An exploration of Cherenkov Radiation including a background on the discovery and current applications in Astrophysics and Particle Physics.

1. Cherenkov
Radiation
S T E P H E N O ’ R I O R D A N
US Department of

2. Outline of Presentation
EXPLANATION
OF CHERENKOV
RADIATION
FORMALISM
HISTORY OF THE
DISCOVERY
USES IN
CREATIVE
MEDIA
APPLICATIONS

3. Explanation of Cherenkov Radiation
• Electromagnetic radiation emitted when a charged particle passes through a
dielectric medium at a speed greater than the phase velocity of light in that medium.
• Basically it’s electrons moving FTL.
• The upper limit of c still exists but the speed of light in a medium could be 0.75c
(water) or 0.66c (glass). Charged particles can enter the medium at a higher speed
still.
• Cherenkov radiation is an effect that occurs as a result of the particle slowing down
• A common analogy is the sonic boom of an aircraft.

4. History of the Discovery
Cherenkov Radiation as conical wave front
was predicted by Heaviside & Sommerfeld
but forgotten due to restrictions imposed by
relativity.
1888
Discovered by Pavel Cherenkov who was
doing his doctorate thesis on luminescence of
uranium salt solutions.
1934
A theory was later developed by Igor Tamm
and Ilya Frank.
1937
The three won the Nobel Prize for it.
1958

5. History of the Discovery
• Cherenkov identified a blue glow around a radioactive preparation in water during
experiments. This was because he was using gamma rays to excite the uranium
salts rather than visible light.
• Marie Curie overserved a pale blue light in highly concentrated radium in 1910 but
didn’t investigate.
• In 2019 a team of researches discovered Cherenkov light from the vitreous humor of
patients undergoing radiotherapy. This explained patients reporting “flashes of bright
or blue light”.

6. The Actual Physics of it
• Classical physics shows that an accelerating charged particle emits EM waves.
• These waves form a spherical “wavefront” which will propagate at the phase velocity
of light in that medium. This phase velocity is given by c / n.
• When the charged particle enters the medium it polarises particles around it.
• These particles get excited and when they return to their ground state they emit the
gained energy as a photon.

8. We have two cases to consider
𝑣𝑝 <
𝑐
𝑛
• In this case the polarisation field is
symmetric around the moving
particle.
• This means the emitted wavefront
may be bunched up but they don’t
coincide or cross.
• We don’t have any interference
effects.
𝑣𝑝 >
𝑐
𝑛
• The polarisation field is asymmetric
along the direction of motion.
• This arises because the particles can’t
recover to their “normal” randomised
states.
• This results in constructive interference
and an observed cone-like light signal.

9. Maritime-Executive

10. https://thenavalarch.com

11. Wikipedia Commons

13. Cherenkov Radiation in a Vacuum
• So far we’ve talked about Cherenkov radiation occurring in mediums as the phase
speed of light is slower. This should mean it’s impossible in a vacuum, right?
• A vacuum is actually filled with empheral/virtual particles that momentarily move in
an out of existence.
• When a strong electromagnetic field is applied it can imbue the vacuum with an
effective anisotropic refractive index.
• This can allow Cherenkov Radiation to occur in a vacuum.

14. Godzilla: King of the Monsters

15. Creative Media • Radiation is commonly portrayed as green
in movies. This is a myth.
• As mentioned before Cherenkov
Radiation actually emits a blue glow in
Nuclear Pools.
• The green colour likely comes from a few
radioactive compounds which glow green
including radium that was used in clock
dials and uranium glass
• Uranium and Plutonium for example are
just grey metals.

16. Applications
• Cherenkov Radiation is used in open pool reactors to identify the remaining
radioactive of spent fuel rods.
• Astrophysics Experiments
• High-Energy Gamma Rays or Cosmic Rays
• Particle Physics Experiments
• Identifying particles in a particle accelerator.

17. Nuclear Reactor Maria

18. Astrophysics
Experiments
• When high energy cosmic rays hit the Earth’s
atmosphere Cherenkov Radiation occurs. This
creates a cone of light on the ground that can be
detected.
• Additionally the high energy particle interacts with
the atoms in the air which can create a cascade of
additional particle.
• Each of these particles can also create Cherenkov
light.
• Current instruments that detect this are:
• H.E.S.S.
• MAGIC
• VERITAS

19. Imaging Atmospheric Chernekov Telescope
• Used to detect photons with energys in the range of 50GeV to 50TeV
• Space based detectors are generally ineffective due to size limitation but IACTs
essentially use the Earth’s Atmosphere as the detection medium.
• The air shower produces a flash of radiation lasting between 5 and 20ns.
• The instrument usually comprises a large mirror which reflects the light onto an array
of photomultiplier tubes
• The most effective method is to have an array of telescopes spaced far apart. This is
because muons that are produced by the air shower create Cherenkov light which
can be difficult to filter out. This light cone is extremely narrow though.

20. Particle
Physics
Experiments
• Particle Accelerators like the LHC in CERN can
accelerate particles up to speeds very close to c.
• When collisions occur and new fundamental
particles are attempting to be identified,
Cherenkov Radiation can be used to help identify
them.
• If the particle can be directed to a medium of a
known refractive index, then the velocity can be
categorised based on whether it exhibits
Cherenkov Radiation or not. This is a threshold
counter.
• The HMPID in ALICE which is one of the six
experiments at the LHC.

21. HMPID-ALICE
• Used to identify charged particles such as pions, kaons, protons.
• HMPID is a Ring Imaging Cherenkov detector with two main parts:
• Radiator medium
• Photon detector
• It has 7 RICH counters each 1.4m x 1.3m each.
• It measures the angle of emission 𝜃 that we discussed before to calculate the
velocity.

22. Summary • History of the discovery
• Explanation and formalism
• Hollywood not understanding radiation
• Applications
• Open Pool Reactors
• Astrophysics Experiments (IACTs)
• Particle Physics Experiments (HMPID-ALICE)