Failed Compton scattering?

Hi all, I mentioned this trouble I’m having here already:


Essentially in my simulation, it appears that my 3MeV gamma ray under goes Compton scattering almost exclusively.

For every scattering position, GEANT4 does not however always produce an electron it appears. I’ve been thinking this is more down to my understanding of the physics, but if a photon undergoes compton scattering, shouldn’t an electron always be produced (assuming it is a loosely bound electron)?

I understand that in the energy region 2-4 MeV this corresponds to a region where the absorption and scattering probabilities for compton scattering are close together as in the picture below:

So why does this happen? Any help, either GEANT4 related or Physics related, would be super appreciated to clarify my confusion. Thank you so much!

_Geant4 Version:_11.0.0
_Operating System:_MacOS
_Compiler/Version:_Apple clang version 14.0.0
_CMake Version:_3.25.1

How do you have your production cuts set? If the secondary electron (from “ionization”, maybe not from Compton scattering) is too low in energy, it may have an estimated range in your target below your production cut. In that case, it will not be created as a “tracked particule”, but it’s energy will be assigned to the energy deposit of the photon’s step.

Thanks @mkelsey, so my electron has a production cut of 1m, so it shouldn’t be this I assume. I show an example event to illustrate the problem (gamma = green, electron=red, opticalphotons=grey)

What is your material? A production cut of 1 m is almost certainly killing nearly all your secondary electrons before they ever get tracked.

If you reduce your electron production cut to 10 cm, what do you see? 1 cm?

@mkelsey water is the material. Ahhh I see, smaller is better! I moved the cut down to micrometer scale and they appear now! Thanks so much @mkelsey! :slight_smile:

Please, notice that, by default, production cut is applied only to electrons from ionization and photons from Bremstrahlung. Compton scattering itself in independent of production cut.

Below, a macro for example TestEm14, its printout, and the plot of e- spectrum.
It shows that Compton scattering works as expected, even with a cut of 1 meter.

There is an option (/process/em/applyCuts) to generalize production threshold to all electromagnetic processes. This option is not by default, and you should have a strong reason to use it.

compton.mac.txt (811 Bytes)
compton.out.txt (2.9 KB)

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