CN109030529B

CN109030529B - Monochromatic excitation X-ray fluorescence spectrometer

Info

Publication number: CN109030529B
Application number: CN201811272585.7A
Authority: CN
Inventors: 徐晓东; 倪虹; 赵金刚
Original assignee: Shanghai East Electronic Co ltd
Current assignee: Shanghai East Electronic Co ltd
Priority date: 2018-10-30
Filing date: 2018-10-30
Publication date: 2021-12-21
Anticipated expiration: 2038-10-30
Also published as: CN109030529A

Abstract

The invention discloses a monochromatic excitation X-ray fluorescence spectrometer, which comprises: (1) an X-ray source selected to produce an X-ray beam, (2) optics to focus the X-ray beam produced by the X-ray source onto the sample and select X-rays of a desired energy, (3) an energy resolving X-ray detector. The monochromatic excitation X-ray fluorescence spectrometer is configured as follows: (i) the X-ray source uses a low power X-ray tube of no more than 50W that emits X-rays that are divergent and have a defined spot size, (ii) the optical element uses a logarithmic spiral rotating hyperboloid intraocular lens to completely and exclusively focus X-rays of a desired energy emitted by the X-ray source and have an extremely high diffraction efficiency, (iii) the energy resolving X-ray detector preferably uses a silicon drift semiconductor detector. The spectrometer adopting the configuration is a small, simple and low-cost monochromatic excitation X-ray fluorescence spectrometer with extremely low detection limit.

Description

Monochromatic excitation X-ray fluorescence spectrometer

Technical Field

The invention relates to a spectrometer, in particular to a monochromatic excitation X-ray fluorescence spectrometer comprising a logarithmic spiral rotating hyperboloid intraocular lens. The ultra-low detection limit achieved by the spectrometer is particularly suitable for detecting trace elements in materials, such as low-concentration silicon, sulfur, chlorine and the like in fuel, oil and additives.

Background

In recent years, with the basic achievement of the well-being goal of China, the people are greatly moving towards the developed countries, and the awareness of the common people on environmental protection is more and more improved. To reduce the emission of pollutants harmful to the environment, the quality standard of the vehicle fuel oil is improved by the nation, wherein the limit requirement of the sulfur concentration is reduced from 50ppm (vehicle gasoline and diesel nation IV) to 10ppm (vehicle gasoline and diesel nation V, VI). The current regulatory limits in the united states, japan and the european union are the same, also 10 ppm. Experts predict that future will fall to 5ppm or less in the near future. Silicon-containing reagents or catalysts are used in the refining process of gasoline and diesel oil, and silicon deposits can cause failures such as automobile oxygen sensor failure, so the detection of silicon elements in gasoline and diesel oil is listed in the national standard established range. Chloride is one of the biggest problems in the refining industry, and the limited requirement for chloride concentration should be 1ppm or less from a corrosion point of view, with rising chloride concentrations leading to extreme corrosion and fouling in the crude distillation unit distillate and/or naphtha hydrotreating unit.

Common microanalysis methods include chemical techniques such as titration, ion and gas chromatography, microcoulomb methods and combustion spectroscopy. These methods all have complex sample preparation and handling requirements and require frequent calibration and cumbersome maintenance, which are difficult methods to use. Two other types of ultra-trace analysis methods: namely atomic absorption/emission spectroscopy (AAS or AES) and Inductively Coupled Plasma (ICP) based methods. Both methods, although having extremely low detection limits, far below the above-mentioned requirements for detection limits, have significant disadvantages: such as high power (-6 kw), high vacuum or inert ambient gases, complex sample preparation and handling requirements, etc., and are expensive.

The X-ray fluorescence spectrometer has the advantages of simple structure, no damage to the sample, easy overcoming of the absorption and enhancement effects of the matrix, high precision, rapid determination, no need of sample preparation or simple sample preparation and the like. However, conventional XRF, either EDXRF or WDXRF, is considered to be useful only for analysis of a large number of samples, typically only up to the ppm level for EDXRF and sub-ppm level for WDXRF, but is typically high power (-4 kw) and heavy (400 to 550 kg).

One way to lower the detection limit of XRF spectrometers is to increase the effective radiation and reduce the background signal. Recently, x-ray optics have been greatly developed. One existing x-ray optic, the doubly-curved crystal DCC, provides focusing of x-rays from a source to a point target in all three dimensions based on bragg diffraction of x-rays on an optical crystal (e.g., a germanium (Ge) or silicon (Si) crystal), thereby collecting divergent radiation from the x-ray source over a large solid angle while monochromating. The detection limit of a low-power X-ray source EDXRF spectrometer using the DCC can reach a sub-ppm level, such as 0.2ppm for elemental sulfur.

Such prior doubly-curved crystals are based on john geometry, one limitation being low radiation collection angle, and methods of overcoming this limitation have been disclosed in U.S. patent No.5,127,028 and U.S. patent No.7,035,374, but have limitations. These 2 patents all use a method of splicing a plurality of diffraction crystals, so that it is difficult to manufacture; although this 2 patent overcomes this limitation, it is not geometrically perfectly focused due to the characteristics of john geometry itself. Such existing doubly-curved crystals also employ natural crystals (e.g., germanium (Ge) or silicon (Si) crystals), which have poor integral diffraction efficiency.

