weighted average of the appropriate elemental mass attenuation coefficients. The second method used was to directly measure the attenuation of 60 keV gamma rays from a 241 known thickness of soil am source through a The chemical analysis showed that the primary component of Enewetak soil is calcium carbonate with calcium contributing approximately 30-40% by weight, oxygen approximately 40-50% by weight, and carbon 10-12% by weight. There were a number of trace elements also identified; the most significant was magnesium which contributed approximately 1-2% by weight. Several trace elements such as sodium, strontium, chlorine and sulfur contributed a few tenths of a percent. The other trace elements generally contributed less than a tenth of a percent, with only a few exceptions. In one area on Mary both samples contained approximatey 4.5% iron. Iron also contributed approximately 1% by weight in one area on Enewetak. For more than half the samples, however, iron only contributed a few hundredths of a percent. Silicon and aluminum, which are two primary components of continental soil, were present in only trace amounts in the Enewetak soil. To help insure that no significant elements were missed in the chemical analysis, 20 samples were analyzed through emission spectroscopy. This analysis showed that nothing of significance was missed in the chemical analysis. The soil samples were also analyzed for organie content. Although the organic content varied from 0.5% to 25% by weight, most samples were in the range from 1% to 8% with an average of approximately 4% for all samples. The in situ or wet soil mass attenuation coefficient for each of the 124 samples were obtained using the elemental plus organic analysis combined with the in situ soil moisture measured at each location with the nuclear moisture gauge. Elemental mass attenuation coefficients were based on National Bureau of Standards (NBS) cross section data.* The mass attenuation coefficient for organic material was estimated by using the value derived for cellulose. The results are summarized in Figure B-22-4. The average value obtained was 0.333 + 0.12 em2/g. The average value for the dry, organic free component was 0.365 em“/g compared to 0.37 em“/g for pure calcium carbonate. Eleven of the samples sent for chemical analysis were split with half of the sample going to Las Vegas for direct attenuation measurements. These samples were from 11 of the 12 locations on Janet where sand-cone comparisons were performed. For each sample, approximately 12 em in diameter by 2.5 em thick were filled with soil. two petri dishes Rocks greater than approximately 1 em were not included. Otherwise, the samples were representative of the in situ soil including organic material, roots and small aggregate. Soil was packed into the petri dish to provide a density typical of the in situ densities which were measured at Enewetak--typically 1.4-1.6 g/em3, The volume of each petri dish was obtained by weighing the amount of water required to fill the dish. The attenuation of gamma rays of a given energy through a given medium is given by N = Noes p/p py. By measuring the net photopeak counts through an empty petri dish (Ng), the net photopeak counts through the dish full of soil (N), the soil density within a given petri dish ( p ) and the soil thickness (x), the soil mass attenuation coefficient (u/p) can be determined. Three independent measurements were made for each of the 11 soil samples —- one with each of the petri dish samples separately and one for both petri dishes stacked together. A115 Ci 241 am source was placed approximately 50 em in front of a side-looking coaxial high purity germanium detector. Table B-22-3 gives the average of the three measurements for each of the 11 samples. Also shown are the results obtained from the soil sample analysis for each of the samples. As can be seen, the two approachesyield results which agree quite well with each other. In addition to the 11 Enewetak samples, three soil samples obtained near Las Vegas were also analyzed in the same manner. The results for these samples are also shown in Table B-22-3. It can be seen that the mass attenuation coefficient for Las Vegas soil is significantly different from that for Enewetaksoil. *Photon Cross Sections, Attenuation Coefficients, and Energy Absorption Coefficients from 10 keV to 100 GeV (NSRDS-NBS29), 1969. B-22-3