Considerations in Measuring Trace Radionuclides in Soil Samples by L X-Ray Detection

Several aspects pertaining to the measurements of trace radionuclides (such as plutonium) in soil samples by L x-ray detection have been considered. A 3 × 3 multiplexed array of edge-mounted Si detectors with a total detection area of 50 cm2 and FWHM resolution of 300 eV at 17 keV has been conceived. The fractional transmission-weighted solid angle for a distributed, self-absorbing source was calculated to be 0.25, which is one half the value of that for a lossless sheet source. To determine the minimum detectable activity, intensities of 59 L x-ray lines from the decay chains of 238U and 232Th have been calculated. A total of 130 x rays ranging from 9-20 keV emanate from soil per 100 alpha decays each of 232Th and 238U, compared with 4.6 for 239Pu. The minimum Pu activity detectable with the Si array in such contaminants as worldwide fallout, weapons material, and Rocky Flats soil is expected to approach 1 pCi in 1 g of soil. This concentration is comparable to the natural activity in soil. The minimum Pu activity detectable with NaI detectors is 100 times higher.