Physics of Electron Transport

Dear Experts,
Recently I have simulated total energy deposition for Pm-147 source under 180 mg/cm2 of Polystyrene and under 14 mg/cm2 of polystyrene in CaSO4. Total energy deposited in CaSO4 under 14 mg/cm2 was found to be 3.4346E-11 MeV/cm3 per unit primary electron and under 180 mg/cm2 was found to be 2.7374E-11 MeV/cm3 per unit primary electron. But the thickness of 180 mg/cm2 is enough to stop all the electrons from Pm-147 source having Emax of 0.210 MeV. But the results shows that the energy deposited under 14 mg/cm2 and that under 180 mg/cm2 are near to each other. Hence I would like to know how electrons contributed to the energy deposition even when it is under a shielding, which is higher than the range of the max energy of the electrons. Even range straggling should not contribute to energy deposition under 180 mg/cm2 of polystyrene. In such case I would also like to know the details of the physics treatment done by FLUKA for electron transport.

With regards,
Sujoy