TABLE 6-3. ISOTOPIC ABUNDANCE FOR MIKE EXPLOSION PLUTONIUM Isotope Isotopie Abundance (Atom %) Fractional Alpha Half-Life (Yrs) Activity (%) 238 low 239 70.3 24,100 42.5 240 25.5 6,540 57.3 24] 87.8 2.74 low 6.10x10° @) 6.6x1074 14.4 (8 242 1.34 3.87x 109 5.1x1072 244 0.083 8.27x10% 1.4x107° It becomes clear from the foregoing discussion that one can expect some variability in the 241 py isotopic abundance in various samples taken at Enewetak Atoll. Thus, the amount of Am alpha activity that has grown into these samples, relative to the plutonium content of the samples, will show a corresponding variability. During the nominal 20-30 year decay time for these samples, there has been opportunity for appreciable chemical fractionation between plutonium and americium, depending upon individual sample history. For coralline soil samples that were exposed mainly to rainwater, the evidence seems to show that the migration rates downward through the soil for plutonium and americium are slow and not very different from each other. (Lynch, 1973) In Table 6~4 are listed activity ratios, total TRU for various types of plutonium as a function of time. The two examples of plutonium with known abundances, “weapons grade" and Mike explosion material, are compared with the median values for the total TRUe/241 Ame ratio from each island of the atoll; the lowest and highest values are listed in Table 6-4. A useful, although coincidental, correlation develops that the extremes in the range of median values for Enewetak samples are approximately equal to values for the known Pu examples. TABLE 6-4. ACTIVITY RATIOS FOR TOTAL TRU ALPHA ACTIVITY TO 241 4m ALPHA ACTIVITY. Activity Ratio Atom Ratio (Total TRUa/ 24lAma) 241 py / 239,240py 20yr 50yr Tyr 100yr 1000 yr "Weapons-grade" Pu (Table 6-1) 0.0055 7.7 5.5 5.3 5.5 17.7 Mike Explosion Pu (Table 6-3) 0.0286 2.8 2.2 2.2 2.2 5.8 Enewetak 1972-73 survey soil samples lowest highest 3.0 10. 2.4 7.3 2.3 6.9 2.4 7.3 6.3 26.0 (range of median values for each island). 158