JP2014020874A - Fluorescent x-ray analyzer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a fluorescent X-ray analyzer capable of, when performing the A/D conversion of a voltage signal acquired in energy dispersion type fluorescent X-ray analysis, and creating a histogram divided with predetermined energy width by using a multi-channel analyzer, reducing the non-uniformity of the number of counts in each energy band.SOLUTION: The fluorescent X-ray analyzer includes: an A/D converter 31 for converting a voltage signal to be output from an X-ray detector into a digital signal value in energy dispersion type X-ray analysis; a signal processing section 3 for calculating the energy of an incident X-ray from a difference between the means values of the predetermined number of digital signal values respectively acquired before and after the point of time of X-ray incidence among the digital signal values to create a histogram; and a random number superimposition section 34 for superimposing random numbers on the digital signal value converted by the A/D converter 31, and for outputting the digital signal value to the signal processing section.

Description

  The present invention relates to a fluorescent X-ray analyzer. In particular, the present invention relates to an energy dispersive X-ray fluorescence analyzer.

  X-ray fluorescence analyzers irradiate solid samples, powder samples, or liquid samples with X-rays, and detect the fluorescent X-rays that are excited and emitted by the X-rays. Used for qualitative analysis and quantitative analysis. X-ray fluorescence analyzers are roughly classified into two types: wavelength dispersion type (WDS: Wavelength Dispersive X-ray Spectrometer) and energy dispersion type (EDS: Energy Dispersive X-ray Spectrometer). In a wavelength dispersive X-ray fluorescence analyzer, fluorescent X-rays having a specific wavelength are selected by an X-ray spectrometer combining a spectroscopic crystal and a slit and then detected by a detector.

  On the other hand, in an energy dispersive X-ray fluorescence analyzer, fluorescent X-rays are directly detected by a detector such as a semiconductor detector without performing wavelength selection, and then a detection signal is separated for each energy (wavelength). When obtaining a fluorescent X-ray spectrum, it is necessary to perform wavelength scanning in the wavelength dispersion type, whereas in the energy dispersion type, information on a large number of wavelengths can be obtained simultaneously. Therefore, a fluorescent X-ray spectrum can be acquired in a short time.

  FIG. 1 is a schematic configuration diagram of an energy dispersive X-ray fluorescence analyzer described in Patent Document 1 and the like. The energy dispersive X-ray fluorescence analyzer includes an X-ray optical system / sample chamber 1, an X-ray detector 2, a signal processor 3, and the like. When the sample 12 is irradiated with X-rays emitted from the X-ray irradiation unit 11 in the X-ray optical system / sample chamber 1, the electrons of the substance constituting the sample 12 are excited, and the excited electrons return to the low energy orbit. X-ray fluorescence is emitted.

  In the X-ray detector 2, the fluorescent X-ray quanta emitted from the sample 12 enter the semiconductor detector 21 and are detected as a charge signal, and converted into an amount of charge proportional to the energy of the X-ray quanta. This charge is converted into a stepped voltage signal by the integrating circuit in the preamplifier 22. When the next fluorescent X-ray quantum enters the semiconductor detector 21, the voltage signal increases by the energy of the X-ray quantum. When fluorescent X-ray quanta are incident multiple times in this way, a stepped voltage signal having multiple steps is formed. Accordingly, the height (step) of each step of the stepped voltage signal output from the preamplifier 22 corresponds to the energy (wavelength) of the X-ray quantum incident on the semiconductor detector 21.

  The preamplifier 22 takes into consideration the maximum energy Emax of fluorescent X-ray quanta that can be incident on the semiconductor detector 21, and a reset voltage Vmax larger than the step of the voltage signal generated by the incidence of one fluorescent X-ray quantum having the maximum energy Emax. Is set. When the voltage signal exceeds the reset voltage Vmax, the preamplifier 22 resets the voltage signal to 0 and newly creates a stepped voltage signal. An example of the voltage signal output from the preamplifier 22 is shown in FIG.

The signal processing unit 3 includes an A / D converter 31 and a multichannel analyzer 32. The stepped voltage signal output from the preamplifier 22 is converted into a digital signal value by the A / D converter 31. The quantization unit of the digital signal value converted by the A / D converter 31 is determined by the reset voltage Vmax of the preamplifier 22 and the number of bits of the A / D converter 31 described above. For example, when using a 16-bit A / D converter, the quantization unit will be Vmax / 2 ^16. Further, when the energy corresponding to the reset voltage Vmax and Emax +, quantization unit of converted digital signals by the A / D converter 31 becomes Emax + / 2 ^16.

