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Wavelength Dispersive X-ray Fluorescence Imaging Kouichi Tsuji,* Takashi Ohmori, and Makoto Yamaguchi Department of Applied Chemistry & Bioengineering, Graduate School of Engineering, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan ABSTRACT: A new wavelength-dispersive X-ray fluorescence (WD-XRF) imaging spectrometer equipped with a two-dimensional X-ray detector was developed in the laboratory. Straight polycapillary optics was applied instead of a soller slit, which is used in conventional WD-XRF spectrometers. X-rays were guided through the straight polycapillary to the exit of the optics by X-ray external total reflections. X-ray fluorescence was dispersed by an analyzing crystal (LiF(200)), keeping the information of elemental distribution on the surface of the sample. The energy resolution of the developed spectrometer was 130 152 eV at the Zn KR peak. X-ray elemental images of Cu KR and Ni KR were successfully obtained by an X-ray CCD detector at the corresponding diffraction angles. The analytical performance of this technique, and further improvements are discussed.
-ray fluorescence (XRF) is a well-known analytical method, which has been used in fundamental research and process control of industrial materials. In the initial stage of XRF study, wavelength-dispersive X-ray spectrometry (WDXRS) was applied.1 WDXRS is based on Bragg diffraction phenomena. The sample is irradiated with primary X-rays. XRF emitted from the sample is introduced into an entrance soller slit for collimation. Then, XRF is dispersed by a crystal, which has a specific dvalue. The dispersed XRF passes through an exit soller slit and is detected by a counter. Here, the soller slit is composed of a stack of thin metallic plates, so that XRF is introduced into the crystal at a limited (controlled) angle (θ). To obtain an XRF spectrum, the angle is scanned using a goniometer. Alternatively, an energy-dispersive X-ray spectroscopy (EDXRS) has been developed.1,2 In EDXRS, XRF from a sample is introduced into a semiconductor such as silicon, where electron hole pairs are created. The charged electric pulses are analyzed by a pulse height analyzer, and then an X-ray spectrum is obtained. Since the EDXRS system has no moving mechanism, it is very compact. After the EDXRS methodology was developed and improved, it has been used in a variety of X-ray analytical instruments, including a hand-held X-ray analyzer. However, WDXRS is still being used. The reasons for this are its high-energy resolution and high sensitivity, especially for light elements (low-Z elements). The energy resolution of normal EDXRS is more than 130 eV at the Mn KR line, while the energy resolution of WDXRS is a few tens of electron volts. One of the research trends in XRF study is 2D elemental imaging.3,4 Elemental distribution in 2D provides detailed information about the sample. In the fundamental research of biological samples, developments of semiconductor materials, and nanotechnology, such chemical 2D information is highly essential.5 To obtain the 2D elemental distribution, two types of
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techniques have been studied and applied. The first technique is a method known as micro X-ray beam scanning.3,6 In practice, the sample is scanned with a fixed X-ray beam. At each position, the XRF spectrum is measured and saved, usually via EDXRS. In this case, the resolution of obtained elemental image depends on the size of the X-ray beam. Therefore, the development of X-ray focusing optics is a key technology in the scanning method. An X-ray microbeam of 10 μm or less is used in the laboratory, while a nano X-ray beam several nanometers in size has been reported using synchrotron radiation (SR) as an X-ray source.7,8 The drawback of this scanning method is the time-consuming measurement as the area under analysis increases. Another elemental imaging technique is a full-field method using an X-ray CCD detector. Sakurai et al. developed a rapid X-ray chemical imaging technique.9,10 The sample was irradiated with the primary X-rays at a glancing angle. The XRF from the sample passed through a two-dimensional (2D) collimator and was detected by an X-ray CCD detector. The experiment was performed at the SR facility (Spring-8). Changing the energy of the primary X-rays enabled the 2D detector to obtain elemental images for a specific element. However, it is difficult to change the energy of X-rays that have an intensity strong enough for X-ray analysis in the laboratory. XRF imaging was performed using the straight polycapillary optics by the author’s group.11,12 In this previous research, total reflections on the inner wall of each capillary in the straight polycapillary were used for energy-filtering of XRF. The critical angle for X-ray total reflection is ∼0.2° at Cu KR (8.0 keV). The X-rays taken into the capillary at angles greater than the critical angle were absorbed in the capillary without total reflection. Received: June 2, 2011 Accepted: July 13, 2011 Published: July 13, 2011 6389
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Figure 1. Concept of WD-XRF imaging.
