Gamma absorption at increasing energy levels

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  • Thread starter snorkack
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  • #1
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Which are main absorption modes of matter for gamma rays under about 500 MeV? And how strong is the absorption?

For low energies (under 20 MeV) the mechanisms are:
1) Photoelectric absorption - all energies from eV range onwards
2) Compton scattering - all energies enough to displace the electron
3) Electron-positron pair production - starting at 1022 keV
4) Photonuclear absorption - beginning at 1,7 MeV for Be-9, and edges going on to about 20 MeV for He-4

But then on? How absorptive is matter in the range of 40...400 MeV?
The next edges should be:
5) N+γ→N+π, from 140 MeV
6) Muon pair production, from 212 MeV+recoil, which is a lot
7) N+γ→Δ, about 300 MeV, but a broad one because Δ is a resonance, thus wide

Is any of these edges distinguished by a significant intensity compared to the background level of absorption due to 1...4, between the 30...100 MeV range?
 

Answers and Replies

  • #2
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Electron pair production stays the dominant process in regular matter. The LHC produces electrons with more than 1 TeV and they still form regular electromagnetic showers.
 
  • #3
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Found estimates for muon pair production. The ratio of electron and muon pair production above threshold turns out to be proportional to inverse square of mass - so 40 000 times less.
But this does not estimate the cross-section for pi meson production - directly or via delta resonance.
How much is that, compared to electron pairs?
 
  • #4
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Pair production is typically several barns (increasing with Z). Inelastic processes with a nucleus are smaller than a barn, and specific reactions are even smaller. I don't have numbers but it is negligible in most cases.
 
  • #5
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Pair production is typically several barns (increasing with Z). Inelastic processes with a nucleus are smaller than a barn, and specific reactions are even smaller. I don't have numbers but it is negligible in most cases.

Well, pion production edge due to blueshifted relic radiation is claimed to be a major limit on cosmic ray range/energy.
How prominent is that pion production edge over positron pair production in order for this claim to be plausible?
 
  • #6
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Well, pion production edge due to blueshifted relic radiation is claimed to be a major limit on cosmic ray range/energy.
How prominent is that pion production edge over positron pair production in order for this claim to be plausible?
That limits the range of protons. Protons have a weaker electric field than all other nuclei and the photon-electron interactions are completely missing in this scenario. In addition the energy loss from pion production is much larger than the energy loss from electron/positron pair production in this scenario - something that doesn't apply if we care about the photon.
 

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