  The signals converted into digital values are sequentially input to the multi-channel analyzer 32 and processed as follows. First, a point in time where a stepped step occurs in a digital value is detected, and that point is used as a reference point, and a voltage value (rear value) input after the reference point in time before the reference point The step voltage is calculated by subtracting the input voltage value (front value). This corresponds to the energy of incident fluorescent X-ray quanta. By subtracting the front value from the calculated rear value, a peak as shown in FIG. 2C is obtained at each reference time point. The height of this peak represents information regarding the energy of X-rays incident on the detector 21. However, fluorescent X-rays with energy higher than the energy corresponding to the generated step are incident at the time of reset processing (Treset), but the energy cannot be specified. Do not acquire.

  Here, in order to calculate the energy of the incident fluorescent X-ray quantum with high resolution, an average of a plurality of voltage values is taken as a voltage value before and after the reference time point. That is, an average value of voltage values of a predetermined number of points input before the reference time (hereinafter referred to as “front average value”) and an average value of voltage values of the predetermined points input after the reference time ( Hereinafter, “rear average value”) is calculated. This data processing time is called peaking time. FIG. 2 (b) shows an example of calculating the time average value of the front 96 points and the rear 96 points at the reference time T3 of the stepped voltage signal shown in FIG. 2 (a).

The peaking time is determined in consideration of the X-ray incidence frequency and energy resolution. If the peaking time is increased, the average value can be calculated using a large number of voltage values, so that the energy resolution is improved. However, if the peaking time is too long, X-rays are incident multiple times during the peaking time, and the X-ray energy cannot be calculated. This phenomenon is called pile-up. Therefore, the peaking time is set in consideration of the X-ray incidence frequency.
When the peaking time is determined, the predetermined number of points is determined from the processing speed of the A / D converter 31. For example, when the peaking time is 4.8 μs and the processing speed of the A / D converter 31 is 20 MHz (50 ns), the above score is 96 points (4,800 ns / 50 ns).

  Finally, the peak data shown in FIG. 2 (c) is tabulated for each peak height (X-ray energy) to create a histogram. The energy width of the histogram is less than one tenth of the energy resolution of the semiconductor detector 21 (usually about 140 eV), and is large enough to distinguish sharp peaks such as the characteristic X-ray peak (for example, 10 eV). Set to. In this way, a histogram as shown in FIG. 3 is obtained. FIG. 3 is an example of a histogram created by detecting more fluorescent X-rays than the example shown in FIGS. 2 (a) and 2 (c).

JP-A-10-318946

  As described above, in the energy dispersive X-ray analysis, the voltage signal is quantized by the A / D converter 31, and the difference between the front average value and the rear average value at a predetermined number of points is calculated by the multi-channel analyzer and is constant. Create a histogram divided into energy widths.

  The following problems occur in relation to the above-described series of procedures. Here, in order to facilitate understanding, specific numerical values are used as an example to explain the problem. Consider a case where the quantization unit when the A / D converter 31 converts a voltage signal into a digital value is 47.0 eV, and the number of points used for calculating the front average value and the rear average value is 96 points. In this case, the values that the front average value and the rear average value can take, and the values that the difference between the rear average value and the front average value can take are multiples of 0.4896 eV (= 47.0 eV / 96).

Here, if X-rays having uniformly dispersed wavelengths are incident on the detector with a constant intensity and a histogram is created with a predetermined energy width (for example, 10 eV), it is shown in FIG. As such, the counts in each energy band should be the same.
However, as described above, the value that can be taken by the difference between the rear average value and the front average value is a multiple of 0.4896 eV. Therefore, when a histogram totaled for each energy width of 10 eV is created, FIG. As shown in FIG. 5, the count number is non-uniform. As shown in FIG. 5, this is divided into a location where the multiple of 0.4896 eV included in the energy width of 10 eV is 20 and a location where it becomes 21 in the histogram (10 eV / 0.4896 eV = 20.43). Because.

  FIG. 6 shows an X-ray spectrum generated using a rhodium tube. Although the bremsstrahlung X-rays appearing in the region of 5 keV or more have a wide energy width and the intensity changes smoothly, the above-described fractionation due to the non-uniformity of the count number appears remarkably. The lower diagram of FIG. 6 is an enlarged view of the range of 11.0 keV to 11.4 keV. Such a fractionation should naturally appear also in the characteristic X-rays of rhodium on the low energy side (around 2.5 to 3.0 keV). That is, when such a phenomenon occurs, there is a problem that it is difficult to distinguish a minute peak.

  The problem to be solved by the present invention is to A / D convert the voltage signal acquired in the energy dispersive X-ray fluorescence analysis, and create a histogram divided by a predetermined energy width using a multichannel analyzer. To provide a fluorescent X-ray analyzer capable of reducing the non-uniformity of the count number in each energy band.