Only the X-rays obtained less than a critical angle for total reflection were guided in the polycapillary, and then they were detected by the CCD detector. This XRF imaging gave an energy-filtered X-ray image; however, elementally selective XRF imaging was difficult. The purpose of our present research is the development of a full field-type elemental imaging method in the laboratory. In this paper, we propose a new technique that combines a WD-XRF spectrometer with an X-ray CCD detector.
’ CONCEPT OF WD-XRF IMAGING In a conventional WD-XRF spectrometer, an entrance soller slit is used to collimate the X-ray fluorescence. The incident angle of X-ray fluorescence to the analyzing crystal is important for achieving Bragg’s diffraction law. The diffracted X-rays are also collimated by an exit soller slit and then are detected by an X-ray detector such as a proportional counter. Since the soller slit is a stack of thin metal plates, the incident angle to the analyzing crystal can be specified. However, the X-rays collimated by the soller slit lose the information on elemental distribution, even if the sample has a spatial elemental distribution. Figure 1 shows the concept of WD-XRF imaging proposed in this paper. We propose the application of straight polycapillary optics, instead of the soller slit. X-rays are guided in the straight polycapillary by X-ray external total reflections. The collimated X-ray beams are dispersed by the analyzing crystal in the same way as the conventional WD-XRF spectrometer. This experimental setup enables the analysis of X-ray fluorescence having the positional information of elemental distribution on the surface of the sample, leading to XRF imaging. ’ EXPERIMENTAL SECTIONS Figure 2 shows a schematic drawing of the experimental setup developed in the laboratory. The X-ray tube (Mo target) was operated at 40 kV and 40 mA. The sample was irradiated with the primary X-rays through a simple stainless steel collimator with an inner diameter of 8 mm. A straight polycapillary optic, specially developed by XOS, Ltd., was applied. This optic consists of several million capillaries in an enclosure length of 5.5 mm and an outer diameter of 8.3 mm with an open area of 60%. The outer diameter of the polycapillary determined the observation area.
Figure 2. Experimental setup for WD-XRF imaging.
Each capillary has a channel diameter of 10 μm and a thickness of a few micrometers. The collimated X-rays were analyzed by an analyzing crystal of LiF(200) (2d = 0.40273 nm at 20 °C, 76.6 mm 33 mm 7.4 mm), which was attached to the center of the inner rotation stage, as shown in Figure 2. Although an exit straight polycapillary was initially designed, as shown in Figure 1, it was not applied for the present imaging not to reduce the XRF intensity. Instead, the analyzed X-rays were directly detected by an X-ray detector without reduction of the XRF intensity. To investigate the fundamental performance of the developed instrument, an energydispersive X-ray spectroscopy (EDX) detector (Si-PIN detector, XR-100CR, Amptek, USA; sensitive area: 7 mm2, energy resolution: 188 eV at 5.9 keV) was attached to the outer rotation stage. In addition, two types of X-ray CCD detectors were applied: an indirect type scintillation detector (PIXIS-XF 1024F, Princeton Instruments, USA, phosphor: Gd2O2S:Tb, 13 μm 13 μm pixels, 1024 1024 imaging array, Be window thickness: 250 μm, cooling temperature: 40 °C) and a direct X-ray detection type, back-illuminated detector (PIXIS-XB, Princeton Instruments, USA, 13 μm 13 μm pixels, 1024 1024 imaging array, Be window thickness: 250 μm, operated cooling temperature: 36 °C). These CDD detectors were applied to confirm the elementally selective XRF imaging. In Figure 2, the distance (d1) 6390
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Figure 3. The ROI intensities of Zn KR measured by the Si-PIN detector were plotted as a function of 2θ. X-ray spectrum of Zn KR obtained with (a) one (entrance) polycapillary and (b) two (entrance and exit) polycapillary optics. The Gaussian curves (dashed-curves) were fitted to the experimental curves.