There is a need to provide an x-ray optic that does not require stitching, is fully focused geometrically, employs an intraocular lens with extremely high integrated diffraction efficiency to provide a monochromatic x-ray beam of greater collection angle and higher intensity than provided by the prior art, and is easy to manufacture.

It is desirable to provide a low power X-ray source X-ray fluorescence spectrometer that employs the X-ray optics provided by the present invention to provide a lower detection limit than previously reported results.

Disclosure of Invention

To achieve the above object, the present invention provides a monochromatic excitation X-ray fluorescence spectrometer, comprising: selecting an X-ray source for generating an X-ray beam; x-ray optics that focus an X-ray beam produced by an X-ray source onto a sample and select X-rays of a desired energy; and an energy-resolving X-ray detector for directly collecting the X-ray beam reflected by the sample; the X-ray optic is designed as a logarithmic spiral rotating hyperboloid crystal according to the Bragg diffraction principle of X-rays, using a C-intraocular lens with the largest integrated reflectivity of all known crystals.

Wherein the X-ray source can be selected to have a spot size of ≦ 1mm, preferably ≦ 0.5mm, and more preferably ≦ 100 μm in diameter. The X-ray source may be an X-ray tube having an X-ray target and operable to produce characteristic X-rays of intense radiation produced by fluorescent X-rays superimposed on the bremsstrahlung spectrum.

The X-ray target may be, without limitation, one of a molybdenum target, a copper target, a rhodium target, a palladium target, a silver target, a gold target, or a tungsten target. In embodiments using a silver target, the X-ray tube may be operated at a voltage between 5keV and 50 keV. At a maximum power of 50w, the light pipe preferably has a high voltage of 22kV and a current of 2.27mA to produce a characteristic AgL α X-ray of 2.984keV with maximum intensity. In the X-ray tube, a domestic side window X-ray tube is suitable for use in consideration of price and focusing effect.

Wherein the X-ray optics are used to monochromate the X-ray light pipe emission spectrum, i.e. to substantially reduce the unwanted parts of the emission spectrum, such as continuous bremsstrahlung and interfering characteristic lines, while reducing as little as possible or even increasing the intensity of the monochromated line or narrow energy band required to excite the X-rays.

The X-ray optics are based on the principle of Bragg diffraction of X-rays, using a C-lens with the maximum integrated reflectivity of all known crystals, designed as a logarithmic spiral rotating hyperboloid, which can focus the point source completely within a sufficiently small area, allowing to compensate for the loss of useful radiation due to reflection by collecting the primary radiation within a large acceptance solid angle, with a very high geometric reflection efficiency.

The logarithmic spiral hyperboloid crystal is based on a logarithmic spiral geometric shape, crystal lattices are parallel to the surface, a logarithmic spiral segment is arranged on an axial plane of a connecting line of a source and a focus, and the logarithmic spiral segment is rotated relative to the axial plane of the connecting line of the source and the focus to form the logarithmic spiral hyperboloid.

The principle of determining the size of the crystal block is to collect the majority of the X-ray source radiation in the smallest possible size.

The logarithmic spiral hyperboloid crystal is logarithmic spiral rho ═ ae in the axial plane^φctgθ(polar coordinates). The pole is the X-ray source point and theta is taken as the Bragg angle theta_bTheta is calculated from the crystal spacing d and the energy E of the desired monochromated line_bThus, θ is determined.

We generally choose the X-ray source to radiate a centerline phi-theta_bThe line of time, the X-ray source radiation angle is gamma, then the independent variable phi is in the range-theta_b+γ/2～-θ_b- γ/2. Logarithmic spiral split X-ray tubePoint P of₁Select the nearest point, then P₁Determine, i.e. p₁Also determines the corresponding phi₁＝-θ_b- γ/2, so that a is determined. After θ and a are determined, the logarithmic spiral is determined. Then the center point P of the logarithmic spiral crystal segment₀Then determine, and then line segment P₁P₀Is also determined. The inventor selects the central point P₀Is the center of the crystal segment length and not the center of the angle, the logarithmic spiral crystal segment length P₁P₂＝2P₁P₀It is determined.

Several incident and emergent rays were directly and accurately drawn on a computer as shown in fig. 3. We find that all the reflected rays are concentrated in p₁、ρ₀、ρ₂Triangle F intersected by reflection rays of three incident rays₁F₂F₃The center point reflection line is closest to the source point S. Thus, in a triangle from F₂Center F of the perpendicular of the point-to-center reflection line₀The spot acts as a focus point. Pole S passes through F₀And (4) making an X-axis, and rotating the logarithmic spiral crystal segment obtained in the step (a) along the X-axis to obtain the logarithmic spiral hyperboloid of the X-ray optical device.

The required rotation angle beta around the axis can be obtained according to the radiation angle gamma of the X-ray tube, and the rotation arc length w, namely the width of the logarithmic spiral crystal, is further determined.

