Gamma Dose Distributicns at and near the Interface of Different Materials

Energy deposition profiles in aluminum and gold adjacent to materials of different atomic number were measured with a multiple parallel plate ionization chamber. The results show that significant variations in dose distributions occur at high photon energies where the mass energy absorption coefficients differ relatively little from one material to another. At these energies (e.g., 1.25 MeV) the perturbation in the bulk dose is no longer a simple function of atomic number alone. The enhancement or reduction in dose becomes very sensitive to the direction of the incident photon beam. The energy deposited in a material is enhanced when it is next to one of lower atomic number provided the gamma beam penetrates the lower atomic number material first. A reversal in bean direction reduces the dose to a level below what it would be for either material alone.