Design and characteristics of parallel plate type ionization chamber for the measurement of high energy photon and electron beam

The dosimetric system consisting of parallel plate type of air-filled ionization chamber and charge to voltage converter was designed to do constancy check of the high energy photon and electron beam of clinical linear accelerator. The ionization chamber had been fabricated using an acrylic plate for the air cavity and two circuit boards for electrical configuration. The air gap between two electrodes ranged from 3 mm, 6 mm and 10 mm. The sensitive volume of the chambers were 0.9 cc, 1.9 cc and 3.1 cc respectively. The major parameters of the chamber characteristics such as zero drift current, leakage current, saturation voltage, reproducibility, linearity, EPM (Effective Point of Measurement), dose rate effect and polarity effect were measured. The experimental results are as followings. Zero drift currents were 0.23 pA for the volume of 0.9 cc, 1.9 cc and 0.45 pA for 3.1 cc. Leakage currents were 0.38 pA for the volume of 0.9 cc, 1.9 cc and 0.56 pA for 3.1 cc. Saturation voltage were 300 V, 400 V and 500 V for three kinds of chambers with volume of 0.9 cc, 1.9 cc and 3.1 cc respectively. Standard deviation of reproducibility was less than 0.003, and the linearity was measured within 0.5% for all kinds of chambers. EPM (effective point of measurement) shifted towards the geometric center of the chamber as the plate separation increased. These data were comparable to those of commercially available dosimetric system for QA-purpose. This dosimetric system consisting of the ionization chamber, the PCB, and RF coaxial cables is satisfactory for the purpose of the constancy check of the high energy photon and electron beam from the medical linear accelerator. (author)