X-ray lenses, application development of lenses

The X-ray lens of Beijing Normal University X-ray Laboratory, and the application and achievements of X-ray lens in the field of X-ray analysis are introduced.
The X-ray lens is composed of an X-ray tube, which can collect the divergent X-rays generated by the X-ray source and form a high-power-density converging and parallel X-ray beam, thus becoming a broadband X-beam regulation method.

X-ray optics emerged in the second half of the 20th century. At first, it mainly referred to X-ray optical path design and some X-ray application technologies. After the emergence of synchrotron radiation sources in the 1970s, X-ray optics has undergone great development, and now it has become a recovering technology. It is also a new category and new hotspot of modern science with interdisciplinary nature. It not only includes basic and applied basic research such as X-ray and matter interaction, modern X-ray source and X-ray detection technology, but also provides various X-ray instruments that are continuously improved and updated for some basic disciplines, interdisciplinary disciplines and various fields of national economy. equipment. In the 1990s, the former Soviet Union scientist Kumakhov proposed the multi-duct X-ray beamforming technology. Since 1990, the X-ray Laboratory of the Institute of Low Energy Nuclear Physics, Beijing Normal University has independently conducted research on the catheter X-ray optics and the X-ray beamforming system, and in 1994 In 2008, he successfully developed the integral X-ray lens; carried out the application development of the lens, and published the application results of the X-ray lens in the field of X-ray fluorescence analysis and deep submicron X-ray lithography. Now that the X-ray lens has entered the international market, this paper focuses on the application achievements of the X-ray lens in the field of X-ray analysis technology.

The X-ray lens is composed of an X-ray pipe. The X-ray pipe is a very thin (3-100 μm) glass tube with a very smooth inner surface. When the X-ray is shot at a glancing angle less than the critical angle θ_c of the external total reflection of the glass material When the X-ray tube is straight, it will propagate from the radiation end of the tube to the output end with high reflectivity through multiple reflections, thereby increasing the X-ray power density at the output end. For the curved tube, as long as the reflection is always maintained. When the incident angle is smaller than θ_c, the transmission efficiency is still very high. For example, for a cylindrical curved pipe, when the inner radius r_ch of the pipe satisfies the following formula

(R is the radius of curvature of the circular tube), all the beams incident parallel to the tube axis will be reflected The theoretical and experimental results are consistent. The research shows that the X-ray pipe produced has good performance, the transmission efficiency is close to the theoretical expected value, and the influence of the inner surface roughness of the pipe can be ignored.

Straight or curved X-ray tubes can transmit X-rays efficiently, or change the direction of X-ray propagation, but capture very little X-rays.

A reasonable combination of a large number of X-ray tubes constitutes an X-ray beamer system, or X-ray lens. The lens that combines a large number of X-ray tubes and directly processes and shapes in a wire drawing furnace is called an integral X-ray lens. This is the main type of lens sold in my country and the international market. At present, there are three types of integral X-ray lenses that have matured, and the schematic diagram is shown in Figure 1.

Figure 1 Schematic diagram of three types of integral X-ray lens

Among them (a) is a parallel beam lens, which can capture and convert the divergent X-ray emitted by the X light source into a parallel beam; (b) is a converging beam. The lens, which converges the captured X-rays to the focal spot at the output end, forms a micro-X beam spot with extremely strong power density; (c) is a micro-converging parallel beam lens, which forms a slightly convergent quasi-parallel thin X beam , on the one hand, a higher power density can be obtained due to its converging effect, and on the other hand, the quasi-parallel X beam can ensure that its divergence is small enough to be used for diffraction analysis.

The main characteristics of X-ray lens are transmission efficiency, beam spot (converging lens) and illumination field (parallel beam lens) size, gain factor and angular dispersion. Below is a brief description of each.

(1) Transmission efficiency η (E)

The transmission efficiency of a lens with X-photon energy E (keV) is

Among them, I₁(E) is the total intensity of the X-beam with energy E emitted by the X light source to the entrance end of the lens, and I₂(E) is the total intensity of the X-beam with energy E emitted by the lens. η(E) is an important parameter that marks the quality of the X-ray lens, and it is related to the energy.

(2) Beam spot size (converging lens) or field size (parallel beam lens) φ (mm)

The beam spot is usually circular, and its radial power density distribution is roughly Gauss distribution. The focal spot size is defined as the full width at half maximum (FWHM) of the Gauss distribution. FWHM gives 93.94% of the normalized area of the focal spot. For the converging lens, the beam spot size is an important parameter, and people always try to get the beam spot as small as possible so that Carry out the analysis of micro-materials or micro-samples. Now the beam spot of the converging lens in our laboratory can be ≤ φ30μm. For the parallel beam lens, the size of the field of view can vary slightly with the distance from the sample to the exit end. The size of the field of view of the lens in our laboratory generally, it is 3-10mm. If micro-beam diffraction analysis experiment is required, a micro-converging parallel beam lens can be used or a limiting diaphragm can be added at the exit end of the parallel beam lens.

