JPH06249804A - Fluorescent x-ray spectroscopic device - Google Patents

Abstract

PURPOSE:To reduce scattering X-rays being a factor of background and enhance detection lower limit by inserting an absorption plate composed of an element having an atomic number smaller than a constituent element of a secondary target between the secondary target and an X-ray tube. CONSTITUTION:Primary X-rays (a) generated 1 pass through an absorption plate and are opplied to a secondary target 3a. Absorption plate switching mechanism 2 is provided with the absorption plate composed of an element having an atomic number smaller by five or more than a constituent element of a target plate. Secondary X-rays (b) generated in the secondary target 3a are applied to a sample 4. A constituent element of the sample 4 is excited by the secondary X-rays (b) to generate fluorescent X-rays (c), which are incident upon an X-ray detector 5 of energy dispersion type, converted into an electric signal (d) and detected as an X-ray spectrum by a wave motion analyzer 6. A computer 7 reads the detected X-ray spectrum and carries out qualitative/ quantitative analysis.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to improvement of the lower limit of detection for trace analysis in fluorescent X-ray analysis.

[0002]

2. Description of the Related Art Fluorescent X by a direct excitation method in which a sample is irradiated with primary X-rays having a wide energy range generated from an X-ray tube.
In the line analysis device, in addition to the characteristic X-rays generated by the elements in the sample being excited, scattered X-rays in which the primary X-rays are scattered by the sample are generated. X-ray generated from the sample is X
The scattered X-rays form a background with respect to the peak caused by the characteristic X-rays (fluorescent X-rays) on the X-ray spectrum detected by the line detector and subjected to the pulse height spectroscopy. This background limits the lower limit of detection in fluorescent X-ray analysis. In this direct excitation system, in order to lower the background, a primary filter made of an element that largely absorbs in a specific energy range is arranged between the X-ray tube and the sample. The primary filter irradiates the sample with the primary X-rays that are largely absorbed and attenuated in the specific energy range of the primary X-rays. When the energy of the characteristic X-ray of the element contained in the sample is within the energy range, the lower limit of detection is improved.

In addition to the direct excitation method, a target plate composed of a single element is irradiated with primary X-rays generated from an X-ray tube, and a characteristic X-ray of the single element generated from the target plate is irradiated to the sample. Then, there is a secondary target excitation method for exciting the elements in the sample. In this secondary target method, in order to monochromeize Kα rays generated from the target plate, an absorption plate composed of an element having an X-ray absorption edge between Kα and Kβ rays is used as the target plate. It is arranged between the samples to remove Kβ rays. In this secondary target method, the sample is mostly excited by the Kα characteristic X-rays generated from the target plate,
Since the background is lowered and the sample is excited with monoenergetic X-rays, the analytical model for quantitative analysis is simplified.

[0004]

Since the primary filter used in the direct excitation method utilizes the X-ray absorption edge of the constituent elements forming the filter, the high energy side of the absorption edge is the sample. X-rays that reach the position are greatly attenuated, and the attenuation rate becomes smaller as the energy becomes higher. Therefore, the background decreases for several KeV from the energy at the absorption edge to the high energy side.
There has been a problem that the lower limit of detection is improved only for the measurement of elements having characteristic X-rays in that range.

Further, when there is a characteristic X-ray energy of a trace element on the low energy side of the characteristic X-ray energy of the main element contained in the sample, from the upper limit of the count rate of the energy dispersive X-ray detection system, X There is a problem in that the setting of the tube current of the wire tube is determined by the peak of the main element, and the characteristic X-ray intensity of the trace element cannot be sufficiently obtained.

On the other hand, in the secondary target excitation method,
By preparing a target plate having a plurality of different elements for excitation by the characteristic X-rays generated from the target plate and switching the target plate according to the measurement element, it becomes possible to measure from low energy to high energy, and Improvement of the lower limit of detection can be expected.

However, since the target plate also scatters a small amount of primary X-rays generated from the X-ray tube, this scattering process has been a problem when performing an extremely small amount of analysis.

[0008]

Between the X-ray tube and one selected target plate among a plurality of switchable target plates, an atomic number of an element constituting the target plate is used. As a mechanism for selecting an absorbing plate composed of elements having an atomic number smaller than at least 5 from the absorbing plate switching mechanism,
The intensity of the X-rays generated by exciting the target plate with the primary X-rays generated from the X-ray tube is reduced to 10% to 90% as compared with the intensity when there is no absorption by the absorption plate. Such an absorbing plate is selected to absorb the primary X-rays. The measurement sample is irradiated with X-rays generated from the target plate, and characteristic X-rays generated from the sample are detected. When the target plate is selected, the tube voltage and the tube current of the X-ray tube are set to specified values in advance according to the type of the target plate and are applied to the X-ray tube.

Further, there is a mechanism capable of switching between the excitation method by the secondary target (secondary target excitation) and the method of directly irradiating the sample with the primary X-ray (direct excitation). In the direct excitation method, X A collimator with micro-holes is placed to limit the dose of radiation.

