89
J. Synchrotron Rad. (2000). 7, 89±94

Rapid and sensitive XAFS using a tunable X-ray undulator at
BL10XU of SPring-8
Hiroyuki Oyanagi,a* Masashi Ishii,b Chul-Ho Lee,a Naurang L. Saini,c Yuji
Kuwahara,d Akira Saito,d Yasuo Izumie and Hideki HashimotofaElectrotechnical Laboratory, 1-1-4 Umezono, Tsukuba 305-8568, Japan, bJASRI, Kamigori,
Ako-gun, Hyogo 678-12, Japan, cUniversita di Roma `La Sapienza', Dipartimento di Fisica,
00185 Roma, Italy, dOsaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan,eTokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan, andf
TORAY Research Center, 337 Sonoyama, Otsu-shi 520-8567, Japan.
E-mail: oyanagi@etl.go.jp(Received 25 November 1999; accepted 22 December 1999 )The design and performance of the high-brilliance XAFS facility at BL10XU of SPring-8, aimed at
rapid and sensitive measurement of X-ray absorption ®ne structure (XAFS), is reported. Bothundulator gap and double-crystal monochromator have been successfully controlled covering a wide
energy range (5±30 keV). A versatile goniometer system, consisting of two independent highprecision goniometers, is capable of polarized XAFS in ¯uorescence mode and surface-sensitive
experiments using a grazing-incidence geometry. By sharing major components, i.e. a monolithic Ge
100-pixel array detector and a closed-cycle He cryostat, both polarized XAFS and X-ray standing
wave (XSW) experiments can be performed at low temperature (15±300 K). The performance of thespectrometer has been evaluated by recording XAFS spectra in transmission mode.
Keywords: tunable undulators; pixel array detectors; rapid and sensitive XAFS.

1. Introduction
Tunable X-ray undulators inserted into straight sections ofultralow-emittance (<10 nm rad) storage rings haveopened up new opportunities for local structure studies
beyond the limitations in concentration and/or measurement time. Rapid and sensitive X-ray absorption ®nestructure (XAFS) (Chance, 1994) is particularly useful forin situ studies where the number of excited species of atom
is strictly limited. For example, it has been demonstrated
that pump-and-probe XAFS (Oyanagi et al., 1998) can
provide a `snapshot' or `frozen' radial distribution, serving
as a probe of the local lattice distortion, relaxation andbond alternation as a result of optical excitation (Kolobov
et al., 1997). The observation that the time-averaged andspatially averaged coordination number of chalcogenatoms in chalcogenide glasses such as Se or As2Se3
increases during illumination at low temperature showedthat the dominant photo-induced defect state is a pair ofneutral C03 states (C03±C03) and not a charged defect pairsuch as C+3±Cÿ1 which would give the same averagecoordination number as that of the original state (Kolobov
et al., 1997).
Rapid and sensitive XAFS based on ¯uorescence
detection and a tunable undulator, discussed in terms ofmere feasibility in the early 1990s (Oyanagi, 1993), is now
becoming a routine tool (Rogalev et al., 1998). In this paper# 2000 International Union of Crystallography
Printed in Great Britain ± all rights reserved

we turn our attention to efforts to combine a tunable
undulator and a high-ef®ciency X-ray detector. We havedesigned and built a high-brilliance XAFS apparatus at oneof the undulator beamlines, BL10XU, of SPring-8. A novel
100-pixel array detector (PAD) has been recently developed as a high-throughput X-ray detector (Oyanagi, 1999).
By computer control of the undulator de¯ection parameterand a double-crystal monochromator, a continuous scanover a wide energy range (5±30 keV) with a high energy
resolution (E/E < 2  10ÿ4) has been achieved. Anotherfeature of the spectrometer is a versatile goniometersystem which allows various polarization geometries or
sample orientations.
Fluorescence-detected polarized XAFS experiments forcuprate superconductors demonstrated that the local
structure of the conducting (CuO2) plane is microscopicallyinhomogeneous, i.e. distorted `localized' and undistorted`itinerant' domains form charge and lattice stripes (Bianconi et al., 1996). In order to investigate the role of stripes
in high-Tc superconductivity, however, more ef®cientmeasurements of polarized XAFS are required. Morerecently, a large local lattice distortion (Jahn±Teller
distortion) in manganites was found upon the charge andorbital ordering (Argyriou et al., 1996). Much attention has
been paid to the microscopic origin of colossal magnetoresistance (CMR) which arises from a competition
between the lattice distortions and charge dynamics for the
Journal of Synchrotron Radiation
ISSN 0909-0495 # 2000

