optimizing the illumination in a hard xray microscope with...

Optimizing the Illumination in a Hard Xray Microscope with IntegratedTalbot InterferometerK. Uesugi, A. Takeuchi, M. Hoshino, and Y. Suzuki Citation: AIP Conf. Proc. 1365, 309 (2011); doi: 10.1063/1.3625366 View online: http://dx.doi.org/10.1063/1.3625366 View Table of Contents: http://proceedings.aip.org/dbt/dbt.jsp?KEY=APCPCS&Volume=1365&Issue=1 Published by the American Institute of Physics. Related ArticlesDirectional waveguide coupling from a wavelength-scale deformed microdisk laser Appl. Phys. Lett. 100, 061125 (2012) Tunneling and filtering characteristics of cascaded -negative metamaterial layers sandwiched by double-positivelayers J. Appl. Phys. 111, 014906 (2012) Measurement of phase retardation of waveplate online based on laser feedback Rev. Sci. Instrum. 83, 013101 (2012) Design of narrow band photonic filter with compact MEMS for tunable resonant wavelength ranging 100 nm AIP Advances 1, 042171 (2011) Simple alignment technique for polarisation maintaining fibres Rev. Sci. Instrum. 82, 125103 (2011) Additional information on AIP Conf. Proc.Journal Homepage: http://proceedings.aip.org/ Journal Information: http://proceedings.aip.org/about/about_the_proceedings Top downloads: http://proceedings.aip.org/dbt/most_downloaded.jsp?KEY=APCPCS Information for Authors: http://proceedings.aip.org/authors/information_for_authors

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Optimizing the Illumination in a Hard X-ray Microscope with Integrated Talbot

Interferometer

K. Uesugi, A. Takeuchi, M. Hoshino, and Y. Suzuki

Japan Synchrotron Radiation Research Institute, JASRI / SPring-8, Hyogo 679-5198, Japan

Abstract. A high spatial resolution x-ray phase imaging system was developed at BL47XU in SPring-8, which uses a full-field x-ray microscope with a Fresnel zone plate (FZP) as an objective and a condenser zone plate (CZP) for pseudo-Köhler illumination. For quantitative phase contrast imaging, a grating interferometer (Talbot type interferometer) was developed. The combination of these optics is suitable for phase contrast imaging with high spatial resolution. The optimal conditions for phase tomography were found.

Keywords: synchrotron, phase contrast image, x-ray microscope PACS: 07.85.Tt, 42.87.Bg, 68.37.Yz, 87.59.-e, 87.64.mh

INTRODUCTION

X-ray phase contrast imaging can be more sensitive than absorption contrast imaging under many measurement conditions. Synchrotron radiation based phase imaging has been developed during the last few decades. Crystal and grating interferometers were used for x-ray phase tomography in those developments [1, 2]. These phase imaging techniques were successfully applied to weak absorbing biological specimens in the hard x-ray region. However, these techniques have a spatial resolution of only tens of micrometers.

Recently full-field x-ray microscopes using a Fresnel zone plate (FZP) have been developed at some facilities [3-5]. These microscopes achieved a spatial resolution of better than 1 micrometer. One of the simplest ways to obtain highly sensitive phase imaging with x-ray microscope optics is to use a Zernike phase plate [6-9]. However, it is impossible to obtain quantitative values of the phase shift by materials with this system. The combination of x-ray microscope optics and grating interferometry is promising for satisfying both requirements for quantitative measurement of phase shift and a spatial resolution better than 1 micrometer.

In grating interferometry, several interferograms are required for quantitative phase retrieval (fringe scan method) without losing spatial resolution. The minimum number of steps is three for the fringe scan when the carrier fringe profile is an ideal sine wave. As Momose et al. [10] discussed in their paper, the number of steps should be five for suppressing harmonics from a rectangular-shaped grating, because the carrier fringe profile becomes a triangle wave under highly coherent illumination. This means a five times longer time period is necessary for a phase image compared to a simple projection image. Such a long scan time could be a fatal problem for phase tomography because a drift or deformation of the specimen could occur during the scan.

