SPL Set ID: 493fa2ab-4eb0-4434-9739-3079b2f0e272

Sodium Iodide I 123 (Na 123 I) for diagnostic use is supplied in capsules for oral administration. The capsules are available in strengths of 3.7 and 7.4 megabecquerels (MBq) (100 and 200 μCi) I-123 at time of calibration. The radionuclidic composition at calibration is not less than 97.0 percent I-123, not more than 2.9 percent I-125 and not more than 0.1 percent Te-121. The radionuclidic composition at expiration time is not less than 87.2 percent I-123, not more than 12.4 percent I-125 and not more than 0.4 percent Te-121. The ratio of the concentration of I-123 and I-125 changes with time. Graph 1 shows the minimum concentration of I-123 as a function of time and Graph 2 shows the maximum concentration of I-125 as a function of time. Graph 1. Radionuclidic Concentration of I-123 PERCENT OF TOTAL RADIOACTIVITY: IODINE-123 Graph 2. Radionuclidic Concentration of I-125 PERCENT OF TOTAL RADIOACTIVITY: IODINE-125 Graph 1 Graph 2 Physical Characteristics Iodine-123 decays by electron capture with a physical half-life of 13.2 hours Kocher, David C., Radioactive Decay Data Tables. DOE/TIC-11026, 122 (1981). . The photon that is useful for detection and imaging studies is listed in Table 1. Table 1. Principal Radiation Emission Data Radiation Mean % Disintegration Energy (keV) Gamma-2 83.4 159 External Radiation The specific gamma ray constant for I-123 is 1.6 R/hr-mCi at 1 cm. The first half-value thickness of lead (Pb) for I-123 is 0.005 cm. A range of values for the relative attenuation of the radiation emitted by this radionuclide that results from the interposition of various thicknesses of Pb is shown in Table 2. For example, the use of 1.63 cm of lead will decrease the external radiation exposure by a factor of about 1,000. Table 2. Radiation Attenuation by Lead Shielding Shield Thickness (Pb), cm Coefficient of Attenuation 0.005 0.10 0.88 1.63 2.48 0.5 10 -1 10 -2 10 -3 10 -4 Note that these estimates of attenuation do not take into consideration the presence of radionuclidic contaminants. To correct for physical decay of I-123, the fractions that remain at selected intervals after the time of calibration are shown in Table 3. Table 3. Iodine I-123 Decay Chart: Half-Life 13.2 Hours Hours Fraction Remaining Hours Fraction Remaining *Time of Calibration 0* 3 6 9 12 15 1.000 0.854 0.730 0.623 0.533 0.455 18 21 24 27 30 0.389 0.332 0.284 0.242 0.207