[Gate-users] variance in the number of optical photons produced per gamma interaction in PET

[Pedro Correia]

Dear Gate users

I’m simulating a cylindrical Pet, where my crystals are LYSO, with a light yield of 28000 photons/MeV, each one with ~ 3eV, and my source are gamma rays with fixed energy of 511 keV.

When I analise the root files, I saw that the actual number of hits of each detected interaction of 511 keV is not ~14000, as I would expect since 28000*0.511=14308. Actually, what I see in the root file is that some events produced ~10000 hits, while others produced ~15000. 

Is this related with the random seed that I’m using for simulations? In my point of view, the number of hits per each gamma photon absorbed should be the same, for the same energy, am I wrong?

I can attach the root output if someone ask to. 
Thank you all. Best regards.

Pedro Correia
PhD student
University of Aveiro