Figure 4. X-ray spectra obtained by Si-PIN detector at different 2θ angles for the stainless steel sample.
between the sample and the entrance polycapillary was 17.5 mm, the distance (d2) between the entrance polycapillary and the crystal was 35.0 mm, and the distance (d3) between the crystal between the crystal and the detector was 95.0 mm. The two rotation stages equipped with stepping motors were controlled by a personal computer (PC) using motor drivers and a motor controller (NT-2400, Laboratory Equipment Co., Japan).
’ RESULTS Energy Resolution. To evaluate the energy (wavelength) resolution of the developed WD-XRF spectrometer, a pure Zn plate was measured by the Si-PIN detector. Figure 3a shows the X-ray spectrum measured for Zn KR. The ROI intensity of Zn KR was plotted as a function of the 2θ angle. This spectrum was obtained with only the entrance polycapillary in the experimental setup shown in Figure 2. The energy resolution was evaluated from the full width at half maximum (fwhm) value of the peak and found to be 152 eV, which was considerably larger than that in conventional WD-XRF. This broadening of the peak would occur in the collimating process in the polycapillary. As mentioned above, each capillary has an inner diameter of 10 μm and a length of 5.5 mm. This dimension produces a divergence of 0.208° from the collimation angle, corresponding
Figure 5. Experimental setup for confirmation of positional information in XRF images. A Si-PIN detector with a Pb slit was adjusted at the corresponding angles for Cu KR or Zn KR. Then, the Si-PIN detector was scanned for 12 mm.
to 80 eV. The measurement was performed without the exit polycapillary to produce a strong intensity for elemental imaging. After the second (exit) polycapillary was installed, the peak of Zn KR was measured again for the same sample. The energy resolution was improved to 130 eV, as shown in Figure 3b, which was still larger than the expected value (80 eV). Since the thickness of each capillary was very thin, there was a possibility that some X-ray beams penetrated the walls between the capillaries, leading to the deterioration of the energy resolution. The peak intensity of Zn KR in Figure 3b was ∼65% of the peak intensity of Zn KR in Figure 3a. This decrease in the intensity was reasonable for an open area of 60% in the polycapillary optics. WD-XRF of Stainless Steel. A stainless steel plate (SUS-304, Fe 10%Ni 18%Cr, 20 mm 40 mm, thickness: 1 mm) was measured by the WD-XRF setup using the EDX to check the performance of the developed spectrometer with only entrance polycapillary. XRF spectra were measured for 60 s at different rotation angles (2θ). The results are shown in Figure 4. XRF peaks of Cr KR (5.41 keV), Cr Kβ (5.95 keV), Fe KR (6.40 keV), Fe Kβ (7.06 keV), and Ni KR (7.47 keV) from the plate were 6391
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Analytical Chemistry clearly observed, confirming that each characteristic peak was separated by the present spectrometer.
Figure 6. XRF profiles of Cu KR and Zn KR measured for Zn Cu plates. Compared with the sample shown in Figure 5, a reversed profile was obtained (left to right).
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Positional Information of XRF Emitted from the Sample. Zn and Cu plates (20 mm 20 mm, thickness: 1 mm) were placed on the sample holder at the left and right sides, respectively, as shown in Figure 5. A Pb slit (width: 2 mm) was attached to the EDX system. After the crystal and the EDX were positioned at the corresponding θ 2θ angles for Zn KR and Cu KR, the EDX was scanned from the origin for a distance of 12 mm, using a translation stage. As expected, Figure 6 indicates that the Cu KR signal was first observed in the scanned range from 2 mm to 8 mm, while the Zn KR signal was observed from 6 mm to 11 mm in the scanned range. The relationship of the intensity profile was reversed from the sample position. This reversed profile can easily be understood from experimental geometry. WD-XRF Image. Finally, WD-XRF images were observed using the CCD X-ray detector. The sample used was composed of Ni and Cu plates (thickness: 1 mm), as shown in Figure 7a. After the crystal and the CCD X-ray detector were positioned at the corresponding 2θ angle (45.0°) for Cu KR, an X-ray image was obtained after an exposure time of 60 min, as shown in Figure 7b. Similarly, an X-ray image of Ni KR was obtained at a 2θ angle of 48.8°. In both cases, a semicircular image was
Figure 7. (a) Photograph showing a Ni Cu plate sample. (b,c) XRF images of Cu KR (panel b) and Ni KR (panel c), measured at 2θ angles of 45.0° and 48.7°, respectively, for the Ni Cu plate samples depicted in panel a. An indirect-type scintillation X-ray CCD detector was applied. 6392
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Figure 8. XRF images of (a) Cu KR and (b) Ni KR measured for a triangle of Ni film (50 μm thick) on a Cu plate (1 mm thick). The exposure time was 10 min. The X-ray intensity profiles of Cu KR and Ni KR in the square region in panels a and b were analyzed, as shown in panels c and d, respectively. A direct X-ray, detection-type, back-illuminated X-ray CCD detector was applied.
