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Performance of assembled X-ray optics consisted of a polycapillary X-ray optics and a monocapillary X-ray optics for micro X-ray fluorescence spectrometry
Spectrochimica Acta Part B 165 (2020) 105770
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Performance of assembled X-ray optics consisted of a polycapillary X-ray optics and a monocapillary X-ray optics for micro X-ray fluorescence spectrometry
T
Xuepeng Suna,b, , Xiaoyun Zhanga,b, Yabing Wanga,b, Shangkun Shaoa,b, Yufei Lia,b, Shiqi Pengc, ⁎ Zhiguo Liua,b, Tianxi Suna,b, ⁎
a b c
Key Laboratory of Beam Technology of Ministry of Education, College of Nuclear Science and Technology, Beijing Normal University, Beijing 100875, China Beijing Radiation Center, Beijing 100875, China Nuclear Power Institute of China, Chengdu 610000, China
ARTICLE INFO
ABSTRACT
Keywords: Capillary X-ray optics Laboratory X-ray source Micro X-ray focusing Micro X-ray fluorescence
A micro X-ray fluorescence (μ-XRF) spectrometry instrument based on an assembled X-ray optics was constructed in the laboratory. The assembled X-ray optics consisted of a polycapillary slightly focusing X-ray lens and a tapered monocapillary X-ray lens (TMXRL). The output X-ray beam of the polycapillary slightly focusing Xray lens was focused again by the TMXRL. A TMXRL with output aperture of 12.01 μm was drawn by a nested glass tube. To evaluate the performance of the instrument, a performance comparison between different capillary X-ray optics was conducted. The intensity of the Fe Kα from an iron plate was used to indicate the analysis efficiency of the μ-XRF instrument. The relative gain of the instrument was corrected by comparing the intensity per unit area for a pinhole aperture. Compared with TMXRL single used, the assembled X-ray optics also had a micro focal spot. Furthermore, the relative gain of the assembled X-ray optics and the intensity of the Fe Kα from the iron plate obtained by the assembled X-ray optics were approximately 5 times those of the TMXRL single used. This highly intense micro X-ray beam with several diameters obtained by the assembled X-ray optics will be useful for μ-XRF analysis.
1. Introduction Micro X-ray fluorescence (μ-XRF) is an efficiently nondestructive analysis technology that is used to obtain the two-dimensional elemental distribution of the surface of a sample. Compared with several competing techniques such as Inductive coupled plasma mass spectrometry, scanning electron microscope coupled to energy dispersive spectrometry, nano secondary ion mass spectrometry and micro particle induced X-ray emission, μ-XRF can be used in an air environment on large samples and requires little sample preparation. According to the generation method of the X-rays, μ-XRF can be divided in two main groups: synchrotron radiation μ-XRF and μ-XRF based on laboratory Xray tube. Compared with a laboratory X-ray source, synchrotron radiation has many advantages for μ-XRF, such as: high X-ray flux, collimated and monochromatic X-ray beam and wide selective of X-ray focusing optics [1–3]. However, considering the limitations on machine time and access possibilities to synchrotron radiation facilities, the study μ-XRF based on laboratory X-ray tubes remains significant.
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With the development of the manufacture of X-ray focusing optics, μ-XRF based on laboratory X-ray source has been applied in several research fields, such as forensic science [4], archaeometry [5,6], medicine [7], geology [8], cement and concrete research [9,10] and applications to environmental sciences were recently reviewed [11]. In practice, the polycapillary focusing X-ray lens (PFXRL) and the monocapillary X-ray lens, which provide a highly intense and small focal spot, have been significant in development of the μ-XRF instrument [4,5,12–16]. The PFXRL was fabricated by drawing a bundle of glass capillary tubes in a constant temperature furnace. On the input side, the apertures of the glass capillary tubes all point to the input focal spot to receive the diverging X-rays emitted from the X-ray source. Similarly, on the output side, the apertures of all glass capillary tubers are directed to the focal spot. Therefore, the PFXRL can capture X-rays with a large solid angle and can focus the divergent X-rays from X-ray tubes in a small area. Generally, the size of the output focal spot of the PFXRL is tens of microns, and the magnitude of the gain in power density can be as high as 103. Although a high flux X-ray beam can be obtained in the
Corresponding authors. E-mail addresses: [email protected] (X. Sun), [email protected] (T. Sun).
https://doi.org/10.1016/j.sab.2020.105770 Received 2 July 2019; Received in revised form 18 January 2020; Accepted 20 January 2020 Available online 22 January 2020 0584-8547/ © 2020 Published by Elsevier B.V.
Spectrochimica Acta Part B 165 (2020) 105770
X. Sun, et al.
output focal spot of the PFXRL, an output focal spot of PFXRL with a diameter of less than ten microns was difficult to achieve due to the “halo effect” [17]. Compared with the PFXRL, the monocapillary X-ray lens can provide a micro X-ray focusing beam with smaller focal spot size (micron to tens of nanometers) and divergence angle [12,18,19]. However, the capture angle of the monocapillary X-ray lens is much smaller than that of the PFXRL, resulting in the X-ray flux in the focal spot of the monocapillary X-ray lens being lower than that of the PFXRL. Thus, the monocapillary X-ray lens and the PFXRL have advantages and disadvantages. Recently, to acquire a suitable micro X-ray beam for μ-XRF, X-rays from a laboratory X-ray tube focused by assembled optics were studied. Tsuji et al. has proposed several types of assembled X-ray optics system for laboratory X-ray tube focusing [20–22]. In 2002, a cone-shaped Mo pinhole assembled with a straight single capillary was used to restrict the diameter of the primary X-ray beam to 230 μm [20]. In the study, the pinhole was placed very close to the sample surface to conduct Grazing-exit X-ray spectrometry. In 2009, they developed a μ-XRF instrument by a PFXRL combined with a glass conical pinhole [21]. In this instrument, the size of the obtained X-ray micro beam was 25 μm. In 2013, they combined a precision manufactured tungsten conical pinhole with a PFXRL for μ-XRF analysis and realized a small spot with diameter of 2.8 μm [22]. We have reported on the small X-ray beam that was obtained by two assembled lens based on laboratory X-ray source: one was based on a PFXRL and a single-bounce ellipsoidal capillary and the other was based on a polycapillary parallel X-ray lens and a single-bounce parabolic capillary [23]. In this study, we will report a new assembled X-ray optics of a polycapillary slightly focusing Xray lens (PSFXRL) and a tapered monocapillary X-ray lens (TMXRL) which was manufactured by a nested glass tube. In the assembled X-ray optics, the drawback of lower intensity of the micro focusing beam obtained by the TMXRL single used in μ-XRF analysis has been improved. The manufacture and configuration of the TMXRL will be described in detail and the analytical performance of the assembled X-ray optics will be compared with other capillary X-ray optics.
