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Cherenkov Radiation

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Re: Cherenkov Radiation

#91

Post by physicsdude »

RubberbandSoul wrote: post #88
Sharlin wrote: post #70

Earlier quoted context omitted.

Maybe the coolest (heh) application of Cherenkov radiation is neutrino telescopes like IceCube, detecting tiny flashes of light in cubic kilometers of ultra-clear Antarctic ice, caused by vanishingly rare interactions between neutrinos and ordinary matter – and to exclude non-neutrino interactions, they look down , using the entire Earth as a shield guaranteed to stop anything that’s not a neutrino!

While I knew that IceCube existed it didn't occur to me until know that I have no idea how they filter out collisions from particles that comes from above or sideways. Is the resulting flash of light not omnidirectional? Is it polarized differently?

One of the amazing things about Cherenkov radiation is that it is not omnidirectional. There are processes (fluorescence, or scintillation, for example) which result in isotropic emission of light from materials. But Cherenkov light is emitted along a conical surface about the axis of the charged particle's direction of travel. As a sister comment said, muons tend to move linearly, so this light stacks up in a particular direction. Electrons, particularly at lower energy, scatter more, and that stacking gets washed out a bit.

That directionality is very useful. The directionality of a muon or electron produced from a neutrino interaction is correlated with the direction of the incident neutrino. That's interesting for the astrophysical kind of questions that IceCube is addressing, and it's also useful in other contexts (background rejection for fundamental physics searches, monitoring of nuclear reactors via neutrinos, ...).

Cherenkov radiation is indeed polarized, but to my knowledge that hasn't ever been made use of.

Tangentially: there's also some really cool work being done on detectors which utilize both Cherenkov and scintillation light at the same time, exploiting the directional (and other) differences to extract the most information possible out of each event.

Re: Cherenkov Radiation

#92

Post by gattr »

loopies wrote: post #76
post-it wrote: post #55

Earlier quoted context omitted.

We know light is not being absorbed and re-emitted, because re-emission sends light in a random direction.

That is interesting. But then what is the explanation? Medium is not really anything concrete, we call that a collection of atoms isn't it? By default there is only one medium, the vacuum of space, in which there can be various densities of "stuff", particles/atoms/molecules. So then, it becomes some sort of quantum thing, a probability of photons bumping/being captured/emitted or?

Simplifying from memory, light propagation in medium can be treated on two levels (some phenomena require the 2nd, quantum, level):

- classical (Maxwell): electrons in the medium oscillate out of phase in response to the incident wave; this results in a new wave, propagating somewhat slower

- quantum (QED): incident photon couples to matter excitation (an exciton), producing a pseudo-particle called a polariton; it has a non-zero effective mass, resulting in propagation slower than c

As mentioned above, it absolutely isn't "absorbing and re-emitting". If that was the case, transparent materials would not exist (only milky/clouded ones), as re-emission is in random direction. However, photon propagation in the Sun is just that, a (very long, counting from the core to photosphere) random walk.

Re: Cherenkov Radiation

#94

Post by teravor »

lowestprimate wrote: post #93

Can Cherenkov radiation carry information? If it can does this mean information can travel faster than light? Question from a physics newbie

it's only faster in a medium, which technically isn't even light but rather electron interactions.

Re: Cherenkov Radiation

#95

Post by maxnoe »

lowestprimate wrote: post #93

Can Cherenkov radiation carry information? If it can does this mean information can travel faster than light? Question from a physics newbie

Cherenkov radiation is UV to visible blue light, it travels at the speed of light in the medium, which is lower than the speed of light in vacuum.

Only the charged particle that induces the emission travels faster than the local speed of light.

Re: Cherenkov Radiation

#96

Post by jacquesm »

hakonjdjohnsen wrote: post #50

A fun fact about Cherenkov radiation is that research on making efficient Cherenkov detectors in the 1960s led to the development of optical principles still used to design illumination systems and solar concentrators today. The late professor Roland Winston worked on this problem and discovered a geometry that could concentrate the light from a diffuse source like Cherenkov radiation to a detector with near-ideal pe…

I worked for a little bit with a group making optical sieves, this stuff is mindblowing to a non-trained-in-the-art outsider. I can easily see losing a couple of decades in that rabbit hole.

