Calibration of 60Co and 137Cs exposure standards

The significance is stressed of primary standardization of exposure to 60Co and 137Cs gamma radiation. A standardization equipment was built at the Laboratory of Radiological Dosimetry of the Czechoslovak Academy of Sciences for the exposure to photon radiations of energies up to 3 MeV. The principal parameters are given of the pressurized-air ionization chamber. Part of the equipment are two 60Co sources and one 137Cs source placed in the same housing (wall thickness 0.27 m Pb) as one of the 60Co sources. A portable radionuclide source of constant irradiation was used for making the said standard compatible with those of some of the CMEA countries (Hungary, the German Democratic Republic, Poland, the Soviet Union). The source comprises 381 MBq (=10.3 mCi) of 90Sr+90Y deposited evenly on the inner wall of a cylindrical surface (50 mm in dia and 20 mm in height). A commercial VA-K-253 spherical ionization chamber (16 mm in dia) was used for relative ionization measurements, connected to a semiconductor d-c amplifier made in the laboratory. In international comparison the average mean deviation of +-2.1% was measured for the 60Co beam and a deviation of 0.005% was found for the 137Cs beam. (B.S.)