- Email: [email protected]
Towards nanotomography with asymmetrically cut crystals
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 551 (2005) 119–124 www.elsevier.com/locate/nima
Towards nanotomography with asymmetrically cut crystals Marco Stampanonia,, Gunther Borchertb, R. Abelaa a
Swiss Light Source, Paul Scherrer Institut, CH-5232, Villigen, Switzerland b FRM II, Technische Universita¨t Mu¨nchen, D-85747, Garching, Germany Available online 9 August 2005
Abstract The most used detection system in state-of-the-art synchrotron-based microtomography devices usually collects light produced by a thin scintillator and conveys it to charge coupled device (CCD) through a microscope optics. This detection system is intrinsically limited by scintillation properties, optical coupling and CCD granularity to a practical limit of about 1 mm spatial resolution and efficiency of a few percent. A novel detector, called Bragg magnifier, is a method recently proposed to efficiently exceed the micrometer barrier. It exploits two-dimensional asymmetric Bragg diffraction from flat crystals to produce X-ray images with magnification factors up to 150 150 and pixel sizes less than 100 100 nm2 . This work presents its functional principle, some applications and future developments. r 2005 Elsevier B.V. All rights reserved. PACS: 07.85.Qe; 87.59.Fm; 61.10.i; 87.59.Ls Keywords: X-ray imaging; Synchrotron microtomography; Asymmetric Bragg diffraction
1. Introduction The combination of X-ray microscopy with tomographic techniques as well as the exceptional properties of third-generation synchrotron radiation sources allow to obtain volumetric information of a specimen at micron or sub-micron scale with minimal sample preparation. Microtomographic investigations tend nowadays towards the analysis of millimeter-sized specimens at Corresponding author.
E-mail address: [email protected] (M. Stampanoni).
micrometer resolution within minutes. The requirements on the detectors in terms of spatial resolution and efficiency are therefore very high and tremendous efforts have been made all over the world. The most established detection method consists of converting X-rays into visible light with a scintillator and projecting them onto a charge coupled device (CCD) with the help of suitable optics [1–4]. It has been shown that the spatial resolution of the visible-light-based approach is intrinsically limited to about 1 mm by scintillation properties, optics efficiency and CCD granularity [5]. Different approaches have been proposed to efficiently exceed the micrometer barrier: Fresnel
0168-9002/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2005.07.046
ARTICLE IN PRESS 120
M. Stampanoni et al. / Nuclear Instruments and Methods in Physics Research A 551 (2005) 119–124
zone plates [6–9], parabolic compound refractive lenses [10] and Kirkpatrick–Baez setups [11] have demonstrated to reach submicrometer resolution with sufficient efficiency but with a limited field of view. This article describes a novel device, called Bragg magnifier, which achieves a breakthrough in this respect, satisfying the requirements for efficient imaging at submicron resolution at high energies. The novel instrumentation performs 2D magnification in the X-ray regime exploiting the well-known principle of asymmetrical Bragg diffraction from two crossed flat crystals: for the prototype described in this work, a pair of Si(2 2 0) crystals with an asymmetrical cut of 81 has been used. The integrated reflection power is higher than 90% for these energies, which means that almost no flux is lost when passing through the crystals. The magnifying optics collect the intensity distribution of the incoming beam, which contains absorption and phase information about the sample, and magnifies it, acting therefore as a full field microscope. The enlarged image can be converted to visible light in a much more efficient way (thicker scintillator) since the larger spread will be compensated by the X-ray magnification. As a consequence, the efficiency will be enhanced without deterring the spatial resolution.
