Electron Depth-Dose Distribution Measurements in Two-Layer Slab Absorbers

Electron depth-dose distribution measurements were made using the Chalkley-McLaughlin dye film dosimeter in two-layer slab absorbers of polystyrene-copper, polystyrene-tin, and aluminum-gold. A plane-parallel 2.0-MeV electron beam was used. Data were obtained for 12 cases by varying the location of the interface and by reversing the order of the slab components. Reproducibility of the measurements was ±6% (2 σ). An investigation of stopping-power ratio evaluation methods based on the treatment of the film dosimeter as a nonperturbing cavity demonstrated that a constant value could be used for each material in a two-layer absorber independent of the thickness of each layer. The error in absorbed dose determination introduced by the use of a constant value ranged up to 8%. A film placed at the interface between the two components of the slab was used to estimate the dose at the interface surfaces. Corresponding theoretical calculations were made using a multilayer electron transport computer code developed by Berger. Agreement between the measured and the calculated doses was within 10% for metal layers and within 15% for polystyrene. The discrepancies for polystyrene can be attributed to the omission of energy loss straggling in the calculation.