obtained in the diameter of the cylindrical polycapillary optics. The X-ray images could be observed after an exposure time of 5 min. However, a long exposure time was necessary under the present conditions to improve the signal-to-background intensity ratio. The lateral resolution of X-ray images depends on the X-ray optical system and the X-ray detector. In order not to reduce the XRF intensity, no exit polycapillary was applied in this measurement. This could be one of the reasons that the obtained image was obscure. In addition, the phosphor (Gd2O2S:Tb) of the X-ray CCD detector had a thickness of 100 μm. Therefore, the XRF beam would produce an optical emission in a volume larger than the original XRF beam size, resulting in deterioration of the lateral resolution. Thus, a direct-detection type CCD X-ray detector was applied. Figure 8 shows elemental X-ray images obtained for a triangle of Ni film (50 μm thick) on a Cu plate (1 mm thick). The exposure time was 10 min. Clear elementally selective images could be obtained for this short exposure time. The circular image shown in Figure 8a corresponds to the diameter of the entrance polycapillary optics. The intensity profiles of Cu KR and Ni KR in the square region shown in Figures 8a and 8b were analyzed. The results are shown in Figures 8c and 8d, where the X-ray intensity in the Y-axis was the averaged intensity for the data of 100 pixels. A sigmoid curve was fitted to the experimental data and is shown as a solid (red) line. The height differences in the fitted curves for Cu KR and Ni KR were ∼90 and 125 counts, respectively. This difference could be
caused by the energy dependence of sensitivity of the CCD detector. To evaluate the lateral resolution of the elemental image, the differential curve of the fitted curve was analyzed. The fwhm values of the differential curves for Cu KR and Ni KR were 214 and 373 μm, respectively. The evaluated values were larger than the pixel size of the CCD detector. They could be reduced if the polycapillary optics were improved.
’ CONCLUSION X-ray elemental images were successfully obtained by a developed wavelength-dispersive X-ray fluorescence (WD-XRF) imaging instrument. Unfortunately, the energy resolution was not the value expected in the conventional WD-XRF spectrometer. We believe that the proposed WD-XRF imaging spectrometer has a potential of having the energy resolution closed to that of the conventional WD-XRF spectrometer. The primary reason for obscure X-ray images is the weak XRF intensity after the XRF was dispersed by the crystal. Therefore, a high-power X-ray generator with a rotating anode tube would be necessary in the laboratory to improve the quality of the X-ray elemental image. This improvement will be undertaken. Recently, a color X-ray CCD detector has been developed and studied.13,14 Energy-dispersive X-ray spectroscopy (EDX) analysis is possible at each pixel of the CCD detector.15,16 Therefore, X-ray elemental images are available without any scanning 6393
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Analytical Chemistry mechanism. This detector is very appealing from the point of view of analytical applications. Since a photon counting technique is applied, a small counting rate might be the limitation of this technique. In the history of X-ray spectrometry, both EDS and WDS have been studied and applied for various samples, based on their advantages. We believe that WD-XRF imaging proposed in this work could also be an important technique, considering its potential for high-energy resolution elemental imaging for light-to-heavy elements with suitable analyzing crystals and synthetic multilayers.
