Gamma absorption at increasing energy levels

  • I
  • Thread starter snorkack
  • Start date
  • #1
1,621
230
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
35,267
11,530
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
1,621
230
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
35,267
11,530
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
1,621
230
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
35,267
11,530
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.
 

Related Threads on Gamma absorption at increasing energy levels

  • Last Post
Replies
13
Views
3K
Replies
1
Views
4K
Replies
4
Views
604
  • Last Post
Replies
3
Views
782
Replies
1
Views
3K
  • Last Post
Replies
7
Views
2K
  • Last Post
Replies
8
Views
3K
  • Last Post
Replies
1
Views
3K
  • Last Post
Replies
2
Views
4K
Replies
5
Views
2K
Top