90

Rapid and sensitive XAFS

charge carriers. Polarized XAFS studies provide important
information on the symmetry and magnitude of local lattice
distortions in cuprates and manganites. In order to study
the temperature dependence of anisotropic radial distributions, a high-precision goniometer system with a cryostat
is needed. In this paper we report a newly designed
goniometer system which provides all the requirements
described above.

2. Undulator and beamline optics
We brie¯y describe the optics of BL10XU at SPring-8 in thefollowing. Major optical components are a rotated-inclined
double-crystal monochromator (Ishikawa, 1995) and a

Figure 1

On-axis brilliance of an in-vacuum-type undulator for BL10XU at
SPring-8 with a beam current of 100 mA (Kitamura, 1994). Two
spectra with de¯ection parameter values (Ky = 0.3, 2.3) are shown
on a logarithmic scale.

Figure 2

double ¯at mirror (Uruga et al., 1995). In Fig. 1, undulator
brilliance spectra for a standard in-vacuum-type undulator(U032V) (Kitamura, 1994) with de¯ection parameters Ky =0.3 and 2.3 are shown. Ky depends on the magnetic ®eld of
the magnet array which is given as a function of the magnet
gap. On varying the undulator gap from 9.6 mm to 22 mm awide energy range (5±30 keV) is covered, using the ®rst andthird higher harmonics (L mode, n = 1; H mode, n = 3). On
decreasing Ky, the peak positions shift to higher energy,
giving smooth envelope functions shown by the solid line.
Since a typical energy range of a routine extended X-ray
absorption ®ne structure (EXAFS) scan is 1 keV, bothmonochromator and undulator should be controlled duringa scan. Higher harmonics and background radiation areminimized by a double ¯at mirror with a variable criticalenergy.
In Fig. 1 the on-axis brilliance of the in-vacuum undulator at SPring-8 is plotted as a function of photon energy,
assuming a beam current of 100 mA. The brilliance, whichis greater than that of a multipole wiggler at secondgeneration storage rings by more than four orders ofmagnitude, is a clear advantage of an undulator providing ahigh photon ¯ux for a single-crystal (typically 1 mm indimension) experiment or a grazing-incidence geometry(100 mm aperture). Note that owing to the highest energyof a storage ring (8 GeV) among the third-generation
facilities the envelope functions have the most ¯at intensity
distribution over a wide energy range. The principle of atuning undulator is brie¯y described in the following. First,
the whole energy range was split into two regions, i.e. Lmode and H-mode. In the lower energy range (5±15 keV)
the fundamental radiation (n = 1) is used while in the

Flow chart of undulator gap control. Prior to monochromator control, a gap to be varied in mm (G) is evaluated. When G exceeds theuser-programmable allowance value, G0 (10±30%), the undulator gap is tuned.

Hiroyuki Oyanagi et al.

higher energy range (15±30 keV) the third higher harmonic
radiation (n = 3) is tuned. Since the speed of positioning amagnet array for an in-vacuum-type undulator wasmechanically limited (1 mm minÿ1), the undulator gap istuned when the intensity is reduced beyond a userprogrammable allowance parameter G0 (10±30%). In thelower energy range a ¯at double mirror is inserted which
can vary the cut-off energy so that the higher harmonics
content is minimized.
There could be several approaches for undulator gap
control. Rogalev et al. (1998) scanned the undulator

