Cherenkov radiation detection

[stargazer3]

For a ground-based Cherenkov radiation detection, what are the theoretical/practical constraints on the gamma-ray energies we can detect? Simply speaking, if we have something like an IACT observatory, how do we know it's low and high energy detection limits? Is the higher one due to Cherenkov radiation shifting away from optical/UV region as incident gamma-ray is more energetic (meaning that such a bust still can be detected by X-ray observatories)? What about the lower one? For IACT, it's typically about 50-100 GeV, any particular reason for that?

And do we encounter pair-production mechanisms other than γ → β- + β+? Also, what happens to the positron? Does it produce Charenkov radiation or is it annihilated too fast for that?

Just to be clear, I'm asking about photon-caused Cherenkov radiation only, it'll be fun to hear any of these neutrinos/protons/electrons/whatever else scenarios too.

 

[Bobbywhy]

stargazer3, I found this material. Hopefully it addresses your questions.

"Photon interactions with matter.

Different interactions dominate for different photon energies, as shown in Figure 2. In rough order of increasing energy they are:

1. Coherent elastic scattering ( COH). This comprises Rayleigh scattering from atomic electrons together with Thompson scattering from nuclear charge. Such processes do not excite atoms or cause energy loss, so they are not useful for particle detection.

2. Photo-excitation. The photon may be absorbed by an atom, exciting it to a higher state. This process shows strong absorption resonances for photon energies which correspond to atomic transitions. The cross section is not shown but would be dominantly in the low energy region.

3. The photoelectric effect (). The photon is absorbed by an atom and expels an electron. The cross section depends strongly on atomic charge number Z and at high energies varies roughly as Z5. It may be seen that for 1 MeV photons it is much higher for lead than for carbon.

4. Compton scattering ( INCOH). The photon scatters from an electron which recoils and carries off a fraction of the photons energy. A scattered photon also will leave the interaction (unlike the photoelectric process) but with reduced energy. The cross section is shown as INCOH and is significant for energies well above the electron binding energy, so the atomic electrons may be treated as effectively free.
The kinetic energy T of an electron of mass me, recoiling when a photon of energy E is scattered at an angle , is


The cross section is calculated per atomic electron, so the cross section per atom is  Z. It may be seen from the figure that the cross sections INCOH for Pb and Carbon are in the ratio 82:6.

5. Pair production (Kn). When a photon has energy greater than twice the rest mass of an electron it has enough energy to create an electron and its anti-particle, a positron. This is a sort of photoelectric effect, but instead of the electron being bound in an atom, it is bound with the positron in the vacuum. A photon cannot create an electron-positron pair in free space, as the process cannot conserve momentum and energy. It happens near a nucleus which absorbs some the surplus momentum. A heavier nucleus takes less recoil energy, so the threshold for the process, Kn in the figure, is higher for carbon than lead as carbon nuclei carry off more energy. The surplus momentum from pair production can also be removed by an electron (Ke in the figure) but this has a higher threshold because of the low electron mass.

6. Photonuclear absorption (PH,N). This is a form of photoelectric effect where the photon is absorbed in a nucleus. Photons with energy of 10 MeV or more ( rays) may excite resonant states in the nucleus. The cross section is generally small but peaks in the region of the nuclear “giant resonance”.

As noted, a fast charged particle is surrounded by a cloud of virtual photons and whether these will interact with atoms depends partly on the interaction between the photons and the atoms. It will also depend on the propagation of the photons in the medium, and it is this to which we must next turn our attention."

http://www.google.com/url?sa=t&rct=...PFkWunnPk9TJlZbSA&sig2=-mwQI-hBhT15iVSBWjzuhA

 

[mfb]

The spectrum of Cherenkov radiation does not depend on the energy of the charged particle (at least not in first order), you get a lot of blue and near UV light for all energies. At higher photon (not particle!) energies, the refractive index is too small to get Cherenkov radiation.

For IACT, it's typically about 50-100 GeV, any particular reason for that?

Probably the sensitivity limit. Lower energy means lower number of fast particles and lower number of Cherenkov photons.
I don't think there is a real upper limit for detection. However, very high-energetic particles are extremely rare, so the rate gets too small to catch more than a few.

And do we encounter pair-production mechanisms other than γ → β- + β+?

