How to calculate mass attenuation coefficient?


I am a physic student. I am interested in Geant4. I want to build a gamma transmission technique simulation. But i cannot count in specific gamma ray energies which i have written in (fParticleGun->SetParticleEnergy(1.*MeV)). I want to count reached gamma rays to scoring volume. I tried several times in, but i don’t achieve.

Who can help me? Thank you