CORRECTION FACTOR FOR THE IMP 241AM DATA DATED: APRIL 1980 DOE/ERSP TECH NOTE Nu. 23.0 AUTHOR: W. John Tipton, EG&G Conversion factors for the IMP system, which relate measured photopeak count rate data to source activity in the ground, depend on certain properties of the soil in which the radioactivity is distributed. Specifically, a knowledge of the elemental composition of the soil, including soil moisture and organie content, and the in situ soil density is required to determine the gama ray attenuation properties of the soil matrix. In addition, the soil density is required to convert activity per unit volume to activity per unit mass. The conversion factors used in the IMP field program were based on soil mass attenuation coefficients given by Beck, et al (Beck, 1972). (This report provides a detailed summary ofin situ measurement techniques and contains numerous reference tables which are used quite extensively by vagious groups conducting these types of measurements.) The value used for the soil density, 1.2 g/em’, was based on measurements made by EIC during the initial soil sampling effort. It was pointed out in the fall of 1979 that the soil mass attenuation coefficients given in Beck were based on a silicate soil instead of a calcium carbonate soil as exists at Enewetak. The difference in mass attenuation coefficients between Si and Ca is insignificant for gamma ray energies greater than a few hundred keV. As an example, for 137Cs with a gamma ray energy of 662 keV, the difference is 0.7%. This is the reason why soil composition is not a critical factor or a factor of concern for most types of in situ measurements. However, at low gamma ray energies there is a significant difference. In particular, for the 60 keV gamma ray from 241 am there is a factor of two difference in mass attenuation coefficients between Si and Ca. The actual attenuation coefficients required for deriving in situ conversion factors are those based on the complete soil matrix, including moisture content and organic materials. The detailed in situ soil composition data required did not exist for Enewetak soils. In order to obtain this type of data, a total of 124 soil samples were collected from nine islands in December 1979. These samples were analyzed for base elemental composition, moisture content, and organic content. The results led to an average value of 0.333 + 0.012 em“/g for the soil mass attenuation coefficient at 60 keV, compared to the value of 0.248 em2/g which was used for deriving the original 241am conversion factor. Tech Note 22 discusses these measurements and the results in detail. for 13%Cs and 69Co energies were essentially the same as those used originally. As expected, results In addition to the lack of detailed data on soil composition, it was felt that the data available for in situ density were also rather limited and should be expanded. During December, 1979, in situ soil density and soil moisture measurements were taken at 182 locations on nine islands using a nuclear density/moisture gauge. The results indicated an average value of 1.53 + 0.14 g/em” for the in situ soil density and 16+ 5%, by weight, for the soil moisture. Details of these measurementsare also contained in Tech Note 22. The revised values for the soil mass attenuation coefficient and the soil density lead to a new conversion factor for “41Am of 8.95 pCi/g per cps. This necessitates a 16% increase in all lam IMP data obtained during the cleanup project, which were based on the original conversion factor of 7.7 pCi/g per eps. (Note tnat 8.95/7.7 = 1.16.) In addition to this 16% correction, another 4% increase should be applied to account for a small shielding effect caused by the IMP being within the detector's field-of-view. This rather small systematic error had been neglected in the original conversion factor. All 24lAm data obtained with the IMP system during the actual cleanup were low by 20%. However,all final data in the final report and on the island-by-island certification documents reflect the 1.20 correction factor. tech notes are also in error by 20%. It should be pointed out that all IMP data contained in previous B-23-1