Gamma-ray transport through material layers

The transmitted photon energy fluence was calculated as a function of layer thickness for 1.25 MeV photons in Al, Ag, and Pb using the CEPXS/ONELD coupled electron-photon code. The energy fluence vs. layer thickness was also calculated as a function of beam radius using the CYLTRAN Monte Carlo code. We compare these results with predictions using the photon "energy absorption coefficient attenuation law" equation recommended in ASTM Standard E666-91 and suggest replacing this equation with more realistic expression.