X-ray fluorescence spectrometer

Abstract

By regarding total precision of an X-ray intensity as counting precision due to statistical fluctuation and counting loss and by regarding the counting precision as a product of precision of an uncorrected intensity, which is an intensity before counting loss correction is performed, and a gradient of a corrected intensity with respect to the uncorrected intensity, a counting time calculation unit (13) included in an X-ray fluorescence spectrometer of the present invention calculates a counting time from specified total precision of the X-ray intensity, a given counting loss correction coefficient, and a given corrected intensity for each measurement line (5).

Claims

1. An X-ray fluorescence spectrometer which obtains a quantitative value of a content of a component in a sample and/or a quantitative value of a thickness of the sample, based on corrected intensities obtained by irradiating the sample with primary X-rays, measuring intensities of generated secondary X-rays, and performing counting loss correction, the X-ray fluorescence spectrometer comprising a counting time calculation unit configured to calculate a counting time for each of measurement lines which are secondary X-rays to be measured for intensities, wherein by regarding total precision of an X-ray intensity as counting precision due to statistical fluctuation and counting loss, and by regarding the counting precision as a product of precision of an uncorrected intensity, which is an intensity before the counting loss correction is performed, and a gradient of the corrected intensity with respect to the uncorrected intensity, the counting time calculation unit calculates the counting time from specified total precision of the X-ray intensity, a given counting loss correction coefficient, and a given corrected intensity for each measurement line.

2. An X-ray fluorescence spectrometer which obtains a quantitative value of a content of a component in a sample and/or a quantitative value of a thickness of the sample, based on corrected intensities obtained by irradiating the sample with primary X-rays, measuring intensities of generated secondary X-rays, and performing counting loss correction, the X-ray fluorescence spectrometer comprising a total precision calculation unit configured to calculate total precision of an X-ray intensity for each of measurement lines which are secondary X-rays to be measured for intensities, wherein by regarding the total precision of the X-ray intensity as counting precision due to statistical fluctuation and counting loss, and by regarding the counting precision as a product of precision of an uncorrected intensity, which is an intensity before the counting loss correction is performed, and a gradient of the corrected intensity with respect to the uncorrected intensity, the total precision calculation unit calculates the total precision of the X-ray intensity from a specified counting time, a given counting loss correction coefficient, and a given corrected intensity for each measurement line.

3. The X-ray fluorescence spectrometer as claimed in claim 1, wherein instead of regarding the total precision of the X-ray intensity as the counting precision due to statistical fluctuation and counting loss, the counting time calculation unit regards a square of the total precision of the X-ray intensity as a sum of a square of the counting precision due to statistical fluctuation and counting loss and a square of hardware reproducibility precision due to hardware reproducibility of the X-ray fluorescence spectrometer.

4. The X-ray fluorescence spectrometer as claimed in claim 2, wherein instead of regarding the total precision of the X-ray intensity as the counting precision due to statistical fluctuation and counting loss, the total precision calculation unit regards a square of the total precision of the X-ray intensity as a sum of a square of the counting precision due to statistical fluctuation and counting loss and a square of hardware reproducibility precision due to hardware reproducibility of the X-ray fluorescence spectrometer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In any event, the present invention will become more clearly understood from the following description of preferred embodiments thereof, when taken in conjunction with the accompanying drawings. However, the embodiments and the drawings are given only for the purpose of illustration and explanation, and are not to be taken as limiting the scope of the present invention in any way whatsoever, which scope is to be determined by the appended claims. In the accompanying drawings, like reference numerals are used to denote like parts throughout the several views, and:

(2) FIG. 1 is a schematic diagram showing an X-ray fluorescence spectrometer of a first embodiment of the present invention;

(3) FIG. 2 is a schematic diagram showing an X-ray fluorescence spectrometer of a second embodiment of the present invention; and

(4) FIG. 3 is a diagram showing the relationship between relative precision and corrected intensity by various precision calculation methods.

