ERRORS AND ERROR PROPAGATION IN COMPUTED TRU ACTIVITY DATED: April, 1980 DOE/ERSP TECH NOTE NO. 22.0 AUTHOR: W. John Tipton, EG&G, Ine. Introduction Conversion factors relating measured photopeak count rate data (as obtained with the IMP system) to source activity in the ground depend on certain properties of the soil in which the radioactivity is distributed. In particular, it is necessary to know the in situ soil density and soil moisture as well as the elemental composition of the soil, These parameters are required to obtain the linear attenuation coefficient (the inverse of the gamma ray mean free path) in soil for a given energy gamma ray. The soil density is also required to convert activity per unit volume to activity per unit mass. A series of measurements were made between November 28 and December 11, 1979 over 9 islands to expand the rather limited data base which previously existed for these parameters. Using a nuclear density/moisture gauge, in situ measurements were taken at 182 locations in 73 areas over the 9 islands. A total of 124 soil samples were also obtained and sent to LLL for elemental composition analysis. An additional 11 samples were returned to EG&G in Las Vegas, NV for direct measurementsof the linear attenuation coefficient. Procedures Direct in situ soil density and soil moisture measurements were made using a Troxler Model 3411 nuclear density/moisture gauge. The instrumentation and procedures employed were those specified by the American Society for Testing and Materials (ASTM). Standard procedures for measuring soil density by nuclear methods are given in ASTM D 2922-71 and for soil moisture in ASTM D 3017-72. Briefly, the in situ or wet density of soil is determined by measuring the attenuation of 662 keV gamma rays from a !37Cs source through a given depth of soil. The moisture content, by weight, 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. percent moisture is obtained by dividing the moisture content by the dry density. The In the Troxler Model 3411 gauge both the 137Cs and the Am-Be sources are located in a probe which can 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. Four independent measurements were made at each of the 182 locations sampled. Measurements was repeated after rotating the detectors through an angle of 90°. Each measurement gives the average wet density and moisture content for that volume of soil lying between the sources and the detectors. ow were made with the sources located at a depth of 15 em, 10 em and 5 em. The 5 em measurement The standard procedure was to measure three locations within a given area to obtain an area average. Measurements were made 5 meters N, 5 meters SE and 5 meters SW of a given reference point, generally chosen to be one of the IMP measurement locations. This procedure was followed for 54 of the 73 different areas which were measured. Only a single location was measured in the other 18 areas, density~-the sand-cone method. In the sand-cone methodsoil is carefully removed down to a given depth. The resulting hole is then filled with fine sand having a known density. Measuring the weight of sand required to fill the hole gives the total volume of soil removed. The apparatus used B-22-1 we son Of the 18 areas where only a single location was measured, 13 were areas where a cross-calibration was performed between the nuclear density/moisture gauge and another technique for measuring soil