0.625 μm on each axis. The silicon drift detector was placed approximately 25 mm from the sample. The TMXRL was attached to a motorized X-Y-Z-θ1-θ2 stage (7STA1280 X-Y-Z stage and 7SRA1100 θ1-θ2 rotating stage; Sofn Instruments Co., Ltd., China) for the accurate adjustment of the TMXRL. The step resolution of the rotating θ1-θ2 stage was 0. 00125° for each axis. 3. Results and discussion 3.1. Capillary X-ray optics in the setup 3.1.1. PSFXRL Table 1 presents the parameters of the PSFXRL in the assembled Xray optics used in this study. The reason for not choosing the PFXRL as the pre-focusing optics is that the PSFXRL has a longer working distance (f2) and a smaller divergence of the output beam compared with the PFXRL. Therefore, the PSFXRL can combine with a common TMXRL and not a conical pinhole [21,22]. LF and f1 are the length and input focal distance of the PSFXRL, respectively. d1 and d2 are the diameter of the input section and output section of the PSFXRL, respectively. F is the focal spot size of the PSFXRL, which was measured by the knife scanning method [24]. The gain in power density (G) and the transmission efficiency (G) of the PSFXRL were measured by a proportional counter tube and a pinhole with a diameter of 100 μm. 3.1.2. TMXRL Fig. 2(a) is the photograph of the TMXRL used in the μ-XRF instrument in the study. The length of the TMXRL is 41 mm. The TMXRL was fabricated by drawing a nested glass tube. Fig. 2(b) shows the cross profile of the nested glass tube. The inner glass tube of the nested glass tube was prepared by drawing a common silicate glass tube until the outer diameter of the glass was equal to the inner diameter of the outer glass tube. Then, the silicate glass tube after drawing was slipped into the outer glass tube. After putting one glass tube into the other, a nested glass tube with large ratio of the outer diameter to the inner diameter was obtained. In the drawing process, the outer diameter of the monocapillary X-ray optics is proportional to the inner diameter [25]. The focal spot of the assembled X-ray optics is determined by the diameter of the output aperture of the TMXRL. To obtain an assembled Xray optics with a small focal spot, the output aperture of the TMXRL must be compressed to few microns. The nested glass tube with a larger ratio of the outer diameter to the inner diameter could insure that the tip of the TMXRL has a proper diameter, which is beneficial to manufacture of the optics. Furthermore, the nested glass tube has a large outer diameter which is convenient for drawing a TMXRL with a larger slope. Fig. 3 shows cross-sections of the input side and the output side of the TMXRL. The outer diameter and inner diameter of the input side of the TMXRL are 6.48 mm and 28.23 μm, respectively. The outer diameter of the input side of the inner glass tube of the TMXRL is 821.27 μm. The outer diameter and the inner diameter of the output side of the TMXRL are 3.09 mm and 12.01 μm, respectively. The outer diameter of the output side of the inner glass tube of the TMXRL is 382.5 μm. The ratio of the outer diameter to the inner diameter of the TMXRL is approximately 230. In this study, the quality of the TMXRL was evaluated by an optical measurement [26]. A digital micrometer [LS-7601, Keyence, Japan] with a motorized X stage was used to measure the outer profile of the TMXRL and the inner profiles that are presented in Fig. 4(a) were calculated from the outer profiles by scaling the ratio between the tube inner diameter and outer diameter [25]. Fig. 4(b) plots the design figure (red line) versus the measured data (scatter line) and the measured data are very close to the design figure. To observe the deviation more clearly, the absolute deviation is plotted in Fig. 4(c). After calculation, the root-mean-square deviation of the measured inner profile of the TMXRL from the ideal profiles was approximately 0.079 μm. Fig. 4(d)
2. Experimental Fig. 1 illustrates the laboratory constructed μ-XRF setup, which is coupled with an assembled X-ray optics consisted of a PSFXRL and a TMXRL. The X-ray source used in this study is a Mo rotating anode Xray generator (RIGAKU RU-200, Rigaku Corporation, Japan) with a focal spot size of 300 × 300 μm. In the experiment, the Mo target X-ray source was operated at 30 kV and 10 mA. In the detecting channel, a silicon drift detector (AXAS-M, Amptek, Germany) with an energy resolution of < 136 eV at 5.9 keV was used to measure the X-ray fluorescence. The maximum count rate of the detector system was 2 × 106 counts/s. The sensitive area of the detector system was 80 mm2. The PSFXRL and TMXRL used in this study were designed and manufactured by the Key Laboratory of Beam Technology of Ministry of Education. The silicon drift detector, PSFXRL and sample were adjusted by a motorized X-Y-Z stage (7STA1280, Sofn Instruments Co., Ltd., China), respectively. The step resolution of the motorized X-Y-Z stage was
Fig. 1. Schematic diagram of the laboratory μ-XRF setup with an assembled Xray optics consisted of a PSFXRL and a TMXRL. 2
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Table 1 Parameters of the PSFXRL. Parameter
LF
f1
f2
d1
d2
F
G
E
Value
101.3 (mm)
57.5 (mm)
150 (mm)
4.3 (mm)
5.1 (mm)
370 (μm)
966
31.5%
presents the measured straightness of the TMXRL and a deviation of < ± 0.07 μm was obtained. These results represent a large improvement compared with measurements previously reported [25]. The progress of the quality of the monocapillary X-ray optics might benefit from the nested drawing method. Because, in the drawing process of the nested tube glass, the outer tube glass of the nested tube glass functioned as a mould to compress the inner tube glass. In this compression, the deviation of the inner profile of the TMXRL might be decreased.
TMXRL 4 mm from the end. The full-width at half-maximum of the red line in Fig. 6 represents the measured beam. Furthermore, the diameter of the output beam of the TMXRL at various distances from the end of the TMXRL was measured, as plotted in Fig. 7. According to the diameter curve of the output beam of the TMXRL in Fig. 7, the output Xrays from the tip of the TMXRL converge at a distance of 0–4 mm from the end of the TMXRL and diverge after 4 mm. Therefore, the minimum size of the output beam of the assembled X-ray optics was approximately 9.3 μm at 4 mm away from the end of the TMXRL. According to the slope of the divergent part of the curve in Fig. 7, the divergence of the output beam of the TMXRL was approximately 4.3 mrad.