DESCRIPTION OF THE EMBODIMENTS

(5) Hereinafter, a spectrometer of a first embodiment of the present invention will be described with reference to the drawing. As shown in FIG. 1, this spectrometer is an X-ray fluorescence spectrometer that, based on corrected intensities obtained by irradiating a sample 1 (including both an unknown sample and a standard sample) with primary X-rays 3, measuring intensities of generated secondary X-rays 5, and performing counting loss correction, obtains a quantitative value (an analytical value) of a content of a component in the sample 1 and/or a quantitative value of a thickness of the sample 1, and includes a sample stage 2 on which the sample 1 is placed, an X-ray source 4 such as an X-ray tube which irradiates the sample 1 with the primary X-rays 3, a spectroscopic device 6 which monochromates the secondary X-rays 5 such as fluorescent X-rays generated from the sample 1, and a detector 8 on which secondary X-rays 7 monochromated by the spectroscopic device 6 are incident and which detects an intensity of the secondary X-rays 7. An output of the detector 8 is input to a control unit 11 such as a computer which controls the entire spectrometer, via an amplifier, a pulse height analyzer, a counting unit, etc., which are not shown.

(6) This spectrometer is a wavelength dispersive and sequential X-ray fluorescence spectrometer including an interlocking unit 10 (i.e. a so-called goniometer) configured to interlock the spectroscopic device 6 and the detector 8 so as to change the wavelength of the secondary X-rays 7 entering the detector 8. When the secondary X-rays 5 enter the spectroscopic device 6 at a certain incident angle θ, an extension line 9 of the secondary X-rays 5 and the secondary X-rays 7 monochromated (diffracted) by the spectroscopic device 6 define a spectroscopic angle 2θ therebetween, which corresponds to twice the incident angle θ. The interlocking unit 10 is configured to rotate the spectroscopic device 6 by a rotation angle about an axis O, which passes through a center of a surface of the spectroscopic device 6 and which is perpendicular to the sheet of the drawing, and to rotate the detector 8 by an angle twice as wide as the rotation angle of the spectroscopic device 6 about the axis O along a circle 12, so that the spectroscopic angle 2θ is changed so as to change the wavelengths of the secondary X-rays 7 to be monochromated while the monochromated secondary X-rays 7 is allowed to enter the detector 8. The value of the spectroscopic angle 2θ (angle of 2θ) is input into the control unit 11 from the interlocking unit 10.

(7) For each of measurement lines that are the secondary X-rays 5 to be measured for intensity, the control unit 11 stops the interlocking unit 10 at the corresponding spectroscopic angle 2θ for a determined counting time and obtains a measured intensity. If necessary, the control unit 11 performs counting loss correction on the measured intensity and obtains a corrected intensity. The counting loss correction may be performed before input to the control unit 11. For each measurement line, a gross intensity obtained by measuring only a peak may be used as the measured intensity, or a net intensity obtained by measuring the peak and its background(s) and performing background subtraction may be used as the measured intensity. In addition, the quantitative calculation method for obtaining a quantitative value based on the measured intensities or the corrected intensities may be either a calibration curve method or a fundamental parameter method (hereinafter, also referred to as FP method).

(8) The spectrometer of the first embodiment includes a counting time calculation unit 13 which calculates a counting time for each measurement line 5, as a program incorporated in the control unit 11. In the calculation by the counting time calculation unit 13, as in the following equation (3), a square of total precision σ.sub.Total of the X-ray intensity is regarded as a sum of a square of counting precision σ.sub.Count due to statistical fluctuation and counting loss and a square of hardware reproducibility precision σ.sub.Inst due to hardware reproducibility of the X-ray fluorescence spectrometer.
σ.sub.Total.sup.2=σ.sub.Count.sup.2+σ.sub.Inst.sup.2  (3)

