Dosimetry at interfaces: measurements by polypropylene electrets at and near plane interfaces between different materials (C, Al, Cu, Sn, Pb) irradiated with cobalt 60 gamma rays

The electret surface charge density σ deposited by a corona effect is a function of the gamma photon exposure. The relative variations of σ=q/q₀ are a function of the ionization created in the air cavity which contains the electret (if the initial and final charges of the electret are q₀ and q). A multilaminar system made with two materials (metal and carbon) allowed measurements to be made of the ionization relative to the carbon at all points. The same evolution was obtained with a traditional ionization chamber used under the same experimental conditions. The results show that significant variations in dose distributions occur at high photon energy where the mass energy absorption coefficients differ relatively little from one material to another. The enhancement or reduction in ionization 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 beam direction reduces the ionization to a level below that which it could be for either material alone