[0010]

When the absorption plate is inserted, the X-ray intensity generated from the target plate and irradiated on the measurement sample is reduced from 10% to 90%. I often choose the one that produces lines. At this time, if the constituent element of the absorption plate is composed of an element smaller than the constituent element of the target plate by 5 or more, there is no absorption edge above the absorption edge energy of the constituent element of the absorption plate, and the absorption edge energy of the absorption plate. The closer it is to, the more it absorbs. This effect changes with an exponential function, and the X-ray intensity generated from the target plate is that the energy X-rays at or below the absorption edge of the elements constituting the target plate do not contribute to the generation and Contributes to the generation thereof, from the absorption edge energy of the element forming the absorption plate to the characteristic X-ray energy of the element forming the target plate, without significantly impairing the X-ray dose irradiated to the measurement sample. Largely absorbs the primary X-rays.

Therefore, the X-rays in the energy range scattered by the target plate are also extremely small, and the scattered X-rays generated from the measurement sample are also small. Therefore, the background is small in the measured X-ray spectrum, and the lower limit of detection is improved.

[0012]

Embodiments of the present invention will be described below with reference to FIG. The X-ray tube 1 generates a primary X-ray a. The generated primary X-ray 1a passes through the absorption plate 2a (filter) and irradiates the secondary target 3a. The absorption plate 2a is held by the absorption plate switching mechanism 2, and the absorption switching mechanism 2 includes a plurality of absorption plates 2a made of different materials. By rotating the absorption switching mechanism 2, the absorption plate 2a through which the primary X-ray 1a passes can be arbitrarily switched.

The secondary target 3a is held by the secondary target switching mechanism 3. Secondary target switching mechanism 3
Includes a plurality of secondary targets 3a made of different materials. By rotating the secondary target switching mechanism 3, the secondary target 3a arbitrarily irradiated with the primary X-ray 1a
Can be switched.

The secondary X-ray 1b emitted from the secondary target 3a irradiates the sample 4 to be measured. The elements forming the sample 4 are excited by the secondary X-rays 1b and generate fluorescent X-rays 1c. The generated fluorescent X-ray 1c enters the energy dispersive X-ray detector 5, is converted into an electric signal d, and is detected by the wave height analyzer 6 as an X-ray spectrum.

The computer 7 reads the X-ray spectrum detected by the wave height analyzer 6 and performs qualitative / quantitative analysis. With the secondary target mechanism 3, Si plate, Ti plate, Cr
Plate, Ni plate, Zn plate, Zr plate secondary target 3a is made selectable, Be 200 μm plate, Cr10 by absorption mechanism 2
0 μm plate, Al 50 μm, Al 100 μm, Al200
A description will be given of the present embodiment in which the absorption plate 2a of μm, Al of 300 μm, and Pb of 5 mm or the blank can be selected.

For example, in the secondary target switching mechanism 3, Si
When the secondary target 3a of (atomic number 14) plate is selected, the absorption mechanism 2 sets the absorption plate 2a of Be (atomic number 4) 200 μm plate. At this time, the secondary X-ray intensity 1b is B
It is reduced to 57% as compared with the case without the absorption plate 2a of e.

On the other hand, the attenuation rate of the primary X-ray 1a absorbed by the Be absorption plate 2a at the energy corresponding to the AlKα ray is 0.14%. This means that the intensity ratio (P / B ratio) between the characteristic X-rays and the scattered X-rays generated from the sample 4 is improved by two digits, and the lower limit of detection is improved by one digit. The detection limits of the other elements in Table 1 are also improved by one digit or more. Table 1 shows an example.

[0018]

[Table 1]

In Table 2, Ti is used as the secondary target 3a.
An example in which Al (atomic number 13) 50 μm is set as the absorption plate 2a when the (atomic number 22) plate is selected is shown. From Table 2, the secondary X-ray intensity 1b decreases to 61% due to the absorption effect of the absorption plate 2a, while the background equivalent to CaKα decreases to 0.2%, so that the P / B ratio is 2
The digit is improved, and the lower detection limit is increased by one digit.

[0020]

[Table 2]

In Table 3, when a Cr (atomic number 24) plate is selected as the secondary target 3a, Al is used as the absorption plate 2a.
An example in which (atomic number 13) is set to 100 μm is shown. Table 3
The secondary X-ray intensity 1b decreases to 55% due to the absorption effect of the absorption plate 2a, while the background equivalent to TiKα decreases to 0.08%, so that the P / B ratio is 2
It is improved by more than one digit and the lower limit of detection is improved by one digit.

[0022]

[Table 3]

When a Ni (atomic number 28) plate is selected as the secondary target 3a in Table 4, Al is used as the absorption plate 2a.
An example in which (atomic number 13) is set to 200 μm is shown. Table 4
The secondary X-ray intensity 1b decreases to 58.2% due to the absorption effect of the absorbing plate 2a, while the background equivalent to MnKα decreases to 0.13%.
The ratio is improved by two digits and the lower limit of detection is improved by one digit.