It should be noted that the above method of determining a logarithmic spiral crystal mass is only applicable to X-ray tubes with a radiation angle γ smaller than θ_bThe case (1).

In certain embodiments, a monochromatic excitation X-ray fluorescence spectrometer may include two or more optics to concentrate as much of the excitation X-rays of the X-ray tube within two or more selected energy ranges onto a sufficiently small area of the sample as possible.

Wherein the energy-resolving X-ray detector can be a proportional counter tube, a Si-PIN detector, a silicon drift detector SDD and the like. Preferred are silicon drift detectors, wherein the effective diameter of the detector is about 3 to 8 mm.

The energy resolving X-ray detector preferably comprises processing electronics adapted to process output signals obtained from the respective detector.

Due to the adoption of the technical scheme, the monochromatic excitation X-ray fluorescence spectrometer does not need splicing, is completely focused geometrically, adopts an artificial crystal with extremely high integral diffraction efficiency, provides a monochromatic X-ray beam with a larger collection angle and higher intensity than the prior art, and is easy to manufacture; the monochromatic excitation X-ray fluorescence spectrometer of the low-power X-ray source can provide a detection limit lower than the previously reported result by adopting the X-ray optical device provided by the invention.

Drawings

FIG. 1 is a schematic diagram of a monochromatic excitation X-ray fluorescence spectrometer according to one embodiment of the present invention;

FIG. 2 is a schematic diagram of a semi-focused (Johann) spectroscopy principle;

FIG. 3 is a schematic diagram of a full focus (Johansson) spectroscopy principle;

FIG. 4 is a schematic diagram of the crystal components of the optics used in the monochromatic excitation X-ray fluorescence spectrometer of FIG. 1 in an axial plane;

FIG. 5 is a schematic view for identifying the proximal end of the crystal assembly of FIG. 4;

fig. 6 is a schematic diagram for determining the pivoting angle of the crystal assembly shown in fig. 4.

Detailed Description

Although the examples described in the following detailed description relate to the detection of the concentration of trace elements such as silicon (Si), sulfur (S), chlorine (Cl) in oil, it should be understood that the present invention can also be applied to the detection of the concentration of trace elements in various materials. In addition, the invention can be used for portable field analysis of trace elements in various materials.

Referring first to FIG. 1, a schematic diagram of a monochromatic excitation X-ray fluorescence spectrometer 100 according to one embodiment of the present invention is shown. Note that the relative positions of each component are not to scale. In this example, monochromatic excitation X-ray fluorescence spectrometer 100 is designed to determine the concentration of elements such as silicon (Si), sulfur (S), chlorine (Cl) in a quantity of oil.

Monochromatic excitation X-ray fluorescence spectrometer 100 includes X-ray source 104, X-ray optics 108, sample presentation device 112, and energy resolving X-ray detector 120.

The limit of detection LOD (limit of detection) of a monochromatic excitation X-ray fluorescence spectrometer is expressed in terms of concentration and refers to the minimum analysis signal X that can be reasonably detected by a specific analysis procedure_LDThe lowest concentration C obtained_LD. By signal-to-noise ratio, we mean the corresponding amount of the 3-fold value of the standard deviation of the instrument background signal generated by the matrix blank, i.e.:

in the formula, R_bAs background (background) count intensity, N is the count intensity of a low concentration sample of known concentration C, and T is the measurement time.

The detection limit is related to the measurement time, and it is meaningless to not give the detection limit of the measurement time. The comparison of detection limits between different instruments is carried out under the condition of the same measurement time, the detection of trace elements in oil is generally assumed to be T300 seconds, and the detection limits given in the patent text are all assumed to be T300 seconds. The detection limit is the most important index for a micro or trace monochromatic excitation X-ray fluorescence spectrometer, and the smaller the detection limit is, the better the instrument is.

From the formula (1), the detection limit and sensitivity (N-R) can be seen_b) Is inversely proportional to/C, and is related to background R_bIs proportional to the square root of. To lower the detection limit for a given measurement time, the sensitivity must be increased and/or the background reduced. One of the main reasons that conventional XRF fails to achieve a lower detection limit is that the continuous scattering background of the fluorescence spectrum is high due to the continuous bremsstrahlung scattering in the X-ray tube exit spectrum.

The X-ray source 104 is selected based on one or more elements in the sample to be detected. In this example, the X-ray source 104 is in the form of a microfocus X-ray tube that emits X-rays 106 having a spectrum that includes a continuous portion (called bremsstrahlung) and a few prominent peaks, i.e., characteristic X-rays of the target material. The X-ray optics 108 is a diffractive crystal that diffracts the characteristic X-rays in the spectrum, i.e. the characteristic X-rays as X-rays 110 that excite the sample, in order to obtain high intensity. The closer the characteristic X-ray energy exciting the sample is to the absorption limit of the desired analytical element, the higher its excitation efficiency. At the same time, to be effective, the best characteristic X-ray energy is sufficiently higher than the fluorescence line of the desired detection element to ensure that elastically scattered and non-scattered X-rays in the sample do not overlap with the fluorescence line of the desired detection element.

