High energy gamma rays

mathman

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Is there any on-line reference which describes the interaction of high energy (> 100 Mev) gamma rays with matter. These are gamma rays from gamma ray bursts, etc. in contrast to low energy from nuclear radiation.
 
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I am not sure what you are looking for, but the NIST website has a bunch of databases of photon interaction cross sections with elemental targets and compounds.

http://physics.nist.gov/PhysRefData/contents-xray.html [Broken]
 
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Astronuc

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mathman

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The nist data is for nuclear radiation gamma rays (up to about 10 Mev). It does not have anything for 100 Mev and higher.

The Astronuc references describe the sources of these (cosmic origin) gamma rays, but not the interactions, such as the nist data for the nuclear radiation.

Mt gut feeling is that no one has worked it out in detail. I suspect the principal reactions are Compton scattering and pair production. The main difference from nuclear radiation is that the pair production might give pairs other than electron-positron, since there is much more energy available.
 
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The nist data is for nuclear radiation gamma rays (up to about 10 Mev). It does not have anything for 100 Mev and higher.
This is incorrect.

From the NIST website:
NIST said:
This paper describes a web program called XCOM which carries out this task quickly for any element, compound or mixture, at energies between 1 keV and 100 GeV.
You can run the XCOM software through your browser and get both text data and plots.

In addition, in the link I posted it has the bibliography of photon attenuation experiments. The energy range covered is from 10 eV to 13.5 GeV.
 
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mathman

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Norman
You're right. I looked at it too quickly. Most of the references are for nuclear energy gamma rays, which I was already aware of. I missed the references you cited. Thank you.
 
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Astronuc

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The nist data is for nuclear radiation gamma rays (up to about 10 Mev). It does not have anything for 100 Mev and higher.

The Astronuc references describe the sources of these (cosmic origin) gamma rays, but not the interactions, such as the nist data for the nuclear radiation.

Mt gut feeling is that no one has worked it out in detail. I suspect the principal reactions are Compton scattering and pair production. The main difference from nuclear radiation is that the pair production might give pairs other than electron-positron, since there is much more energy available.
I looked at the NIST data, and it seemed that they were extrapolating much above 100 MeV. I'm not sure how they get 1 GeV (and greater) gammas, but I'll have to look closer.

I know the papers/sources didn't elaborate on the interactions, but I figured one could contact the authors who would hopefully know the interactions or someone who does, or at least where one can find such a discussion.

I believe Compton and pair-production are the reactions, and I think I saw one mention of muon pair-production - at very high energies of course (> 2(105.7 MeV) = 211.4 MeV).

Of course, there could be photo-neutron interactions as well, e.g. photo-disintegration of deuterons.
 
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You all are correct. The most dominant effect is pair production, which begets more pairs, etc., called an electromagnetic cascade. Compton scattering dominates below about 10 MeV, but pair production dominates above say 20 MeV. The maximum of the electromagnetic cascade is usually several radiation lengths from the start, and trails off after maybe 15 radiation lengths. The shower is roughly half photons and half electrons. Positrons can annihilate in flight and produce two more gammas.The radiation length in air is about 37.15 grams per cm^3.
In addition, there are two photo-neutron reactions in nitrogen and oxygen; (gamm,n) in nitrogen leads to a free neutron and N^13, and (gamma,n) in oxygen leads to a free neutron and O^15. These reactions peak at about 12-15 MeV with a cross section of roughly 15 barns. The threshold may be about 6 or 8 MeV.
 
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I looked at the NIST data, and it seemed that they were extrapolating much above 100 MeV. I'm not sure how they get 1 GeV (and greater) gammas, but I'll have to look closer.
At the lab we get them from coherent bremsstrahlung in a crystal, and bending out the electrons. Coherent gammas come in sharp peak, and they can also be polarized.
 

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