The present invention made in order to solve the above-mentioned problems, in energy dispersive X-ray analysis, an A / D converter that converts a voltage signal output from an X-ray detector into a digital signal value, and the digital signal X-ray fluorescence analyzer having a signal processing unit that calculates the energy of incident X-rays from the difference between the average values of digital signal values of a predetermined number of points acquired before and after the X-ray incident time, and creates a histogram Because
A random number superimposing unit that superimposes a random number on the digital signal value converted by the A / D converter and outputs it to the signal processing unit is provided.

In the X-ray analyzer according to the present invention, a random number is superimposed on the digital signal value after A / D conversion. As a result, the digital signal value after A / D conversion changes from a quantized value to a continuously distributed value. Thereby, the non-uniformity of the count number in each energy band of the histogram, which has occurred due to the quantization of the voltage signal, can be reduced.
As the random number, various random numbers such as a uniform random number having a uniform occurrence probability, a random number having a generation probability following a normal distribution (normal random number), and the like can be used.

If the generation width of the random number is too wide, the fluctuation of the value due to the random number superimposition becomes too large, and there is a possibility that the histogram is counted in an energy band different from the energy of the incident X-ray. On the other hand, if the random number generation width is too narrow, the quantized digital signal value cannot be changed to a continuously distributed value, and the non-uniformity of the number counted in each energy band of the histogram can be sufficiently reduced. It cannot be resolved.
Therefore, it is preferable that the random number is a random number generated with a predetermined width that reduces the non-uniformity of the count number in each energy band of the histogram. The predetermined generation width can be determined by checking whether or not the X-ray spectrum as shown in FIG. 6 is generated by changing the generation width of the random number.

  In the X-ray analysis apparatus according to the present invention, random numbers are superimposed on the quantized digital signal values after A / D conversion, and the digital signal values after A / D conversion are continuously distributed from the quantized values. Change to value. Thereby, the non-uniformity of the count number in each energy band of the histogram, which has occurred due to the quantization of the voltage signal, can be reduced.

The principal part block diagram of the conventional fluorescent-X-ray-analysis apparatus. The figure explaining the signal processing procedure in a signal processing part. The figure which shows an example of the histogram obtained in a fluorescent X ray analysis. The figure explaining the non-uniformity of the count number in each energy band, which is a problem of signal processing by a conventional X-ray fluorescence analyzer. Another figure explaining the non-uniformity of the number of counts in each energy band, which is a problem of signal processing by a conventional X-ray fluorescence analyzer. Results (upper figure) and partial enlarged view (lower figure) of X-rays generated using a rhodium tube measured with a conventional X-ray fluorescence spectrometer. The principal part block diagram of one Example of the fluorescent-X-ray-analysis apparatus which concerns on this invention. The figure explaining the example which reduced the nonuniformity of the count number in each energy band with the fluorescent X-ray-analysis apparatus of a present Example.

An embodiment of an X-ray fluorescence analyzer according to the present invention will be described with reference to the drawings.
The configuration of the X-ray fluorescence analyzer of this example is shown in FIG. The same components as those in FIG. 1 are denoted by the same reference numerals, and description thereof is omitted. The X-ray fluorescence analyzer of the present embodiment is different from the conventional X-ray fluorescence analyzer in that a random number superimposing unit 34 and a smoothing filter 35 are provided between the A / D converter 31 and the multichannel analyzer 32. As in the example of the conventional X-ray fluorescence analyzer described above, the quantization unit of the digital signal value converted by the A / D converter 31 is 47.0 eV, and the predetermined points used for calculating the front average value and the rear average value are 96 points.

  A series of flows related to fluorescent X-ray analysis in this example will be described. Since the flow in the X-ray optical system / sample chamber 1 and the X-ray detector 2 is the same as that of the conventional fluorescent X-ray analyzer described above, description thereof is omitted here. Further, since the flow in the signal processing unit 3 is the same as that of the conventional fluorescent X-ray analyzer except for the signal processing (random number superimposition) performed by the random number superimposing unit 34, the description thereof is omitted.