In this example, the absorption limits of the detecting elements silicon (Si), sulfur (S) and chlorine (Cl) are 1.840keV, 2.470keV and 2.819keV, respectively, and the fluorescence lines are 1.739keV, 2.307keV and 2.621keV, respectively. We choose the X-ray source 104 to target silver (Ag), which is a good choice. The lalpha-based fluorescence of silver is a characteristic X-ray with an energy of 2.984keV, greater than and close to the absorption limit of the desired detection elements silicon (Si), sulfur (S), chlorine (Cl), and can excite silicon, sulfur, chlorine fluorescence very efficiently, and the elastically and inelastically scattered X-rays of this energy are sufficiently higher than the energy of the highest chlorine fluorescence line 2.621keV to ensure minimal overlap with the silicon, sulfur, chlorine fluorescence lines when the energy resolving detector 120 is configured as a high resolution silicon drift detector.

Since the intensity of the excitation radiation is high and its scattering on the sample is not low, we must also consider the effect of its escape peak here, although the escape peak is only a few hundredths of the incident intensity. Scattered X-rays with an energy of 2.984keV have an escape peak energy E in the silicon drift detector with which we are equipped_{Escape from the body}＝E_{Incident light}-E_Si(E_Si1.739keV to 2.984-1.739 keV to 1.245keV, much less than the energy of the lowest silicon phosphor line of 1.739keV, is also sufficient to ensure minimal overlap with the silicon, sulfur, chlorine phosphor lines.

After the target material is selected by the X-ray light pipe, the maximum characteristic X-ray intensity can be achieved by reasonable combination of high voltage (kV) and current (mA) of the light pipe under the condition of a certain maximum power. In this example, the X-ray source 104 may be operated at a voltage between 5keV and 50keV to produce a characteristic AgL α X-ray having an energy of 2.984keV, with a light pipe high voltage of 22kV and a current of 2.27mA at a maximum power of 50W to achieve a maximum characteristic Ag L α X-ray intensity. Since with point-to-point focusing the X-ray source should be point-outwardly diverging, an X-ray tube with a micro focal spot must be used, in this case the source spot size has a diameter of 100 μm or less. Since the characteristic X-ray energy of the target is low, thin beryllium window X-ray tubes must be used.

In summary, in the present embodiment, the X-ray source 104 is configured as a 100 μm micro-focal spot Ag target 50w thin beryllium window X-ray tube, and is used under the conditions of 22kV high voltage and 2.27mA current of the light pipe.

The X-ray optics 108 are used to monochromate the X-ray tube emission spectrum, i.e. to substantially reduce the unwanted parts of the emission spectrum, such as continuous bremsstrahlung and interfering characteristic lines, and the like, thereby reducing the spectral background below the fluorescence line of the element to be detected in the fluorescence spectrum, while reducing as little as possible or even increasing the intensity of the monochromated line or narrow energy band of the selected excited X-ray to improve the sensitivity, thereby reducing the detection limit of the X-ray fluorescence spectrometer.

There are many methods for monochromating the emission spectrum of an X-ray tube, including a filter method, a secondary target method, a Barkla scattering method, and the like. However, all of these methods greatly reduce the intensity of the main X-ray tube radiation and thus the sensitivity of the X-ray fluorescence spectrometer, so the detection limit of the X-ray fluorescence spectrometer for a given X-ray tube power cannot be greatly reduced.

One method for monochromating the exit spectrum of an X-ray tube is diffraction, which must satisfy Bragg's law:

nλ＝2dsinθ(2)

wherein λ is the wavelength of X-ray fluorescence; n is a natural number; d is a lattice distance or a multilayer film spacing; θ is the angle of incidence. That is, the wavelength of the radiation emitted from the source must satisfy the formula (2) to be diffracted, so that it has excellent monochromatization.

As with other methods, because the use of X-ray optics keeps the X-ray source away from the sample, the intensity of X-rays received per unit area of the sample surface is inversely proportional to the square of the source-to-sample distance, according to inverse square law, and because the reflection coefficient is always less than 100%, the intensity of the monochromated line or narrow energy band is reduced. Such Bragg diffractive X-ray optics can be designed as doubly Curved crystal dccs (double Curved crystals) of appropriate geometry that can focus a point source, allowing for extremely high geometric reflection efficiency by collecting the primary radiation over a large acceptance solid angle, to compensate for distance and loss of useful radiation by reflection. In addition, focusing also enables a small spot of light to be applied to the sample, thereby enabling a small area semiconductor detector Si-PIN or SDD to receive a large portion of the fluorescent radiation 116 in a smaller face of the sample, i.e., DCC also improves detection efficiency. Therefore, DCC in the diffraction method is best monochromatized and most efficient.

The DCC is further divided into a half focus (Johann) rotation, a full focus (Johansson) rotation, a logarithmic spiral rotation, and the like according to its curved surface. Fig. 2, 3 show the principle of half-focus and full-focus, respectively, and fig. 4 shows the principle of a logarithmic spiral.