3.2. Beam size and beam divergence of the assembled X-ray optics For a μ-XRF spectrometer, the size of the incident X-ray beam, which determines the scanning resolution of the setup, is a significant parameter. For the μ-XRF setup combined with the assembled X-ray optics consisted of the PSFXRL and the TMXRL in this study, the scanning resolution of the instrument is determined by the output X-ray beam of the TMXRL. Before measuring the diameter of the micro X-ray beam, the PSFXRL and the TMXRL must be aligned. At certain distance between the PSFXRL end and the TMXRL end, by adjusting the motorized stage of X-Z-θ1-θ2, the optical axis of the PSFXRL coincided with the axis of the TMXRL and the maximum intensity was obtained. To measure the intensity of the X-ray beam focused by the TMXRL, the silicon drift detector was placed 10 cm from the end of the TMXRL. To weaken the X-rays that directly radiated from the TMXRL, a Perspex sheet with a thickness of 2 mm was fixed before the silicon drift detector. To optimize the distance between the PSFXRL end and the TMXRL end, the intensity of Mo Kα was measured for the TMXRL at various distances from the PSFXRL. In the measurement process, the distance between the TMXRL end and the silicon drift detector was fixed. Fig. 5 presents the relationship between the distance of the PSFXRL end from the TMXRL end and the Mo Kα intensity. As shown in Fig. 5, the maximum intensity was obtained at approximately 156 mm. The optimized distance between the PSFXRL end and the TMXRL end exceeded the output focal distance of the PSFXRL by 6 mm. There is the balance between the high flux density of the PSFXRL and the absorbed X-rays of the receiving angle of the TMXRL [21]. In the following experiment, the distance from the TMXRL end to the PSFXRL end was fixed at 156 mm. In this study, the diameter of the output beam of the TMXRL was measured by the knife edge scanning method [27]. A steel ruler with a thickness of 1 mm fixed on the sample stage was scanned perpendicularly to the output X-ray beam of the TMXRL (X-axis in Fig. 1). The step size of the sample stage was 2 μm and the measuring time was 20 s for each step. Fig. 6 presents the scanning curve of the output beam of the
3.3. Comparison performance of capillary X-ray optics To evaluate the performance of the assembled X-ray optics for micro X-ray fluorescence spectrometry, a multifaceted comparison of capillary X-ray optics was conducted. Three parameters that have great significance to the performance of the μ-XRF instrument were selected for comparison: the spot size, the relative gain and the intensity of the Fe Kα from an iron plate obtained by the capillary X-ray optics. The spot size which determines the spatial resolution of the μ-XRF instrument is the minimum diameter of the focusing beam of the capillary X-ray optics. To directly compare the beam focusing ability of the capillary Xray focusing optics, the relative gain was defined as a ratio of the intensity per unit of the focal spot of the capillary X-ray optics to the intensity per unit of the pinhole. A pinhole with a diameter of 100 μm was used as a criterion. The relative gain of the pinhole was 1, and the relative gain of the capillary X-ray optics was the ratio of the intensity per unit of the focal spot of the capillary X-ray optics to the intensity per unit of the pinhole. The intensity of the Fe-Kα from an iron plated obtained by the capillary X-ray optics, which is related to the total number of photons in its focal spot, can directly reveal the efficiency of the μ-XRF instruments. The comparison results of the capillary X-ray optics are presented in Table 2. First, the performances of the assembled X-ray optics (PSFXRL+ TMXRL) and the TMXRL single used were compared. The assembled Xray optics and TMXRL both have a small focal spot with a size of approximately 9 μm. However, the relative gain and intensity of the Fe Kα from an iron plate obtained by the assembled X-ray optics are approximately 5 times those of the TMXRL single used, as shown in Table 2. The assembled X-ray optics has a higher relative gain and higher photon flux benefit from the incident X-rays of the TMXRL of the assembled X-ray optics pre-focused by the PSFXRL. The photon flux at the input aperture of the TMXRL in the assembled X-ray optics could be
Fig. 2. (a) Photograph of the TMXRL acquired by drawing a nested tube glass. (b) Cross profile of the nested glass tube. 3
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Fig. 3. Photographs of cross-sections of the TMXRL. (a) Input cross-section of the TMXRL. (b) Input cross-section of the nested glass tube of the TMXRL. (c) Input aperture of the TMXRL. (d) Output cross-section of the TMXRL. (e) Output cross-section of the nested glass tube of the TMXRL, (f) Output aperture of the TMXRL. (Figures (a) and (d) were captured by a low- powered digital microscope [GE-5, Aigo, China]. Figures (b), (c), (e) and (f) were captured by a high-powered digital microscope [VHXe500F, Keyence, Japan].)
Fig. 4. Optical test results for the TMXRL: (a) Measured curve of the inner profile of the TMXRL, (b) Design diameter (red line) and measured data (scatter curve). (c) Absolute diameter deviation from the design. (d) Straightness of the TMXRL. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 4
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Leq =
L G
(1)
where L is the distance from the X-ray source to the output beam of the optics. G is the gain in power density of the output beam of the optics, which is the ratio of the radiation intensity with and without the capillary X-ray optics. The gain in power density of the X-ray beam at the input aperture of the TMXRL in the assembled X-ray optics was 120. Thus, Leq in the input aperture of the TMXRL in the assembled X-ray optics was 25 mm. When the single TMXRL was used, the TMXRL was placed 60 mm from the X-ray source. The intensity of the X-ray from the X-ray source is inversely proportional to the square of the distance from the X-ray source. Therefore, the ratio of photons incidence into the aperture of the TMXRL in the assembled X-ray optics and the aperture
( )
60 2
of the TMXRL single used was 25 = 5.67 , which is in agreement with the relative gain and photon flux improvement of the assembled X-ray optics. Therefore, compared with TMXRL single used, the assembled Xray optics could effectively increase the photon flux of the focal spot. Second, compared with a commonly used PFXRL with a focal spot of 32.4 μm, the assembled X-ray optics has a smaller focal spot. However, the relative gain and the intensity of the Fe Kα from an iron plate obtained by the assembled X-ray optics are both significantly lower than those by PFXRL (Table 2), which indicate μ-XRF instrument based on assembled X-ray optics would take longer time for the example analysis than an μ-XRF instrument based on PFXRL. The main reason for this is that the divergence angle of the PSFXRL used in the assembled X-ray optics was 7.8 mrad, which is much larger than the receiving angle of the TMXRL of 0.4 mrad. Therefore, the TMXRL in the assembled X-ray optics receives few X-rays from the PSFXRL. For the assembled X-ray optics that consists of the PSFXRL and the TMXRL, the diameter of the input aperture of the TMXRL was 28.23 μm and the diameter of the output beam of the PSFXRL at the entrance of the TMXRL was approximately 680 μm. Only approximately 0.17% of the X-ray photons from the PSFXRL were transmitted by the TMXRL. To improve the transmission efficiency of the assembled X-ray optics, a TMXRL with a receiving angle of 3.49 mrad (TMXRL-2) was manufactured in our laboratory. The length and the diameters of the input and output apertures of the TMXRL-2 were 45 mm, 180 μm and 23 μm, respectively. In the assembled X-ray optics consisted of TMXRL-2 and PSFXRL, the ratio of the photons used by the TMXRL is approximately 6.61%. The progress of the newly designed TMXRL is reflected by the measurement results presented in Table 2. Compared with the previously used TMXRL, both the relative gain and the intensity of the Fe Kα from an iron plate obtained by the TMXRL-2 used in the assembled X-ray optics and single used increased dramatically, while the size of the focal spot increased by approximately 50%. Therefore, by fabricating a TMXRL with a higher receiving angle and a smaller output aperture, the intensity of the Fe Kα from an iron plate obtained by of the assembled Xray optics will increase and the focal spot will further condense.
