Experimental determination of scaling factors C/sub pl/ to convert ranges measured in plastic into water ranges, for electron beam dosimetry

摘要

Most dosimetry protocols recommend the use of water phantoms to determine the absorbed dose to water in electron beams. However, at energies below 10 MeV, plastic phantoms are most commonly used to improve the detector position reproducibility. In order to determine the absorbed dose to water in plastic materials, following the Code of Practice TRS-381, (IAEA, 1997) recommendations, it is necessary to know the scaling factor C/sub pl/, which allows to find the equivalent plastic depth to water, and the material density. In this work have been determined the C/sub pl/ for PMMA and Solid Water (RMI-457) materials from depth ionization and absorbed dose curves in these materials by comparison with depth distributions in water. The measurements have been performed in a linear accelerator in a range of nominal energies 6 to 20 MeV. A cylindrical ionization chamber and two plane-parallel ionization chambers were used. A good agreement was found between the density and scaling factor values determined in this work for PMMA and the values given in TRS-381. For the Solid Water the determined density value was different. Using the ratio /spl rho//sub user///spl rho//sub table/ together with the value of C/sub pl/ given in TRS-381, an excellent agreement was obtained with the experimental factor determined by us to convert the depth curves measured in plastic to water. This provides an experimental confirmation of the Monte Carlo calculated values given in TRS-381. In the light of these results, the need for measuring the density of the materials becomes evident.