[0024]

[Table 4]

In Table 5, when a Ni (atomic number 28) plate is selected as the secondary target 3a, Al is used as the absorption plate 2a.
An example in which (atomic number 13) is set to 300 μm is shown. Table 5
From the above, the secondary X-ray intensity 1b is reduced to 57% by the absorption effect of the absorption plate 2a, while the background equivalent to CoKα is reduced to 0.2%, so that the P / B ratio is 2%.
The digit is improved, and the lower detection limit is increased by one digit.

[0026]

[Table 5]

When a Zr (atomic number 30) plate is selected as the secondary target 3a in Table 6, Cr is used as the absorption plate 2a.
An example in which 100 μm is set is shown. From Table 6, while the secondary X-ray intensity 1b decreases to 37% due to the absorption effect of the absorption plate 2a, for example, the background equivalent to PbLα is 0.
Since it decreases to 019%, the P / B ratio is improved by three digits and the lower limit of detection is improved by one digit.

[0028]

[Table 6] When other elements are used as the secondary target 3a, the element and the thickness of the absorbing plate 2a are similarly experimentally determined and used. When the measurement is completed, use P as an absorption plate.
When b5 mm is selected, X-rays are completely cut and the measurement sample can be converted. Of course, in addition to the absorption plate switching mechanism 2 having the shutter function, the shutter mechanism may be arranged independently. Secondary target switching mechanism 3
When a light element such as an alloy or graphite is used as a secondary target in addition to the metal plate, some characteristic X-rays are generated as the secondary X-ray 1b in the case of an alloy, and a light element such as graphite is generated. In this case, since many X-rays are scattered, the atoms of the sample 4 are excited by the scattered rays.

At this time, the absorbing plate switching mechanism 2 is provided with a mechanism that can be moved in and out of the X-ray path, or a blank can be selected by setting the absorbing plate switching mechanism 2. In addition to such a secondary target excitation method, a mechanism for switching between a direct excitation method of directly irradiating the measurement sample 4 with the primary X-ray a generated from the X-ray tube 1 is provided. In the case of this direct excitation method, the primary X-ray a ′ is irradiated onto the measurement sample 4 through the limiting collimator 8 having the micro holes. This is because the primary X-ray a7 is so strong that it is necessary to reduce the tube current flowing in the X-ray tube 1. However, considering the linearity of the tube current, the variable range from 2 to 3 digits is usually the maximum limit. Is. Therefore, the limiting collimator 8 is arranged to further weaken the primary X-ray a ′.

[0030]

EFFECTS OF THE INVENTION The measured intensity of fluorescent X-rays is absorbed by several 10% due to the absorption effect of the absorption plate, but the background is absorbed by 2 to 3 digits or more, so that the P / B ratio is improved and the lower limit of detection is lower. Go down. That is, the detection sensitivity is improved.

[Brief description of drawings]

FIG. 1 is a schematic perspective view of an embodiment of the present invention.

[Explanation of symbols]

 1 X-ray tube 2 Absorption plate switching mechanism 3 Secondary target switching mechanism 4 Sample 5 Energy dispersive X-ray detector 6 Wave height analyzer 7 Computer 8 Micro-perforated limited collimator 1a Primary X-ray (secondary target excitation method) 1a 'Secondary X-ray (direct excitation method) 1b Secondary X-ray passing through the absorption plate 1c Fluorescent X-ray d Electric signal

Claims (4)

[Claims] 1. An X-ray tube that generates primary X-rays, target plates of different materials that are irradiated with the primary X-rays and generate secondary X-rays, and a target plate that irradiates the primary X-rays. Of energy for detecting fluorescent X-rays generated from elements contained in the measurement sample by irradiating the measurement sample with a secondary X-ray generated from the target plate, and a target switching mechanism that enables arbitrary switching In a fluorescent X-ray analyzer comprising a dispersive X-ray detector and a plurality of absorption plates of different materials arranged switchably between the X-ray tube and the target plate, the absorption plate is A fluorescent X-ray analysis apparatus comprising an element having an atomic number that is at least 5 or more smaller than the atomic number of the target plate constituent element. 2. In the energy dispersive X-ray fluorescence analyzer, when one of the plurality of target plates is selected, a voltage determined in advance for each type of the target is applied to the X-ray tube. The thickness of the secondary X-ray that has a mechanism to be applied and that is generated under the conditions of the selected target and the voltage is attenuated by absorption by the absorption plate to a range of 10% to 90%. The fluorescent X-ray analysis apparatus according to claim 1, which is determined for each target type and is switchable. 3. In the energy dispersive X-ray fluorescence analyzer, in addition to the plurality of absorption plates, one is a shield plate that absorbs almost all the primary X-rays and one is equipped with an absorption plate. Not having a blank, selecting the shielding plate when not measuring, and selecting one of the plurality of absorbing plates when measuring
The X-ray fluorescence analyzer according to claim 1, wherein the blank can be selected individually. 4. The energy dispersive X-ray fluorescence analyzer further comprises a shutter having a drive mechanism between the X-ray tube and the sample, in addition to the X-ray fluorescence excitation system, The fluorescent X-ray analysis apparatus according to claim 1, wherein a limiting plate having a large number of minute holes that can be switched to be directly excited by the primary X-rays is arranged between the X-ray tube and the sample.