Setup of an 8 keV laboratory transmission x-ray
microscope
S. Baumbach1, B. Kanngießer2, W. Malzer2, H. Stiel3,
S. Bjeoumikhova4and T. Wilhein 1
1Institute for X-Optics, RheinAhrCampus Remagen, University of Applied Sciences Koblenz,
Joseph-Rovan-Allee 2, 53424 Remagen, Germany
2Berlin Laboratory for innovative X-ray Technologies (BliX), Institute for Optics and Atomic
Physics, Technical University of Berlin, Hardenbergstrasse 36, 10623 Berlin, Germany
3Berlin Laboratory for innovative X-ray Technologies (BliX), Max-Born-Institut,
Max-Born-Strasse 2A, 12489 Berlin, Germany
4IFG, Institute for Scientific Instruments GmbH, Rudower Chausse 29/31, 12489 Berlin,
Germany
E-mail: [email protected]
Abstract. This article presents a concept and the first results for the setup of an 8 keV
laboratory transmission x-ray microscope with a polycapillary optic as condenser at the BliX in
Berlin. The incentive of building such a microscope is that the penetration depth for hard x-rays
is much higher than in the soft x-ray range, e.g. the water window. Therefore, it is possible to
investigate even dense materials such as metal compounds, bones or geological samples. The
future aim is to achieve a spatial resolution better than 200 nm.
1. Introduction
Full field laboratory hard x-ray microscopes have been developed in the last decade and the
achieved resolution is below 40 nm [1, 2]. One of the challenges of setting up a hard x-ray
transmission microscope in the lab is the long focal length of several millimeters of the objective
lens, which leads to very long image distances to get a high detector resolution. Additionally,
in order to get a high photon flux in the image plane, the numerical apertures of the condenser
and the objective lens should match. This leads to further implications for the design of the
condenser lens as well as for a suitable detector system with proper spatial resolution to avoid
long image distances. In this article we are showing the first results obtained with a polycapillary
optic as condender in our laboratory hard x-ray transmission microscope.
2. Experimental Setup
The experiment setup, see figure 1, consists of a copper x-ray tube (A), a polycapillary optic
(B), the object within a pinhole (C), a zone plate as objective lens and central stop (D) and a
x-ray CCD (E). In the following the different components are described.
2.1. X-ray source
The x-ray source of the laboratory x-ray microscope is a micro focus tube. The tube has a
copper target to get the characteristic line emission at 8.05 keV and a source size of about 50
µ
m
22nd International Congress on X-Ray Optics and Microanalysis IOP Publishing
Journal of Physics: Conference Series 499 (2014) 012005 doi:10.1088/1742-6596/499/1/012005
Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution
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Figure 1. Schematic setup of the transmission microscope.
(FWHM)[3]. Reducing radiation damages to the CCD the maximum acceleration voltage of the
tube has been set to 30 kV.
2.2. Condenser
A polycapillary is used as condenser. This optic consists of very thin and bowed hollow glas
capillaries acting as wave guide based on total external reflection [4]. All capillaries are pointing
to the focal spot resulting in 50
µ
m spot size (FWHM). A proper condenser should have a high
acceptance angle at the entry, a divergence angle at the exit of the capillary matched to the
numerical aperture of the zone plate and a high efficiency. In practice, not all conditions can
be realized in one device and one has to make a compromise. In our setup the aperture of the
condenser optic does not match to the zone plate aperture, i.e. light collected by the condenser
does not completely contribute to image formation. But due to the focusing properties of
the polycapillary, an increase of the intensity in the object plane is reached and the object is
illuminated under different angles. In figure 3 one can see the illumination cone of the capillary
optic in a distance of 405 mm to the focal spot.
Figure 2. Schematic drawing of
a polycapillary optic. Image taken
from [4].
