Perturbation correction factors for some ionization chambers commonly applied in electron beams

Values for the fluence perturbation correction factor were determined in a number of electron beams for the commonly applied PTW/Markus plane-parallel chamber and the cylindrical NE 2571 Farmer chamber. These data were determined relative to the NACP plane-parallel chamber as a function of the mean energy at depth, E(z)BAR, calculated according to the method recommended in modern dosimetry protocols. For the cylindrical chamber the results are in agreement with the data recommended in these protocols. For the PTW/Markus chamber a small but significant fluence perturbation correction is necessary: up to about 3% for a value of E(z)BAR = 2 MeV. In order to check the value of these fluence perturbation correction values, dose measurements were performed ionometrically and compared with values determined with the Fricke dosimetry system. The ratio of dose values determined with the NACP and PTW/Markus chambers, applying the NCS Code of Practice and dose values determined with the Fricke dosimetry system, was 1.007 +/- 0.5% on the average for a number of electron beams. This result is in agreement with the conclusions of other investigators if for the Fricke dosimeter system a value of 352 10(-6) m-2 kg-1 Gy-1 for epsilon(m)G is applied.