In cosmic rays or in accelerators? Proton/antiproton pair production is common in accelerators (but usually not by photons), and should happen in cosmic rays, too. Pions are produced, too, but not always in pairs. Heavier particles can be produced, too, but that is rare.

Also, what happens to the positron? Does it produce Charenkov radiation or is it annihilated too fast for that?

Produces Cherenkov radiation and Bremsstrahlung, too - as high-energetic particles, electrons and positrons are very similar. The annihilation cross-section is small for high-energetic collisions, so most positrons will slow down first.

 

[stargazer3]

Thank you both for the answers, I was a bit afraid that the topic will sink and I'll have to look elsewhere. I'll look into the Cherenkov light wavelength dependence on particle speed, it looks surpising that the relation between two is so weak.

In cosmic rays or in accelerators? Proton/antiproton pair production is common in accelerators (but usually not by photons), and should happen in cosmic rays, too. Pions are produced, too, but not always in pairs. Heavier particles can be produced, too, but that is rare.

In cosmic rays with photon as an initial particle. The reason I'm asking this is that in simulations of photon cosmic ray air showers (like http://astro.uchicago.edu/cosmus/projects/veritasshowers/) the whole ray seems to be electron/positron only.

Produces Cherenkov radiation and Bremsstrahlung, too - as high-energetic particles, electrons and positrons are very similar. The annihilation cross-section is small for high-energetic collisions, so most positrons will slow down first.

Hm. So the Cherenkov radiation resulting from pair production results in the interference pattern, correct?

Edit: Oops, I forgot to write the most important part: these detectors DO have an upper energy detection limits, any ideas on why they are there?

 

[mfb]

Pair production and [electron or positron] -> [electron or positron] + photon are the dominant effects, so I think the other contributions are neglected.

Hm. So the Cherenkov radiation resulting from pair production results in the interference pattern, correct?

Which interference pattern, and which "Cherenkov radiation resulting from pair production"?

Edit: Oops, I forgot to write the most important part: these detectors DO have an upper energy detection limits, any ideas on why they are there?

Can you provide a source for that?

 

[stargazer3]

Which interference pattern, and which "Cherenkov radiation resulting from pair production"?

Oh, I'm just asking if the newly created electron-positron pair is radiating at the same frequency, I didn't imply that we should observe the resulting interference.

Can you provide a source for that?

Yes, I think I can.
HESS IACT telescope has an upper limit of ~10 TeV.
VERITAS actually has same 10 TeV threshold for optimal performance.
MAGIC has 30 TeV, and the reason is, as Wikipedia claims, a larger mirror (but I still don't get the reason behind it)

 

[mfb]

Hmm, not very convincing.
The VERITAS publication gives some hints why they might quote an upper limit:

For a successful detection, they want to see the showers in multiple telescopes. A higher energy decreases the width of the showers, so the detection probability might go down. If you look at figure 4, the collection area has its peak at ~5 TeV and might go down a bit afterwards.

Another interesting hint is figure 3: The sensitivity of flux measurements as function of energy. The sensitivity improves with increasing energy, but the spectrum of a real source ("Markarian 421") goes down even quicker, so the relative uncertainty hits 100% somewhere at ~20 TeV.

 

[stargazer3]

At higher photon (not particle!) energies, the refractive index is too small to get Cherenkov radiation.

Pardon me, can you please explain this bit in more detail?

Also, I think I've found the right reference for a higher energy limit cause. mfb, you're right, the more energetic gamma-ray is, the smaller the width of the shower. The emitted light is very faint, and Cherenkov radiation doesn't last till the surface. For example, if we consider an electron at the altitude of 10 km, the Cherenkov radiation conic angle would be about 1° due to very small lateral deviation of the pair production products. That somehow corresponds to a detection area of only 120 square meters at 2 km altitude, the reason for that being low flux density of Cherenkov photons (~hundreds per square meter for 1TeV γ-ray shower), atmospheric absorption and scattering. Low flux density also explains why each telescope in the array has such a large reflecting area.

 

[mfb]

Cherenkov radiation occurs when the particle is faster than the speed of light in the medium - but that speed is frequency-dependent!

Therefore:
Cherenkov radiation at a specific frequency occurs when the particle is faster than the phase velocity of light with this frequency in the medium

Especially: If the phase velocity exceeds the speed of light in vaccum, no particle will emit Cherenkov radiation at that frequency.