2. Theoretical background For s-polarized X-rays generated by insertion devices of third generation synchrotron facilities, the width of the diffraction pattern for a nonabsorbing perfect crystal is given by oS ¼ ð2= sinð2yB Þ r0 l2 =pV jF h jeDW where r0 ¼ 2:818 1015 m is the classical electron radius, l is the wavelength of the incident radiation, V is the volume of the unit cell, yB is the Bragg angle, F h is the crystal structure factor and eDW is the Debye–Waller factor. Defining the magnification factor as m ¼ sinðyB þ aÞ= sinðyB aÞ where a is the asymmetry angle, i.e. the angle between the crystal surface and its lattice planes, simple geometrical arguments lead to Sd ¼ jmj Si , where S i and Sd are the spatial cross-sections of the incident and diffracted beam. Following p Liouffiffiffiffiffiffiffi ville’s theorem, it can be deduced that oi ¼ jmj
oS where oi and od are the angular divergences of the incident and diffracted beam, respectively. Quantitatively, dynamical theory states that oi ¼ pffiffiffiffiffiffiffi jmj oS and od ¼ ð1=jmjÞoi . It follows that if jmj41 the range of total reflection for the emergent beam is1=jmj times smaller than that of the incident beam, while its spatial cross-section is jmj times greater. If two subsequent asymmetrical diffractions with respect to two equal but perpendicular to each other lattice planes occur, we obtain a 2D enlargement of the incoming beam (2D ‘‘Fankuchen’’ effect), as depicted in Fig. 1. Si(2 2 0) crystals with an asymmetry angle of a ¼ 8 have been used as magnifying optics. The crystals have been cut with an angular accuracy of better than 1 arcminute and have been fixed by optical contacting to a precision glass support, the thermal expansion coefficient of which is similar to silicon. The glass support acts as mechanical interface between the silicon crystal and a steel support, which is in turn fixed to a double swivel, ensuring that no mechanical stress is applied to the crystal when the unit is screwed to an highresolution goniometer. Swivel’s pitch and roll accuracy of better than 200 as well as goniometer’s angular resolution of 0:0500 perfectly cope with the narrow rocking curves of Si(2 2 0), numerically obtained with the help of dynamical X-ray diffraction calculations [12,13] and summarized in Table 1. More extensive theoretical considerations have been described in Ref. [14] and more technical details concerning the manufacturing of the crystals, their characterization and the mechanics have been recently presented in Ref. [15]. The successful combination of the Bragg magnifier with tomographic techniques has been demonstrated in Ref. [16].
3. Results: operations with Bragg magnifier In this article we present the first results of two experiments recently performed at the Materials Science Beamline of the SLS. The beamline optics have been set to keep the beam divergence below 1000 for energies between 21 and 23 keV. The comparison with the relevant rocking curve,
ARTICLE IN PRESS M. Stampanoni et al. / Nuclear Instruments and Methods in Physics Research A 551 (2005) 119–124
121
Fig. 1. (a) Functional principle of the Bragg magnifier depicting both crystals, the object and the image (detector) planes. ‘‘i’’ describes the incident beam coordinates, ‘‘d’’ the diffracted one. The inset shows the principle of coplanar asymmetric Bragg diffraction: m is the magnification factor, yB is the Bragg angle and a is the asymmetry angle. and (b) experimental setup installed at the XTM station of the Materials Science beamline: visible are both crystals, mounted on their swivels, fixed to the high-resolution goniometers. The beam comes from the right. On the left side, the entrance of the 1:1 optic is also visible. Table 1 Relevant parameters for Si(2 2 0), a ¼ 8 , at different energies Energy (keV)
Bragg Angle (deg.)