Re: Cherenkov Radiation

#97

Post by RubberbandSoul »

physicsdude wrote: post #91
RubberbandSoul wrote: post #88

Earlier quoted context omitted.

While I knew that IceCube existed it didn't occur to me until know that I have no idea how they filter out collisions from particles that comes from above or sideways. Is the resulting flash of light not omnidirectional? Is it polarized differently?

One of the amazing things about Cherenkov radiation is that it is not omnidirectional. There are processes (fluorescence, or scintillation, for example) which result in isotropic emission of light from materials. But Cherenkov light is emitted along a conical surface about the axis of the charged particle's direction of travel. As a sister comment said, muons tend to move linearly, so this light stacks up in a partic…

Right, so without knowing much about physics I'm guessing that momentum is preserved and IceCube works a little bit like a giant cloud chamber but looks for trails of Cherenkov radiation rather than an ionized particle trail.

Re: Cherenkov Radiation

#98

Post by generuso »

RubberbandSoul wrote: post #88
Sharlin wrote: post #70

Earlier quoted context omitted.

Maybe the coolest (heh) application of Cherenkov radiation is neutrino telescopes like IceCube, detecting tiny flashes of light in cubic kilometers of ultra-clear Antarctic ice, caused by vanishingly rare interactions between neutrinos and ordinary matter – and to exclude non-neutrino interactions, they look down , using the entire Earth as a shield guaranteed to stop anything that’s not a neutrino!

While I knew that IceCube existed it didn't occur to me until know that I have no idea how they filter out collisions from particles that comes from above or sideways. Is the resulting flash of light not omnidirectional? Is it polarized differently?

The cone of Cherenkov radiation produces arcs of "bright pixels" in the detector.

There are very good illustrations of this if you search for "Super-Kamiokande Cherenkov ring." That's a Japanese neutrino telescope with a bunch of huge photomultipliers lining the walls of a giant underground cistern filled with water. Ice Cube has similar light detectors, probably from the same Japanese manufacturer, that were lowered on strings into wells melted into ice.

Re: Cherenkov Radiation

#99

Post by scotty79 »

maxnoe wrote: post #86
scotty79 wrote: post #53

Earlier quoted context omitted.

Sonic boom doesn't explain what causes sound only how it piles up. Similarily I don't see how it explains the glow. Photons get generated regardless of whether they pile up or not. It's a consequence of particles bumping into atoms not the whatever speed of light might be in this medium. How is piling up important?

Very shortly summarized, only if the particle is faster than the local speed of light, you get constructive interference between many atoms that were polarized by the moving particle. For a slower than light particle, you also get emission, but it is completely random and thus does not give the well defined emission in a cone of Cherenkov radiation. For a faster than light particle, the spherical suddenly line up to…

Thank you. That's a very good explanation.

Re: Cherenkov Radiation

#100

Post by loopies »

gattr wrote: post #92
loopies wrote: post #76

Earlier quoted context omitted.

That is interesting. But then what is the explanation? Medium is not really anything concrete, we call that a collection of atoms isn't it? By default there is only one medium, the vacuum of space, in which there can be various densities of "stuff", particles/atoms/molecules. So then, it becomes some sort of quantum thing, a probability of photons bumping/being captured/emitted or?

Simplifying from memory, light propagation in medium can be treated on two levels (some phenomena require the 2nd, quantum, level): - classical (Maxwell): electrons in the medium oscillate out of phase in response to the incident wave; this results in a new wave, propagating somewhat slower - quantum (QED): incident photon couples to matter excitation (an exciton), producing a pseudo-particle called a polariton; it h…

Thank you, that is very interesting! I remember seeing something about absorption/emission and seemed to make some logical sense. But indeed if emission is in a random direction then transparent materials wouldn't make sense.
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