CORRECTION FACTORS FOR FRICKE DOSIMETRY IN HIGH-ENERGY ELECTRON-BEAMS

This paper presents Fricke-to-water dose conversion and wall correction factors for Fricke ferrous sulphate dosimetry in high-energy electron beams. The dose conversion factor has been calculated as the ratio of the mean dose in water to the mean dose in the Fricke solution with a water-walled vessel and the wall correction factor accounts for the change in the Fricke dose due to the presence of the non-water wall material. The EGS4 (Electron Gamma Shower version 4) Monte Carlo code system has been employed in this work together with the application of a correlated sampling variance reduction technique. The results show that for a Fricke dosimeter of 1.354 cm diameter and 5.5 cm length the dose conversion factor is nearly constant at 1.004 (within 0.1%) for electron energies of 11-25 MeV if the dosimeter is placed at the depth of maximum dose but can vary by a few per cent if the dosimeter is placed on the descending portion of the depth-dose curve. The wall corrections are smaller than 0.3% for 0.1 cm thick polystyrene vessels. For 0.1 cm thick Pyrex glass vessels the wall correction factor varies from 0.989 for 11 MeV at 2.75 cm depth to 0.997 for 25 MeV at 7 cm depth. This confirms recent experimental findings that Fricke doses obtained with glass-walled vessels were up to 1% higher than those with polystyrene-walled vessels.