Magn.
o00s
o00i
o00d
21.10 21.75 22.10 22.46 22.75
8.801 8.536 8.400 8.265 8.159
20 30 40 60 100
1.90 1.84 1.82 1.78 1.76
8.63 10.15 11.52 13.89 17.66
0.42 0.34 0.29 0.23 0.18
os , oi , od are the FWHM of the rocking curves for the symmetric case, and for the impinging and reflected beam of the asymmetrical case, respectively.
suggests that the first Si(2 2 0) of the Bragg magnifier will accept more than 95% of the impinging intensity. 3.1. Edge-enhanced imaging In a previous experiment [15] a gold mesh with nominal aperture of 11 mm and wire diameter of 5 mm was imaged while tuning the energy from 21.1 up to 22.6 keV, producing magnification factors of 20 20 up to 80 80, corresponding
ARTICLE IN PRESS 122
M. Stampanoni et al. / Nuclear Instruments and Methods in Physics Research A 551 (2005) 119–124
to pixel sizes of 700 700 nm2 down to 175 175 nm2 . Fresnel’s diffraction effects were clearly observed, but no quantitative consideration could be grasped from that data. The aim of a more refined experiment presented here, was to experimentally demonstrate the theoretical suggestion of Spal [17], i.e. the Bragg magnifier optics can form magnified in-line near-field holograms at submicrometer resolution, with high magnification and high efficiency. For this purpose, a boron fiber of 100 mm outer diameter and an inner tungsten core of 14 mm has been imaged at an energy of 22.64 keV, corresponding to a magnification factor of 60 60 and a pixel size of 235 235 nm2 . A flat- and dark-field corrected image is illustrated in Fig. 2(a). On a line profile, at least four fringes are visible, see Fig. 2(b), and this means that the optics preserve the phase information carried by the beam, i.e. can produce phase contrast images of good quality. In a second step, we recorded a tomogram of the fiber, see Fig. 2(c). It appears clear that the ‘‘edgeenhancement’’ is quite strong and the geometrical information of the sample is partially washed out, see the line profile showed in Fig. 2(d). On the other hand, however, Bronnikov [18] recently
Fig. 2. (a) and (b) X-ray projection (and corresponding line profile) of a boron fiber with an outer diameter of 100 mm and an inner tungsten core of 14 mm. Dark gray levels correspond to low intensity, i.e. high absorption. (c) and (d) Tomographic reconstruction (and corresponding line profile) of the same fiber.
presented a reconstruction algorithm that reconstructs the refractive index of a pure phase object directly, without the necessity of acquiring several projections at different sample-to-detector distances. Since the Bragg magnifier has been designed to work at energies above 20 keV, and since, at these energies, soft tissue (for instance, cartilage or single cells) can be assumed to be a pure phase object, the boundary conditions for Bronnikov’s algorithm seems to be satisfied. We expect therefore to be able to fully verify the predictions of Spal and to generate phase contrast tomograms with the Bragg magnifier. 3.2. Bragg magnifier and pixel detector In a second experiment, the 1:1 X-ray-to-visiblelight optics of the Bragg magnifier has been replaced by a module of the PILATUS pixel detector [19]. The module features 366 157 pixels with 217 217 mm2 pitch, offering an active area of 79 34 mm2 and performing direct X-rays detection. Because of the single-photon counting nature of the device, the sensitivity of the whole system increased dramatically. Thank to that, the PILATUS module easily detected the X-rays transmitted through the optics of the Bragg magnifier with exposure time of a few ms, even at magnification factors of 100 100 or more. Fig. 3 shows the experimental setup as well as some results. The module of the PILATUS detector is clearly visible on the left side, where it substitutes the 1:1 optic of the Bragg magnifier, see as comparison Fig. 1(b). The sensitive area is protected by an aluminum plate and is not visible in this figure. A gold mesh with nominal aperture of 11 mm and wire diameter of 5 mm has been used as a test sample. Tuning the energy from 22.75 up to 23.02 keV resulted in a change of the magnification factor from 100 100 up to 250 250. Considering the pitch of the pixel detector, this yields a theoretical pixel size ranging from 2:17 2:17 mm2 down to 0:87 0:87 mm2 . The mesh pattern can be easily identified. It has to be considered that the Si(2 2 0) crystal pair has not been designed for being operated at this extreme magnifications. When running at 250 250 the incoming angle is only 0.06221: any surface
ARTICLE IN PRESS M. Stampanoni et al. / Nuclear Instruments and Methods in Physics Research A 551 (2005) 119–124
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crystals. For this prototype, 160 mm long crystals have been used, in order to magnify 1 1 mm2 at least 40 40. Upcoming experiments will try to fully determine the Optical Transfer Function of the system, i.e. to quantitatively characterize the imaging characteristic of the device. Future developments could consider the design of ad hoc crystals pairs for a given energy or field of view.
