Measurement of Trace Radionuclides in Soil by L X-Ray Spectrometry

L x-ray spectrometry is shown to be a promising cost efficient technique for assaying several radionuclides in soil simultaneously without prior chemical separation. Analysis is performed with a unique Si(Li)-NaI(Tl) spectrometer. The Si(Li) detector, with thin windows on both surfaces, is mounted on its edge to permit counting x rays from two samples simultaneously. With a thin soil sample on each of its sides this detector is sandwiched between two large NaI(Tl) scintillators. When the Si(Li) detector is operating in anticoincidence with the NaI(Tl) crystals the γ background in the x-ray spectrum is reduced by 50%. Thus, this spectrometer has four times the sensitivity of a conventional Si(Li) spectrometer. Initial measurements were made with a prototype spectrometer consisting of a 3 cm2 Si(Li) detector having a resolution of 350-400 eV and two 5"×4" naI(Tl) scintillators. A 30 cm2 Si(Li) detector array with two 12"×6" NaI(Tl) crystals is under development. Measurements of Pu in soil containing 241Am, from Rocky Flats, and Pu in soil containing 137Cs, from a waste-discharge site, are in good agreement with radiochemical measurements. Activity as low as 1 pCi/g of the natural U and Th in soil has been measured. With the array the minimum detectable Pu activity is expected to be 1 pCi/g which corresponds to a concentration of 10-7 - 10-5 ppm by weight.