effect on the observedglow in pure solvents

⇒ The glow must be a prompt emission compared to normalluminescence time scales

The light was mostly polarized parallel to the gamma-raydirection

Measurements with color filters showed the light wasmostly blue

These results were published in 1934, with Vavilovpostulating the glow was due to bremsstrahlung ofelectrons

The Discoveryof CherenkovRadiation

Roger Huang

9/18

Beginnings of the Theory

Further experiments with a β source confirmed the effectwas caused by the electrons

A measurement with the vessel under a magnetic fieldshowed the light’s polarization direction and intensity bothchanged with the magnetic field

⇒ The light is highly directional, related somehow to theelectrons’ motion

Ilya Frank, who had assisted Cherenkov with some of themeasurements, mentioned this directionality to Igor Tamm

The Discoveryof CherenkovRadiation

Roger Huang

10/18

The Theory

Tamm immediately remarked that there must be coherentemission over the electron path comparable to thewavelength of the light wave

The angular spread of radiation has order of magnitudegiven by ∆φ = λL

This inspired Frank’s development of a qualitative modelusing the Huygens principle

This explained the directionality of the light by predicting aconical emission given by cos θ = cnv

The Discoveryof CherenkovRadiation

Roger Huang

11/18

Developing the Theory

Tamm and Frank fully developed this theory usingMaxwell’s Equations and submitted the paper in early 1937

Direction of the radiation given by

cos θ =c

n(ω)v

Intensity given by

I (ω)dω = ve2

c2[1 − c

2

v2n2(ω)ωdω]

These relations give rise to the condition that theradiation only occurs for v > cn(ω)

The Discoveryof CherenkovRadiation

Roger Huang

12/18

Cherenkov Radiation’s Angular Spread

An aside: later on, in the1950s V P Zrelov did aclean demonstration of thecos θ = cn(ω)v relationusing a proton beam andphotographic film

The Discoveryof CherenkovRadiation

Roger Huang

13/18

Confirming the Theory

In 1937 Cherenkov performed additional measurementsthat matched the Frank-Tamm theory’s predictions

These results were submitted to Nature and rejected, butwere published in Physical Review

The existence of Cherenkov radiation was confirmed a yearlater by American physicists Collins and Reiling, usingaccelerator electrons, although they seemed to rejectFrank and Tamm’s theory

They wrote “Electrons constantly lose energy as they passthrough a medium. The resultant acceleration isresponsible for Cherenkov’s radiation.”Collins and Reiling also established the term “CherenkovRadiation”, though some in Russia call it“Vavilov-Cherenkov Radiation”

The Discoveryof CherenkovRadiation

Roger Huang

14/18

Afterword

Despite the difficulty of humaneye-based measurements, none ofCherenkov’s work was found tobe in error by any latermeasurements

Vavilov, Cherenkov, Frank, andTamm all received the StalinPrize of First Degree in 1946 forthis work

Tamm, Frank, and Cherenkovwere awarded the Nobel Prize in1958 “for the discovery and theinterpretation of the Cherenkoveffect”

The Discoveryof CherenkovRadiation

Roger Huang

15/18

Prior Work

On the experimental side: in 1910 Marie Curie noticedblue glows from vials with concentrated radium, but paidno special attention to them

In 1926-1929 the French physicist Mallet measured thisglow had a continuous spectrum, but he was unable todetermine its nature and abandoned the investigation

Vavilov’s particular expertise allowed him to guideCherenkov’s experiments that determined the manner inwhich Cherenkov radiation was distinct from luminescence

The Discoveryof CherenkovRadiation

Roger Huang

16/18

Prior Work

On the theoretical side: in 1904 Sommerfeld calculatedthat a charged particle moving faster than light wouldradiate directional EM waves

But special relativity was developed shortly after, and thiswork was forgotten

In 1889, Heaviside calculated a charged particle movingfaster than the speed of light in a medium would radiateEM waves, even getting the angle of the radiation correct

But he did not take dispersion into account, and so did nothave the correct total radiation lossMore importantly, no charged particles had even beendiscovered at the time, and near-light-speed movementseemed inconceivable

The Discoveryof CherenkovRadiation

Roger Huang

17/18

Cherenkov Radiation Today

Cherenkov radiation’s universality and directionality havemade it a very useful tool in detectors since thedevelopment of photomultipliers

Famously was key to the discovery of neutrino oscillationsin Super-Kamiokande

The Discoveryof CherenkovRadiation

Roger Huang

18/18

References

Boris M Bolotovskii 2009 Phys.-Usp. 52 1099. “VavilovCherenkov radiation: its discovery and application”

Elena P Cherenkova 2008 Nucl. Instrum. Meth. A 595 (1)8-11. “The discovery of the Cherenkov radiation”