(3) Gain factor

The gain factor k(E) refers to the power density magnification of the lens, and k(E) is defined as

Among them, j₃(E) is the power density of the X-beam directly incident on the focal spot (converging lens) or field (parallel beam lens) by the X light source without a lens, and j₄(E) is the output end of the lens after using the lens The power density hitting the focal spot or field. The gain factor characterizes the gain of the lens, and is also an important parameter of the lens. For the converging lens k(E)≈1×10²~3×10³, for the parallel beam lens, k≈5~100. It can be seen that the work after using the lens The increase in efficiency is staggering.

(4) Angle dispersion

In applications such as X-ray diffraction and lithography, there are strict requirements on the angular dispersion of parallel X beams, and the angular dispersion is expected to be as small as possible. Generally speaking, the X beams emitted from different parts of the lens have different angular dispersions, which It is related to the grazing angle of the X-ray incident in different channels and the curvature radius of the channel, and it is also related to the size of the X light source. The angular dispersion of the lens is represented by the half-width angle Δθ of the beam. At present, the angular dispersion of a good lens can be 0.15°~0.20°. The geometric parameters and physical properties of the three series of products successfully developed by the X-ray Laboratory of the Institute of Low Energy Nuclear Physics, Beijing Normal University are shown in Tables 1 and 2.

Table 1 Main geometric parameters of X-ray lens

Table 2 Physical properties of X-ray lenses

It is well known that X-ray analysis technology is widely used in scientific research and production. At present, the application of X-ray lens in X-ray fluorescence analysis and X-ray diffraction analysis has been successful.

XRF analysis is used to measure the elemental content of samples. Microbeam XRF analysis has developed rapidly in recent years. It is used for the analysis of the spatial distribution of elements in ultra-small samples of micromaterials and bulk materials. There are distinct advantages to using a converging X-ray lens in this analysis. The X-ray lens is placed between the X-ray source and the sample, the X-ray source The X-ray lens and the sample are placed on the entrance and exit focal spots of the X-ray lens, respectively. The micro-X beam formed by the convergence of the X-ray lens forms a strong power density and a very small beam spot on the focal spot. An example of the measured Mylar film The detection limits of various elements on the mixed standard samples are shown in Table 3.

Table 3 Microbeam X-ray Fluorescence Analysis Detection Limit (MDL) Using X-ray Lenses Measurement conditions: Mo anode X-ray tube, 27kV, 36μA; measurement time 100 s

Table 3 Microbeam X-ray Fluorescence Analysis Detection Limit (MDL) Using X-ray Lenses Measurement conditions:
Mo anode X-ray tube, 27kV, 36μA; measurement time 100 s

The measurement results show that the detection limit is in the order of pg to sub-pg, which is 4 to 5 orders of magnitude lower than the limit of conventional measurements. The experiments conducted by the Micro and Trace Analysis Center of the University of Antwerp in Belgium using the lens of this laboratory show that the power density of the X-beam obtained by the rotating anode target X-ray tube and X-ray lens is equivalent to that of the synchrotron radiation source. The power density is equivalent to the power density at a distance of 2~3mm (equivalent distance) from the X light source when there is no lens.

It can be seen that the application of the converging X-ray lens has brought the microbeam X-ray fluorescence analysis technology to a new level, which can simultaneously give a high power density, a high spatial resolution of <30μm and a detection limit of pg level, giving Two-dimensional spatial distribution map of microsamples. Now the analysis of ultra-small samples is positioned in the order of 10 μm. This is the need for some disciplinary and applied research development. To understand the mechanisms of macroscopic systems, we need information on microscopic details, because most natural or artificial systems are inherently heterogeneous. For example, understanding human growth, disease, and aging requires information about elements, chemical composition, and structure at the cellular and macromolecular levels. For example, the size of cancer cells is on the order of 10 μm. In environmental research, airborne dust can drift a long distance and bring harm to human health. However, dust particles >10 μm will not drift too far and can be retained in the nasal cavity when a person breathes, and will not enter the lungs, while finer particles (<10 μm) can be inhaled into the lungs and cause harm. Ultra-pure materials and ultra-clean workshops are used in the contemporary semiconductor industry. Impurities of 10⁹~10¹⁰ atoms/cm² are required to be measured, and the measured beam spot size is required to be ≤10 μm. At present, there is no good method, and X-ray lenses may fill this gap.