Fig. 5. Relationship between the X-ray fluorescence intensity of Mo Kα and the distance of the PFXRL end from the TMXRL end (the y axis is as shown in Fig. 1).
Fig. 6. Scanning curve of the output beam of the TMXRL 4 mm from the end.
3.4. Two-dimensional mapping image by μ-XRF instruments based on different capillary X-ray optics To test the performances of the μ-XRF instruments, a stainless steel mesh (500 mesh/in.) with the interval of approximately 30 μm and the diameter of the stainless steel wire approximately 26 μm, as shown in Fig. 8(a), was measured. To evaluate the performance of the assembled X-ray optics, μ-XRF instruments based on a PFXRL with a focal spot of 32.4 μm and the TMXRL were constructed for the contrast test. Fig. 8(b), (c) and (d) are the elemental mapping images of Fe Kα in the stainless steel mesh by the μ-XRF setup based on the TMXRL, assembled X-ray optics and PFXRL, respectively. The scanning step size was 5 μm for all setups. The measuring time was 100 s per point for the μ-XRF setup based on TMXRL and assembled X-ray optics, and 20 s per point for the μ-XRF setup based on a PFXRL. The mapping images were measured at 4 mm from the end of the TMXRL (Fig. 8(b), (c)). The
Fig. 7. Diameter of the output beam of the TMXRL at various distances from the end of the TMXRL.
estimated by the equivalent distance (Leq). Leq is the distance at which the density of the radiation created by the X-ray source without a lens is analogous to the radiation created by the lens with the same source [28,29]. Leq can be expressed as:
5
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Table 2 Performance of the μ-XRF instrument with various capillary focusing X-ray optics. Optics
Pinhole
PSFXRL+ TMXRL
TMXRL
PSFXRL+ TMXRL-2
TMXRL-2
PFXRL
Spot size/μm Relative gain/Fe Kα Intensity/cps (Fe Kα)
100 1 –
9.3 130 23
9.6 25 4
14 1800 460
16 320 90
32.4 2900 3750
Fig. 8. (a) Photograph of the stainless steel mesh. (b) (c) (d) Mapping image of the stainless mesh obtained by μ-XRF setup based on different capillary X-ray optics.
image sharpness can be characterized by the contrast (C) of the image, which is defined as [27]:
C=
Imax Imin Imax
Foundation of China (Grant No.11675019, No.11875087) and the Project of Beijing Excellent Talents (Young Core). References
(3)
where Imax and Imix are the maximum and minimum intensity at the red line in Fig. 8(b), (c) and (d), respectively. The contrast of Fig. 8(b), (c) and (d) were 95.4%, 98.6% and 32.5%, respectively. The mapping images of the μ-XRF instruments coincide with results of the performance test presented in Table 2. The TMXRL and assembled X-ray optics have the focal spot of almost the same size, and accordingly, the mapping images obtained by the μ-XRF instruments based on TMXRL and assembled X-ray optics have nearly the same contrast. However, compared with TMXRL single used, μ-XRF instruments based on assembled X-ray optics is more efficient. While with high relative gain, the μ-XRF instrument based on the PFXRL cannot obtain a clear structure of the stainless mesh like the assembled X-ray optics and the TMXRL. Hence, the spatial resolution is a critical factor for the resolution of the X-ray fluorescence image.
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4. Conclusions An assembled X-ray focusing optics consisted of the PSFXRL and the TMXRL was constructed in our laboratory. This assembled optics system produced a micro X-ray beam with a focal spot of 9.3 μm at 4 mm from the end of TMXRL and a small divergence angle of 4.3 mrad. Using a nested glass tube can easily fabricate a high-quality TMXRL with a small output aperture and large receiving angle, which facilitates the production of a micro beam with small focal spot and high intensity by the assembled optics. Compared with the TMXRL single used, the micro beam of the TMXRL in the assembled X-ray optics not only has a micro focal spot but has a higher relative gain and higher photon flux. This micro beam of the assembled X-ray optics with small focal spot and high relative gain will be useful for μ-XRF analysis. Declaration of Competing Interest None. Acknowledgements This work was supported by the National Natural Science 6
Spectrochimica Acta Part B 165 (2020) 105770
X. Sun, et al. [21] A. Matsuda, K. Nakano, S. Komatani, S. Ohzawa, H. Uchihara, K. Tsuji, Fundamental characteristics of polycapillary X-ray optics combined with glass conical pinhole for micro x-ray fluorescence spectrometry, X-Ray Spectrom. 38 (2009) 258–262. [22] S. Komatani, K. Nakamachi, K. Nakano, S. Ohzawa, H. Uchihara, A. Bando, K. Tsuji, Fundamental characteristics of hybrid X-ray focusing optics for micro X-ray fluorescence analysis, Nucl. Instrum Meth. B 309 (2013) 260–263. [23] X. Sun, Z. Liu, T. Sun, L. Yi, W. Sun, F. Li, B. Jiang, Y. Ma, X. Ding, Combined optic system based on polycapillary X-ray optics and single-bounce monocapillary optics for focusing X-rays from a conventional laboratory X-ray source, Nucl. Instrum Meth. A 802 (2015) 5–9. [24] T. Sun, X. Ding, Measurements of energy dependence of properties of polycapillary x-ray lens by using organic glass as a scatterer, Journal of applied physics, J. Appl. Phys. 97 (2005) 1–7 124904.
