J.R. Swider, E.B. Steel, T. Jach
Objective: To create, using a laboratory X-ray tube and capillary optics, a flexible, portable X-ray fluorescence instrument capable of sub-nanogram detection with a spatial resolution less than 50 micrometers.
Problem: Although many techniques for microanalysis exist, few are totally nondestructive, easy to maintain, or portable. Although techniques such as Electron Probe Microanalysis (EPMA) deliver sub-micrometer beams, they can be destructive in sample preparation and in analysis, costly, and not portable. Other bulk techniques, such as X-ray Fluorescence (XRF) possess poor beam resolution (millimeter-size) and have detection limits in the µg/g range. Recently the use of synchrotron radiation for XRF has overcome some of these drawbacks in XRF and in charged particle techniques. Synchrotron micro-XRF produces sub-micrometer beams with pg/g detection but still has to be performed at a synchrotron facility. The problem remains how to construct a micro-XRF instrument that utilizes the benefits of X-ray analysis yet is free of the constraints of a large, costly instrument.
Approach: To create a micro-XRF instrument that has low detection limits and is also flexible and portable, the beam must not be wasted with spatial collimation but concentrated with X-ray focusing. Of the many methods used for X-ray focusing, capillary optics best suits a micro-XRF instrument. Capillary optics is based on the premise that X-rays can be reflected at small grazing angles with little loss in intensity. Capillary optic devices are compact, easy to implement, focus a divergent beam well, and conserve the beam brilliance. A polycapillary optic lens ("Kumakhov lens") can be used with a typical laboratory sealed X-ray tube in a relatively small radiation enclosure. The optic is positioned in x,y,z, pitch, and yaw directions to maximize X-ray capture and transmission. The focused beam location and attributes are determined with an X-ray imager. Samples replace the imager at the focal distance and fluorescent radiation is detected with a Si(Li) detector. Motion control, data acquisition, and image acquisition and processing are accomplished on a single PC.
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Figure 1. Microfluorescence s-ray spectrum from 15 micrometer Sphere Multicomponent Glass K919 |
Results and Future Plans: The capillary micro-XRF instrument has successfully analyzed spherical particles around 15 micrometers in diameter as shown in Figure 1. Detection is in the range of 0.1 ng/g for elements in silicate glasses fluoresced with W tube radiation. The instrument is easily manipulated to accommodate samples in a variety of shapes and sizes and to analyze samples in situ. Implementation of a Mo tube will allow more transition and actinide elements to be examined. In order to bring the instrument to a portable stage, the positioners will be replaced with holders fixed to the tube and a small Si-PIN detector will be used for X-ray detection. |
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Last Updated
March 5, 2002
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Web Contact micro@nist.gov
