Real-time assays of soil constituents can be performed when the instrument system is properly calibrated. Calibration of the instrument for a particular element and observation of the number of counts appearing in a specific fluorescence range of the energy spectrum results in a quantitative determination of the concentration of the element in the soil. Multi-element calibration is possible. The source photon collides with the target atom and causes an electron vacancy in, for instance, the K shell. The competition between the two processes is described by the fluorescence yield.
Several factors affect the minimum detectable concentration of an element.
The excitation process is increased when the source and fluorescence energies are closely matched. Third, the detector quantum efficiency depends on the atomic number of the anticipated target atom. In 1992, a commercial downhole probe manufactured by Scitec Corporation was employed for measurements of chromium concentration in three 9-in diameter boreholes, each 100 ft deep. These boreholes were in a landfill lined with high-density polyethylene material and a nylon cloth outer liner. The probe was demonstrated in the same boreholes as before, and much improved contaminant detection capability was obtained, with chromium detectable at around the 50 to 100 ppm level.