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Dosimetric calibration of a panoramic 60Co gamma-ray source

The panoramic 60Co source at the Ruđer Bošković Institute is the only facility in Croatia suitable for variety of applications: from medium range of doses used in radiobiology and accident dosimetry to the high doses pertain to radiation processing and research in radiation chemistry. In November 2015 thanks to IAEA support the source was upgraded with additional 3.5 PBq. The results of precise dosimetric calibration and determination of the dose distribution curves typical for the cylindrical gamma ray panoramic source geometry are presented. Because of the cylindrical shape of the source, the radiation field also has a cylindrical symmetry and is best described by cylindrical coordinates. Radiation field mapping was performed with ethanol-chlorobenzene (ECB) dosimetry system which was discovered 50 years ago in Radiation Chemistry and Dosimetry Laboratory and developed later for scientific radiation research and dosimetry in radiation processing (ISO/ASTM 51538:2009). The commercial 5 cm3 pharmaceutical ampoules were used for irradiations. Irradiated ECB dosimeters were analysed for chloride ion concentration by two methods: oscillometry before opening ; after opening aliquots were taken for mercurimetric titration. Oscillometry was performed by oscillotitrator Model OK-302/1 (Radelkis Electrochemical Onstruments, Budapest). After measurements, the doses were evaluated from the previously determinated calibration curves. For mercurimetric titration, the standardization of Hg+2 solution used for titration with standard NaCl solutions was performed before the analysis of irradiated dosimetric solutions. All doses were expressed as “absorbed dose to water”, obtained from doses absorbed in ECB solutions multiplied with appropriate ratios of mass energy absorption coefficients for water and ECB. For dosimetry calibration and “transit” dose measurements an ionization chamber type 2581 and a Farmer Dosimeter type 2570 (NE Technology Limited, England) were used. Dose mapping in horizontal and vertical planes was performed with ECB dosimetry system, as it follows: (a) The check for the angular symmetry of the radiation field was performed inside the annular space enclosed by the source rack. (b) The dependence of the dose rate on the radius r from the plane axes of the cylinder, was measured at two heights: at h = 0 cm, which is in the horizontal plane passing through the centre of the source cylinder and at h = 100 cm. (c) The dependence of dose rate on the height h was measured at two radii: at r = 50 cm from the axis of the source, and at r = 100 cm from the axis, both in a vertical plane through the axis. The “transit dose” was measured at different radii at h = 0. In this work it was shown that the radiation field of the cylindrical gamma ray panoramic source geometry is fully described by using only two parameters, a radius r and distance h from the reference horizontal plane. Acknowledgment: The support of the IAEA in ensuring the timely supplies of the radioactive 60 Co over the years, especially through the recent Technical Co-operation Project CRO/1/006 (2014-2015) is gratefully acknowledged.

gamma radiation ; ethanol-chlorobenzene dosimetry system ; panoramic gamma source

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