Ionization chamber system for calibration of Co -ray high dose rate at radiation processing level

Kojima, Takuji; Tachibana, Hiroyuki; Haneda, Noriyuki; Kaneko, Hirohisa; Haruyama, Yasuyuki; Tanaka, Ryuichi*

Plaque type Co -ray sources having wide radiation field with high absorbed dose rates ranging of about 10010,000Gy/h is widely used in radiation processing e.g. radiation sterilization of health care products. Calibration of such radiation processing doses or dose rates is not easy to be performed at the present standard dosimetry laboratories, since characteristics of most of their radiation field can provide collimated gamma-rays from a point source and/or lower dose rates, which are quite different from those of panoramic (uncollimated) radiation fields used in radiation processing. The high-dose calibration technique using a parallel-plate ionization chamber system has developed in the JAERI dose calibration facility. The facility equips two Co -ray plaque sources covering wide dose-rate range of 520,000 Gy/h which have overlapping dose-rate region with that at the National Institute of Advanced Industrial Science and Technology(AIST), the national standard Dosimetry laboratory in Japan. Consistency of dose calibration results at JAERI and those at AIST was verified without additional correction regarding to difference in characteristics of radiation fields involving current/exposure -rate conversion coefficients given in the dose rate level of 10100 Gy/h at the AIST and high-dose rates of 520,000 Gy/h at JAERI. It demonstrates our dose traceability to the AIST from the technical point of view. The dose calibration at JAERI covers the high-dose range of 0.5 to Gy with uncertainty of 2.2% at 95% confidence level.