Pacific Northwest Laboratory
Description
Downhole X-ray fluorescence spectroscopy is a method for detecting and quantifying inorganic (i.e., metal) contaminant concentrations in soils above the water table using a photoelectric process. The X-ray fluorescence (XRF) instrument is a downhole probe consisting of an X-ray source and a photon detector. The instrument probe is placed in a lined borehole. The surrounding soil and the detector are then irradiated with the source X-rays for a specified period of time. The detector receives a combination of Compton backscatter photons, as well as fluorescence photons emitted by certain atoms in the soil. Real-time assays of soil constituents can be performed when the instrument system is properly calibrated. The system also includes an analog-to-digital converter, a multichannel analyzer, and a computer processor. Calibration of the instrument for a particular element and observation of the number of counts appearing in a specific fluorescence range of the energy spectra results in a quantitative determination of the concentration of the element in the soil.
Fluorescence occurs when the source X-ray energy is greater than the electron binding energy of the K or L shell in the target atom. The source photon collides with the target atom and causes an electron vacancy in the K shell. This vacancy is filled by a transition of an L electron into the K shell and the emission of either a Ka X-ray photon (especially in heavy elements) or an Auger electron (especially in light elements). The competition between the two processes is described by the fluorescence yield. The probability that a Ka X-ray will be emitted approximates unity in high atomic number (Z) elements and approaches zero in low-Z elements. Typically, X-ray fluorescence is useful for elements with a Z > 20.
Technical Performance Data
Several factors affect the minimum detectable concentration of an element. First, the source
X-ray energy must be greater than the electron binding energy in the K or L shell of the target atom. The excitation process is increased when the source and fluorescence energies are closely matched. Second, greater atomic number elements (higher-Z elements) have increased probabilities of Ka (photon) emission compared to Auger electron emission that dominates in lower-Z elements. Third, the detector quantum efficiency depends on the atomic number of the anticipated target atom. Fourth, the energy band resolution becomes increasingly important to achieve signal discrimination when neighboring elements are present in the soil media (e.g., chromium Z=24, iron Z=26). Finally, attenuation of low energy X-rays limits the volume of soil that can be probed, but when the element under consideration increases in Z, then a greater volume of soil can be analyzed with increased accuracy. The accuracy is directly related to the minimum detectable contaminant concentration level.
A field test of an XRF system was conducted at Sandia National Laboratories (SNL) Chemical Waste Landfill (CWL) by Pacific Northwest Laboratories (PNL) in 1992. A commercial downhole probe manufactured by Scitec Corporation was employed for measurements of chromium concentration in three boreholes, 100-ft deep, 9-in. diameter, lined with high-density polyethylene material and a nylon cloth outer liner in a landfill. The X-ray instrument reliably detected chrome and copper. Naturally occurring iron was also detected and shown to overlap with the chrome signal. Therefore, a finer resolution detector is needed (resolution of 0.2 or 0.3 keV) to resolve different elements better and establish a more reliable instrument calibration.
Cost for a commercial instrument is estimated to be $50K.
Projected Performance
In 1993, PNL will be testing the XRF system with a new higher resolution detector at Sandia CWL. The instrument will have an outside diameter of 4 in. and a length of 5 ft. The new detector will be a cryogenically cooled SiLi detector that will have an increased quantum efficiency, a better resolution, and an improved count rate. The resolution will be 0.2 or 0.3 keV (full width at half maximum, FWHM). The improved efficiency and increased size of the new SiLi detector will result in a factor of five increase in count rate. As a result, the new system is expected to detect elements in the 50 to 100 ppm range in soil, a ten-fold increase in the minimum detection contaminant concentration limit of the present system.
In 1994 an instrument will be developed that has a diameter of less than 1.5
in. and a length of 3 ft. It will be capable of analyzing any soil type, but
boreholes must be bare or lined with thin membranes such as
SEAMIST (i.e., not steel or PVC cased holes).
Waste Applicability
The XRF spectroscopy method of detection and quantification of contamination is appropriate for high-Z metals and all other elements where Z > 20. The penetration thickness into the soil is limited by X-ray attenuation for low-energy radiation. The low-energy X-rays are likely to be used when the target atom has a low atomic number (lower-Z). However, when higher-Z atoms are to be detected, higher energy X-rays will be used and the volume of probed/analyzed soil is increased. The applicability of this technology is largely dependent on the desired minimum detectable concentration, the atomic number of the contaminant, the site characteristics (such as high levels of an element with a similar atomic number), and the resolution of the detecting device.
Status
This is a developing technology with respect to low concentration detection (ppm concentrations) and lower-Z element detection such as chromium, Z=24. The technology has been demonstrated on a field scale with some success, but significant improvements have been suggested for the next field demonstration.
Regulatory Considerations
Compliance with the Occupational Safety and Health Administration regulatons is required for hazardous waste operations and protection of occupational workers from ionizing radiation. In addition, permits may be required for drilling at hazardous waste sites.
Potential Commercial Applications
This technology could be used to detect metallic contamination near industrial sites. Examples would be Environmental Protection Agency (EPA) required testing, post-closure monitoring, site investigation, or follow-up soil analysis after structural lead paint stripping. XRF could also be used in experimental situations to determine concentrations of metals in an aerosol or aerosol filter (radioactive spent fuel aerosol experiments). In countries that have dated steel processing facilities, such as Poland, the soil surrounding an industrial plant can be analyzed for metallic contamination, specifically lead. Municipal solid waste processing and/or disposal facilities can be monitored for undesirable, toxic, or hazardous metallic waste. Other applications may include decontamination/decommissioning and post-closure monitoring for all types of industrial sites (i.e., nuclear, coal , diesel, natural-gas fired-power plants, decommissioned transformers and others).
Baseline Technology
The baseline technology for analysis of heavy metals is conventional laboratory analysis such as inductively coupled plasma spectroscopy or atomic absorption. Each require laboratory sample preparation and data evaluation to detect contaminants in soil. XRF has been used previously in the mining industry to detect soil constituents in concentrations greater than 1 percent. The utilization of XRF in environmental site characterization is a recent application. XRF provides a qualitative indication of heavy metal content with minimal sample preparation and data evaluation.
Intellectual Property Rights
PNL and Scitec have entered into a Cooperative Research and Development Agreement (CRADA) to develop the probe. All intellectual property rights will be shared between the Department of Energy (DOE), PNL, and Scitec.
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References
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