The semi-focusing rotary DCC is based on John geometry, crystal lattice is parallel to surface, and on the axis plane of the source and focus connecting line is a circular arc with radius 2R, and the centre point of the circular arc is tangent to the circle with radius R, and said circle is called focusing circle, namely Rowland circle A. The rotation of the arc relative to the axis connecting the source and the focus is the semi-focusing hyperboloid of revolution. Light emitted by the point light source S positioned on the circle is diffracted by the crystal and then is converged near the symmetrical point F of the circumference. In theory, the semi-focusing method only has the crystal center strictly meeting the Bragg condition, the rest part of the method utilizes the divergence characteristic of the crystal and considers that the crystal meets the Bragg condition, and the diffraction line is only converged near the F point, so that the effective area and the diffraction efficiency of the semi-focusing crystal surface participating in diffraction are lower than that of full focusing, and the focusing is also lower than that of full focusing. The radiation collection angle is small, since the fraction of the available crystal divergence is small. In order to increase the radiation collection angle, in the published U.S. patent No.5,127,028, a method of splicing a plurality of diffraction crystals in a step shape is adopted, so it is difficult to manufacture, and the more steps (i.e., crystal blocks) the worse the focus is, so only limited splicing can be performed; in the published U.S. Pat. No.7,035,374, a method of continuously splicing a plurality of diffraction crystals in a Rowland circle is used, but the crystal lattice of the other crystal blocks must be at an angle to the surface except for the center block, which requires grinding of the crystals and is therefore more difficult to manufacture, and only natural crystals can be used since grinding of the artificial crystals is impossible.

The full focus rotary DCC is based on the johnson geometry with a crystal lattice that is 2R circle from the surface and is a circular arc of a rowland circle of radius R on the axial plane that connects the source and the focal point. The rotation of the arc relative to the axis connecting the source and the focus is the full-focusing hyperboloid of rotation. Light emitted by the point light source S positioned on the circle is diffracted by the crystal and is completely converged at the symmetrical point F of the circumference. Ideally full focusing is fully satisfactory for diffraction conditions and is point-to-point focused, and is therefore best. However, the manufacturing process of the full-focusing DCC is very complicated, a process of grinding the crystals into a 2R curved surface is required except bending, the artificial crystal cannot be ground, and natural crystals such as Si, Ge and the like are very fragile and are not easy to grind, so that the successful manufacturing of the commercial full-focusing DCC crystal is not reported at home and abroad at present.

The logarithmic spiral rotating DCC is based on the logarithmic spiral geometry, the crystal lattice is parallel to the surface, a logarithmic spiral segment is arranged on the axial plane of the connecting line of the source and the focus, and the rotation of the logarithmic spiral segment relative to the axial line of the connecting line of the source and the focus is the logarithmic spiral rotating hyperboloid. The light emitted by the point light source S at the pole is diffracted by the crystal and then is converged in one area. Theoretically the same as full focus, for a point source the effective area of the crystal surface that participates in diffraction is theoretically 100%, except that it is focused on one area, unlike full focus which is focused on one point. But this area is very small, typically only around 2mm, and can be considered point-to-point for X-ray optics used as an excitation light source monochromator in an X-ray fluorescence spectrometer. In addition, it can be manufactured with all lenses, including intraocular lenses, and is generally easy to manufacture without splicing.

Natural crystals generally have large reflectance values, but they reflect X-rays in a very narrow spectral region. These crystals have a high spectral resolution with a spectral width δ E ═ 10^-4～10^-5) E, spectral width δ E of the characteristic X-ray of the target (10)^-3～10^-4) E is narrower resulting in a low integrated reflectivity with only a fraction of the target characteristic X-rays being diffracted. Natural crystals are not suitable candidates for applications where monochromatization is desirable and where the intensity of reflected radiation is highly desirable, such as X-ray optics used as monochromators of excitation light sources in X-ray fluorescence spectrometers.

The C crystal is an artificial crystal structure and has unique X-ray reflection characteristics. In the energy region of 2.5keV to 30keV, the C-crystal shows good peak reflectivity. However, its most prominent feature is large mosaicism, which is typically around 0.4 °, and the target feature X-rays are all diffracted, which results in it having the maximum integrated reflectivity of all known crystals. And the C crystal has excellent thermal conductivity, thermal stability and radioactivity stability, and is very suitable for an X-ray fluorescence spectrometer.

In summary, in the present invention, the X-ray optic 108 is configured as a logarithmic spiral hyperboloid of revolution, employing a C-lens. It is fully focused and can be considered point-to-point with the maximum integrated reflectivity in a known crystal.

A specific method of designing the X-ray optics 108 is given below in connection with this embodiment.

The mathematical expression of the logarithmic spiral in the polar coordinate system (ρ, φ) is:

ρ＝ae^φctgθ (3)

in the formula, rho represents a polar diameter, phi represents a polar angle, e is the base of a natural logarithm, theta is an included angle between a connecting line of a pole S and any point P on the curve and a tangent line of the curve at the point P, and a and theta are constants.

Theta is a constant which is a unique property of a logarithmic spiral. We use this unique property of the logarithmic spiral to take θ in equation (3) as the bragg angle θ b with the pole being the X-ray source point, so that each ray impinging on the logarithmic spiral crystal is at a bragg angle with the logarithmic spiral crystal. The diffracted X-rays converge in a region, rather than being fully focused, whose focus range is related in size to the diffraction angle θ b and other optical path parameters.