Acknowledgements
Fig. 3. (a) Experimental setup at the Materials Science beamline: the PILATUS detector sits where usually a 1:1 X-ray-tovisible light converter is. X-rays emerging from the optics are directly detected. (b–d) Gold mesh (nominal aperture 11 mm, wire diameter of 5 mm) imaged at different magnifications.
imperfection, for instance a scratch, is of course dramatically reproduced in the magnified image, see Figs. 3(c) and (d).
4. Discussion and outlook The first prototype of the Bragg magnifier has been successfully operated and produced highresolution, aberration free X-ray microradiographs. For the first time the Bragg magnifier has been tested in conjunction with the in-house developed PILATUS pixel detector, demonstrating the feasibility of the approach. It was striking to observe the extreme sensitivity of the combined devices and the high quality of the images. Reliability, reproducibility and stability of the Xray optics have been showed to be excellent [14] and high-resolution tomograms have been recorded. It appears clear however that the optics produce strong edge-enhanced images affecting therefore the final spatial resolution. The effect is well pronounced due to the relatively large distance between the sample and the scintillator mounted on the 1:1 optic (350 mm). This large distance is itself determined by the size of the
The authors thank G. Hu¨elsen, E. Eikenberry and C. Bro¨nimann of PSI for the setup of the PILATUS detector and data analysis. Authors are also indebted to D. Meister and M. Lange, also from PSI, for designing and manufacturing important mechanical parts.
References [1] U. Bonse, F. Busch, Progress Biophys. Molec. Biol. 65 (1–2) (1996) 133. [2] R. Lee, B. Lai, W. Yun, D.C. Mancini, Z. Cai, X-ray microtomography as a fast three-dimensional imaging technology using a CCD camera coupled with a CdWO4 single-crystal scintillator, in: U. Bonse (Ed.), Developments in X-ray Tomography, Proceedings of SPIE, vol. 3149, 1997, p. 257. [3] B.A. Dowd, G. Campbell, R. Marr, V. Nagarkar, S. Tipnis, L. Axe, D. Siddons, Developments in synchrotron X-ray computed microtomography at the National Synchrotron Light Source, in: U. Bonse (Ed.), Developments in X-ray Tomography II, Proceedings of SPIE, vol. 3772, 1999, p. 224. [4] T. Weitkamp, C. Raven, A. Snigierv, An imaging and microtomography facility at the ESRF beamline ID 22, in: U. Bonse (Ed.), Developments in X-ray Tomography II, Proceedings of SPIE, vol. 3772, 1999, p. 311. [5] A. Koch, K. Raven, P. Spanne, A. Snigirev, J. Opt. Soc. Am. A 15 (7) (1998) 1940. [6] G. Schneider, D. Hambach, B. Niemann, B. Kaulich, J. Susini, N. Hoffmann, W. Hasse, Appl. Phys. Lett. 78 (13) (2001) 1936. [7] D. Weiss, G. Schneider, B. Niemann, P. Guttmann, D. Rudolph, G. Schmal, Ultramicroscopy 84 (3–4) (2000) 185. [8] B. Lai, W. Yun, Y. Xiao, D. Legnini, Z. Cai, A. Krasnoperova, F. Cerrina, E.D. Fabrizio, L. Grella, M. Gentili, Rev. Sci. Instrum. 66 (1995) 2287. [9] B. Lai, K.M. Kemmer, J. Maser, M.A. Schneegurt, Z. Cai, P.P. Ilinski, C.F. Kulpa, D.G. Legnini, K.H. Nealson, S.T.