Table 1. Parameters of the
polycapillary optic. Values are
denoted in mm.
f1Din Lf2Dout
62.3 2.2 45.4 18 1.2
2.3. Objective lens and central stop
A tungsten zone plate with a focal length of about 32.5 mm at 8.05 keV and an outermost zone
width of 50 nm is used as objective lens. The zone plate produces a magnified image of the
illuminated object on a CCD with 20
µ
m·20
µ
m pixel size. The central stop (CS) is mounted
in front of the zone plate and the illumination cone of the polycapillary forms a shadow of the
CS on the CCD preventing background illumination. In this shadow the magnified image of the
object is formed by the objective. Ideally, the position of the CS is at the exit of the condenser
optic. But due to the beam divergency of each capillary a CS on a polycapillary would not form
a sharp shadow on the CCD.
22nd International Congress on X-Ray Optics and Microanalysis IOP Publishing
Journal of Physics: Conference Series 499 (2014) 012005 doi:10.1088/1742-6596/499/1/012005
2
3. First results
In the first steps a 30
µ
m in diameter Pt/Ir-pinhole was used as an object. The resulting image
with 108-fold magnification is shown in figure 4. An aluminium aperture with 4 mm in diameter
in front of the detector blocks all the light around the image area.
To get a first valuation of the achievable spatial resolution a gold mesh with 5.5
µ
m bar width
and 10.5
µ
m spaces was used. The resulting 143-fold image is shown in figure 5. For a 143-fold
magnification and a pixel size of 20
µ
m the resulting effecitve pixel size is 140 nm and therefore
the Shannon-Nyquist-theorem gives a maximum spatial resolution of 4 ·140 nm = 560 nm. The
corresponding line plot through the edge of a mesh bar is shown in figure 6 and the measurement
from 10% to 90% edge profile gives an estimated spatial resolution of about 580 nm, i.e. the
resolution of the system is mainly detector limited.
Figure 3. Image of the polycapillary
illumination cone. Focal spot to CCD
distance is 405 mm.
Figure 4. Image of a 30
µ
m in diameter
Pt/Ir-pinhole. 108-fold magnification.
Figure 5. Image of a gold mesh with 1500
line pairs per inch. 143-fold magnification.
Figure 6. Line plot through the egde of a
mesh bar, (black box in figure 5).
4. Summary and Outlook
We have shown that a polycapillary optic can be used as an x-ray condenser in a laboratory
8 keV x-ray transmission microscope. The resolution of the current system is in the sub micro
22nd International Congress on X-Ray Optics and Microanalysis IOP Publishing
Journal of Physics: Conference Series 499 (2014) 012005 doi:10.1088/1742-6596/499/1/012005
3
meter region and is still detector limited. A better detector resolution would be achieved with a
higher magnification, respectively a longer image distance. But a detector system with smaller
pixel sizes, e.g. an image intensifier and a phosphor screen with grain sizes about 2
µ
m would
be the proper choice. In future it is planned to optimize the condenser optic in order to increase
the photon flux in the object plane and to get a more adapted output angle to the zone plate.
With the described improvements a resolution below 200 nm would be feasible.
Acknowledgements
A special thanks to Mr. Pambos Charalambous (www.ZonePlates.com) for the fabrication of
the zone plates.
References
[1] A Tkachuk, M Feser, et al 2006 Proc. of SPIE Vol. 6318 Developments in X-Ray Tomography V
[2] Y S Chu,J M Yi,et al 2008 Applied Physics Letters 92, 103119
[3] IFG 2012 X-ray source and power supply manual IFG Institue for Scientific Instuments GmbH Rudower
Chaussee 29/31, 12489 Berlin, Germany
[4] A Bjeoumikhov, S Bjeoumikhova, R Wedell 2006 Part. Part. Syst. Charact., Vol. 22 p. 384-390
[5] R Goergl, P Wobrauschek, Ch Streli, H Aiginger, M Benedikt 1995 X-Ray Spectrometry, Vol. 24, 157-162
22nd International Congress on X-Ray Optics and Microanalysis IOP Publishing
Journal of Physics: Conference Series 499 (2014) 012005 doi:10.1088/1742-6596/499/1/012005
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