The conversion relationship between the wavelength and the energy of the X-ray is as follows:

where E is the X-ray energy in keV and λ is the wavelength in nm.

Combining (3) and (4), and taking n as 1, obtaining the Bragg angle as follows:

where E is the X-ray energy in keV and d is the crystal spacing in nm.

In this example, a C-intraocular lens (d-0.3355 nm) was used, and in order to monochromate 2.984keVL α X-rays from the rays emitted from the Ag target X-ray tube, the bragg angle θ b was 38.27 °.

After the constant θ is determined, we determine the range of the argument φ and the constant a.

The X-ray tube is windowed, and only the part of the point light source emitted from the source point on the target can be used by us, and the window is circular, so that the exit part is an infinite cone with the source point as the vertex, and the solid angle of the exit part is larger as the source point is closer to the window. The X-ray tube is divided into 2 types of end windows and side windows, and the distance between the source point of the X-ray tube of the side window and the window is larger than that of the end window, so that the solid angle of an emergent part is smaller than that of the end window, and the X-ray tube with the end window is better in theory. However, one of the objectives of the present invention is low cost, so we must choose a domestic X-ray tube, because the price of the domestic X-ray tube is much lower than that of the foreign X-ray tube. However, no X-ray tube with an end window is seen in China at present, so that only a domestic side window X-ray tube can be used. The size of the crystal is limited, a large solid angle requires a very large crystal, the price of the C-lens is not inexpensive, it accounts for a large fraction of the total cost of the X-ray fluorescence spectrometer, and the price of the crystal is proportional to its area. In addition, if the crystal size is too large, the aggregation effect is poor. Therefore, it is appropriate to select a window X-ray tube having a large opening angle, for example, an opening angle of 20 ° to 30 °.

The X-ray tube of KYW2000A type produced by Yiwei electron of Shanghai is selected for the implementation case. The X-ray radiation angle is 27.5 degrees, and the distance between the source point and the window surface is 36.5 mm.

As shown in fig. 4, when phi is equal to-theta_bAt the point P where the X-ray hits the logarithmic spiral₀We refer to the center point where the tangent to the curve is parallel to the x 'axis and the intersection of its reflection lines on the x' axis is the shortest distance from the pole S, given by the relationship 2ae^θbctgθbcosθ_bGiven that the angle is greater or less than-theta_bThe X-ray hits the intersection of the reflection lines of the logarithmic spiral at the X' axis at a distance greater than the pole S. Therefore, we generally choose the center line of the X-ray radiation as phi ═ theta_bLine of time, then the argument φ is in the range of- θ_b+γ/2～-θ_b- γ/2. In this embodiment, the argument φ is in the range of-38.27 ° +27.5 °/2-38.27 ° -27.5 °/2, or-24.52 ° -52.02 °.

In the case of a certain solid angle, the smaller the crystal area, i.e. the smaller the logarithmic spiral segment, the better, the closer to the source point. As shown in fig. 5, the window plane of the X-ray source is perpendicular to the centerline of the cone of X-ray radiation, i.e., perpendicular to SP0, and the source point is at a distance H from the window plane. Due to the space required for installing the X-ray tube, the point of the logarithmic spiral segment closest to the X-ray tube, that is, the proximal end P1 of the logarithmic spiral segment must be further away from the X-ray tube by 5mm to 10mm, and if h is 8.5mm, the extreme length SP of the proximal end of the logarithmic spiral segment is set to be very long₁＝ρ₁Is (H + H)/cos (γ/2) ═ 36.5+8.5)/cos (27.5 °/2) ═ 46.33 mm. After ρ 1 is determined, by substituting equation (3), a:

46.33＝SP₁＝ρ₁＝ae^φctgθb＝ae^{-52.02°ctg38.27°}

a＝46.33e^{52.02°ctg38.27°}＝46.33e^{52.02°π/180°ctg38.27°}＝46.33e^{0.90792*1.2676}＝146.57

after θ and a are determined, the logarithmic spiral is determined.

Then another 1 important point of the logarithmic spiral crystal segment, the center point P₀It is also determined that:

ρ₀＝SP₀＝ae^φctgθb＝146.57e^{-38.27°ctg38.27°}＝62.86

the parametric form of a logarithmic spiral with point S (origin) as pole x' axis as axis is:

x'＝a cosφe^φctgθb，r’＝a sinφe^φctgθb (6)

where φ is the polar angle measured from the selected x 'axis, x' is the distance to the origin measured along that axis, and r 'is the distance to the x' axis.