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[10]
[11]
[12] [13]
M. Stampanoni et al. / Nuclear Instruments and Methods in Physics Research A 551 (2005) 119–124 Pratt, W. Rodigues, M.L. Tischler, W. Yun, Highresolution X-ray imaging for microbiology at the advanced photon source, AIP-Conference-Proceedings, vol. 506, 2000, pp. 585–589. C. Schroer, J. Meyer, M. Kuhlmann, B. Benner, T. Gu¨nzler, B. Lengeler, C. Rau, T. Weitkamp, A. Snigirev, I. Snigireva, Appl. Phys. Lett. 81 (8) (2002) 1527. O. Hignette, G. Rostaing, P. Cloetens, A. Rommeveaux, W. Ludwig, A. Freund, Submicron focusing of hard X-rays with reflecting surfaces at the ESRF, Proceedings of SPIE, vol. 4499, 2001, pp. 105–116. A. Authier, Dynamical Theory of X-ray Diffraction, Oxford University Press, New York, 2001. S. Brennan, P. Cowan, Rev. Sci. Instrum. 63 (1992) 850.
[14] M. Stampanoni, G.L. Borchert, R. Abela, P. Ru¨egsegger, J. Appl. Phys. 92 (12) (2002) 7630. [15] M. Stampanoni, G.L. Borchert, R. Abela, Two-dimensional asymmetrical Bragg diffraction for submicrometer computer tomography, in: T. Ishikawa, A.T. Macrander, J.L. Wood (Eds.), Crystals, Multilayers and Other Synchrotron Optics, Proceedings of SPIE, vol. 5195, 2003, pp. 54–62. [16] M. Stampanoni, G.L. Borchert, R. Abela, P. Ru¨egsegger, Appl. Phys. Lett. 82 (17) (2003) 2922. [17] R.D. Spal, Phys. Rev. Lett. 86 (14) (2001) 3044. [18] A.V. Bronnikov, J. Opt. Soc. Am. A 19 (3) (2002) 472. [19] E.F. Eikenberry, C. Bro¨nimann, G. Hu¨lsen, H. Toyokawa, R. Horisberger, B. Schmitt, C. Schulze-Briese, T. Tomizaki, Nucl. Instr. and Meth. 501 (2003) 260.
Nuclear Instruments and Methods in Physics Research A 551 (2005) 119–124 www.elsevier.com/locate/nima
Towards nanotomography with asymmetrically cut crystals Marco Stampanonia,, Gunther Borchertb, R. Abelaa a
Swiss Light Source, Paul Scherrer Institut, CH-5232, Villigen, Switzerland b FRM II, Technische Universita¨t Mu¨nchen, D-85747, Garching, Germany Available online 9 August 2005
Abstract The most used detection system in state-of-the-art synchrotron-based microtomography devices usually collects light produced by a thin scintillator and conveys it to charge coupled device (CCD) through a microscope optics. This detection system is intrinsically limited by scintillation properties, optical coupling and CCD granularity to a practical limit of about 1 mm spatial resolution and efficiency of a few percent. A novel detector, called Bragg magnifier, is a method recently proposed to efficiently exceed the micrometer barrier. It exploits two-dimensional asymmetric Bragg diffraction from flat crystals to produce X-ray images with magnification factors up to 150 150 and pixel sizes less than 100 100 nm2 . This work presents its functional principle, some applications and future developments. r 2005 Elsevier B.V. All rights reserved. PACS: 07.85.Qe; 87.59.Fm; 61.10.i; 87.59.Ls Keywords: X-ray imaging; Synchrotron microtomography; Asymmetric Bragg diffraction
1. Introduction The combination of X-ray microscopy with tomographic techniques as well as the exceptional properties of third-generation synchrotron radiation sources allow to obtain volumetric information of a specimen at micron or sub-micron scale with minimal sample preparation. Microtomographic investigations tend nowadays towards the analysis of millimeter-sized specimens at Corresponding author.
