Standardization of 125I by the coincidence method and practical applications

A new coincidence system, based on two thin NaI(Tl) crystals, 50×2 mm2, with beryllium windows, 1.5 mg cm−2, positioned approximately symmetrically from the source holder (at various distances) was constructed. The electronic chains of a classical 4πPC-γ system were utilized. A multichannel analyzer was employed for energy calibration, and an oscilloscope for visualization of the pulses. After calibration, the system was used for the standardization of an 125I solution. An energy window of 12-95 keV containing the entire spectrum (single and sum peak) was employed, and a simple demonstration of Taylor's equation [1967. X-ray-X-ray coincidence counting methods for the standardization of 125I and 197Hg. In: Standardization of Radionuclides: Proceedings of a International Atomic Energy Agency Symposium, Vienna, 1966, pp. 341-353, SM-79/65] for whole spectrum counting, was developed. The method and system were tested during the calibration of a LKB 2104 multi-gamma counter, by comparing the detection efficiency values obtained for this prepared standard solution and that utilized in the CIPM-CCRI(II) key comparison. The efficiency of the equipment was calculated as the mean of the two individual values. The result validates the method and demonstrates the accomplishment of the national traceability assurance in nuclear medicine for 125I.