91

whenever the gap value to be changed, G, exceeded theminimum step of gap control, Gmin. The advantage of thismethod is that one can obtain the most smooth intensity
distribution, tracing the envelope functions of peak intensity. However, this would take a long time since the gap ismost frequently tuned. In the present case anotherapproach, i.e. a variable tuning condition, has been adopted
because of a mechanical limitation in the speed of themagnet array. As illustrated by the schematic ¯ow chart
shown in Fig. 2, G (variation of gap value) is calculated
each time the monochromator angle B is requested tochange. The undulator gap is tuned when the stored accumulated value exceeds the allowance parameter G0
calculated from the width of the undulator peak as afunction of energy (Sasaki, 1994). In Fig. 3(a), calculated
and observed gap values giving a maximum intensity arecompared. The empirical gap values giving the maximum
intensity were ®tted by a quadratic polynomial F(B) whereB denotes the Bragg angle of the monochromator.
Fig. 3(b) shows the deviations of gap values (error) fromthe calculated ones as a function of B. The calculated full
width at half-maximum (FWHM) values for n = 1 and n = 3(Sasaki, 1994) were ®tted by a simple formula: G(B) =B , where and are free parameters and the magnitudeof allowance parameter was obtained from G(B) after thecoef®cient was normalized. Using F(B) and G(B) the
undulator was successfully tuned using the Bragg angle as
the single parameter. If a smoothly varying intensity
distribution is needed, one can change the coef®cient .

3. Goniometer system
Figure 3(a) Calculated (open diamonds) and observed (closed diamonds)
undulator gap values giving the peak intensity as a function of
Bragg angle for the ®rst (L-mode) and third (H-mode) higher
harmonic radiation. (b) Difference between the calculated andobserved gap values.

Figure 4

For anisotropic single crystals, radial distribution functions
along a particular direction are obtained by choosing asample orientation such that the electrical ®eld vector
becomes parallel to a crystallographic axis. For high-Tc
superconductivity perovskites with an orthorhombic

Schematic set-up of the two goniometers.  and ' rotations in the ®rst goniometer are used to orient a single crystal with respect to theelectrical ®eld vector E.

92

Rapid and sensitive XAFS

symmetry, the electrical ®eld vector should be oriented
along the a-, b- or c-axis. In temperature-dependent
polarized XAFS experiments a dominant systematic error
arises due to the beam inhomogeneity and the uncertainty
of a sample geometry and/or beam positions on the sample
which de®nes the probing area. Both geometry and positions should be precisely controlled and kept stable duringa scan. In order to meet those requirements a cryostat
should have at least '- and -axis rotations and additionaltranslational motions (X and Z) as shown in Fig. 4. Tiltingof the sample to set up a grazing-incidence angle is alsoessential in surface-sensitive XAFS experiments (Oyanagi
et al., 1995). In a grazing-incidence geometry, vertical andhorizontal sample orientations provide information on theradial distributions parallel and perpendicular to thesurface normal, respectively. A grazing-incidence geometryis also useful in pump-and-probe XAFS for minimizing
mismatches between X-ray probe and optical pumping(Oyanagi et al., 1998).
Thus we have designed a three-axis goniometer system
consisting of two separate goniometers as illustrated inFig. 4. A sample in a closed-cycle He cryostat mounted on
the upstream goniometer (Huber Eulerian cradle 511.1)
can be oriented allowing any orientation geometry withrespect to the electrical ®eld vector using '- and -axis
rotations. The goniometer is useful for temperaturedependent polarized XAFS experiments. An ionizationchamber as a transmitted or a re¯ected beam monitor canbe rotated around a 2 axis of the downstream goniometer(Huber 420). The Ge PAD (Oyanagi, 1998) is mounted

through the base plate of the 2 goniometer and detects the¯uorescence from a sample held in a cryostat. For a horizontal orientation of the cryostat one can also perform
X-ray standing wave (XSW) experiments using the samedetector for which the re¯ected beam intensity is monitored by a downstream ion chamber mounted on the 2
axis. ',  and 2 axes, and Z and X linear motions are
remotely controlled. For two polarization geometries(E k ab and E k c) for an orthorhombic crystal, a grazing
incidence angle 0 (usually taken as 1.6 to minimize thediffractions) is adjusted by tilting the '-axis. Fig. 5 shows
the front and plan views of the goniometer system.
In structural studies of surfaces and interfaces, XSWexperiments provide complementary information withpolarized XAFS on the atom positions along the surface
normal direction (Zegenhagen, 1993). Once a sample ismounted on the goniometer which allows a conventional
two-circle diffractometer geometry, XSW experiments canbe performed for the same sample.

