FIELD INVESTIGATION OF SOIL SAMPLE TO IMP RESULTS DOE/ERSP TECH NOTE NO.8.0 DATED: Draft - May 1978 Final - August 1978 AUTHOR: Z. Burson, EG&G B. Friesen, DRI I. Introduction For the coarse grid survey of 241Am on Enewetak Atoll, surface soil samples are taken in every four hectare parcel of each of the 17 larger northern islands. However, no island is sampled in less than four locations. The locations chosen always coincide with an IMP measurement. Table B-8-1 lists the measured soil sample to IMP ratio results for the islands surveyed. The weighted average ratio of soil to IMP is 1.23 + 0.21 using the number of composites per island as the weighting factor. The range in values shown in Table B-8-1 is 0.18 to 3.21. In view of the fact that the measurement errors are a larger percentage of the measured value for low activity levels than for higher activity levels, a better indicator of agreement differences could be derived using the activity level as a weighting factor. This result is obtained by using the ratio of the means instead of the mean of the ratios as given above. The ratio of the means for all 17 islands is 1.25. (The computational proceedure is to sum the soil sample results for all samples, sum the IMP value for all soil sample locations, divide each sum by the number of observations, then divide soil by IMP to obtain the ratio of the means.) The ratio of the means does not readily convert to graphie form so Figure B-8-1 is included to show the distribution of individual ratios using the same input as was used to compute the ratio of the means. Rather than arbitrarily correct the IMP results to match the soil sample results or vice versa, it seemed appropriate to investigate some of the factors that contribute to the comparisons. Il. Factors Influencing Comparisons There are a number of factors that influence the comparison of soil sample and IMP readings. Some of these are listed below and briefly discussed. A. Background subtraction in 241m photopeak IMP readings. The background subtraction routine in the IMP data reduction program considers channels on both sides of the 24l1Am photopeak. The influence of this routine in the calibration data as related to the actual field conditions should be investigated. B. Soil Density. Does the fact of different soil densities affect the IMP and soil sample calibration? C. 241m vertical distribution in the soil. What is the vertical distribution of 241Am in the soil and how does this influence the soil sample-IMP comparisons? D. Field-of-View. Does the soil sampling procedure adequately sample the IMP's field-of-view? Several items in this category are: 1. Effect of rocks in the field-of-view. 2. What is the variability from point to point? Are enough soil samples being taken? 3. Whatis the effect of changing the sampling board and rope knots? 4, What are the roadway effects? 5. What is the influence of the IMP and boom in the field-of-view of the detector? B-8-1