PIXE (Particle/Proton Induced X-ray Emission) : a Powerful Quantitative Tool for Mineral Exploration
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概要
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Proton induced X-ray emission (PIXE) uses proton beam to excite the atoms in the specimen resulting in the emission of the characteristic X-rays of its elemental constituents. The low continuum background radiation in PIXE results in much higher sensitivity with detection limits in the ppm regime, even when the X-ray spectrum is recorded in energy-dispersive detectors. The large penetration depth (>30 microns) of 2-4 MeV energy protons typically used for PIXE also simplifies the X-ray yield calculations and self-absorption corrections, facilitating quantification of the results without resorting to standards. PIXE conducted with broad beam of protons, often referred to as bulk-PIXE, can be used effectively for trace element distribution measurements in drill cores, and for measurements of the minute quantities of elements present in geogas emanations captured on a collector foil. When the beam is focused to the order of micrometer, the method is known as micro-PIXE. Micro-PIXE is a powerful tool in its readily quantified, non-destructive, in situ multielement analytical capability, with ppm sensitivity and microns spatial resolution. It opens up new windows of opportunity as demonstrated in many applications. In mantle petrology and diamond exploration (GRIFFIN et al., 1990) it leads to the development of a single mineral thermometry resulting in a novel and powerful method of assessing diamond prospectivity. Applications to sulfide mineralogy reveal the residence of gold in refractory ores, and the general distribution of precious metal and trace elements in coexisting phases in a deposit for exploration, ore genesis studies and beneficiation purposes (SIE et al, 1989, 1991). A recent study on kuroko-analogue ore from the Okinawa Trough reveals that rare metal distribution and is dependent on the occurrence mode of ore, and within each ore, it is regulated by the crystal structure (MURAO et al., 1996). The usefulness of micro-PIXE is further demonstrated in studies of complex and fine-grained tin-polymetallic ore to delineate the distribution of high-tech and medical rare metals in the deposits (MURAO et al., 1995a). The large penetration depth of the proton beam is exploited in analysis of fluid inclusions without decrepitation (HEINRICH et al., 1992). The non-destructive nature of micro-PIXE is invaluable in studies of and small melt inclusions. The use of EDS defines some of the limitations of PIXE, specifically in detection of light elements and the REE. The development of new detectors with ultra thin windows lowers the limit of detectable atomic number. REE detection can be improved by using Ge detector or WDS detectors. One growth area for future work includes two-dimensional mapping in minerals and channeling analysis for determining the state of an impurity in crystals. The case histories within the past few years demonstrated that the benefits of PIXE far outweighs the limitations.
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