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(15) St€ruder, L.; B€rauninger, H.; Meier, M.; Predehl, P.; Reppin, C.; Sterzik, M.; T€rumper, J.; Cattaneo, P.; Hauff, D.; Lutz, G.; Schuster, K. F.; Schwarz, A.; Kendziorra, E.; Staubert, R.; Gatti, E.; Longoni, A.; Sampietro, M.; Radeka, V.; Rehak, P.; Rescia, S.; Manfredi, P. F.; Buttler, W.; Holl, P.; Kemmer, J.; Prechtel, U.; Ziemann, T. Nucl. Instrum. Methods Phys. Res., Sect. A 1990, 288, 227–235. (16) Tsunemi, H.; Wada, M.; Hayashida, K.; Kwai, S. Jpn. J. Appl. Phys. 1991, 30, 3540–3544.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT The part of this work was supported by JST (Japan Science and Technology Agency) Adaptable and Seamless Technology Transfer Program (A-STEP). ’ REFERENCES (1) Helsen, J. A.; Kuczumow, A. In Handbook of X-Ray Spectrometry; Van Grieken, R., Markowicz, A. A., Eds.; Marcel Dekker: New York, 2002; pp 95 198. (2) Kanngiesser, B.; Haschke, M. In Handbook of Practical X-ray Fluorescence Analysis; Beckhoff, B., Kanngiesser, B., Langhoff, N., Wedell, R., Wolff, H., Eds.; Springer: Berlin, 2006; pp 433 474. (3) Janssens, K. H. A., Adams, F. C. V., Rindby, A., Eds. Microscopic X-Ray Fluorescence Analysis; Wiley: New York, 2000. (4) Tsuji, K.; Nakano, K.; Takahashi, Y.; Hayashi, K.; Ro, C.-U. Anal. Chem. 2010, 82, 4950–4987. (5) Ide-Ektessabi, A. Applications of Synchrotron Radiation; Springer: Berlin, 2007. (6) Nakano, K.; Tsuji, K. X-Ray Spectrom. 2009, 38, 446–450. (7) Mimura, H.; Handa, S.; Kimura, T.; Yumoto, H.; Yamakawa, D.; Yokoyama, H.; Matsuyama, S.; Inagaki, K.; Yamamura, K.; Sano, Y.; Tamasaku, K.; Nishino, Y.; Yabashi, M.; Ishikawa, T.; Yamauchi, K. Nature Physics 2010, 6, 122–125. (8) Mimura, H.; Kimura, T.; Yumoto, H.; Yokoyama, H.; Nakamori, H.; Matsuyama, S.; Tamasaku, K.; Nishino, Y.; Yabashi, M.; Ishikawa, T.; Yamauchi, K. Nucl. Instrum. Methods Phys. Res., Sect. A 2011, 635, S16–S18. (9) Sakurai, K.; Eba, H. Anal. Chem. 2003, 75, 355–359. (10) Sakurai, K.; Mizusawa, M. Anal. Chem. 2010, 82, 3519–3522. (11) Tsuji, K.; Nakano, K.; Yamaguchi, M.; Yonehara, T. Proc. SPIE 2008, 7077, 70770W-1–1 70770W-8. (12) Yonehara, T.; Yamaguchi, M.; Tsuji, K. Spectrochim. Acta, Part B 2010, 65, 441–444. (13) Str€uder, L.; Epp, S.; Rolles, D.; Hartmann, R.; Holl, P.; Lutz, G.; Soltau, H.; Eckart, R.; Reich, C.; Heinzinger, K.; Thamm, C.; Rudenko, A.; Krasniqi, F.; K€uhnel, K.-U.; Bauer, C.; Schr€oter, C.-D.; Moshammer, R.; Techert, S.; Miessner, D.; Porro, M.; H€alker, O.; Meidinger, N.; Kimmel, N.; Andritschke, R.; Schopper, F.; Weidenspointner, G.; Ziegler, A.; Pietschner, D.; Herrmann, S.; Pietsch, U.; Walenta, A.; Leitenberger, W.; Bostedt, C.; M€oller, T.; Rupp, D.; Adolph, M.; Graafsma, H.; Hirsemann, H.; G€artner, K.; Richter, R.; Foucar, L.; Shoeman, R. L.; Schlichting, I.; Ullrich, J. Nucl. Instrum. Methods Phys. Res., Sect. A 2010, 614, 483–496. (14) Scharf, O.; Ihle, S.; Ordavo, I.; Arkadiev, V.; Bjeoumikhov, A.; Bjeoumikhova, S.; Buzanich, G.; Gubzhokov, R.; G€unther, A.; Hartmann, R.; K€uhbacher, M.; Lang, M.; Langhoff, N.; Liebel, A.; Radtke, M.; Reinholz, U.; Riesemeier, H.; Soltau, H.; Str€uder, L.; Th€unemann, A. F.; Wedell, R. Anal. Chem. 2011, 83, 2532–2538. 6394
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