E-mail address: [email protected] (M. Stampanoni).
micrometer resolution within minutes. The requirements on the detectors in terms of spatial resolution and efficiency are therefore very high and tremendous efforts have been made all over the world. The most established detection method consists of converting X-rays into visible light with a scintillator and projecting them onto a charge coupled device (CCD) with the help of suitable optics [1–4]. It has been shown that the spatial resolution of the visible-light-based approach is intrinsically limited to about 1 mm by scintillation properties, optics efficiency and CCD granularity [5]. Different approaches have been proposed to efficiently exceed the micrometer barrier: Fresnel
0168-9002/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2005.07.046
ARTICLE IN PRESS 120
M. Stampanoni et al. / Nuclear Instruments and Methods in Physics Research A 551 (2005) 119–124
zone plates [6–9], parabolic compound refractive lenses [10] and Kirkpatrick–Baez setups [11] have demonstrated to reach submicrometer resolution with sufficient efficiency but with a limited field of view. This article describes a novel device, called Bragg magnifier, which achieves a breakthrough in this respect, satisfying the requirements for efficient imaging at submicron resolution at high energies. The novel instrumentation performs 2D magnification in the X-ray regime exploiting the well-known principle of asymmetrical Bragg diffraction from two crossed flat crystals: for the prototype described in this work, a pair of Si(2 2 0) crystals with an asymmetrical cut of 81 has been used. The integrated reflection power is higher than 90% for these energies, which means that almost no flux is lost when passing through the crystals. The magnifying optics collect the intensity distribution of the incoming beam, which contains absorption and phase information about the sample, and magnifies it, acting therefore as a full field microscope. The enlarged image can be converted to visible light in a much more efficient way (thicker scintillator) since the larger spread will be compensated by the X-ray magnification. As a consequence, the efficiency will be enhanced without deterring the spatial resolution.
2. Theoretical background For s-polarized X-rays generated by insertion devices of third generation synchrotron facilities, the width of the diffraction pattern for a nonabsorbing perfect crystal is given by oS ¼ ð2= sinð2yB Þ r0 l2 =pV jF h jeDW where r0 ¼ 2:818 1015 m is the classical electron radius, l is the wavelength of the incident radiation, V is the volume of the unit cell, yB is the Bragg angle, F h is the crystal structure factor and eDW is the Debye–Waller factor. Defining the magnification factor as m ¼ sinðyB þ aÞ= sinðyB aÞ where a is the asymmetry angle, i.e. the angle between the crystal surface and its lattice planes, simple geometrical arguments lead to Sd ¼ jmj Si , where S i and Sd are the spatial cross-sections of the incident and diffracted beam. Following p Liouffiffiffiffiffiffiffi ville’s theorem, it can be deduced that oi ¼ jmj
oS where oi and od are the angular divergences of the incident and diffracted beam, respectively. Quantitatively, dynamical theory states that oi ¼ pffiffiffiffiffiffiffi jmj oS and od ¼ ð1=jmjÞoi . It follows that if jmj41 the range of total reflection for the emergent beam is1=jmj times smaller than that of the incident beam, while its spatial cross-section is jmj times greater. If two subsequent asymmetrical diffractions with respect to two equal but perpendicular to each other lattice planes occur, we obtain a 2D enlargement of the incoming beam (2D ‘‘Fankuchen’’ effect), as depicted in Fig. 1. Si(2 2 0) crystals with an asymmetry angle of a ¼ 8 have been used as magnifying optics. The crystals have been cut with an angular accuracy of better than 1 arcminute and have been fixed by optical contacting to a precision glass support, the thermal expansion coefficient of which is similar to silicon. The glass support acts as mechanical interface between the silicon crystal and a steel support, which is in turn fixed to a double swivel, ensuring that no mechanical stress is applied to the crystal when the unit is screwed to an highresolution goniometer. Swivel’s pitch and roll accuracy of better than 200 as well as goniometer’s angular resolution of 0:0500 perfectly cope with the narrow rocking curves of Si(2 2 0), numerically obtained with the help of dynamical X-ray diffraction calculations [12,13] and summarized in Table 1. More extensive theoretical considerations have been described in Ref. [14] and more technical details concerning the manufacturing of the crystals, their characterization and the mechanics have been recently presented in Ref. [15]. The successful combination of the Bragg magnifier with tomographic techniques has been demonstrated in Ref. [16].
