Operational characteristics of ionization chambers for a radiation monitoring

Ionization chambers for a Radiation Monitoring System (RMS) are required to cover a wide range of dose rates from 10^-1 μSv/h to 10⁶ μSv/h. For a RMS, two cylindrical ionization chambers, which had active volumes of 11.8 L and 1L, were fabricated in consideration of an electronic equilibrium and a guard electrode structure. Preliminary tests such as saturation currents and leakage currents were performed with and without a 925 MBq ²⁴¹Am gamma source. Linearity against low dose rates was also performed by using a conventional shadow shielding technique with NIST certified 33.52 MBq ²²⁶Ra source in a calibration room. A field test with 11.8 L was also performed at the Yong-kwang power plant. A Parallel Plate Ionization Chamber (PPIC) was also developed not only to measure the proton beam intensity but also to derive relevant parameters with other reference ionization chambers aimed at assisting in the quality control of a proton accelerator. Operational characteristics were investigated at high dose rates by using the X-ray generator located at the Korea Research Institute of Standards and Science (KRISS). Ion recombination, which is one of the characteristics of an ionization chamber, was evaluated with the experimental results and a theoretical model. The collection efficiencies of a PPIC were calculated by a experimental two-voltage method when the filling gases and the distance of the two electrodes were varied.