(9) Here, the hardware reproducibility is reproducibility due to all fluctuation factors other than a variation due to the counting of the X-ray fluorescence spectrometer, and includes reproducibility due to mechanical factors, the reproducibility due to electrical factors, and reproducibility due to sample preparation. The reproducibility due to mechanical factors is reproducibility due to drive or replacement of mechanical elements, and includes, for example, angle reproducibility due to goniometer drive, position reproducibility due to replacement of the spectroscopic device, a slit, a primary X-ray filter, and a sample holder, etc. In addition, the reproducibility due to electrical factors is reproducibility due to fluctuations of a tube voltage, a tube current, etc., and the reproducibility due to sample preparation includes reproducibility due to pellet preparation with a powder press and reproducibility due to sample preparation with glass beads or the like. Since the hardware reproducibility precision σ.sub.Inst does not depend on the X-ray intensity, when each precision in equation (3) is divided by the average value of the X-ray intensity in all the measurement lines, the following equation (4) holds for total relative precision σ.sub.RelTotal of the X-ray intensity, counting relative precision σ.sub.RelCount, and hardware reproducibility relative precision σ.sub.RelInst.
σ.sub.RelTotal.sup.2=σ.sub.RelCount.sup.2+σ.sub.RelInst.sup.2  (4)

(10) By transforming equation (4), the hardware reproducibility relative precision σRelInst is expressed as the following equation (5).
σ.sub.RelInst=(σ.sub.RelTotal.sup.2−σ.sub.RelCount.sup.2).sup.1/2  (5)

(11) Here, as the total relative precision σ.sub.RelTotal of the X-ray intensity, an arbitrary sample is used for each kind of sample, and relative precision of the X-ray intensity experimentally obtained by repeatedly performing measurement, for example, about 50 times for each measurement line in a state where the mechanical elements of the X-ray fluorescence spectrometer are driven or replaced according to analysis conditions including a provisional counting time, can be used. In addition, as the counting relative precision σ.sub.RelCount, relative precision of the X-ray intensity experimentally obtained by similarly repeatedly performing measurement for each measurement line in a state where no mechanical element of the X-ray fluorescence spectrometer is driven or replaced, can be used. Instead, the counting relative precision σ.sub.RelCount obtained by calculating the counting precision σ.sub.Count from a given counting loss correction coefficient τ, a given corrected intensity I.sub.C, and a provisional counting time T or a specified counting time T, for example, based on equation (10) described later, and dividing the counting precision σ.sub.Count by the corrected intensity I.sub.C, may be used.

(12) As for the counting loss correction, various correction equations are known, and for example, the following equation (6) based on an extended dead time model can be used.
I.sub.C=I.sub.0×exp(τI.sub.C)  (6) I.sub.C: corrected intensity I.sub.0: uncorrected intensity (measured intensity itself before the counting loss correction is performed) τ: counting loss correction coefficient (dead time)

(13) In equation (6), since the corrected intensity I.sub.C is included on both sides, the corrected intensity I.sub.C is obtained by repeatedly performing calculation by the Newton method, based on the uncorrected intensity I.sub.0 as measured and the given counting loss correction coefficient τ, until the corrected intensity I.sub.C for which the uncorrected intensity I.sub.0 is the initial value converges.

(14) Since the corrected intensity I.sub.C is larger than the uncorrected intensity I.sub.0, the counting precision σ.sub.Count which is precision of the corrected intensity has a larger value than precision σ.sub.0 of the uncorrected intensity and is worse as precision. As can be understood from the previous equation (6), the corrected intensity I.sub.C is a value obtained by multiplying the uncorrected intensity I.sub.0 by a coefficient corresponding to this intensity I.sub.C, and thus the counting precision σ.sub.Count can be obtained from the precision σ.sub.0 of the uncorrected intensity. Based on this idea, in the calculation by the counting time calculation unit 13, as in the following equation (7), the counting precision σ.sub.Count is regarded as a product of the precision σ.sub.0 of the uncorrected intensity and a gradient ∂I.sub.C/∂I.sub.0 of the corrected intensity with respect to the uncorrected intensity.
σ.sub.Count=σ.sub.0(∂I.sub.C/∂I.sub.0)  (7)

