Number of events with the same process different within the same material

Dear all,

I’m trying to understand how to interpret the results I’m getting from my simulation, and I think I don’t quite understand how Geant4 is processing tracks. I have an 8x8 pixel detector simulations with scintillating, square-based parallelepiped crystals. All crystals have the same length. In the first simulation, the area of the base of the crystal is 2x2 mm2, and in the second one it’s 3x3 mm2. I want to count which processes are happening in the first step of gammas interacting with the crystals when I’m shooting a 0.511 MeV gamma per event, in the middle of one of the central pixels in the pixel detector.

Here is what is bothering me: since Geant4 generates tracks that “see” basically the same geometry in these two simulations - all the gammas interact along the same line in the crystals - which means the same material regardless of the base area, why do I get very different results regarding the first processes in the crystals?

For example, if I shoot 10^5 gammas into the crystals, here are the numbers I get for how many of the first interactions in the crystal are Compton interactions:

2x2: 21 210
3x3: 54 539

I expected to see some divergence, but is there a reason why these numbers are so very different?

Thank you very much!

you must run few events interactively, with visualisation and tracking/verbose 2;
and double check what you count

1 Like

@maire, thank you for your reply! I will turn on verbose 2 and double-check. Every other test we tried was successful, and it seems that physics is working properly, except for this counting. Thanks again!