[25] B. Jiang, Z. Liu, X. Sun, T. Sun, B. Deng, F. Li, L. Yi, M. Yuan, Y. Zhu, F. Zhang, T. Xiao, J. Wang, R. Tai, Single bounce ellipsoidal glass monocapillary condenser for X-ray nano-imaging, Opt. Commun. 398 (2017) 91–94. [26] X. Zeng, F. Duewer, M. Feser, C. Huang, A. Lyon, A. Tkachuk, W. Yun, Ellipsoidal and parabolic glass capillaries as condensers for x-ray microscopes, Appl. Opt. 47 (2008) 2376–2381. [27] T. Sun, C.A. MacDonald, Full-field transmission x-ray imaging with confocal polycapillary x-ray optics, J. Appl. Phys. 113 (2013) 053104. [28] M.A. Kumakhov, Capillary optics and their use in X-ray analysis, X-Ray Spectrom. 29 (2000) 343–348. [29] S. Peng, Z. Liu, T. Sun, Y. Ma, X. Ding, Spatially resolved in situ measurements of the ion distribution near the surface of electrode in a steady-state diffusion in an electrolytic tank with confocal micro X-ray fluorescence, Anal. Chem. 86 (2013) 362–366.
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Spectrochimica Acta Part B journal homepage: www.elsevier.com/locate/sab
Performance of assembled X-ray optics consisted of a polycapillary X-ray optics and a monocapillary X-ray optics for micro X-ray fluorescence spectrometry
T
Xuepeng Suna,b, , Xiaoyun Zhanga,b, Yabing Wanga,b, Shangkun Shaoa,b, Yufei Lia,b, Shiqi Pengc, ⁎ Zhiguo Liua,b, Tianxi Suna,b, ⁎
a b c
Key Laboratory of Beam Technology of Ministry of Education, College of Nuclear Science and Technology, Beijing Normal University, Beijing 100875, China Beijing Radiation Center, Beijing 100875, China Nuclear Power Institute of China, Chengdu 610000, China
ARTICLE INFO
ABSTRACT
Keywords: Capillary X-ray optics Laboratory X-ray source Micro X-ray focusing Micro X-ray fluorescence
A micro X-ray fluorescence (μ-XRF) spectrometry instrument based on an assembled X-ray optics was constructed in the laboratory. The assembled X-ray optics consisted of a polycapillary slightly focusing X-ray lens and a tapered monocapillary X-ray lens (TMXRL). The output X-ray beam of the polycapillary slightly focusing Xray lens was focused again by the TMXRL. A TMXRL with output aperture of 12.01 μm was drawn by a nested glass tube. To evaluate the performance of the instrument, a performance comparison between different capillary X-ray optics was conducted. The intensity of the Fe Kα from an iron plate was used to indicate the analysis efficiency of the μ-XRF instrument. The relative gain of the instrument was corrected by comparing the intensity per unit area for a pinhole aperture. Compared with TMXRL single used, the assembled X-ray optics also had a micro focal spot. Furthermore, the relative gain of the assembled X-ray optics and the intensity of the Fe Kα from the iron plate obtained by the assembled X-ray optics were approximately 5 times those of the TMXRL single used. This highly intense micro X-ray beam with several diameters obtained by the assembled X-ray optics will be useful for μ-XRF analysis.
1. Introduction Micro X-ray fluorescence (μ-XRF) is an efficiently nondestructive analysis technology that is used to obtain the two-dimensional elemental distribution of the surface of a sample. Compared with several competing techniques such as Inductive coupled plasma mass spectrometry, scanning electron microscope coupled to energy dispersive spectrometry, nano secondary ion mass spectrometry and micro particle induced X-ray emission, μ-XRF can be used in an air environment on large samples and requires little sample preparation. According to the generation method of the X-rays, μ-XRF can be divided in two main groups: synchrotron radiation μ-XRF and μ-XRF based on laboratory Xray tube. Compared with a laboratory X-ray source, synchrotron radiation has many advantages for μ-XRF, such as: high X-ray flux, collimated and monochromatic X-ray beam and wide selective of X-ray focusing optics [1–3]. However, considering the limitations on machine time and access possibilities to synchrotron radiation facilities, the study μ-XRF based on laboratory X-ray tubes remains significant.
⁎
With the development of the manufacture of X-ray focusing optics, μ-XRF based on laboratory X-ray source has been applied in several research fields, such as forensic science [4], archaeometry [5,6], medicine [7], geology [8], cement and concrete research [9,10] and applications to environmental sciences were recently reviewed [11]. In practice, the polycapillary focusing X-ray lens (PFXRL) and the monocapillary X-ray lens, which provide a highly intense and small focal spot, have been significant in development of the μ-XRF instrument [4,5,12–16]. The PFXRL was fabricated by drawing a bundle of glass capillary tubes in a constant temperature furnace. On the input side, the apertures of the glass capillary tubes all point to the input focal spot to receive the diverging X-rays emitted from the X-ray source. Similarly, on the output side, the apertures of all glass capillary tubers are directed to the focal spot. Therefore, the PFXRL can capture X-rays with a large solid angle and can focus the divergent X-rays from X-ray tubes in a small area. Generally, the size of the output focal spot of the PFXRL is tens of microns, and the magnitude of the gain in power density can be as high as 103. Although a high flux X-ray beam can be obtained in the
Corresponding authors. E-mail addresses: [email protected] (X. Sun), [email protected] (T. Sun).
https://doi.org/10.1016/j.sab.2020.105770 Received 2 July 2019; Received in revised form 18 January 2020; Accepted 20 January 2020 Available online 22 January 2020 0584-8547/ © 2020 Published by Elsevier B.V.