In the fluorescent X-ray analysis apparatus of the present embodiment, the random number superimposing unit 34 superimposes a random number on the voltage signal digitized by the A / D converter 31. In the present embodiment, the random number superimposing unit 34 generates a pseudo random number sequence using a linear congruential method and superimposes it on the voltage signal. In the linear congruential method, random numbers are generated according to the following recurrence formula.
X _{n + 1} = (A × X _n + B) modM
Here, A, B, and M are constants and satisfy the conditions of M> A, M> B, A> 0, and B ≧ 0. The constant M is a value that does not exceed a value (0.4896 eV) obtained by dividing the quantization unit (47.0 eV) of the digital signal value after A / D conversion by the score (96 points). Therefore, in this embodiment, the generation range of random numbers (the value obtained by subtracting the generation lower limit value from the generation upper limit value) does not exceed the value obtained by dividing the quantization unit of the digital signal value after A / D conversion by the above score. It is.
Subsequently, the smoothing filter 35 smoothes the digital value after the random number superimposition. The signal processing after smoothing is the same as the signal processing in the conventional X-ray fluorescence analyzer already described with reference to FIGS.

An example of reducing the non-uniformity of the count using the apparatus of this embodiment will be described with reference to FIG. X-rays with uniformly dispersed wavelengths are incident so that 100,000 beams are counted in each energy band divided for each energy width of 10 eV. As described above, the quantization unit of the digital signal value after A / D conversion of this embodiment is 47.0 eV, the front average value, the score used for calculation of the rear average value is 96 points, so the rear average value and the front average value The value that the part average value can take is a multiple of 0.4896 eV (= 47.0 eV / 96). As already described, if a histogram is created without superimposing random numbers as it is, it will be divided into a location where the multiple of 0.4896 eV included in the energy width of 10 eV is 20 and a location where it is 21. As a result, as shown in Fig. 8 (a), it is divided into an energy band where the count number is 102,500 and an energy band where the count number is 97,500. Sex occurs.
In the fluorescent X-ray analysis apparatus of this embodiment, random numbers are superimposed on the digital value after A / D conversion. As a result, the result shown in FIG. 8B is obtained. The number of counts in each energy band is within the range of about 99,200 to about 100,250. That is, by using the X-ray fluorescence analyzer of this embodiment, the non-uniformity of the count number in each energy band can be reduced to about 1% (= (100,250-99,200) / 100,000). This is a value in which the random number superimposing unit 34 superimposes a random number on a digital signal value that is quantized after A / D conversion and has a multiple of 0.4896 eV, and continuously distributes the digital signal value. This is the effect of changing to.

  In the above embodiment, the random number superimposing unit 34 is configured to superimpose a random number generated using the linear congruence method. However, if the method can generate a random number (a uniform random number or a normal random number), the type is selected. It doesn't matter.

  If the generation width of the random number is too large, the fluctuation of the value due to the random number superimposition becomes too large, and there is a possibility that the histogram is counted in an energy band different from the energy of the incident X-ray. On the other hand, if the generation width of the random number is too small, the quantized digital signal value cannot be changed to a continuously distributed value, and the unevenness of the number counted in each energy band of the histogram is sufficiently eliminated. Can not do it. Therefore, it is desirable that the random number to be used is a random number generated with a predetermined width that reduces the non-uniformity of the count number in each energy band of the histogram. The predetermined generation width can be determined, for example, by changing whether the generation width of the random number is changed and checking whether or not the fractionation occurs in the X-ray spectrum. In both cases of uniform random numbers and normal random numbers, the range of random number generation is such that the parameters that define the random number are changed to obtain an X-ray spectrum, and the fluctuation of the spectrum due to non-uniformity of the count is minimized. It can be optimized by setting to. Further, in the case of uniform random numbers, the random number generation width is set to 30% to 70% of the value obtained by dividing the quantization unit after A / D conversion by the above points, so that each energy band of the histogram The non-uniformity of the count number may be reduced.

DESCRIPTION OF SYMBOLS 1 ... X-ray optical system and sample chamber 11 ... X-ray irradiation part 12 ... Sample 2 ... X-ray detection part 21 ... Detector 22 ... Preamplifier 3 ... Signal processing part 31 ... A / D converter 32 ... Multichannel analyzer 34 ... Random number superimposing unit 34 ... random number generating unit 35 ... smoothing filter

Claims (2)

In energy dispersive X-ray analysis, an A / D converter that converts the voltage signal output from the X-ray detector into a digital signal value, and the digital signal value obtained before and after the X-ray incident time, respectively. A fluorescent X-ray analyzer having a signal processing unit that calculates the energy of incident X-rays from the difference between the average values of the digital signal values of the predetermined number of points, and creates a histogram,
A fluorescent X-ray analysis apparatus comprising: a random number superimposing unit that superimposes a random number on the digital signal value converted by the A / D converter and outputs the random number to a signal processing unit.   2. The fluorescent X-ray analysis apparatus according to claim 1, wherein the random number is a random number generated with a predetermined width that reduces non-uniformity of the count number in each energy band of the histogram.

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