We developed very small parallel plate radiation detector by using our existing experience of making radiation dosimeter and capability of analyzing characteristics of dosimeter. The radiation detector was consisted of microfilm and carbon electrode. The detector was parallel plate type of air-filled ionization chamber. The ionization chamber had been fabricated using an acrylic plate for the air cavity and carbon coated microfilm for electrical configuration. The air gap between two electrodes was 0.48 mm. The diameters of collect electrode and guard electrode were 3.3 mm, 5 mm respectively. The diameter of high voltage electrode was 5 mm. Nominal sensitive volume of the chamber was 0.016 cm³. The major parameters of the chamber characteristics such as leakage current, reproducibility, dose rate effect, and polarity effect were measured. The experimental results were as followings. Leakage current was 0.1 pA. Standard deviation of reproducibility was less than 0.1%. Dose rate effect was less than 1.5%. Polarity effect was less than 2.4%. These data were comparable to those of commercially available dosimetric system for QA-purpose. As the result, we found that the radiation detector consisting of the ionization chamber, microfilm and carbon electrode, was satisfactory for the purpose of the small field dosimetry in size and characteristics. In the future, We will try to refine the dosimeter for use in very small volume.

KeywordsMicrofilm, Carbon electrode, Parallel plate ionization chamber