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output focal spot of the PFXRL, an output focal spot of PFXRL with a diameter of less than ten microns was difficult to achieve due to the “halo effect” [17]. Compared with the PFXRL, the monocapillary X-ray lens can provide a micro X-ray focusing beam with smaller focal spot size (micron to tens of nanometers) and divergence angle [12,18,19]. However, the capture angle of the monocapillary X-ray lens is much smaller than that of the PFXRL, resulting in the X-ray flux in the focal spot of the monocapillary X-ray lens being lower than that of the PFXRL. Thus, the monocapillary X-ray lens and the PFXRL have advantages and disadvantages. Recently, to acquire a suitable micro X-ray beam for μ-XRF, X-rays from a laboratory X-ray tube focused by assembled optics were studied. Tsuji et al. has proposed several types of assembled X-ray optics system for laboratory X-ray tube focusing [20–22]. In 2002, a cone-shaped Mo pinhole assembled with a straight single capillary was used to restrict the diameter of the primary X-ray beam to 230 μm [20]. In the study, the pinhole was placed very close to the sample surface to conduct Grazing-exit X-ray spectrometry. In 2009, they developed a μ-XRF instrument by a PFXRL combined with a glass conical pinhole [21]. In this instrument, the size of the obtained X-ray micro beam was 25 μm. In 2013, they combined a precision manufactured tungsten conical pinhole with a PFXRL for μ-XRF analysis and realized a small spot with diameter of 2.8 μm [22]. We have reported on the small X-ray beam that was obtained by two assembled lens based on laboratory X-ray source: one was based on a PFXRL and a single-bounce ellipsoidal capillary and the other was based on a polycapillary parallel X-ray lens and a single-bounce parabolic capillary [23]. In this study, we will report a new assembled X-ray optics of a polycapillary slightly focusing Xray lens (PSFXRL) and a tapered monocapillary X-ray lens (TMXRL) which was manufactured by a nested glass tube. In the assembled X-ray optics, the drawback of lower intensity of the micro focusing beam obtained by the TMXRL single used in μ-XRF analysis has been improved. The manufacture and configuration of the TMXRL will be described in detail and the analytical performance of the assembled X-ray optics will be compared with other capillary X-ray optics.
0.625 μm on each axis. The silicon drift detector was placed approximately 25 mm from the sample. The TMXRL was attached to a motorized X-Y-Z-θ1-θ2 stage (7STA1280 X-Y-Z stage and 7SRA1100 θ1-θ2 rotating stage; Sofn Instruments Co., Ltd., China) for the accurate adjustment of the TMXRL. The step resolution of the rotating θ1-θ2 stage was 0. 00125° for each axis. 3. Results and discussion 3.1. Capillary X-ray optics in the setup 3.1.1. PSFXRL Table 1 presents the parameters of the PSFXRL in the assembled Xray optics used in this study. The reason for not choosing the PFXRL as the pre-focusing optics is that the PSFXRL has a longer working distance (f2) and a smaller divergence of the output beam compared with the PFXRL. Therefore, the PSFXRL can combine with a common TMXRL and not a conical pinhole [21,22]. LF and f1 are the length and input focal distance of the PSFXRL, respectively. d1 and d2 are the diameter of the input section and output section of the PSFXRL, respectively. F is the focal spot size of the PSFXRL, which was measured by the knife scanning method [24]. The gain in power density (G) and the transmission efficiency (G) of the PSFXRL were measured by a proportional counter tube and a pinhole with a diameter of 100 μm. 3.1.2. TMXRL Fig. 2(a) is the photograph of the TMXRL used in the μ-XRF instrument in the study. The length of the TMXRL is 41 mm. The TMXRL was fabricated by drawing a nested glass tube. Fig. 2(b) shows the cross profile of the nested glass tube. The inner glass tube of the nested glass tube was prepared by drawing a common silicate glass tube until the outer diameter of the glass was equal to the inner diameter of the outer glass tube. Then, the silicate glass tube after drawing was slipped into the outer glass tube. After putting one glass tube into the other, a nested glass tube with large ratio of the outer diameter to the inner diameter was obtained. In the drawing process, the outer diameter of the monocapillary X-ray optics is proportional to the inner diameter [25]. The focal spot of the assembled X-ray optics is determined by the diameter of the output aperture of the TMXRL. To obtain an assembled Xray optics with a small focal spot, the output aperture of the TMXRL must be compressed to few microns. The nested glass tube with a larger ratio of the outer diameter to the inner diameter could insure that the tip of the TMXRL has a proper diameter, which is beneficial to manufacture of the optics. Furthermore, the nested glass tube has a large outer diameter which is convenient for drawing a TMXRL with a larger slope. Fig. 3 shows cross-sections of the input side and the output side of the TMXRL. The outer diameter and inner diameter of the input side of the TMXRL are 6.48 mm and 28.23 μm, respectively. The outer diameter of the input side of the inner glass tube of the TMXRL is 821.27 μm. The outer diameter and the inner diameter of the output side of the TMXRL are 3.09 mm and 12.01 μm, respectively. The outer diameter of the output side of the inner glass tube of the TMXRL is 382.5 μm. The ratio of the outer diameter to the inner diameter of the TMXRL is approximately 230. In this study, the quality of the TMXRL was evaluated by an optical measurement [26]. A digital micrometer [LS-7601, Keyence, Japan] with a motorized X stage was used to measure the outer profile of the TMXRL and the inner profiles that are presented in Fig. 4(a) were calculated from the outer profiles by scaling the ratio between the tube inner diameter and outer diameter [25]. Fig. 4(b) plots the design figure (red line) versus the measured data (scatter line) and the measured data are very close to the design figure. To observe the deviation more clearly, the absolute deviation is plotted in Fig. 4(c). After calculation, the root-mean-square deviation of the measured inner profile of the TMXRL from the ideal profiles was approximately 0.079 μm. Fig. 4(d)
2. Experimental Fig. 1 illustrates the laboratory constructed μ-XRF setup, which is coupled with an assembled X-ray optics consisted of a PSFXRL and a TMXRL. The X-ray source used in this study is a Mo rotating anode Xray generator (RIGAKU RU-200, Rigaku Corporation, Japan) with a focal spot size of 300 × 300 μm. In the experiment, the Mo target X-ray source was operated at 30 kV and 10 mA. In the detecting channel, a silicon drift detector (AXAS-M, Amptek, Germany) with an energy resolution of < 136 eV at 5.9 keV was used to measure the X-ray fluorescence. The maximum count rate of the detector system was 2 × 106 counts/s. The sensitive area of the detector system was 80 mm2. The PSFXRL and TMXRL used in this study were designed and manufactured by the Key Laboratory of Beam Technology of Ministry of Education. The silicon drift detector, PSFXRL and sample were adjusted by a motorized X-Y-Z stage (7STA1280, Sofn Instruments Co., Ltd., China), respectively. The step resolution of the motorized X-Y-Z stage was
Fig. 1. Schematic diagram of the laboratory μ-XRF setup with an assembled Xray optics consisted of a PSFXRL and a TMXRL. 2
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Table 1 Parameters of the PSFXRL. Parameter
LF
f1
f2
d1
d2
F
G
E
Value
101.3 (mm)
57.5 (mm)
150 (mm)
4.3 (mm)
5.1 (mm)
370 (μm)
966
31.5%
presents the measured straightness of the TMXRL and a deviation of < ± 0.07 μm was obtained. These results represent a large improvement compared with measurements previously reported [25]. The progress of the quality of the monocapillary X-ray optics might benefit from the nested drawing method. Because, in the drawing process of the nested tube glass, the outer tube glass of the nested tube glass functioned as a mould to compress the inner tube glass. In this compression, the deviation of the inner profile of the TMXRL might be decreased.