By reducing the spatial coherence of the incident beam, the profile of the carrier fringe produced by a rectangular-shaped grating can be modified to a sine-like wave at the expense of the visibility of the carrier fringe. Therefore, the optimum illumination condition is important for this kind of phase imaging.

We have developed a phase contrast micro-imaging system using x-ray microscope optics with pseudo-Köhler illumination by sector-type condenser zone plates (CZPs) and a Talbot-type interferometer. In this study, we investigated the optimum conditions for phase contrast imaging; it has a minimum number of steps for the fringe scan and visibility as high as possible, which are achieved by controlling the coherence of the incident beam.

The 10th International Conference on X-ray MicroscopyAIP Conf. Proc. 1365, 309-312 (2011); doi: 10.1063/1.3625366

© 2011 American Institute of Physics 978-0-7354-0925-5/$30.00

309


METHODS AND RESULTS

The experiments were carried out at BL47XU in SPring-8. A schematic illustration is shown in Figure 1. The system consists of an “in-vacuum type” undulator, a Si (111) double-crystal monochromator cooled with liquid nitrogen, high-precision stages, x-ray optics for image magnification, and an x-ray image detector. The x-ray energy of the fundamental harmonic peak of the undulator was tuned to 8 keV. High-precision stages were made by Kohzu precision Co., Ltd. The accuracy of the translation stages is better than 1 m. The CZP and FZP parameters are summarized in Table 1. Details of the x-ray microscope optics are described elsewhere [4, 10].

FIGURE 1. Schematic illustration of x-ray phase contrast imaging system at BL47XU. The undulator, double-crystal monochromator, and a pair of vertical deflecting mirrors suppressing the higher harmonics are not shown.

TABLE 1. Parameters of CZP and FZP. Both zone plates are made of Tantalum on SiC membrane (2-m thickness). The ZPs were fabricated by means of the electron-beam

lithography technique at NTT-AT, Japan. CZP (sector type) FZP Number of zones 875 388 Diameter (m) 1000 155.038 Outermost zone width (nm) 200 100 Focal length at 8 keV (mm) 800* 100 Thickness of zones (m) 1.65 1.0 * This number shows the distance from the CZP to the sample, which has some uncertainty because of

the size of the beam stop in front of the CZP. The gratings were made by electron beam lithography at NTT-AT. The phase grating (G1) was designed for /2

modulation at 8 keV and the absorption grating (G2) was inclined at 60 degrees to increase the effective optical path length. The grating pitch is 5 μm (2.5-m line and space), and the materials of the gratings are 0.96-μm-thick and 4.5-μm-thick tantalum, respectively. The patterned area is 5 mm × 10 mm. The distance between G1 and G2 is 8 cm for 8 keV. The yaw and roll angles of G1 were adjusted to match those of G2. A piezo flexure stage (P-752.21C, Physik Instrumente GmbH, Germany) was used as a translation stage for fringe scans with G2.

The CZP consists of eight sectors of constant pitch gratings. Diffracted beams from these eight sectors are superposed at the object plane and have five discrete incident angles in the horizontal direction. They are 0 mrad, ± 0.10 mrad, and ± 0.19 mrad (Figure 2). When the full aperture of the CZP is used as shown in Figure 2(a)(1), angular components of illuminating beams consist of all of the above five angles at the object plane. As the horizontal aperture size of the CZP becomes narrower, the incident beams of the angular components of ± 0.19 mrad and ± 0.10 mrad are eliminated in turn. The spatial coherence increases correspondingly. As shown in Figure 2(a)(4), the opening aperture size is limited only to the region of center sectors whose horizontal angular component is 0 mrad; then an object is illuminated nearly coherently.

