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 fluorescent × 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 × 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 gammas, 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 gammas and × 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.
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