3.2.6 Variables Affecting the 24] am Conversion Factor Air Density and Composition As may be inferred from Table 3-2, the conversion factor for 241Am at a detector height of 7.4 meters is relatively insensitive to large changes in the air density. The IMP conversion factor assumes an air density of 1.15 (1073) g/em®, which corresponds to air at a temperature of 85°F (30° C) and a pressure of 750 mm Hg. There is only a +2.5 percent change in the conversion factor by going to the density extremes given in Table 3-2. (A density of 1.30 (107%) g/em® corresponds to air at a temperature of 41°F (5°C) and a pressure of 780 mm Hgand a density of 1.00 (107%) g/em? corresponds to air at a temperature of 125°F (52°C) and pressure of 700 mm Hg.) Changesin air density over the ranges of temperature and pressure which actually occur at Enewetak should not contribute more than a | percent error to the conversion factor. The mass attenuation coefficient for 60 keV gamma rays in air (0.188 em2/g) was derived from standard air composition tables and elemental mass attenuation coefficient tables. Since the corresponding mass attenuation coefficient for water is 0.20, moisture in the air should not significantly affect the air attenuation factor. Soil Density and Composition The in situ or wet soil density and soil composition are both required to determine the attenuation factor for gamma rays of a given energy in soil. Soil composition is required to determine the mass attenuation coefficient. The product of the mass attenuation coefficient and the soil density then gives the linear attenuation coefficient, which is the inverse of the attenuation mean free path. (On the average, 63 percent of the gamma rays traversing a distance of one mean free path in a given medium undergo an interaction which attenuates, i.e., reduces, their energy.) The soil density is also required to convert concentration per unit volume to concentration per unit mass. Soil density and sail composition data used for the final Enewetak conversion factor were obtained in December 1979 (see Tech Note 22). Up to that time the data available for in situ density was somewhat limited. In addition, a question arose in the fall of 1979 about the mass attenuation coefficient which was used in the original conversion factor. (These problems are discussed in detail in Tech Note 23.) Soil density and percent soil moisture were obtained using a Troxler Model 3411 nuclear density/moisture gauge. Density is determined by measuring the attenuation of 662 keV gamma rays from a 187Cs source through a given depth of soil. The moisture content of soil is determined by measuring the moderation or slowing of fast neutrons from an Am-Be neutron source. Dry density is obtained by subtracting the moisture content from the wet density. The percent moisture is obtained by dividing the moisture content by the dry density. In the Troxler gauge, both the 137Cg and the Am-Be sources are located in a probe which ean be inserted to a given depth in the soil. The gamma ray and neutron detectors are placed on the surface at a fixed lateral displacement of 25 em from the sources. After placing the sources at a given depth, gamma ray and neutron counts are accumulated for a period of one minute. The resulting counts are converted to wet density and moisture content using calibration curves supplied by the manufacturer. Measurements were made at 182 locations within 73 different areas over 9 islands. At each location the average wet density and percent moisture were obtained for the top 15 em, the top 10 em and the top 5em. The 5 cm measurements were repeated after rotating the detectors through an angle of 90°, Based on the 364 independent readings taken at the 5 em depth, the mean wet density obtained was 1.53 g/em, with a standard deviation of 0.14 g/em3, The mean value for the percent moisture was 16 percent, with a standard deviation of 5 percent. A wet density of 1.50 g/em? was used for the final conversion factor. This corresponds to an average percent moisture of 14 percent, which is probably closer to the average yearly percent moisture, , 92