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Copyright (C) JCPDS-International Centre for Diffraction Data 1999
Characterization of x-ray optic performance
Jerome Gormley, Terrence Jach, Eric Steel, and *Qi-Fan Xiao
National Institute of Standards and Technology, Gaithersburg, MD 20899
*X-Ray Optical Systems, Inc., 90 Fuller Road, Albany, NY 12205
ABSTRACT
This paper reports the characterization of a high-efficiency polycapillary lens designed for
microanalysis from a point source. The lens is unique in that it has been optimized for use over a broad
energy range from 8 keV (Cu &) to 17.5 keV (MO I&). A n x-ray optical bench, which provides both
high resolution imaging and energy resolving capabilities, was used to test the lens. Images of the
focusing characteristics and transmission quality of the lens were obtained using a magnifying x-ray
camera, and the focal-spot size and overall convergence were determined from these images. The focal
spot size was also measured using a knife-edge technique, and the results compared with those obtained
using the CCD camera. Measurements of the x-ray intensity with and without the lens were used to
determine the x-ray flux gain at various energies.
INTRODUCTION
Curved glass capillaries, with a channel diameter of a few micrometers, can efficiently deflect xrays over a wide energy band through large angles using multiple grazing-incidence reflections [ 1,2].
The polycapillary optic used here consists of tens of thousands of fused capillaries that are shaped such
that all capillaries point toward a common focal point at each end. This optic collects x-rays from a point
source over a large solid angle and forms an intense, focused beam at the output.
An optical bench system was used to align the source, optic, and detector [3]. The x-ray source
was a 50 W microfocus spot tube (250 pm spot diameter) with a molybdenum target. Both the
molybdenum characteristic radiation and the bremsstrahlung continuum radiation were used to
characterize the optic. One of the detectors used was a magnifying CCD imager, which uses a 5 pm thick
layer of Gd202S phosphor sputtered onto a magnifying fiber-optic plate to achieve a spatial resolution of
12 pm. The other detector was a Peltier-cooled Si(Li) detector used to obtain energy-dependent
information.
The purpose of this work was to characterize the performance of a specific optic suitable for
micro-XRF with a tabletop x-ray source. Using a Monte Carlo ray-tracing program, the optic was
designed to achieve the maximum x-ray density at the focus for the integrated energy range of S17.5 keV. The optic performance parameters examined here are the focal spot size (and its variation with
energy), overall beam divergence, and the flux gain.
The variation of these parameters with x-ray energy can be predicted from basic physics. The
focal spot size is expected to become smaller at higher x-ray energies. This occurs because the critical
angle for reflection inside the capillaries gets smaller, leading to less divergence of the transmitted beam.
As divergence decreases, the optic output intensity will be distributed over a smaller area in the focal
plane. On the other hand, the reduction in the critical angle as energy increases reduces the acceptance
angle at the optic input which leads to decreasing flux gain.
EXPERIMENTAL
RESULTS
Figure 1 shows several images recorded at various distances from the output of the optic.
Focusing of the x-ray flux is clear in these images. The conic half-angle of the full-width-at-tenth-
This document was presented at the Denver X-ray
Conference (DXC) on Applications of X-ray Analysis.
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b
Copyright (C) JCPDS-International Centre for Diffraction Data 1999
Figure 1: CCD images of the optic output at varying distances from the end window. Optic is illuminated with a MO spot tube
at 40 kV and varying current (to prevent saturation). Note the presence of a non-transmitting bundle as seen in the lower center
of image (a). Marker represents 120 lrn in length.
maximum (FWTM) of the images was found to be (2.2 f 0.1)“. The focal spot size calculated from
Figure l(e) is (107 f 8) pm (FWTM), but this is a weighted-average because of the CCD energy
response function. The CCD response function is broad and peaks near 8 keV but has significant
response below 8 keV. The focal spot size measured using a knife-edge scan and a Si(Li) detector was
(55 f 1) pm (FWTM) at 17.5 keV and (100 & 2) pm (FWTM) at 8 keV. The larger CCD-measured focal
spot size is a result of the low-energy weighting of the CCD response function.
The profile of the focal spot intensity is important to determine the spatial extent and relative
intensity of target illumination. For an ideal Gaussian distribution, the ratio of the FWHMIFWTM is
0.549. The CCD-recorded focal spot in Figure l(e) has a (52 -f 4) urn full-width-at-half-maximum
(FWHM), giving it a ratio of FWHM/FWTM = (0.486 f 0.074). The ratio of FWHM/FWTM from the
knife-edge experiments was (0.452 ZJX
0.001) at 8 keV and (0.447 +Z0.001) at 17.5 keV. These
measurements show that the focal spot profile has tails that are about 15% to 25% larger than a Gaussian.
