Maryam Azizi - Nuclear Engineering School, Shahid Beheshti University, Tehran, Iran
Ali Biganeh - Physics and Accelerators Research School, Nuclear Science and Technology Research Institute, P.O. Box ۱۴۳۹۵-۸۳۶, Tehran, Iran
Omidreza Kakuee - Physics and Accelerators Research School, Nuclear Science and Technology Research Institute, P.O. Box ۱۴۳۹۵-۸۳۶, Tehran, Iran
Behjat Ghasemi - Nuclear Engineering School, Shahid Beheshti University, Tehran, Iran
Yashar Vosoughi - Physics and Accelerators Research School, Nuclear Science and Technology Research Institute, P.O. Box ۱۴۳۹۵-۸۳۶, Tehran, Iran

Primary standardization of radioactivity is related to the direct measurement of activity in radioactive decay. A large variety of primary standardization techniques have been developed in the past years. The photon-photon coincidence counting is one of the methods for activity determination. This method is particularly applied for the standardization of I-۱۲۵ using the detection of X-ray and gamma-ray coincident counting. In this paper, a ۲D photon-photon coincidence digital system with two similar ‎‎۲'' × ۲''‎‎ NaI(Tl) detectors for absolute activity measurement is developed. The system is established based on a ۱۰۰ MHz CAEN waveform digitizer (DT۵۷۲۴) which directly records the pre-amplifier output signals of the two NaI(Tl) detectors. The sampled signals was transformed to trapezoidal signals using pulse height analyzer firmware and coincidence events were recorded in a list file. The list file was analyzed offline using a Matlab code to realize correlated gama lines of Co-۶۰ source.  The Volkovitsky formulas were used for the activity calculation and the details of the experimental setup were also discussed. Standardization of  the two Co-۶۰ standard sources was performed using this system. Results are in good agreement with the reference activity of Co-۶۰ sources. The presented formula can be modified for absolute calibration of the other medical radioisotopes. The technique can be generalized for absolute activity measurement of I-۱۲۵ which uses for ophthalmic plaque radiation therapy.

Absolute activity, Coincidence counting, Co-۶۰ radioisotope, Metrology