(15) Here, the precision σ.sub.0 of the uncorrected intensity is expressed as the following equation (8) from the previous equations (1) and (6) when the uncorrected intensity I.sub.0 is assumed to follow statistical fluctuation, for example.
σ.sub.0=(I.sub.0/T).sup.1/2=(I.sub.C/(T×exp(τI.sub.C))).sub.1/2  (8)

(16) Moreover, the gradient ∂I.sub.C/∂I.sub.0 of the corrected intensity with respect to the uncorrected intensity is expressed as the following equation (9) by partially differentiating the previous equation (6) with the uncorrected intensity I.sub.0.
∂I.sub.C/∂I.sub.0=exp(τI.sub.C)/(1−τI.sub.C)  (9)

(17) From these equations (7), (8), and (9), the following equation (10) is obtained.
σ.sub.Count=((exp(τI.sub.C)).sup.1/2/(1−τI.sub.C))×(I.sub.C/T).sup.1/2  (10)

(18) From this equation (10) and the previous equation (3), the following equations (11) and (12) are obtained.
T=I.sub.C′/(σ.sub.Total.sup.2−(σ.sub.RelInst×I.sub.C).sup.2)  (11)
I.sub.C′=I.sub.C×exp(τI.sub.C)/(1−τI.sub.C).sup.2  (12)

(19) For each measurement line, the counting time calculation unit 13 calculates the counting time T from the specified total precision σ.sub.Total of the X-ray intensity, the given counting loss correction coefficient τ, and the given corrected intensity I.sub.C, based on the equations (11) and (12). Here, as for the total precision σ.sub.Total of the X-ray intensity, a desired value is specified by an operator using an input unit such as a keyboard or a touch panel which are not shown. The counting loss correction coefficient τ is obtained in advance by a known technique and is stored in the counting time calculation unit 13.

(20) As for the corrected intensity I.sub.C, a standard sample that is of the same kind as an unknown sample to be analyzed and whose composition is known is measured at a provisional counting time, and the corrected intensity I.sub.C obtained as described in the description of the previous equation (6) is used. In addition, when a current value of the X-ray tube is adjusted as an analysis condition related to the magnitude of the corrected intensity I.sub.C, if the corrected intensity I.sub.C with which the total relative precision σ.sub.RelTotal of the X-ray intensity is minimized is desired to be confirmed, the corrected intensity I.sub.C assumed by the operator is input from the input unit. That is, the given corrected intensity I.sub.C includes both the corrected intensity I.sub.C given from another portion of the control unit 11 to the counting time calculation unit 13 as a calculated value based on the measured intensity and the corrected intensity I.sub.C specified directly by the operator via the input unit and given to the counting time calculation unit 13. As for the hardware reproducibility relative precision σ.sub.RelInst, a value calculated based on the previous equation (5) from the total precision σ.sub.Total of the X-ray intensity and the counting relative precision σ.sub.RelCount which are obtained as described in the description of the previous equation (5), is used.

(21) As described above, in the X-ray fluorescence spectrometer of the first embodiment, by regarding the counting precision σ.sub.Count as the product of the precision σ.sub.0 of the uncorrected intensity and the gradient ∂I.sub.C/∂I.sub.0 of the corrected intensity with respect to the uncorrected intensity, the effect of counting loss is appropriately reflected in the counting precision σ.sub.Count. Thus, even when counting loss occurs, the counting time T can be calculated correctly from the specified total precision σ.sub.Total of the X-ray intensity. Therefore, for example, if the total precision σ.sub.Total of the X-ray intensity corresponding to desired analysis precision (precision of an analytical value that is a content and/or thickness) is specified and the calculated counting time T is confirmed and is excessively long, it is possible to measure an unknown sample with an appropriate counting time and appropriate analysis precision by increasing the total precision σ.sub.Total of the X-ray intensity in the permissible range and specifying again the total precision σ.sub.Total.