TMXRL 4 mm from the end. The full-width at half-maximum of the red line in Fig. 6 represents the measured beam. Furthermore, the diameter of the output beam of the TMXRL at various distances from the end of the TMXRL was measured, as plotted in Fig. 7. According to the diameter curve of the output beam of the TMXRL in Fig. 7, the output Xrays from the tip of the TMXRL converge at a distance of 0–4 mm from the end of the TMXRL and diverge after 4 mm. Therefore, the minimum size of the output beam of the assembled X-ray optics was approximately 9.3 μm at 4 mm away from the end of the TMXRL. According to the slope of the divergent part of the curve in Fig. 7, the divergence of the output beam of the TMXRL was approximately 4.3 mrad.
3.2. Beam size and beam divergence of the assembled X-ray optics For a μ-XRF spectrometer, the size of the incident X-ray beam, which determines the scanning resolution of the setup, is a significant parameter. For the μ-XRF setup combined with the assembled X-ray optics consisted of the PSFXRL and the TMXRL in this study, the scanning resolution of the instrument is determined by the output X-ray beam of the TMXRL. Before measuring the diameter of the micro X-ray beam, the PSFXRL and the TMXRL must be aligned. At certain distance between the PSFXRL end and the TMXRL end, by adjusting the motorized stage of X-Z-θ1-θ2, the optical axis of the PSFXRL coincided with the axis of the TMXRL and the maximum intensity was obtained. To measure the intensity of the X-ray beam focused by the TMXRL, the silicon drift detector was placed 10 cm from the end of the TMXRL. To weaken the X-rays that directly radiated from the TMXRL, a Perspex sheet with a thickness of 2 mm was fixed before the silicon drift detector. To optimize the distance between the PSFXRL end and the TMXRL end, the intensity of Mo Kα was measured for the TMXRL at various distances from the PSFXRL. In the measurement process, the distance between the TMXRL end and the silicon drift detector was fixed. Fig. 5 presents the relationship between the distance of the PSFXRL end from the TMXRL end and the Mo Kα intensity. As shown in Fig. 5, the maximum intensity was obtained at approximately 156 mm. The optimized distance between the PSFXRL end and the TMXRL end exceeded the output focal distance of the PSFXRL by 6 mm. There is the balance between the high flux density of the PSFXRL and the absorbed X-rays of the receiving angle of the TMXRL [21]. In the following experiment, the distance from the TMXRL end to the PSFXRL end was fixed at 156 mm. In this study, the diameter of the output beam of the TMXRL was measured by the knife edge scanning method [27]. A steel ruler with a thickness of 1 mm fixed on the sample stage was scanned perpendicularly to the output X-ray beam of the TMXRL (X-axis in Fig. 1). The step size of the sample stage was 2 μm and the measuring time was 20 s for each step. Fig. 6 presents the scanning curve of the output beam of the
3.3. Comparison performance of capillary X-ray optics To evaluate the performance of the assembled X-ray optics for micro X-ray fluorescence spectrometry, a multifaceted comparison of capillary X-ray optics was conducted. Three parameters that have great significance to the performance of the μ-XRF instrument were selected for comparison: the spot size, the relative gain and the intensity of the Fe Kα from an iron plate obtained by the capillary X-ray optics. The spot size which determines the spatial resolution of the μ-XRF instrument is the minimum diameter of the focusing beam of the capillary X-ray optics. To directly compare the beam focusing ability of the capillary Xray focusing optics, the relative gain was defined as a ratio of the intensity per unit of the focal spot of the capillary X-ray optics to the intensity per unit of the pinhole. A pinhole with a diameter of 100 μm was used as a criterion. The relative gain of the pinhole was 1, and the relative gain of the capillary X-ray optics was the ratio of the intensity per unit of the focal spot of the capillary X-ray optics to the intensity per unit of the pinhole. The intensity of the Fe-Kα from an iron plated obtained by the capillary X-ray optics, which is related to the total number of photons in its focal spot, can directly reveal the efficiency of the μ-XRF instruments. The comparison results of the capillary X-ray optics are presented in Table 2. First, the performances of the assembled X-ray optics (PSFXRL+ TMXRL) and the TMXRL single used were compared. The assembled Xray optics and TMXRL both have a small focal spot with a size of approximately 9 μm. However, the relative gain and intensity of the Fe Kα from an iron plate obtained by the assembled X-ray optics are approximately 5 times those of the TMXRL single used, as shown in Table 2. The assembled X-ray optics has a higher relative gain and higher photon flux benefit from the incident X-rays of the TMXRL of the assembled X-ray optics pre-focused by the PSFXRL. The photon flux at the input aperture of the TMXRL in the assembled X-ray optics could be
Fig. 2. (a) Photograph of the TMXRL acquired by drawing a nested tube glass. (b) Cross profile of the nested glass tube. 3
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Fig. 3. Photographs of cross-sections of the TMXRL. (a) Input cross-section of the TMXRL. (b) Input cross-section of the nested glass tube of the TMXRL. (c) Input aperture of the TMXRL. (d) Output cross-section of the TMXRL. (e) Output cross-section of the nested glass tube of the TMXRL, (f) Output aperture of the TMXRL. (Figures (a) and (d) were captured by a low- powered digital microscope [GE-5, Aigo, China]. Figures (b), (c), (e) and (f) were captured by a high-powered digital microscope [VHXe500F, Keyence, Japan].)
