Two new parallel-plate ionization chambers for electron beam dosimetry

Abstract

Two parallel-plate ionization chambers were projected, constructed and evauated for use in high energy electron beams. They were constructed using the two plastic materials recommended for clinical dosimetry protocols, i.e. acrylic and polystyrene. Both chambers have cylindrical shape with entrance windows in aluminized Mylar and they are open to the air. The acrylic chamber has a 2 mm air gap and the polystyrene chamber has a 1 mm air gap. Pre- and post-irradiation leakage, repeatability and long term stability were determined for these two ionization chambers. The ionic recombination and polarity effects, besides angular and energy dependencies, were also verified. The results obtained are within values recommended by IEC (1982) [Medical electrical equipment: dosimeters with ioniz chamber as used in radiotherapy. IEC, Geneva (IEC-731-82)] for this kind of ionization chamber. The ionization chambers were calibrated in a 20 MeV electron beam and gamma radiation of cobalt-60. The wall correction factors for the gamma radiation for cobalt-60 were 1.014 and 1.000 for the acrylic and polystyrene chambers, respectively. The ionization chambers do not present the energy dependence for the 6–20 MeV electron beam range. These results are comparable to commercially available ionization chambers.