(22) Next, an X-ray fluorescence spectrometer of a second embodiment of the present invention will be described. As shown in FIG. 2, the X-ray fluorescence spectrometer of the second embodiment is different from the X-ray fluorescence spectrometer of the first embodiment shown in FIG. 1, only in that instead of the counting time calculation unit 13, a total precision calculation unit 14 which calculates the total precision σ.sub.Total of the X-ray intensity for each measurement line 5 is provided as the program incorporated in the control unit 11. Thus, only the total precision calculation unit 14 will be described.

(23) In the calculation by the total precision calculation unit 14 as well, as in the previous equation (3), the square of the total precision σ.sub.Total of the X-ray intensity is regarded as the sum of the square of the counting precision σ.sub.Count due to statistical fluctuation and counting loss and the square of the hardware reproducibility precision σ.sub.Inst due to the hardware reproducibility of the X-ray fluorescence spectrometer. In addition, as in the previous equation (7), the counting precision σ.sub.Count is regarded as the product of the precision σ.sub.0 of the uncorrected intensity and the gradient ∂I.sub.C/∂I.sub.0 of the corrected intensity with respect to the uncorrected intensity.

(24) As for the total precision σ.sub.Total of the X-ray intensity, the following equation (13) and the previous equation (12) are obtained from the previous equation (10) and the previous equation (3).
σ.sub.Total=(I.sub.C′/T+(σ.sub.RelInst×I.sub.C).sup.2).sup.1/2  (13)
I.sub.C′=I.sub.C×exp(τI.sub.C)/(1−τI.sub.C)  (12)

(25) For each measurement line, the total precision calculation unit 14 calculates the total precision σ.sub.Total of the X-ray intensity from the specified counting time T, the given counting loss correction coefficient T, and the given corrected intensity I.sub.C, based on the equations (13) and (12).

(26) Here, as for the counting time T, a desired value is specified by the operator using the input unit such as a keyboard or a touch panel which are not shown. As for the counting loss correction coefficient τ, the corrected intensity I.sub.C, and the hardware reproducibility relative precision σ.sub.RelInst, the same values as those used in the counting time calculation unit 13 are used.

(27) In the X-ray fluorescence spectrometer of the second embodiment as well, by regarding the counting precision σ.sub.Count as the product of the precision σ.sub.0 of the uncorrected intensity and the gradient ∂I.sub.C/∂I.sub.0 of the corrected intensity with respect to the uncorrected intensity, the effect of counting loss is appropriately reflected in the counting precision σ.sub.Count. Thus, even when counting loss occurs, the total precision σ.sub.Total of the X-ray intensity can be calculated correctly from the specified counting time T. Therefore, for example, if a desired counting time T is specified and the calculated total precision σ.sub.Total of the X-ray intensity or the analysis precision corresponding thereto is confirmed and is excessively large, it is possible to measure an unknown sample with an appropriate counting time and appropriate analysis precision by increasing and specifying again the counting time T.

(28) FIG. 3 shows the relationship between relative precision and corrected intensity by various precision calculation methods. In FIG. 3, what is described as “STATISTICAL FLUCTUATION” is the relationship between the counting relative precision and the corrected intensity when only the counting precision due to statistical fluctuation is considered. The corrected intensity is substantially equal to the true X-ray intensity incident on the detector in the normal measured intensity range where counting loss correction is performed correctly. What is described as “STATISTICAL FLUCTUATION, COUNTING LOSS CORRECTION” is the relationship between the counting relative precision and the corrected intensity when the counting precision due to statistical fluctuation and counting loss is considered. As will be described later, the precision calculation method when the hardware reproducibility precision can be ignored in the present invention corresponds to this case.

