3In 1938, Hertz, Hamilton, and co-workers 1,2 pioneered the use of 128I radionuclide (T1/2 = 25 min) in the investigation of thyroid physiology. 128I radionuclide was prepared by the Szilard-Chalmers reaction through the interaction of neutrons from a plutonium-beryllium source, with ethyl iodide as target, by the 127I (n, γ) 128I nuclear reaction. Because of the importance of the physical properties of y-ray energy and the physical half-life in tracer techniques, other radioisotopes were explored, and the radionuclides of 126I, 130I, and 131I were discovered in 1938. Iodine has about two dozen radionuclides. The physical characteristics of several radionuclides of iodine used in biomedical investigation are listed in Table 1.