Thanks for discussion and my bad I've not given any feedback.
The real problem is that usual detector's sizes are about 3" x 3" and the energies of photons are 1 - 10 MeV. In my now modeling I implement only two types of photo-atomic reactions: compton scattering and pair production. It allows...
The monochromatic gamma rays enter a scintillator (e.g. NaI). Then I compute the deposited energy spectrum. I need a techinque of modeling of the bremsstrahlung produced by the recoil electrons without direct modeling of the electrons trajectory. What could I do?