‘OF F ¢ Short (total) , r 1 r "> Single Intake _ ma = 1.0L S 2 C C wn C a 5 bet = a - By GORDON M. DUNNING Division of Biology and Medicine U.S. Atomic Energy Commission Washington, D.C. oO " a L . So OIF 2 s ec 00! F r i 0. Lop ae tiyl . _ 1.0 Le pe pt id Lp ehigitt- a Lit 100 fpoa, Time from Detonation to Intake (hr) % FIG. 1. Approximate ratio of infinity dose to thyroid from short-lived radioiodine isotopes and I'*! for case of single intake. See Table 2 for sample calculation of these curves Two Waysto Estimate Thyroid Dose from Radioiodine in Fallout CALCULATING RADIATION DOSE from fallout presents unique problems. One of these is how to estimate thyroid dose from intake of the radioisotopes of iodine found in fallout material. Because of uncertainties in the relevant variables, the calculations cannot be made with the precision that is possible in a laboratory or clinical situation. However, it is often essential to make such estimates, even though admittedly based on limited data. There is disagreement as to the principal mode of entry into the body of the radioactive I contained in fallout, i.e., inhalation or ingestion. For low-yield detonations, such as in Nevada, relatively higher concentrations of fallout material are found in the air for only a matter of a few hours with essentially all of calculated intake by inhalation completed within 24 hours for nearby communities. When the detonations occur at 38 the Pacific proving ground the activity in the air maypersist for some- what longer times in the U.S. Thus, for relatively early, short exposures the amount of intake by inhalation may be comparable to that by inges- tion. However, if one considers normal ingestion for a continuing period, it would appear this is by far the dominant factor. This is especially true for grazing animals. In fact, field experiments near the Nevada test site showed there was little I in the thyroid of rabbits, who were restrained so that they could not ingest any material but could continue to inhale during and after fallout. The problem of dosage calculations is complicated by the presence, at early times after detonation, of shortlived I isotopes in addition to I'#! and by tellurium precursors for several. The general approach given below for these problems is to calculate the dose from I?*! and its precursors and then add to this the contribution from the short-lived isotopes of I and their precursors. Due to relative abundance or short half-lives the only isotopes of concern here are: ['%!, [18?, [133, 135) Teltim Te!t!) Tels? and Te?33, Their properties are given in Table 1. Calculating Dose The precise calculation of I?*) intake to the thyroid is difficult because of a. Uncertainties of the percentage of intake into body that reaches the thyroid. b. The tellurium precursors that re- sult in the absolute activity of the I! in the environment remaining roughly constant for about a day followed by a period of increasing decay rate until the precursors no longerplaya significant role and the decay rate then becomes that of I13!. February, 1956 - NUCLEONICS