As shown in Figure 3, the measured visibilities were 60.2%, 65.3%, 68.3%, and 70.9% for the illumination conditions in Figure 2(a), respectively. The calculated visibilities were 54.2%, 54.3%, 60.4% and 68.2%, respectively. The dispersive angle by beam diffuser was about 10 μrad in full-width-at-half-maximum. It was expressed as a Gaussian in the calculation and convolved to a self image formed by G1 in the calculation. The difference between measured and calculated values may be due to the inadequate approximation of the beam profile;

310


however, the measured values have good agreement with calculated ones. Strong harmonics appear in Figure 2(4) which requires a five-step fringe scan for one phase image. However, such harmonics are not seen in Figures 3(1), 3(2), and 3(3), which enables us to obtain the phase image with a three-step fringe scan.

FIGURE 2. (a) Relation between CZP, beam stop, and cross slit. The vertical direction is fully opened in all conditions. (b) The far-field image for each illumination condition without grating. The contrast of each image is adjusted. (c) Interferogram for each illumination condition. Those fields of view correspond to blue boxes in (a) and (b). (d) 2D-FFT image of each interferogram and

its line profile in the horizontal direction. The contrast of each image is adjusted.

FIGURE 3. Measured and calculated profiles in four aperture conditions. The numbers corresponds to the numbers in Figure 2.

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A glass rod and gold ball were measured as a demonstration measurement of phase image (Figure 4). The gold ball was attached to the glass rod with glue. The diameter of the rod and the ball were about 5 m and 3.8 m, respectively. The measured amounts of phase shift were 1.86 rad and 10.6 rad in the image, whereas the calculated amounts were 1.39 rad and 6.90 rad. The glass rod is clearly seen in phase images, while it is not seen in the absorption image.

FIGURE 4. (a) Differential phase image of glass fiber and gold ball obtained under the illumination condition in Fig. 2(a)(3) by a three-step fringe scan. The field of view is 12.85 m 9.18 μm. (b) Line integral image of (a). (c) Absorption image at the

same position of (a) and (b). (d) Line profiles of yellow lines in (a), (b), and (c).

CONCLUSION

The optimal illumination condition for a phase imaging system using x-ray microscope optics and a grating interferometer was found. The visibility in the interferogram was measured and calculated. The results show that the illumination condition (aperture size of the slit before the CZP) obviously affects the coherence and visibility. In the high-coherence condition, the visibility becomes high and the wave profile of the carrier fringe shows a triangle-like shape, which requires a five-step (or more) fringe scan for precise phase retrieval. On the other hand, under the low-coherence condition, the visibility becomes rather low and the wave profile of the carrier fringe becomes a sine-like shape which requires at least a three-step fringe scan. The exposure time is longer in the high-coherence condition.

In tomography measurements we employ condition (3). A three-step fringe scan is approximately acceptable at the expense of a little visibility in this condition. With this condition, not only can the number of the shots be reduced to 3/5, but also the exposure time can be reduced because of a comparably large aperture.

ACKNOWLEDGMENTS

The authors thank Koki Aoyama and Tomoki Fukui for their technical support. The experiments were performed under the approval of SPring-8 committees 2007B1440 and 2008B1483.

REFERENCES

1. A. Momose, Nucl. Instrm. Methods A, 352, 622 (2005). 2. A. Momose et al., Jpn. J. Appl. Phys. 42, L866 (2003). 3. B. Lai et al., Rev. Sci. Instrum. 66, 2287 (1995) 4. K. Uesugi, A. Takeuchi and Y. Suzuki, Proc. SPIE 6318, 63181F (2006). 5. C. Rau et al., Proc. SPIE 6318, 63181G (2006). 6. Y. Kagoshima et al., J. Synchrotron Radiat. 9, 132 (2002) 7. J. C. Andrews et al., J. of Phys.: Conf. Ser. 186, 012002 (2009). 8. M. Stanpanoni et al., J. of Phys.: Conf. Ser. 186, 012018 (2009). 9. A. Takeuchi et al., J. Phys.: Conf. Ser. 186 012020 (2009). 10. A. Momose et al., Jpn. J. Appl. Phys. 45, 5254 (2006).

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