4% 9 areas is generally sparse and the organic content of the soils quite low, so weathering may accelerate the reduction Table 1. BXTERNAL y-RBAY EXPOSURE RATES AND MAJOR CONTRIBUTORS TO THE RADIATION FIELDS AT REPRESENTATIVE LOCATIONS ON BIKINI, ENEU AND NAM 18105 Bikini 1 Near iagoon shore 19:0(77) (iu brush) 18-9(78) 2 50 feet along transect 17-8 (73) $ 50 fect along transect 5 300 feet 8 400 feet 22-8 (61) 27-2 (62) 7 1,800 feet 8 1,410 feet 3:0(12) 28(11) 24-8 24-0 21 (0) $38(14) 248 25-0 113-4 36-8 103-2 36-1 2-4 (10) 11-3 (30) 12-5 (20) 2-7 (12) 35 (9) 40 (9) 88-8 (74) 28-:1(76) 10-5 (17) 4-913) 10-3 (9) 3-8(10) island 8-1(78) 2 1,200 feet north of 1-3-0 (63) @5 (12) =—-1-5 (31) 0-4 (10) 0-3 (6) Eneu 1 $00 feet inland—midNam 1 Near lagoon shore 18-1 (50) 2 Island centre $ Near NE corner 17-2(48) 25-8 (39) 304(50) 60-6 (33) 119-5 (66) 22°9 06 (2) ize the composition of the radiation field on islands where we were unable to obtain field spectra, a large numberof soil samples were collected from throughout the atoil. These samples, usually obtained in several depth incre- 41-2 475 400 48° 35-9 ments, were analysed quantitatively by laboratory Nal(T!} -spectrometry and also qualitatively by Ge(Li) spectrometry. We found that in high activity areas most of the activity (two-thirds or more) was usually in the top 2 or 3 inches of soil. Because of large local variations in 41 5-1 34-1 66:3 1821 soil activity on all the islands, we could not calculate accurate exposure rates in air from the one or two ‘soil 755 2040 The sums of the component exposure rates, obtained with the field spectrotmeter, are compared with the total exposure rates obtained with the jonization chamber (percentages within parentheses). samples obtained per site. The soil samples proved very useful, however, for identifying and determining relative activities of the isotopes present, which were then used to estimate the relative contributions of these isotopes to site due to 187Cs, 15 per cent to Co and 10 per cent to 5 128b. sium) the exposure rates at the various sites*. The relative exposure rate values obtained for the samesites with the Natural emitters (uranium, thorium and potas- were almost Also the greater proportional! contribution of the shorter-lived *°Co and '**8b relative to "Cs to the exposure rates in these areas will also cause these radiation levels to decrease more rapidly with time. To complement our field spectrometry and to character- 22-8 376 43-7 11 (2) 2-0 (1) densely vegetated ureas. entirely undetectable in the field spectra. The composition of the radiation field on Eneu spectrometer-ionization chamber system and from soi! 87Cs with some "Co and '**Sb. even though the exposure sample analysis agreed quite well. Because the field spectrometer and ionization chamber “‘see’’ large arcas of blast areas, including high exposure rates and increased amounts of "Co and !"Sb in the soil relative to 1*’Cs. The composition of the radiation field on Lukoj, a densely vegetated heavy fallout area, where exposure was quito similar to that of Bikini, that is, prodominantly rate levela were much lower. Nam, however, because of its proximity to several test sites, had several properties (approximately 30 feet in diameter), local variations are averaged out and these measurements are very reliable. rates varied from 60 to 200 ur./h, is indicated by the The maximum exposure rates measured on Bikini Atoll in 1967 were in blast areas very near the ground | Ge(Li) spectrum (Fig. 3). Approximately 60 per cent of the exposure rate at the soil sampling site in the high activity interior of the island was from *Co, 30 per cent from '*5Sb and ™Rh and the remainder principally from zeros wf tests. At one isolated area on West Eneman near the ground zero for two surface tests we measured an exposure rate just over 500 ur./h. Vegetation in the blast, yr r t : Mey 865 aeigm 108 6 ean ao? 3M a lf a 379 1235p 1764 JM ce ws Orgy ays VY / | ya fi j zm, Rn 4m | 250] 22: 462 633 | \' ‘C3 666 wm"Rh 767 “ seo Sb ia 1 ;t 3 rR, ~\ toz ™Rh 10aT nos H H | ' = 10"! & ®Co W7d~ *2n i366 | & “Co (32 j; i yl 1 i 10% tr i (I I | 10 J 0 Fig. 3, LUKOJ 4 500 i 1,000 a 1,500 Channel i 2,000 \ 2,600 Ge(Li) spectrumiwith’peak identification and approximate energies In keV of a soil sample taken from Lukoj Island, a heavy fallout area on the south-western rim of the atoll. . 8,000 cee one L: cation of radiation levels in these areas compared with the more Exposure rates (ur.fh) Components Field —_Toniz“Co ‘Sb spectro- ation meter chamber