3. Results: operations with Bragg magnifier In this article we present the first results of two experiments recently performed at the Materials Science Beamline of the SLS. The beamline optics have been set to keep the beam divergence below 1000 for energies between 21 and 23 keV. The comparison with the relevant rocking curve,
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Fig. 1. (a) Functional principle of the Bragg magnifier depicting both crystals, the object and the image (detector) planes. ‘‘i’’ describes the incident beam coordinates, ‘‘d’’ the diffracted one. The inset shows the principle of coplanar asymmetric Bragg diffraction: m is the magnification factor, yB is the Bragg angle and a is the asymmetry angle. and (b) experimental setup installed at the XTM station of the Materials Science beamline: visible are both crystals, mounted on their swivels, fixed to the high-resolution goniometers. The beam comes from the right. On the left side, the entrance of the 1:1 optic is also visible. Table 1 Relevant parameters for Si(2 2 0), a ¼ 8 , at different energies Energy (keV)
Bragg Angle (deg.)
Magn.
o00s
o00i
o00d
21.10 21.75 22.10 22.46 22.75
8.801 8.536 8.400 8.265 8.159
20 30 40 60 100
1.90 1.84 1.82 1.78 1.76
8.63 10.15 11.52 13.89 17.66
0.42 0.34 0.29 0.23 0.18
os , oi , od are the FWHM of the rocking curves for the symmetric case, and for the impinging and reflected beam of the asymmetrical case, respectively.
suggests that the first Si(2 2 0) of the Bragg magnifier will accept more than 95% of the impinging intensity. 3.1. Edge-enhanced imaging In a previous experiment [15] a gold mesh with nominal aperture of 11 mm and wire diameter of 5 mm was imaged while tuning the energy from 21.1 up to 22.6 keV, producing magnification factors of 20 20 up to 80 80, corresponding
ARTICLE IN PRESS 122
M. Stampanoni et al. / Nuclear Instruments and Methods in Physics Research A 551 (2005) 119–124
to pixel sizes of 700 700 nm2 down to 175 175 nm2 . Fresnel’s diffraction effects were clearly observed, but no quantitative consideration could be grasped from that data. The aim of a more refined experiment presented here, was to experimentally demonstrate the theoretical suggestion of Spal [17], i.e. the Bragg magnifier optics can form magnified in-line near-field holograms at submicrometer resolution, with high magnification and high efficiency. For this purpose, a boron fiber of 100 mm outer diameter and an inner tungsten core of 14 mm has been imaged at an energy of 22.64 keV, corresponding to a magnification factor of 60 60 and a pixel size of 235 235 nm2 . A flat- and dark-field corrected image is illustrated in Fig. 2(a). On a line profile, at least four fringes are visible, see Fig. 2(b), and this means that the optics preserve the phase information carried by the beam, i.e. can produce phase contrast images of good quality. In a second step, we recorded a tomogram of the fiber, see Fig. 2(c). It appears clear that the ‘‘edgeenhancement’’ is quite strong and the geometrical information of the sample is partially washed out, see the line profile showed in Fig. 2(d). On the other hand, however, Bronnikov [18] recently
Fig. 2. (a) and (b) X-ray projection (and corresponding line profile) of a boron fiber with an outer diameter of 100 mm and an inner tungsten core of 14 mm. Dark gray levels correspond to low intensity, i.e. high absorption. (c) and (d) Tomographic reconstruction (and corresponding line profile) of the same fiber.