4. Detector and electronics
As a high-ef®ciency X-ray detector, a novel Ge PAD has
been developed. The arrangement of 10  10 Ge pixels is
illustrated in Fig. 6. In a recent design of a conventional
multi-element Ge detector the packing ratio reached 57%(Oyanagi et al., 1998). In a monolithic approach the deadregion between pixels can be as small as 0.3 mm which
dramatically improves the packing ratio to 80±90%. In thisdesign each pixel (pure Ge) is 10 mm thick and has aneffective area of 4.7 mm  4.7 mm. The energy resolution isdesigned to be 190 eV at 5.9 keV at a medium count rate.
We estimate the energy resolution at a high count rate (2 
105 counts sÿ1) to be 240 eV using a 1 ms peaking time for
a commercial digital signal processor.
Each pixel is equipped with a PSC941 preampli®er(Penta FET). Ten ®eld-effect transistor (FET) modules aremounted on a single cooled circuit board and connected to
Ge pixels by mechanical contact. All FET circuit boards
and preampli®er modules were fabricated and tested.
Performance tests for Ge PADs with improved passivatedsurfaces have shown the high energy resolution (215 eV at

Figure 5(a) Plan view and (b) front view of a high-brilliance XAFS
apparatus installed at BL10XU, SPring-8. Using a -circle, asample on a cryostat can be oriented with either a vertical or
horizontal geometry.

Figure 6

Monolithic Ge pixel array detector.

Hiroyuki Oyanagi et al.

5.9 keV). In Fig. 7 the energy spectrum for one channel(pixel #85) is shown. Up to now 60% of the total 100
pixels are in operation while further research is under way
to increase this percentage. Details of the PAD will appear
elsewhere (Oyanagi, 2000). In total, a maximum count rateof 2  107 counts sÿ1 is expected after the optimization of
FET capacitance for high count rate use. For recording aparticular ¯uorescence line, a four-channel digital signal
processor (X-ray Instrumentation Association, DXP
Model 4 T) is used. For 100 channel outputs, 25 DXP
modules are equipped which are controlled via a CAMAC
interface (WieNer, CC16) and a PC (Dell Computer,
Optiplex GXMT5166) running on linux 2.1.30.
5. Performance
Owing to a grazing-incidence angle of 1 in an asymmetricdouble-crystal (+, ÿ) rotated inclined con®guration, thepower density on the ®rst crystal surface, 5 W mmÿ2, isreduced to 1/57.3. We ®nd that the re¯ectivity calculated bydynamical theory for an Si(111) re¯ection becomes larger
than the symmetric case by a factor of two; the full width at
half-maximum (FWHM) values for rocking curves are
14.9 arcsec at 8 keV for Si(111) and 2.3 arcsec at 24 keV forSi(220). However, the observed energy resolution E/E
given as a convolution of rocking-curve width and anangular divergence multiplied by cot B was less than 2 
10ÿ4 because of a small angular divergence of the incident
beam (4.85 mrad). A mechanical slit placed in front of themonochromator limited the beam to 1 mm.
We evaluated the performance in the low-energy region
which is most critical to the quality of the incident beam.
Because of a strict limitation on the heat load of theundulator front-end, the minimum gap is limited to 9.6 mm.
The lowest energy limit in Bragg angle is B = 21.5
(5.39 keV) for which the undulator gap value is 9.61 mm. In

Fig. 8 the Cr K-XANES spectrum for K2CrO4 powder isshown. A sharp transition to a bound state (5992.8 eV)
demonstrates the high energy resolution of the spectrometer. The evaluation was performed during the initialstage of operation with a ring current of 40±50 mA but it
was con®rmed that the same performance is available withthe present ring current, 70±80 mA. For the Si(111)re¯ection the higher limit in energy for the H mode (n = 3)
is 23.1 keV with B = 5 and a gap value of 12.18 mm.
However, since the minimum Bragg angle B of themonochromator is 3 , the upper limit in energy can beeasily extended to 30 keV if the n = 5 peak is used (seeFig. 1).
Fig. 9 shows the I0 intensity variation and Ru K-XAFS
spectrum taken for PrRu4P12 powder as a function of

Figure 8

Cr K-edge XANES spectrum for K2CrO4 powder (Izumi &Nagamori, 1999).

Figure 9
Figure 7

Multichannel analyzer output of pixel #85. S/B = signal-tobackground ratio.

