Measurement of ion recombination correction factors for plane parallel ionization chambers

The work presented focuses on the measurement of Ja e plots and determination of ion recombination factors for plane parallel ionisation chambers used in pulsed electron beams. The ability to measure the Ja e plots accurately is useful when commisioning a new chamber and the ion recombination correction factor needs to be known when the chamber is calibrated in a beam quality di erent from the user beam. In this study the ion recombination factor is measured and Ja e plots are produced for three di erent plane parallel chambers. A Roos chamber type 34001 (PTW Freiburg), a plane parallel NACP chamber type -02 (Scanditronix) and a plane parallel Pitman chamber (631). Experimental results are presented in form of Ja e plots, as well as the ion recombination correction factor, determined both from Ja e plots and by means of the two voltage method as recommended in IAEA TRS-398. The report also presents results dealing with the measurement technique and experimental setup. Drift, leakage and polarity e ect are discussed. The methods of statistical analysis are brought up and discussion is made regarding the validity of the statistical analysis. The measurements were performed on an Elekta linear accelerator at Perth Radiation Oncology, Wembley, Perth WA. Energies between 6 and 15 MeV were used with doses per pulse below 0.02 cGy per pulse. The project goal was to measure the ion recombination correction factor and produce a Ja e plot for a plane parallell Pitman chamber that during various other measurements have shown discrepancy from other chambers. In the goal was also included measurment of the ion recombination factor and Ja e plot for a plane parallel Roos chamber and a plane parallel NACP chamber to allow for a comparison to be made between chambers. The Pitman chamber was found to have a deviation from linearity of the Ja e plot at lower voltages than the other chambers.