Energy deposition of electrons in low-, medium- and high-Z material: comparison of the Monte Carlo transport code EGS4 with experiment

The Monte Carlo electron transport code EGS4 was benchmark tested against early experimental results derived by Freyberger. These consist of absolute depth ionization and depth dose curves measured at a pencil beam with sharp energy definition of nominally 4, 10 and 20 MeV electrons extracted from a Betatron. The Freyberger precision measurements have been made with a wide plane-parallel ionization chamber in a slab phantom for the materials PMMA, C, Al, Cu, and Pb. The bremsstrahlung and, in some experiments, the forward and backward directed contributions had been determined separately. The pencil-beam/wide-chamber geometry is equivalent, in respect to the measurement of depth ionization and depth dose curves, to the more common wide-beam/point-detector geometry. However, it requires the simulation of merely one pencil beam position and practically all particle histories contribute to the ionization in the wide air cavity. Thus a considerable amount of computing time is saved. We applied a prototype of the new electron transport code PRESTA II. The results of our simulation generally agree very well, even in absolute terms, with experiment. Small deviations are found at lower energies and high-Z materials. For low energies they may in part be explained by contamination with bremsstrahlung from the beam guide system and an overestimation of bremsstrahlung-production in the experiment. The simulation of a gas-filled chamber within a high-Z absorber block seems to produce small errors in the calculation of ionization for electrons of approximately 4 MeV. Larger deviations for high-Z materials are attributed to the employment of screened Rutherford cross sections which lead to an underprediction of ionization from backscattered particles. Backscatter was found to be sufficiently accurate in the simulation of the PMMA absorber.