X-ray or gamma ray systems or devices – Specific application – Fluorescence
Reexamination Certificate
1999-12-16
2001-11-27
Dunn, Drew (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Fluorescence
C378S044000, C378S207000
Reexamination Certificate
active
06324251
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a fluorescent x-ray analyzer.
A fluorescent x-ray analyzer of a so-called bottom lighting type is characterized as having an x-ray source disposed below a sample table for supporting a sample thereon such that the primary x-ray emitted from the source will be made incident onto an opening at a specified position on the sample table. If a sample is placed at this specified position and the primary x-ray beam from the source is made incident thereon through the opening, a secondary x-ray beam is generated on the lower surface of the sample and this secondary x-ray beam is analyzed by means of a detector such as a semiconductor detector to obtain measured data.
Some of such fluorescent analyzers are equipped with a sample table capable of not only carrying thereon a plurality of samples to be analyzed but also transporting a selected one of them to a specified position for analysis. A typical example of sample table of this kind comprises a rotatable disk-shaped member referred to as a turret, having a plurality of openings at equal distances from the center of this disk Each of the openings has a sample position associated therewith such that the turret is rotated a sample is selected.
With a fluorescent x-ray analyzer provided with a turret structured as above, the height of the target surface to be measured will generally change with respect to a reference surface, depending on which of the sample positions is disposed at the position for measurement. If this height changes, both the path length of the primary x-ray beam from the source to the bottom surface of the sample and that of the secondary beam from the bottom surface of the sample to the detector will also change. A change in the path length of the primary beam means that the intensity of the primary x-ray beam changes on the bottom surface of the sample and hence that the intensity of the secondary beam will change accordingly. A change in the path length of the secondary beam means that its intensity is not constant as it is received by the detector. In other words, even if primary beams of the same intensity are used to analyze the same sample, there will be variations in the signals outputted from the detector, depending on the position of the sample placed on the turret. For this reason, it has been a common practice in the production of a turret to minimize the differences among the heights of the sample positions.
This may be done, for example, by using primary beams of the same intensity and the same sample to measure the intensities of the signals outputted from the detector as the sample is moved from one sample position to another. If the error is expressed in terms of the ratio between the amplitude of the variations and the average signal intensity, an error as small as about 0.02-0.3% is usually required for a fluorescent x-ray analyzer of a medium capability. In order to reduce the error to this level the differences in height at different sample positions must be within a range of several to several tens in units of &mgr;m. Thus, it has been necessary to use not only an expensive high-precision machine for the production of a sample table with a turret but also a micro-gauge or the like to assemble the produced parts with a high degree of accuracy. It now goes without saying that the use of such apparatus adds to the production cost.
It should be appreciated that a problem of this kind is not unique to fluorescent x-ray analyzers but may well come about with fluorescent x-ray analyzers of different types. In the case of a fluorescent x-ray analyzer with a so-called X-Y stage or an r&thgr; stage adapted to two-dimensionally move the sample position with respect to the x-ray source, the distance between the x-ray source and the target surface for the measurement is likely to vary, depending on the position at which the sample is placed.
SUMMARY OF THE INVENTION
It is therefore an object of this invention in view of the problems described above to provide a fluorescent x-ray analyzer capable of always yielding a correct result of measurement even if the distance between the x-ray source and the target surface of a sample to be measured may change as the sample to be measured is switched or the position of the sample is changed.
A fluorescent x-ray analyzer embodying this invention, with which the above and other objects can be accomplished, may be characterized as comprising a sample table having a movable part, positioning means for moving this movable part such that a sample which is placed at a specified sample position on the movable part of the sample table is at a specified position where it is to be analyzed, an x-ray source for emitting primary x-rays to a specified target measurement plane of the sample, a detector for detecting secondary x-rays emitted from the irradiated sample, and correcting means for correcting the detection signal outputted from the detector for an error caused by the variation in the sample position, or the variations in the distance between the plane of measurement and the x-ray source and/or the detector.
In the description given above, what is referred to as the movable part of the sample table may be a turret of which the position may be specified by an angle, an X-Y stage of which the position may be specified by an X-coordinate and a Y-coordinate, or an r&thgr; stage of which the position may be specified by a radial direction and an angular orientation. With a fluorescent x-ray analyzer provided with a movable part of this type, the distance between the plane of measurement on which the sample is analyzed and the x-ray tube and/or the detector will change, depending on the position of the movable part. According to this invention, such variations due to the positioning of the movable part of the sample table is compensated for, or corrected, according to a preliminarily measured characteristics of the sample positions.
When the analyzer is adjusted, the following measurements are carried out preliminarily to its actual operation by placing a standard sample at each of the plurality of sample positions on the movable part of the sample table. The standard sample may be copper or iron with a sufficient high degree of purity having a high rate of emission of secondary x-rays. For each sample at each sample position, the movable part of the sample table such as a turret is transported to the position for irradiating it with the primary x-rays from an x-ray source, and the corresponding output signal from the detector is analyzed by converting it by means of an analog-to-digital convertor in a well-known manner, obtaining information representing the intensity of this output signal. The intensity data thus obtained from the samples at different sample positions are collected and stored in correlation with the individual sample positions in the form of a correction table. What is stored in the form of a correction table, however, need not be the measured intensity values but may be, for example, their ratios with respect to a selected one of them.
When the analyzer is used thereafter in an actual analysis, such a correction table is conveniently utilized as a database for the correction of actually measured data. Thus, although the components of the sample table is not built or assembled with a sufficient accuracy and there may be variations among the sample positions in the distances between the sample and the x-ray source and/or the detector, such variations are efficiently compensated for by using the data in the correction table stored in the memory device.
REFERENCES:
patent: 4797906 (1989-01-01), Smith
patent: 5062127 (1991-10-01), Sayama et al.
patent: 5457726 (1995-10-01), Miyazaki
Coudert Brothers
Dunn Drew
Shimadzu Corporation
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