Point P₁The distance to the origin measured along the x 'axis is x'₁＝a cosφe^φctgθb＝ρ₁cos phi 46.33cos52.02 DEG 28.51, point P₁Distance to the x 'axis is r'₁＝a sinφe^φctgθb＝ρ_1sinPhi 46.33sin52.02 deg. 36.52. Point P₀The distance to the origin measured along the x 'axis is x'₀＝a cosφe^φctgθb＝ρ₀cos phi 62.86cos38.27 DEG 49.35, point P₀Distance to the x 'axis is r'₀＝a sinφe^φctgθb＝ρ_0sinPhi 62.86sin38.27 deg. 38.93. Here we can treat two points approximately as straight lines, then (P)₁P₀)²＝(x'₁-x'₀)²+(r'₁-r'₀)²＝(28.51-49.35)²+(36.52-38.93)²＝440.13，P₁P₀21. And then the line segment P₁P₀Is also determined.

The inventor selects the central point P₀Is the center of the crystal segment length and not the center of the angle, the logarithmic spiral crystal segment length L ═ P₁P₂＝2P₁P₀42, is determined. The reason for this is that if another boundary value-theta of the argument phi is chosen_b+ gamma/2 is the argument of the boundary of the logarithmic spiral segment, then P₀P₂Will be much larger than P₁P₀. In this case, P₀P₂Is greater than P₁P₀Although of greater length, the collected solid angle is only a small fraction of the angle and at the edges, and as mentioned above, C is not inexpensive and P is a cost effective consideration₂Should not be selected too far. In addition, P₂The farther away the incident ray is at point P₂Is located near the focal point and away from the center point P₀The farther the reflected rays are, that is to say the worse the concentration, so P₀P₂Should not be too large.

The logarithmic spiral crystal segment is now fully defined.

The following describes how the inventors deal with the problem of imperfect focus.

What is the area, previously described was that the logarithmic spiral did not focus the incident X-ray at a point, but rather in a small area? How large the area is, the point of the area will be the focus?

In order to solve the series of problems, a computer is utilized, a plurality of incident rays and emergent rays are directly and accurately drawn on the computer, and the shape, the size and the light distribution of a focusing area are visually observed by enlarging the focusing area, as shown in fig. 4. Through research and analysis, all the reflection lines are concentrated on rho₁、ρ₀、ρ₂Triangle F intersected by reflection rays of three incident rays₁F₂F₃The center point reflection line is closest to the source point S, and the distribution rule cannot be further summarized in this triangle. Thus, in a triangle from F₂Center F of the perpendicular of the point-to-center reflection line₀The point acts as a focus point, and thus a very complex problem is simply solved.

Pole S passes through F₀And (3) taking an x-axis, measuring an included angle alpha between the x-axis and the x '-axis to be 2.51 degrees, wherein the x-axis is the original x' -axis rotated by an angle alpha. The logarithmic spiral crystal segment obtained above is rotated along the X-axis to obtain the logarithmic spiral hyperboloid of the X-ray optic 108. The magnitude of this rotation is determined below, (as shown in fig. 6).

The parametric form of a logarithmic spiral with point S (origin) as pole x axis as axis is:

x＝a cos(φ-α)e^{(φ-α)ctgθb}，r＝a sin(φ-α)e^{(φ-α)ctgθb} (7)

where φ is the polar angle from the x' axis, x is the distance to the origin measured along the x axis, and r is the distance to the x axis.

In the present embodiment, the radiation angle γ of the X-ray tube is smaller than θ_bThe angle β of rotation required about the axis is less than π, and the following can be approximately derived:

ρ₀sin(γ/2)＝r₀ sin(β/2) (8)

when β is calculated according to equation (8), the arc length w is 30.58, and the arc length w is 30.

Thus, the X-ray optics 108 are designed. The logarithmic spiral rotating hyperboloid block is a logarithmic spiral segment which is 42 in length and 30 in width, satisfies the formula (7) in the length direction, takes phi-52.02 degrees as a starting point and is 42 in length, and is an arc segment which is 30 in length and has the radius of r in the formula (7) in the width direction. The thickness of the crystal is 0.4-0.5.

There are many types of energy-resolving X-ray detectors, including proportional counter tubes, Si-PIN detectors, and silicon drift detectors SDD. The resolution of the detector is expressed in terms of the half-width of the full-energy peak, the net count of which is independent of the half-width, but the background count is proportional to the half-width, so the higher the resolution the lower the detection limit. The half width of the proportional counter tube is about 8 times that of the semiconductor detector, so the detection limit is about the square root of 8. The resolution of Si-PIN is slightly worse than that of SDD and its resolution drops dramatically at high count rates, so SDD is the best detector.

The energy-resolving X-ray detector 120 in this embodiment is a VITUS H20CUBE (top grade) silicon drift detector SDD detector manufactured by the germany KETEK company, the resolution of which is less than 123eV, the effective detection area of which is 20mm2, and the highest counting rate of which is 2 Mcps.

Advantageously, the angle between the exit line of the X-ray optics 108 and the sample surface can be greater than 70 °, or as large as possible, in order to keep the focal area, i.e. the spot on the sample, as small as possible. The energy resolving X-ray detector 120 should be as close to the sample as possible, and the angle between it and the sample surface may be larger than 45 °, or as large as possible, to collect more fluorescence radiation.

The detection limits of the X-ray fluorescence spectrometer manufactured by the inventor according to the embodiment of the invention on trace elements such as silicon, sulfur, chlorine and the like in oil are respectively 0.4ppm, 0.1ppm and 0.05ppm, which are half of those of the existing same-power spectrometer, and meanwhile, the size, cost and complexity of the spectrometer are obviously reduced.