Parallel beam X-ray lens can be used for X-ray diffraction analysis. X-ray diffraction is the most important means of material structure analysis. If we can try to increase the diffracted light intensity, the work efficiency will be greatly improved. In the past, the acquisition of parallel X-ray beams in diffractometers was limited by diaphragms and Sora slits, resulting in a large loss of intensity. In recent years, we have used parallel beam lenses to collect X-rays to form parallel beams, which have achieved significant benefits. Since the lens can change the propagation direction of the incident X-ray, it can increase the proportion of X-rays emitted at small angles in the X-beam, the lens has the characteristic of two-dimensional focusing, and the angular dispersion of the X-beam is also improved.

In the experiment, a collimated beam lens was placed at one end of the X light source to form a quasi-parallel X beam. The German Bruker-AXS company made a systematic study on the collimated beam lens developed in our laboratory, and some of the results are now quoted. Figure 2 shows the intensity gain of the X-beam exiting the lens compared with the X-beam limited by the aperture of the same area. As can be seen from the figure, the enhancement is up to 100 times.

Fig. 2 X-ray intensity gain of parallel beam lens Measurement conditions: point-focusing copper anode X-ray tube, rolled steel, 12µm nickel filter, 40kV, 40mA, lens exit end with 5mm aperture

Figure 3 shows a comparison of the textures of zirconium alloys and rolled aluminum measured by the conventional method and the method using a lens. The left side of the figure is the pole figure measured by the conventional method, and the right side is the pole figure measured by using the lens. It can be seen from the figure that the work that can be completed in 70 hours in the past can be completed in only 1 hour when the lens is used, and the obtained pole figure statistics are better and the shape is better.

Fig. 3 Comparison of texture pole figures of zirconium alloy and rolled aluminum measured by conventional method and method using lens

The above examples demonstrate the enormous advantages of using parallel beam X-ray lenses in diffraction analysis. It not only greatly improves the conventional diffraction analysis method, but also it is believed that for some high-parallel diffraction devices, such as twin-crystal or quad-crystal diffractometers, the lens will bring high benefits. Because the lens forms a lot of fine quasi-parallel X beams, it does not have the effect of focusing circle focusing in common goniometers, and can tolerate large positioning errors, and it is very convenient to adjust.

Single crystal diffraction analysis, especially the macromolecular diffraction analysis of biological proteins, is widely used. A small X-beam spot (10~10³ µm) and a tolerable angular dispersion (eg. less than 0.5°) are required in this type of research. That is to say, microbeam diffraction analysis is required. For microbeam diffraction analysis, a specially designed A micro-convergence parallel beam X-ray lens was used. In the experiment, the lens was placed on one side of the X light source, and the X-rays emitted from the X light source were condensed into a micro-convergent collimated beam through the lens, and the sample was placed at the focal spot of the X beam.

Diffraction analysis of biological protein macromolecules using a lens was carried out at the Institute of Biophysics, Chinese Academy of Sciences. The original device was equipped with a Japanese Rigaku X-ray machine (Rigaku RU-200 rotating copper anode, beam spot 0.3mm×0.3mm), Charles Supper, USA Model 7600 double-focusing mirror and German MAR imaging plate, the monochromatic X beam beam spot φ0.9mm hits the sample through the collimator. In the experiment, a micro-beam lens (the distance from the source to the sample is 550mm, the focal spot φ0.7mm) ,  the transmission efficiency of copper K_a line is 19.2%) instead of the American double focusing system, the obtained X-ray intensity is increased by 200 times; the collimation tube with φ0.2mm is used, and the Ni filter of 11μm is added, and the intensity is still increased by nearly 7 times. And the resolution has also improved. Diffraction analysis of samples such as scorpion venom and snake venom was carried out with this device, and the resolution was improved from 1.8~2.7Å to 1.60~2.20Å. The diffraction pattern of snake venom PLA2 (phosphatase A2) analyzed by lens and double focusing mirror is shown in Figure 4. An improvement in the quality of the diffractograms and an increase in sensitivity are clearly seen.

Fig. 4 Analysis of snake venom with (a) lens and (b) double focusing mirror Diffraction pattern of PLA2

Experimental studies at home and abroad have shown that the use of micro-converging parallel beam lenses greatly improves the performance of single crystal diffractometers, improves the intensity of human X-ray beams, improves resolution, and improves signal-to-noise ratio, thereby greatly improving analytical measurement. Speed, sensitivity, experimental studies that were previously difficult to perform in conventional laboratories, such as analyzing imperfect crystals, monitoring the growth process of crystals, and microbeam analysis of small crystals, etc.

The development and application of X-ray lenses was a major breakthrough in X-ray optics in the 1990s. It realizes the regulation of broadband X-ray beams, and opens up a way for the effective use of X-ray sources, improving the performance of X-ray instruments and equipment, and realizing the replacement of X-ray instruments and equipment, and opening up new application fields.