Ultra-high energy photon interactions with matter

[pervect]

While this was inspired by another thread, I think the question is different enough that it can be asked separately. It's also more suited to this forum than the forum where the question that inspired this one was asked.

Wiki gives four possible interactions for interactions of gamma rays with matter in https://en.wikipedia.org/wiki/Gamma_ray_cross_section#Photoelectric_effect_cross_section.

These are described as "photoelectric effect", "compton scattering", "pair production", and "triplet production".

Which of these mechanism (if any of the above) would dominate at unreasonably high photon energies? It appears it should be a function of $k = h v / m_e c^2$, v being the photon frequency, but it's hard for me to tell what the asymptotic behavior of cross sectio vs k is. Note: while I know some physics at greater than the "I" level, particle physics isn't one of the fields where I have advanced knowledge.

By "unreasonably high energies", I mean I'm interested in photons with energies on the order of 1 gram * c^2, similar to the energies of the highest energy cosmic rays.

(My source for cosmic rays with this order of magnitude of energy is the wiki article on the GZK limit, https://en.wikipedia.org/wiki/Greisen–Zatsepin–Kuzmin_limit).

Also of interest is the highest energy photons detected. A google search gives 450 Tev and https://www.science.org/content/article/highest-energy-light-ever-seen-traced-crab-nebula. Is this popularization trustworthy?

 

[DaveE]

Also of interest is the highest energy photons detected. A google search gives 450 Tev and https://www.science.org/content/article/highest-energy-light-ever-seen-traced-crab-nebula. Is this popularization trustworthy?

This? Phys. Rev. Lett. is pretty trustworthy

 

[Vanadium 50]

There are other processes that turn on at higher energy - the base processes you describe + additional radiation and photon-gluon fusion are two.

 

[PAllen]

By "unreasonably high energies", I mean I'm interested in photons with energies on the order of 1 gram * c^2, similar to the energies of the highest energy cosmic rays.

?

Just a side detail - this isn’t the energy you mean. This energy is about the energy of the Hiroshima atomic bomb. You mean this many (c^2 in cm/ sec) e.v., not this many ergs.

 

[mfb]

With enough energy you can produce all the particles we know (and probably some unknown ones, too). For specific cross sections ("probabilities") ask your local theorist, there are too many processes to list.

 

[pervect]

Just a side detail - this isn’t the energy you mean. This energy is about the energy of the Hiroshima atomic bomb. You mean this many (c^2 in cm/ sec) e.v., not this many ergs.

I had just started to wonder about that issue, i.e. if we really had cosmic ray protons of that energy, why were they were not more - erm - impactful. Even if they were not quite like atomic bombs, I would naievely think that the cascade showers would have been fairly destructive. As in "Death rays from space", not just flashes on sky survey instruments. Though I couldn't guess how far the energy might be spread out - the total irradiation of the whole Earth from the solar constant is on the order of kg/sec, last I calculated it.

A factor of (1ev/erg), about 10^-12, would explain the discrepancy though.

 

[pervect]

There are other processes that turn on at higher energy - the base processes you describe + additional radiation and photon-gluon fusion are two.

I suppose what I'm really interested is the behavior of the total cross section vs energy. Does it increase, decrease, or stay the same at high energies? And - how fast.

Based on your comments, I've concluded that I don't know and don't have enough information to figure it out at the moment.

 

[mfb]

In the range of more reasonable energies (~TeV) the cross sections (or interaction length, if we talk about a mixture of targets) don't depend that much on the energy. I don't expect a change at even higher energies.

 

[Vanadium 50]

I suppose what I'm really interested is the behavior of the total cross section vs energy. Does it increase, decrease, or stay the same at high energies? And - how fast.

Every exclusive process falls with energy - that's just dimensional analysis. The number of allowed exclusive processes, however, grows with energy. The net effect is a slow growith with energy - roughly logarithmic.