Accurate calibration of a Co-60 gamma radiographic source

The artificial radioisotope Co-60, produced in nuclear reactors by activation of the natural stable isotope Co-59, is frequently used as a gamma ray source for industrial radiography of steel specimens 25--200 mm (1--8 in.) thick. The exposure time required for a specific radiographic operation can be calculated knowing the activity of the Co-60 source, and the parameters for exclusion of non0radiography personnel can be more accurately established. Activities of Co-60 sources can be determined either by comparing the response of an instrument (not necessarily calibrated) to the unknown source and to a calibrated reference source located at the same point in the radiation field, or by finding the response of a calibrated instrument to the unknown source. If the first technique is applied, then the calibrated source should be fully specified with respect to energy, intensity and spatial distribution of the gamma rays emitted. If the second technique is applied, then a reliable transfer instrument which is traceable to a national standard is required. This transfer instrument must be calibrated by a primary or secondary standards laboratory aiming at overall calibration uncertainties smaller than 3% (NCRP, 1991). Traceable standard instruments are relatively expensive and their use is limited to standardizationsmore » rather than for routine dosimetric activities. Their handling requires great care and attention to many details. The aim of the present work was to calibrate the activity and dose rate of a Co-60 gamma radiographic source, using a portable gamma-responsive field survey monitor and a reference gamma source of the same radionuclide. These two items are available in all nuclear laboratories.« less