93

Expanded Ru K-XAFS spectrum for PrRu4P12 (top) plotted as afunction of photon energy (keV) measured in transmission mode
(Lee et al., 1999). I0 intensity variation as undulator andmonochromator are scanned from 22.9 keV down to 22.1 keV(bottom).

94

Rapid and sensitive XAFS

photon energy (Lee et al., 1999). The saw-tooth shape, i.e.
gradual decrease of I0 intensity followed by a sharp
increase on decreasing photon energy, is clearly seen.
Inspection of the Ru K-XAFS spectrum con®rmed that
there are no glitches related to undulator tuning.

ray tracing of the beamline. This work has been performed
as the implementation program of the high-brilliance
XAFS facility at SPring-8.

6. Summary

Argyriou, D. N., Mitchell, J. F., Potter, C. D., Hinks, D. G.,
Jorgensen, J. D. & Bader, S. D. (1996). Phys. Rev. Lett. 76, 3826±
3829.
Bianconi, A., Saini, N. L., Lanzara, A., Missori, M., Rossetti, T.,
Oyanagi, H., Yamaguchi, H., Oka, T. & Ito, T. (1996). Phys.
Rev. Lett. 76, 3412±3415.
Chance, B. (1994). Synchrotron Radiation in Biosciences, pp. 247±
251. Oxford University Press.
Ishikawa, T. (1995). Unpublished.
Izumi, Y. & Nagamori, H. (1999). Unpublished.
Kitamura, H. (1994). Rev. Sci. Instrum. 66, 2007±2010.
Kolobov, A. V., Oyanagi, H., Tanaka, K. & Tanaka, K. (1997).
Phys. Rev. B, 55, 726±734.
Lee, C. H., Oyanagi, H., Sekine, C., Shirotani, I. & Ishii, M. (1999).
Phys. Rev. B, 60, 13253±13256.
Oyanagi, H. (1993). J. Appl. Phys. Suppl. 32(2), 861±865.
Oyanagi, H. (1998). J. Synchrotron Rad. 5, 48±53.
Oyanagi, H. (2000). In preparation.
Oyanagi, H., Kolobov, A. V. & Tanaka, K. (1998). J. SynchrotronRad. 5, 1001±1003.
Oyanagi, H., Saito, M. & Martini, M. (1998). Nucl. Instrum.
Methods, A403, 58±64.
Oyanagi, H., Shioda, R., Kuwahara, Y. & Haga, K. (1995). J.
Synchrotron Rad. 2, 99±105.
Rogalev, A., Gotte, V., Goulon, J., Gauthier, C., Chavanne, J. &Elleaume, P. (1998). J. Synchrotron Rad. 5, 989±991.
Sasaki, S. (1994). Private communication.
Uruga, T., Kitamura, H., Kohmura, Y., Kuroda, M., Nagasawa, H.,
Ohtomo, K., Yamaoka, H., Ishikawa, T., Ueki, T., Iwasaki, H.,
Hashimoto, S., Kashihara, Y. & Okui, K. (1995). Rev. Sci.
Instrum. 66, 2254±2256.
Zegenhagen, J. (1993). Surf. Sci. Rep. 18, 200±271.

References
A continuous spectroscopic scan over 1 keV and widerange tuning over 5±30 keV have been achieved bycomputer-controlling an undulator gap. For minimizing thetuning time the undulator gap was tuned based on a userde®ned criterion. In spite of broadening of the width of thecrystal rocking curve due to an asymmetric re¯ection
geometry, a high energy resolution better than 2  10ÿ4 at
9 keV was obtained using an Si(111) plane. Two independent high-precision goniometers were combined so that theorientation and position of a sample mounted in a cryostat
can be controlled. This allows polarized XAFS including a
grazing-incidence experiment in ¯uorescence mode and
XSW experiments. A novel Ge pixel array detector
provides a 20 MHz total count rate. The combination of ahigh-brilliance beam and high-throughput X-ray detector is
expected to provide a routine tool for local structures indilute systems. The performance is demonstrated by highspectral quality (energy resolution, statistics and stability)
of XAFS data taken for some powder samples.
We wish to express our thanks to J. Zegenhagen forvaluable comments on the instrumentation related to XSWexperiments. One of the authors (HO) would like to thankS. Sasaki for calculation of the undulator width as a function of gap value. He also thanks M. Jinno and Y. Ueno for