Vol. VII, No 3-4 131 RADICIODINE UPTAKE MEASUREMENT Since all instruments except spectrometers integrate the two peaks, it has been experimentally determined that the difference in this portion of the spectrum causes 2 to 4 per cent difference in the comparative measurement of iodine and mock-iodine by most instruments. In the X-ray portion of the spectrum thereis slightly too much of the 80 key peak and slghtly too little of the 35 kev peak. This causes another 2 to 4 per cent difference in the comparative measurement of iodine and mock-iodine (which may or may not cancel out the medium-energy ditfference depending upon the instrumentused). It has been empirically determined that in a wide variety of instruments, the difference in the measurement of iodine and mock-iodine is from O to 5 per cent When the mock-icdine is used in its raw state, both the barium 133 and the cesium 137 contribute entirely too large a proportion of the very low-energy X rays, however, this can be corrected by partially shielding the raw mock-iodine with a medium Z metal. Figure 2 shows the results when suc- cessive layers of babbit metal (a tin-antimony mixture) are interposed between the mockiodine and the detecting crystal. The very low energies decrease very fast. The higher ener- gies are almost unaffected. Thus when the correct thickness of babbit metal shields the mock-iodine, a compromise absorption is ob- tained. It has been found empirically that a 0.82 mm thickness of babbit metal ts the best compromise filter the raw mock-iodine. All the mock-iodine sources are therefore manufactured and used in a container made of babbit metal. Since barmmm 133 and cesium 137 have widely different half lives, the mock-iodine does not have a true half life. It does. however, have a useful life about 10 years. In Fig. 3 the method of arriving at this useful life is illustrated. The ideal mixture of barium 133 and cesium 137 has been shown to be 10.5 units of barium 133 to 1 unit of ce- sium 137. A millicurie measure is used for the cesium, but since the decay scheme of barium 133 is unknown, an arbitrary milltcurie had to be defined for barium 133. The barium 133 decays with a half life of about 9.5 years. The cesium 137 decays with a half life of about 33 years. Neither of these half lives is exactly known but they THE PHYSICAL DECAY OF THE TWO 3 ¢ MOCK IODINE . 10 YEARS €xcp ! = . ECAY oF a5 vEaa cata? say Excess. . “". i + - cl. 1 , Bias OF 5 | . USEFUL LIFE CF 2 t | ISOTOPES MAKING UP MOCK IODINE - om - 7 Tat STARTING IDEAL, POINT wiTH ���10 YEARS T -SVEARS MIXTURE I 10.571 j or Casta se . ay ‘a res 28 BaF EXPIRATION Balt3-cs!87 POINT WITH. wie MINTURE i___f I : ®ecy ror aA. I t YEARS oP MIXTURE 9200-22 U2. Oe | Lp T F T +S YEARSsIO YEARS +14 YEARY Fic. 3. The method of arriving at the useful life of mock-iodine are approximately true. If one starts out with a mixture containing an excess of barium, the decay will extend through the ideal’ mixture and will eventually show an excess of cesium 137. Many different mixtures of barium and cesium were made up on an experimental basis and were empirically com- pared with identical iodine 131 source. It was found that, with some kinds of instrumentation, when there was an excess of ba- rium 133 (much greater than a 15 to 1 ratio) significant differences in the comparison of mock-iodine with iodine 131 began to appear. When the mixtures contained too little ba- rium 133 (muchless than a ratio of 8 barium 133 to l cesium 137) variations again appeared in the comparison with iodine 131 with different kinds of instrumentation. Therefore, an arbitrary cut-off point was made with a starting mixture of 13.1 to 1, which represented a 5-year decay period before the ideal mixture was reached. An expiration point was arbitrarily set up at a mixture of 8.2 to 1, which represented 5 years of decay after the mixture had gone throughtheideal