The x-ray flux gain is defined here as the ratio of the x-ray flux at the optic output focal spot (the
sample location when using the optic) to the direct flux within the same area at the optic input face (this is
where a sample could be located without the optic). For the optic used here the output intensity was a
maximum when the optic input face was 64 mm from the x-ray point source.
To measure the x-ray flux gain of the optic as a function of energy, experiments were conducted
in which the optic was aligned with the source, and the Si(Li) detector was used to record the spectrum
observed at the focal point. A 100 urn aperture was used at the focal point to minimize the effects of
scattered radiation and limit the detector field-of-view.
The optic was then removed from the detector
field-of-view and another spectrum was recorded. Figure 2 shows the spectra recorded from these
measurements, exhibiting the bremsstrahlung continuum and the MO K, and KP peaks. The gain was
calculated by dividing the two spectra, channel-by-channel, then scaling the result to the front face of the
optic (since this is where a sample would be if the optic were not present). This calculated gain was a
maximum of (46 f 2) at 8 keV, falling to a gain of (12 f 1) at 17.5 keV.
Simulations of these measurements were performed using a Monte Carlo ray-tracing code. While
the code does not account for absorption in the optic end windows or for surface roughness in the
capillaries, the transfer function of the end windows was included by multiplying the Monte Carlo result
by the end-window transmission efficiency. The simulation values for the flux gain were 147 at 8 keV
and 32 at 17.5 keV. The measurements thus show a transmission reduction on the order of 65% from the
simulations. Others have shown that surface roughness and individual capillary misalignment were
responsible for transmission reduction on the order of 50% or more [4].
The absolute flux of x-rays in the MO K, peak at a distance of 64 mm from the source (equal to
the front face of the optic) without the optic present and with a source high voltage of 40 kV was
measured to be 6.3 x 109 /mA-s-cm2. With the optic in place, a flux gain of 12.1 was measured, yielding
7.6 x 1010/mA-s-cm2 at the optic output focal spot.
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Copyright (C) JCPDS-International Centre for Diffraction Data 1999
1.E+02
with optic
2’ l.E+Ol
E:
‘2
4 l.E-01
k3
0.
22
e
1.E-03
5
10
15
Energy (keV)
20
25
30
Figure 2: Spectra recorded with the Si(Li) detector using the MO microfocus source operated at 40 kV and a 100 pm
aperture at the focal spot, with and without the optic present.
CONCLUSIONS
A new x-ray tapered polycapillary optic, designed for use over a broad range of energy from 8 17.5 keV, was tested. Images recorded with a magnifying CCD demonstrate the flux focusing at the optic
output, and measurements of these images show a focal spot size of (107 f 8) pm (FWTM). When
measured using a knife-edge scan and a Si(Li) detector, the measured focal spot size was (55 f 1) pm
(FWTM) at 17.5 keV and (100 f 2) pm (FWTM) at 8 keV. Spectra recorded with and without the optic
in place were used to calculate the energy dependence of the optic flux gain. This gain was measured to
be a maximum of (47 f 2) at 8 keV, falling to (12 * 1) at 17.5 keV. Comparison to simulation shows a
65% transmission reduction, probably due to surface roughness and individual capillary misalignment in
the real optic. Future characterization of the lens will study the spatial variation in the output energy
spectrum from the optic.
ACKNOWLEDGMENTS
Author J. Gormley gratefully acknowledges the assistance of a National Research Council
Research Associateship tenured at the NIST.
REFERENCES
[l] MacDonald, C.A., et al., “Quantitative measurements of the performance of capillary x-ray optics,”
SPIE 2011, p. 275, (1994).
[2] Kumakhov, M.A., “Channeling of photons and new x-ray optics,” Nucl. Instr. and Meth., B48, p.
283, (1990).
[3] Jach, T., et al., “The characterization of x-ray polycapillary optics with a high-resolution x-ray optical
bench,” SPIE 2805, p. 192, (1996).
Copyright (C) JCPDS-International Centre for Diffraction Data 1999
[4] Chapman, H.N., et al., “Capillary X-Ray Optics,” SPIE 1741, p. 40, (1992).
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