M. BruceR 132 point. Thus the useful life of mock-iodine is 10 years. A 15 or 20 year period could easily have been set up that would meet the requirements, of most instrumentation,It is felt, however, that for calibration purposes, Acta Radiolégica Interamericana what instrumentation is used (excepting spec- trometers) the results from measuring any iodine sample should be about the same as the result from measuring an identical mockiodine sample. THE RELATIVE PROPORTIONS OF LOW, MEDIUM AND HIGH ENERGIES IN Ba*—C,187 MIXTURES MAKING UP MOCK IODINE at ere 0% ~ 20%"40% t } G0%e— BOM 100 toot RarMINTURESeer ‘KR~ RAY ey LL. Fic. loKev LOW ENERGY X~RAY —— Ms ENERGY . a LOW ENERGY:, ob seMEDIUM ENERGY =00Kev250 Kev _ HIGH e | i wm | { | §00~-6800 Kev, {. The expected differences between iodine 131 and mock-iodine at the extremes of the accepted intercalibration mixture. the accuracy of the 10 year period should be in excess of the accuray that is obtainable with most instrumentation. In Fig 4 the differences that can be expected between iodine 131 and mock-iodine at the extremes of the accepted intercalibration mixture are shown by dividing the spectrum into 4 energy regions. It is seen that not quite 40 per cent the total energy emit- ted from iodine 131 is in the arbitrarily defined X-ray region. There is a slightly lo- wer percentage in the X-ray region with the 13.1/1 starting mixture and thereis a slight- ly greater percentage in the 8.2/1 expiration mixture. About 20 per cent of the energy from iodine 131 is in the low-energy region. Another 40 per cent is in the medium region. The mock-iodine mixtures have about the same distribution of energy. Only about 1-1/2 per cent of the distribution is in the high-energy region. Thus it can be seen that mock-iodine matches the gamma- ray spectrum of iodine 131, and no matter It was necessary to suspend the mock-io- dine in extended sources of a variety of shapes in order to mimicclinical conditions. To do this the mock-iodine was suspended on an ion-exchange resin. The ion-exchange resin is a tissue-like, unit-density material that not only matches soft tissues but also tightly binds the isotopes so that their use, even when the containers are broken, is re- latively safe. After the material was bound on the ion-exchange resin, it was measured in small lots, against samples calibrated by the National Bureau of Standards in a high- pressure ionization chamber. These small lots of active material were then mixed with large amounts or small amounts of inactive re- sin to obtain a uniform dispersion of activity throughout any required volume of material. The mock-iodine was then put into a calibration manikin. A great deal of trouble was taken to math as exactly as possible a simulated clinical condition. The manikin