Fig. 4. Optical test results for the TMXRL: (a) Measured curve of the inner profile of the TMXRL, (b) Design diameter (red line) and measured data (scatter curve). (c) Absolute diameter deviation from the design. (d) Straightness of the TMXRL. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 4
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Leq =
L G
(1)
where L is the distance from the X-ray source to the output beam of the optics. G is the gain in power density of the output beam of the optics, which is the ratio of the radiation intensity with and without the capillary X-ray optics. The gain in power density of the X-ray beam at the input aperture of the TMXRL in the assembled X-ray optics was 120. Thus, Leq in the input aperture of the TMXRL in the assembled X-ray optics was 25 mm. When the single TMXRL was used, the TMXRL was placed 60 mm from the X-ray source. The intensity of the X-ray from the X-ray source is inversely proportional to the square of the distance from the X-ray source. Therefore, the ratio of photons incidence into the aperture of the TMXRL in the assembled X-ray optics and the aperture
( )
60 2
of the TMXRL single used was 25 = 5.67 , which is in agreement with the relative gain and photon flux improvement of the assembled X-ray optics. Therefore, compared with TMXRL single used, the assembled Xray optics could effectively increase the photon flux of the focal spot. Second, compared with a commonly used PFXRL with a focal spot of 32.4 μm, the assembled X-ray optics has a smaller focal spot. However, the relative gain and the intensity of the Fe Kα from an iron plate obtained by the assembled X-ray optics are both significantly lower than those by PFXRL (Table 2), which indicate μ-XRF instrument based on assembled X-ray optics would take longer time for the example analysis than an μ-XRF instrument based on PFXRL. The main reason for this is that the divergence angle of the PSFXRL used in the assembled X-ray optics was 7.8 mrad, which is much larger than the receiving angle of the TMXRL of 0.4 mrad. Therefore, the TMXRL in the assembled X-ray optics receives few X-rays from the PSFXRL. For the assembled X-ray optics that consists of the PSFXRL and the TMXRL, the diameter of the input aperture of the TMXRL was 28.23 μm and the diameter of the output beam of the PSFXRL at the entrance of the TMXRL was approximately 680 μm. Only approximately 0.17% of the X-ray photons from the PSFXRL were transmitted by the TMXRL. To improve the transmission efficiency of the assembled X-ray optics, a TMXRL with a receiving angle of 3.49 mrad (TMXRL-2) was manufactured in our laboratory. The length and the diameters of the input and output apertures of the TMXRL-2 were 45 mm, 180 μm and 23 μm, respectively. In the assembled X-ray optics consisted of TMXRL-2 and PSFXRL, the ratio of the photons used by the TMXRL is approximately 6.61%. The progress of the newly designed TMXRL is reflected by the measurement results presented in Table 2. Compared with the previously used TMXRL, both the relative gain and the intensity of the Fe Kα from an iron plate obtained by the TMXRL-2 used in the assembled X-ray optics and single used increased dramatically, while the size of the focal spot increased by approximately 50%. Therefore, by fabricating a TMXRL with a higher receiving angle and a smaller output aperture, the intensity of the Fe Kα from an iron plate obtained by of the assembled Xray optics will increase and the focal spot will further condense.
Fig. 5. Relationship between the X-ray fluorescence intensity of Mo Kα and the distance of the PFXRL end from the TMXRL end (the y axis is as shown in Fig. 1).
Fig. 6. Scanning curve of the output beam of the TMXRL 4 mm from the end.
3.4. Two-dimensional mapping image by μ-XRF instruments based on different capillary X-ray optics To test the performances of the μ-XRF instruments, a stainless steel mesh (500 mesh/in.) with the interval of approximately 30 μm and the diameter of the stainless steel wire approximately 26 μm, as shown in Fig. 8(a), was measured. To evaluate the performance of the assembled X-ray optics, μ-XRF instruments based on a PFXRL with a focal spot of 32.4 μm and the TMXRL were constructed for the contrast test. Fig. 8(b), (c) and (d) are the elemental mapping images of Fe Kα in the stainless steel mesh by the μ-XRF setup based on the TMXRL, assembled X-ray optics and PFXRL, respectively. The scanning step size was 5 μm for all setups. The measuring time was 100 s per point for the μ-XRF setup based on TMXRL and assembled X-ray optics, and 20 s per point for the μ-XRF setup based on a PFXRL. The mapping images were measured at 4 mm from the end of the TMXRL (Fig. 8(b), (c)). The
Fig. 7. Diameter of the output beam of the TMXRL at various distances from the end of the TMXRL.
estimated by the equivalent distance (Leq). Leq is the distance at which the density of the radiation created by the X-ray source without a lens is analogous to the radiation created by the lens with the same source [28,29]. Leq can be expressed as:
5
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Table 2 Performance of the μ-XRF instrument with various capillary focusing X-ray optics. Optics
Pinhole
PSFXRL+ TMXRL
TMXRL
PSFXRL+ TMXRL-2
TMXRL-2
PFXRL
Spot size/μm Relative gain/Fe Kα Intensity/cps (Fe Kα)
100 1 –
9.3 130 23
9.6 25 4
14 1800 460
16 320 90
32.4 2900 3750
Fig. 8. (a) Photograph of the stainless steel mesh. (b) (c) (d) Mapping image of the stainless mesh obtained by μ-XRF setup based on different capillary X-ray optics.
image sharpness can be characterized by the contrast (C) of the image, which is defined as [27]:
C=
Imax Imin Imax
Foundation of China (Grant No.11675019, No.11875087) and the Project of Beijing Excellent Talents (Young Core). References
(3)
where Imax and Imix are the maximum and minimum intensity at the red line in Fig. 8(b), (c) and (d), respectively. The contrast of Fig. 8(b), (c) and (d) were 95.4%, 98.6% and 32.5%, respectively. The mapping images of the μ-XRF instruments coincide with results of the performance test presented in Table 2. The TMXRL and assembled X-ray optics have the focal spot of almost the same size, and accordingly, the mapping images obtained by the μ-XRF instruments based on TMXRL and assembled X-ray optics have nearly the same contrast. However, compared with TMXRL single used, μ-XRF instruments based on assembled X-ray optics is more efficient. While with high relative gain, the μ-XRF instrument based on the PFXRL cannot obtain a clear structure of the stainless mesh like the assembled X-ray optics and the TMXRL. Hence, the spatial resolution is a critical factor for the resolution of the X-ray fluorescence image.
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4. Conclusions An assembled X-ray focusing optics consisted of the PSFXRL and the TMXRL was constructed in our laboratory. This assembled optics system produced a micro X-ray beam with a focal spot of 9.3 μm at 4 mm from the end of TMXRL and a small divergence angle of 4.3 mrad. Using a nested glass tube can easily fabricate a high-quality TMXRL with a small output aperture and large receiving angle, which facilitates the production of a micro beam with small focal spot and high intensity by the assembled optics. Compared with the TMXRL single used, the micro beam of the TMXRL in the assembled X-ray optics not only has a micro focal spot but has a higher relative gain and higher photon flux. This micro beam of the assembled X-ray optics with small focal spot and high relative gain will be useful for μ-XRF analysis. Declaration of Competing Interest None. Acknowledgements This work was supported by the National Natural Science 6
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