presented a reconstruction algorithm that reconstructs the refractive index of a pure phase object directly, without the necessity of acquiring several projections at different sample-to-detector distances. Since the Bragg magnifier has been designed to work at energies above 20 keV, and since, at these energies, soft tissue (for instance, cartilage or single cells) can be assumed to be a pure phase object, the boundary conditions for Bronnikov’s algorithm seems to be satisfied. We expect therefore to be able to fully verify the predictions of Spal and to generate phase contrast tomograms with the Bragg magnifier. 3.2. Bragg magnifier and pixel detector In a second experiment, the 1:1 X-ray-to-visiblelight optics of the Bragg magnifier has been replaced by a module of the PILATUS pixel detector [19]. The module features 366 157 pixels with 217 217 mm2 pitch, offering an active area of 79 34 mm2 and performing direct X-rays detection. Because of the single-photon counting nature of the device, the sensitivity of the whole system increased dramatically. Thank to that, the PILATUS module easily detected the X-rays transmitted through the optics of the Bragg magnifier with exposure time of a few ms, even at magnification factors of 100 100 or more. Fig. 3 shows the experimental setup as well as some results. The module of the PILATUS detector is clearly visible on the left side, where it substitutes the 1:1 optic of the Bragg magnifier, see as comparison Fig. 1(b). The sensitive area is protected by an aluminum plate and is not visible in this figure. A gold mesh with nominal aperture of 11 mm and wire diameter of 5 mm has been used as a test sample. Tuning the energy from 22.75 up to 23.02 keV resulted in a change of the magnification factor from 100 100 up to 250 250. Considering the pitch of the pixel detector, this yields a theoretical pixel size ranging from 2:17 2:17 mm2 down to 0:87 0:87 mm2 . The mesh pattern can be easily identified. It has to be considered that the Si(2 2 0) crystal pair has not been designed for being operated at this extreme magnifications. When running at 250 250 the incoming angle is only 0.06221: any surface
ARTICLE IN PRESS M. Stampanoni et al. / Nuclear Instruments and Methods in Physics Research A 551 (2005) 119–124
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crystals. For this prototype, 160 mm long crystals have been used, in order to magnify 1 1 mm2 at least 40 40. Upcoming experiments will try to fully determine the Optical Transfer Function of the system, i.e. to quantitatively characterize the imaging characteristic of the device. Future developments could consider the design of ad hoc crystals pairs for a given energy or field of view.
Acknowledgements
Fig. 3. (a) Experimental setup at the Materials Science beamline: the PILATUS detector sits where usually a 1:1 X-ray-tovisible light converter is. X-rays emerging from the optics are directly detected. (b–d) Gold mesh (nominal aperture 11 mm, wire diameter of 5 mm) imaged at different magnifications.
imperfection, for instance a scratch, is of course dramatically reproduced in the magnified image, see Figs. 3(c) and (d).
4. Discussion and outlook The first prototype of the Bragg magnifier has been successfully operated and produced highresolution, aberration free X-ray microradiographs. For the first time the Bragg magnifier has been tested in conjunction with the in-house developed PILATUS pixel detector, demonstrating the feasibility of the approach. It was striking to observe the extreme sensitivity of the combined devices and the high quality of the images. Reliability, reproducibility and stability of the Xray optics have been showed to be excellent [14] and high-resolution tomograms have been recorded. It appears clear however that the optics produce strong edge-enhanced images affecting therefore the final spatial resolution. The effect is well pronounced due to the relatively large distance between the sample and the scintillator mounted on the 1:1 optic (350 mm). This large distance is itself determined by the size of the
The authors thank G. Hu¨elsen, E. Eikenberry and C. Bro¨nimann of PSI for the setup of the PILATUS detector and data analysis. Authors are also indebted to D. Meister and M. Lange, also from PSI, for designing and manufacturing important mechanical parts.
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