Photoelectric in compounds

I’m trying to study the interaction of gammas with a piece of a thin scintillator (0.5 mm) with GEANT4.10.3.p01. For the moment I’m interested only in the cross section of the different gammas processes and not in the optical properties of the material.
The scintillator in use is Gd2O2S:Tb mixed with epoxy glue (60% and 40% respectively). To create this material, first of all, I create separately Gd2O2S:Tb and epoxy. Finally, I create a compound adding the two previous materials, averaging the density.

G4Material* Gos_base = nistManager-> 
G4Element* Tb  = nistManager->FindOrBuildElement(65);
G4Material* Gos = new G4Material("Gos", density= 7.5*g/cm3, nelements=2); 
Gos->AddElement(Tb, 0.05*perCent); 
Gos->AddMaterial(Gos_base, 99.95*perCent);

G4Material* Glue = new G4Material("Glue", density= 1.231*g/cm3, nelements=3);

G4Material* Gos_Glue = new G4Material("Gos_Glue", density= 4.992*g/cm3, nelements=2);
Gos_Glue->AddMaterial(Glue, 40*perCent); 
Gos_Glue->AddMaterial(Gos, 60*perCent); 

If I put 100% for Gos (instead of 60%) in the last part I expected that the two materials are equal, except the density.
For the physics list I used the user physics list as it is in the example OpNovice.

I look at the spectrum of deposited energy in the compound material with a GPS beam of 500 keV gammas. To validate everything I put 100% for Gos (instead of 60%) in the compound and I compare two densities: 7.5 g/cm3 and 4.99 g/cm3.
I observe a strange behaviour with a density of 4.99 g/cm3 in the photoelectric peak. I have two peaks (with higher density it disappears). It looks after a photoelectric process it produces an electron, as expected, and a secondary gamma that is not absorbed in the thin scintillator. Why a gamma with lower energy survive after a photoelectric event? The gap between the two peaks (red in figure) is constant with the energy of the impinging gamma. With higher density there is no production of this secondary gamma.

I’d suspect it’s the effect of a production cut. They are defined in Geant4 in units of length and get translated to energy for each material. For a higher density material, you get a higher energy value.
You can check the cuts (and how it translates to energy for each material):


And change the settings, e.g. with one of:

/run/setCut 0.1 mm
/run/setCutForAGivenParticle gamma 0.1 mm


I would advice

  1. to print list of materials and compare material of target for two runs

  2. increase size of the target to reduce leak. Leak of gamma depends on how far from a boundary of the target a gamma is produced.


I guess this secondary gamma comes from fluorescence.
Try to desactivate the phenomena : /process/em/fluo false