Doses from external irradiation @ A. BouviLLe Er AL. rate. In Fig. | and Table 1, the gradual decrease of the exposure rate caused by the migration of the deposited activity into deeper layers of soil is not taken into consideration. Most radionuclides penetrate into the soil quite rapidly during the first year after deposition, but the vertical distribution of activity tends to stabilize after the first year. To properly account for the influence of weathering with time, we developed a time-dependent weathering correction factor, W(it—TOA) (Fig. 2). We believe that this weathering correction, which is based on the analysis of actual depth profiles of '°’Cs and *’Sr measured in the soil in the Marshall Islands in 1978 and 1991-1993, reasonably reflects the actual time variation in exposure rates from Bikini/Enewetak fallout in the years from 1948-1970. Mean values of observed relaxation lengths in the Marshall Islands in 1978 were about 5-7 cm and were only slightly greater in 1993-1994 (with of course wide variations). However, as discussed in Beck et al. (2010), '°’Cs was known to have been lost from the soil profile with an effective half life of about 12-20 y comparedwith a physical half life of 30 y. Thus, after about 5 y, when '’’Cs begins to account for most of the external exposure rate, the weathering loss of '’’Cs accounts for most of the reduction in exposurerate. The weathering correction factor, Wit — TOA), was analytically implemented in our calculations of exposure in one of two different ways, depending onthe time after deposition: in the year of deposition, a weatheringrate, X,,. of 0.00018 h', corresponding to a half-time of 5 mo and reflecting the initial weathering correction shown in Fig. 2, was added to each of the A, values; while for subsequent years, the exposure rates obtained using the values presented in Table 1 were multiplied by the 147 time-dependent factors shownin Fig. 2, 1.e., equal to 0.5 and 0.35 in years | and 2 followingthetest, respectively, and decreasing gradually to a value of 0.1 in the twentieth year after the test. Our corrections for weathering impact the estimated exposurerates only after a few weeksand, because of the rapid decrease in fallout exposure rates with time shown in Fig. 1, have only a minor effect on an individual’s integrated exposure as shown in Table 2. The effect on total exposure is greatest for large TOAs, corresponding in general to relatively low fallout. However, as discussed above, weathering does have a significant effect on the small annual doses from residual long-lived activity. The variation with time of the exposurerate, relative to an exposure rate of 1 mR h| at H+12 h, is shown in Fig. | and illustrates that there is little difference in the rate of change of exposure from fallout from different tests. Fig. 3 and Table 2 illustrate that the degree of fractionation also has only a minoreffect on the temporal variations of exposure rate and mainly at very long times after the detonation. At long times, the more volatile nuclides, such as '°Ru and '°’Cs, contribute a greater fraction of the exposure comparedto refractory nuclides. As shown in Fig. 3, weathering also has only a minor effect on exposure rate at early times, when the exposure rate is high and, thus, as shown in Table 2, has only a relatively small effect on the integrated exposure. However, as shown in Fig. 3, weathering does significantly reduce the exposure rate at long times after deposition. Given the expressions for the decay rate as a function of time, modified for weathering, the exposure, E, between any two times of interest, f, and t,, is determined by integrating the normalized exposure rate (either measured or calculated) using eqn (3): 10 2 E(t), to, i, j) = | [E12(i, j) Da,e"W(t — TOA)dt Weathering Correction Factor tt n=1 10 E1204, j) X Diafe Or tt — eH Ont Aw] - n=1 (AF Ay) G) for the first year of exposure. Estimation of the conversion factors from outdoor exposure to tissue dose 5 10! 10° 10° 10° 10° Time post-detonation (h) Fig. 2. Time-dependent correction factor used to take the weathering effect into account. In order to estimate whole-body or organ dose from the integrated exposure, the following factors were considered. First, the exposure rates estimated above correspond to outdoor conditions in the populated areas but do not