B. Soil Sampling Soil samples were taken from almost all of the islands of the atoll. On Bikini Island complete and careful depth profiles were obtained. All the samples were returned to the United States for quantitative gamma-ray spectrometric analysis by the University of Washington Laboratory of Radiation Ecology and by HASL. Selected samples were analyzed by HASL contractors for ®°°Sr. Qualitative lithium drifted germanium Ge(Li) spectrometry of many of the samples was carried out under contract for HASL to determine all the gamma emitting isotopes in the soils. The results of the gamma spectrometric analyses of the depth profiles were used to determine the approximate average relaxation length of the assumed exponential isotopic concen- tration in the soil*’*. A relaxation length of 2 cm was found to be consistent with these data and this number was used in determining the field spectrometer calibration factors*'®, Gamma-ray exposure rates for various isotopes were calculated from the laboratory gamma spectrometric soil analysis data only for the locations where the soil samples were taken from a known depth and area. For these calcu- lations the relaxation length varied from 1 to 3 cm (i.e. about 67% of the activity was in the first 1 to 3 cm of soil). The results of these calculations turned out to be relatively poor (see Section III) due to the difficulty in obtaining a soil sample representative of the area as a whole. Although we could not always calculate accurate absolute exposure rates from the soil data, by making the following plausible assumptions about the distribution of radioisotopes in a given soil sample, we were able to obtain useful quantitative estimates of the relative contributions of each emitter to the total exposure rate. The first assumption is that the percentage of the total gamma-ray activity per gram of soil due to a given isotope does not vary significantly within an area of approximately a 30 ft. radius about the soil sampling site. The second is that there is no significant fraction- - 10 - DOF ARCHIVES