Hi everyone.
My master thesis group and I are analyzing data from a PET simulation (back-to-back 511keV photons written with GATE macro) in order to recover all the single Compton events.
We are currently stucked because of a weird problem with the energy deposition.
Among ~5.5M “good” Compton events, we noticed ~100k events with energy deposition greater than the 511keV Compton edge, i.e. >340keV.
We have already checked that these events are Compton (we are 100% sure).
My question for you: is it possible that this can be due to a physical effect included in the emstandard_opt4 physics list used for the simulation?
below, a macro for example TestEm14, its printout and the 2 plots.
It is the final state of Compton alone. martelli.mac.txt (529 Bytes) martelli.out.txt (1.6 KB)
1- in TestEm14, PhysicsList::AddPhysicsList(), you can replace the call to PhysListEmStandard(name) by G4EmStandardPhysics_option4()
(I did the exercice …)
2- sorry for the question. Are you sure that you select events with a single Compton alone ? no second gamma interaction (photoelectric or Compton) ?
Dear Maire,
I can confirm that we’re selecting only photons where the first hit has process name Compton and the 2nd photoelectric absorption (in a separate hit). However, even if we didn’t have the 2nd hit, the compton energy deposit should not be higher than the compton edge!
Given the exact same filters and simulation, we see some energy depositions between 340 and 360 keV for 511 keV photons (and a very small numbers even higher) with EmStandardPhysics_option3 and option4, but not a single event with option 1.
We read that option4 implements doppler broadening, which might give this extra kick… but what about option 3 that shouldn’t have it?
The auger e- and the gamma fluorescence are not things that I expect to give 20 keV differences (if I’m not mistaken).
Macro below for TestEm11 confirms the observations
of Mattia and Luca. At the level of few per mille, the energy spectrum of Compton electrons overcomes the “theoritical” tmax value (here tmax = 341 keV).
This phenomena happens in any physics list; it seems inherent to G4ComptonScattering process itself. @civanch , @mnovak , please, could you comment ?
I don’t want to throw the discussion too far off the rails, but a bound electron can have a kinetic energy greater than the free electron Compton maximum energy. There is a Doppler broadening component and an atomic recoil component (the latter is approx a few eV so is negligible). Unfortunately the scattering material here is water, so the extra energy from Doppler broadening will be approx +1 to 2 keV for the oxygen atoms which is less than the 1-23 keV that needs to be explained.
Dear @John_McFee
according to the Geant4 documentation only the physics list EM 4 takes Doppler broadening into account (instead, we notice the extra energy for the EM 3 too). Futhermore, this effect should be enabled for >20MeV photons only, while the Monarsh University model (G4LowEPComptonModel) is used in our energy range.