An additional benefit of the present invention is that XRF quantitative analysis based on fundamental parameters is generally more reliable in the case of monochromatic excitation radiation than polychromatic excitation radiation. This is due to the fact that when X-ray tubes, in particular in combination with non-monochromatic optics, the shape of the spectral distribution of the excitation radiation is often less well defined, and monochromatic excitation radiation is simply well defined.

It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the above-described embodiments without departing from the broad general scope of the disclosure. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.

Claims

1. A monochromatic excitation X-ray fluorescence spectrometer, comprising:

selecting an X-ray source for generating an X-ray beam;

x-ray optics that focus an X-ray beam produced by the X-ray source onto a sample and select X-rays of a desired energy; and an energy-resolving X-ray detector for directly collecting the X-ray beam reflected by the sample;

the X-ray optic is designed as a logarithmic spiral rotating hyperboloid crystal according to the Bragg diffraction principle of X-rays, using a C-intraocular lens with the largest integrated reflectivity of all known crystals.

2. The monochromatic excitation X-ray fluorescence spectrometer of claim 1, wherein said X-ray source has a spot size with a diameter ≦ 1 mm.

3. The monochromatic excitation X-ray fluorescence spectrometer of claim 2, characterized in that the diameter of the spot size is ≤ 0.5 mm.

4. The monochromatic excitation X-ray fluorescence spectrometer of claim 2, characterized in that the spot size has a diameter of 100 μm or less.

5. The monochromatic excitation X-ray fluorescence spectrometer of claim 1, wherein the X-ray source is a domestic side window X-ray tube with an X-ray target producing intense characteristic X-ray radiation with a maximum power of 50 w.

6. The monochromatic excitation X-ray fluorescence spectrometer of claim 1, wherein the X-ray optics focus as much of the excitation X-rays within the selected energy range of the X-ray tube onto a sufficiently small area of the sample as possible.

7. The monochromatic excitation X-ray fluorescence spectrometer of claim 1, wherein the logarithmic spiral hyperboloid of revolution crystal is based on a logarithmic spiral geometry, the crystal lattice is parallel to the surface, and on the axial plane of the source-focus line is a logarithmic spiral segment, and the rotation of the logarithmic spiral segment relative to the axis of the source-focus line is a logarithmic spiral hyperboloid of revolution.

8. The monochromatic excitation X-ray fluorescence spectrometer of claim 7, wherein the logarithmic spiral hyperboloid of revolution crystal is logarithmic spiral p = ae in the axial plane^φctgθA section P of₁P₂Where the pole is the X-ray source point, ρ is the pole length, φ is the pole angle, a and θ are constants, P₁、P₂Respectively the near end and the far end of the segment, and parameters a, theta and P are determined₁And P₂The steps are as follows:

(a) calculating the Bragg angle theta according to the crystal spacing d and the energy E of the desired monochromated line_bTaking theta as Bragg angleθ_bThus, θ is determined;

(b) taking the central line of the radiation of the X-ray source as phi = -theta_bLine of time, e.g. with an X-ray source radiation angle of gamma, the argument phi is in the range-theta_b +γ/2～-θ_b- γ/2; the logarithmic spiral separates the nearest point P of the X-ray tube₁Select the nearest point, then P₁It determines the corresponding phi₁=-θ_b- γ/2, substituted into logarithmic spiral polar coordinate form ρ = ae^φctgθA is obtained, thus a is determined;

(c) after theta and a are determined, the logarithmic spiral is determined, and the center point P of the logarithmic spiral crystal segment is determined₀Then determine, and then line segment P₁P₀Also determines, selects the center point P₀Is the center of the crystal segment length, the logarithmic spiral crystal segment length P₁P₂=2P₁P₀Is determined, then P₂It is determined.

9. The monochromatic excitation X-ray fluorescence spectrometer of claim 8, wherein said logarithmic spiral rotating hyperboloid crystal takes two endpoints P₁、P₂Intersection point F of two reflection lines on a point₂Point to center point P₀Center F of perpendicular to reflection line on point₀The spot acts as a focus point.

10. The monochromatic excitation X-ray fluorescence spectrometer of claim 9, wherein said logarithmic spiral rotating hyperboloid crystal is curved by curving said logarithmic spiral crystal P₁P₂Passing F along pole S₀The rotation of the X-axis is obtained, and the required rotation angle beta of the X-axis can be obtained according to the radiation angle gamma of the X-ray tube, so as to determine the rotation arc length w, namely the width of the logarithmic spiral crystal.

11. The monochromatic excitation X-ray fluorescence spectrometer of claim 1, wherein the X-ray optics are two or more to concentrate as much of the excitation X-rays of the X-ray tube within two or more selected energy ranges onto a sufficiently small area of the sample as possible.

12. The monochromatic excitation X-ray fluorescence spectrometer of claim 1, wherein said energy-resolving X-ray detector is a proportional counter tube, a Si-PIN detector or a silicon drift detector SDD.