This has the solution Symbols Used A = A(t) = activity (ue/gm) in. E = average energy of beta particles (Mev) thyroid Ro = initial rate of intake of thyroidal I)! (ue/gm/day) A = 55 Rof (a constant) D, = infinity dose (reps) to thyroid fromsingle I intake D, = total infinity dose (rep) to thyroid from continual intake from i = 0 tot = «. A, = radiological (physical) decay constant A» = biological decay constant é = time A=— (eae — eo Arther) (1) Thus, analyzing the thyroid for its I'3! activity, A, at any time, ¢, one can figure back to the initial rate of intake, Ro. 2. Infinity I'*! dose. The dose to infinity from I"! intake on thefirst day is dD, = K I 7 Arthas dt 0 TABLE 2—Sample Calculations for Figure 1 A B Cc Radio- Halflife Activity? (dpm/?0,00G isolo pe (hr) [isl 192 Te! 30 Tem 0.42 [#32 2.4 Te! 77 [133 21 Tetss 1 [3 6.7 I(ail short-lived) fisstans} 0.014 0.019 1.5 0.026 0.056 QO. 14 2.6 0.88 D E Number of <Atomsoafiodine atoms present per 10,000 = K/(hr + Av) F Average G H Ratio: short Energy Maz, rel. fissians reaching thyrotd per £0,000 fisstons beta energy energies to thyroid 230 49.4 54.3 5.4 bv 205 226 512 57.54 9, 9¢ 13 .6/ 0.39 11.2 48 .5¢ 43.0: 61.66 0.20 0.20 0.20 0.52 0.52 0.45 0.45 0.30 11.5 1.9 2.7 0.16 5.8 21.8 19.4 18.5 (Mev) (Ex Fy : £131 Brergy 0.01 0.36 1.35 1.20 1.15 ~4.07 * Based on Hunter and Ballou tables (Z}. ’ The biological fate of the isotopes of iodine is the same. Thus, the same proportions of the total number of atoms of each are taken into the thyroid and then eliminated according to the biological characteristics of the animal. The loss of an atom of a short-lived isotope means a greater loss of energy to the thyroid than does the loss of an I?3!, However, it is to be expected that the biological half-life of animal thyroids will be much greater than the radiologicai half-life of even the longest shortlived radioiodine isotope (I)4 with 2]1-hr half-life) so that essentially all of these energies will be delivered to the thyroid. For cases where the biological decay constant, x, is significantly large compared with the radiological decay constant, A, of I}4!, (0.0036 hr~!), then the values for energies of I}*! (including Te!#! and Te!*'™ precursors) given in column G should be multiplied by the factor A,/(A, + +) and lhkewise the The infinity dose from a continuing intake that decreases according to the radiological decay is then given by D, K/\y Xe i eh dt dD, K/(A- + Aa)Ar (2) 3. Doses from short-lived isotopes. The additional dose to the thyroid from short-lived isotopes of iodine resulting from a single intake is summarized in Fig, 1. A sample of the calculations used to construct Fig. 1 is given in Table 2 at left. In the case of grazing animals, however, the period of intake maystart at different times after detonation and extend for varying periods of time. An estimation of additional doses to the thyroid from short-lived isotopes of iodine under these conditions is summarized in Fig. 3. A sample of the values for the relative energies in column H for the short-lived isotopes should be multiplied by the factor (A; + As) /Ap. ¢ All of the iodine atoms reaching the thyroid will disintegrate there. Corrections may be necessary according to footnote 6. 425% of the [131 atoms taken into the blood reach the thyroid. ¢20% of Te!?'!" taken into the body reaches the thyroid, 1.e., about 80% would have disintegrated to Te!#! while in the gut of whichall disintegrates to J"! of which 25% reaches the thyroid. f All of the Te'*! atoms taken into the body will disintegrate to I'*! of which 25% will reach the thyroid. . 76% of the [}5 taken into the body reaches the thyroid. The biological half-life of I in the blood of humans may be about 7 hr (2). According to available data to date (¥) the biological half-life of I in the blood of sheep may be 10-12 hrfor the first day, followed bya flattening out of the curve. The proportion of activity reaching the thyroid may be estimated as !4h8/(\s + ,-). The values in Table 1 are calculations used to construct Fig. 3 is given in Table 3. give approximate values for sheep. Assuming a biological half-life of iodine in the blood of sheep of 1] hours over a period of 22 hours only, the ratios given in Fig. 1 concentrations on July 8 were about based on human data (7-hr biological half-life of iodine in the blood), These also differ for sheep as follows: I!8?, about 306% too high; 1133, 10% too low; 145, 10% too Example: Sheep Ingestion About 344 hours after the nuclear detonation at the Nevaca Test Site on May 19, 1953, fallout occurred in an area around Cedar City, Utah, where sheep were grazing. On June 15 some of these sheep were sacrificed and on July 8 the I'8! concentrations were measured in specimens of their thyroids. The highest measured ['* 5 X 1072 ue/gm (c.f. 3). What might low. The ratio of doses from individual short-lived iodine isotopes indicated in Fig. 1 suggests that the ratio of the total short-lived isotopes to I)*! may be underestimated for sheep by a few per cent in the early times after detonation. At later times the ['*? contribution predominates, but also the ratio of infinity doses from the total short-lived isotopes to I%! has decreased significantly. Thus, the method sug- have been the total radiation dose to the thyroids of these sheep from all of the isotopes of radioiodine? First calculate the I!3! dose, then the 3% of the Te!intake reaches the thyroid as I}, i.e., 50% would disintegrate to [1% while in the gut of which 6% will be deposited in the thyroid per footnote g. termine the initial rate of intake of gested here maygive a fair approximation of the total infinity doses for sheep. 19% of the I!" taken into the bodywill be deposited in the thyroid; 25% would be deposited normally, but about 25% of these atoms will decay before deposition’, ? All of the Te'™? taken in will disintegrate into I! while within the bodyof which 19% will reach the thyroid according to footnote 7. * 12% of the [5 intake will be deposited in the thyroid; 25% would be deposited normally, but about 50% will decay before deposition according to footnote g. 40 dose from short-lived isotopes. De- I'3! activity per gram, Mo from Eq. 1 A = (Ro/dalert — ec One] In this case A, = 0.37 uc/gm when sacrificed June 15. (Working back February, 1956 - NUCLEONICS