(29) What is described as “TOTAL PRECISION (STATISTICAL FLUCTUATION, HARDWARE REPRODUCIBILITY)” is the relationship between the total relative precision and the corrected intensity when the counting precision due to only statistical fluctuation and the hardware reproducibility precision are considered. The precision calculation method of the background art exemplified by citing Patent Document 2 corresponds to this case. What is described as “TOTAL PRECISION (STATISTICAL FLUCTUATION, COUNTING LOSS CORRECTION, HARDWARE REPRODUCIBILITY)” is the relationship between the total relative precision and the corrected intensity when the counting precision due to statistical fluctuation and counting loss and the hardware reproducibility precision are considered. The precision calculation methods in the first and second embodiments of the present invention correspond to this case. In the above precision calculation, when the hardware reproducibility precision is included, the value of the hardware reproducibility relative precision is set to 0.0005.

(30) The relative precision of “STATISTICAL FLUCTUATION” is ideally high (small numerical value) precision that is not affected by counting loss and hardware reproducibility, but in actual measurement, the relative precision is always affected by hardware reproducibility, so that, like the relative precision of “TOTAL PRECISION (STATISTICAL FLUCTUATION, HARDWARE REPRODUCIBILITY)”, the relative precision does not become less than the hardware reproducibility relative precision (here, 0.0005). In addition, apart from the effect of hardware reproducibility, in actual measurement, as the true X-ray intensity incident on the detector increases, counting loss occurs more frequently, and even if the corrected intensity is substantially equal to the true X-ray intensity, the deviation between the uncorrected intensity and the true X-ray intensity increases. Thus, like the relative precision of “STATISTICAL FLUCTUATION, COUNTING LOSS CORRECTION”, the relative precision rises diagonally up to the right in a high intensity region.

(31) In the present invention, the counting precision due to statistical fluctuation and counting loss is considered, and as described above, the counting precision is regarded as the product of the precision of the uncorrected intensity, which is the intensity before the counting loss correction is performed, and the gradient of the corrected intensity with respect to the uncorrected intensity. Accordingly, in the present invention, the effect of counting loss is appropriately reflected in the counting precision, and for example, like the relative precision of “TOTAL PRECISION (STATISTICAL FLUCTUATION, COUNTING LOSS CORRECTION, HARDWARE REPRODUCIBILITY)” in FIG. 3, the relative precision does not become less than the hardware reproducibility relative precision, rises diagonally up to the right in the high intensity region, and has a value that is in line with reality.

(32) In the spectrometers of the first and second embodiments, the hardware reproducibility precision is taken into consideration for the total precision of the X-ray intensity, and the square of the total precision of the X-ray intensity is regarded as the sum of the square of the counting precision due to statistical fluctuation and counting loss and the square of the hardware reproducibility precision. However, when the hardware reproducibility precision can be ignored in the present invention, the total precision of the X-ray intensity may be regarded as the counting precision due to statistical fluctuation and counting loss. In this case, the following equations (14), (15), (16), and (17) are used instead of the previous equations (3), (4), (11), and (13), respectively.
σ.sub.Total=σ.sub.Count  (14)
σ.sub.RelTotal=σ.sub.RelCount  (15)
T=I.sub.C′/σ.sub.Total.sup.2  (16)
σ.sub.Total=(I.sub.C′/T).sup.1/2  (17)

(33) Moreover, in the above description of the embodiments, the gross intensity obtained by measuring only the peak is used as the measured intensity, but the present invention can also be applied by a known technique to the case where the net intensity obtained by measuring the peak and its background(s) and performing background subtraction is used as the measured intensity.

(34) Although the present invention has been fully described in connection with the preferred embodiments thereof with reference to the accompanying drawings which are used only for the purpose of illustration, those skilled in the art will readily conceive numerous changes and modifications within the framework of obviousness upon the reading of the specification herein presented of the present invention. Accordingly, such changes and modifications are, unless they depart from the scope of the present invention as delivered from the claims annexed hereto, to be construed as included therein.

REFERENCE NUMERALS

(35) 1 . . . sample 3 . . . primary X-rays 5 . . . secondary X-rays (measurement line) 13 . . . counting time calculation unit 14 . . . total precision calculation unit