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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 printed 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 pAfor the volume of 0.9 cc, 1.9 cc and 0.56 pAfor 3.1 cc. Saturation voltages 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 within0.5% 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.

The activity of gamma-ray emitting nuclides is calculated assuming that each gamma-ray is detected individually; thus, the magnitude of the coincidence summing signal must be considered during activity calculations. Here, the correction factor for the coincidence summing effect was calculated, and the detection efficiencies of two HPGe detectors were compared. The CANBERRA Inc. GC4018 high-purity Ge detector provided an estimate for the peak-to-total ratio using a point source to determine the coincidence summing correction factor. The ORTEC Inc. GEM60 high-purity Ge detector uses EFFTRAN in LVis to obtain the parameters of the detector and source model and the gamma-gamma and gamma-X match estimates, in order to determine the coincidence summing correction factor. Nuclide analyses, radioactivity comparisons, and analyses of reference material samples were performed utilizing certified reference materials to accurately determine the detection efficiencies. For both Co-60 and Y-88, the detection efficiency for a point source increased by an average of at least 12–13%, whereas the detection efficiency determined using LVis increased by an average of at least 13–15%. The calculated radioactivity values of the certified reference material and reference material samples were accurate to within 3% and 6% of the measured values, respectively.