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Location of background soil samples and external gamma-ray exposure rate measurements in the United States Location of background samples and external gamma-ray exposure rate measurements in Alabama 15 3 Location of background samples and external gamma-ray gure Page 32 Location of background samples and external gamma-ray exposure rate measurements in West Virginia 80 33 Location of background samples and external gamma-ray exposure rate measurements in Wyoming 82 34 External gamma-ray exposure rates at 1 m above the ground-State averages 84 35 Concentration of 226 Ra in surface soil samples-State averages 86 36 Concentration of 232 Th in surface soil samples-State averages 88 37 Concentration of 238 U in surface soil samples-State averages 90 vii Summary of state background external gamma exposure rate measurements 83 Summary of state background concentrations of 226 Ra in surface soil 85 Summary of state background concentrations of 232 Th in surface soil 87 Summary of state background concentrations of 238 U in surface soil 89 Ground surveys of background radiation in the United States 91 Comparison of state background external gamma exposure rate measurements 92 Background radionuclide concentrations in surface soil-World averages 94 Correlation statistics for background measurements ..

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An assay technique that directly determines the amount of 238U and 232Th in unprocessed, bulk soil samples has been developed. In this technique, a sample consisting of a 125-mL plastic bottle full of contaminated, moist, unprocessed soil is irradiated with gamma rays from 57Co. Induced U and Th K alpha fluorescent x rays are detected using a high-purity intrinsic germanium planar detector. Because of sample irradiation geometry, the fluorescent x-ray peaks lie on top of a large Compton backscatter peak. Spectral data are first fit to a combination polynomial and ERFc-step-function background that is subtracted from the peak data. The remaining peak data are fit to a Voigt Profile to properly determine peak area. The Voigt Profile, which is the convolution of the Gaussian response of the detector system and the Lorentzian energy distribution of x rays, describes the spectral peak better than a simple Gaussian distribution. A mathematical point-node model of the source-target-detector system, which includes fluorescent x-ray production by singly scattered Compton gamma s, calculates the predicted x-ray peak area per Bq cm-3 of target contamination. Soil attenuation coefficients, which are measured for each sample, are used by the model to properly transport gamma s and x rays through the soil sample. The sample 238U and 232Th concentrations are then calculated by dividing the measured peak areas by the model-predicted peak areas per Bq cm-3. No "soil standards," no assumptions about progeny equilibrium, and no sample preparation are required for this technique.

A technique has been developed for the determination of 227Ac concentration in soil samples. Emanating 219Rn is introduced into an air stream and passed through a two-filter decay tube similar to that reported by Thomas and LeClare for 222Rn [Health Phys. 18, 113 (1970)]. Experimental tests and theoretical calculations indicate the technique to be sensitive at a level of 2 pCi/g with a 95% confidence level. Relevant empirical and theoretical considerations are discussed.