Updated July 2019 to correct dimension of the Soller screen & small typos
Journal Reference AIP Conf. Proc. 2054, 20002. DOI: 10.1063/1.5084562
https://aip.scitation.org/doi/abs/10.1063/1.5084562
https://arxiv.org/abs/1807.03620

Auxiliary Optics for meV-Resolved Inelastic X-Ray
Scattering at SPring-8: Microfocus, Analyzer Masks,
Soller Slit, Soller Screen, and Beam Position Monitor
Alfred Q.R. Baron1,a), Daisuke Ishikawa1,2, Hiroshi Fukui1,3
and Yoichi Nakajima1,4
1Materials Dynamics Laboratory, RIKEN SPring-8 Center, 1-1-1 Kouto, Sayo, Hyogo 679-5148, JAPAN2
SPring-8/JASRI, 1-1-1 Kouto, Sayo, Hyogo 6789-5198, JAPAN3
Graduate School of Material Science, University of Hyogo, 3-2-1 Kouto, Kamigori, Hyogo 678-1297 JAPAN
4
Department of Physics, Kumamoto University, Kumamoto 860-8555, JAPANa)Corresponding author: baron@spring8.or.jp

Abstract. This paper discusses several optical elements now in use at BL43LXU, the RIKEN Quantum NanoDynamics
Beamline, of the RIKEN SPring-8 Center. BL43LXU is dedicated to meV-resolved inelastic x-ray scattering (IXS)
using spherical analyzers operating between 15.8 and 25.7 keV. The work described here is relevant for setups on thehigh-resolution spectrometer (10m two-theta arm) with resolution between 2.8 and 0.8 meV. Specific optics discussed
include a multilayer Kirkpatrick-Baez (KB) mirror pair that focuses the full (~1x3mm2) beam at 17.79 keV to a 4.4 x
4.1 µm2 spot with ~60% throughput, two different types of Soller slits that help reduce backgrounds, masks for the
analyzers that allow increased solid angle to be collected while preserving momentum resolution, and a diamondquadrant beam position monitor (BPM). These elements have been used for experiments in extreme conditions withdiamond anvil cells, and liquid measurements at low momentum transfers, among other work.

INTRODUCTION
Inelastic x-ray scattering (IXS) with meV resolution offers the opportunity to probe atomic dynamics (thedynamic structure factor, S(Q,ω)) in cases when it is not easily accessible using other methods. Specific examples
include the measurement of small (~10 micron) samples and the measurement of large energy transfers at small
momentum transfers. Thus meV-IXS is often used for investigations of samples in extreme (high pressure, hightemperature) conditions in diamond anvil cells (DACs) and investigation of liquids, where the low-Q region canhave unique information. The present paper describes some auxiliary instrumentation used to facilitate thesemeasurements, as implemented at the RIKEN Quantum NanoDynamics beamline, BL43LXU [1], of the RIKEN
SPring-8 Center. A broader introduction to IXS experiments and optics may be found in [2].
The motivation for this paper is primarily to show what has been done and found to be useful. It is not intended
to be an in-depth look at the background for the more typical components (Kirkpatrick-Baez (KB) mirrors andSoller slits) so much as to provide one set of operating parameters. In particular, KB mirrors are in use in many
beamlines, but we were not able to find details on any that were very close in parameter space to what we
considered. Meanwhile, we know of no cases where tailored analyzer masks, as discussed here, or Soller slits, orwhat we call a Soller screen, have been used for IXS. The beam position monitor is mostly a combination ofcommercial components (with some specialization of the mechanics) and we show reasonable, even easy, usage
at the 0.01 mm level, as is sufficient for the way it is used here.

ELLIPTICAL MULTILAYER KB MIRROR PAIR AT 17.793 KEV
Focusing the x-ray beam is highly desirable to increase count rates when small samples are measured.
At first glance, having a smaller beam is generally better, as most experiments using a large beam can also be donewith a small beam, but not the reverse. However, small beam sizes also usually have increased divergence (i.e.:
conserved brilliance), and strong focusing optics often lead to limited clear aperture (free space, without optics)
around the sample, reduced throughput, and higher radiation damage, all of which may negatively impact
experiments. Thus the focal spot size for an experiment is a compromise between competing issues. For thepresent case we targeted a 5 µm beam size (unless otherwise stated, all sizes mentioned will be full width at half
maximum, FWHM) as being a reasonable compromise between what was needed for high-pressure studies, up to~300 GPa, and the other issues mentioned. Previously the SPring-8 IXS beamlines have used compound focusingas this was easiest to implement: the addition of an optic after the usual bent cylindrical mirror was used to reducethe beam from ~50 um to ~15 µm [3]. However, it is difficult to go to a smaller size in that compound geometryas (a) the compound focusing tends to limit the demagnification possible for a given sample clear aperture, and(b) the focusing of the second component must overcome the errors of the first optic. These issues led us to
implement a single focusing elliptical Kirkpatrick-Baez (KB) mirror pair ("single focusing" means just one
focusing element in each direction transverse to the beam). However, this means that there is no upstream
component to help reduce the beam size so the installed optic must accept the full beam size (~1x3mm2, full width)
near the sample, ~100m from the source to avoid large losses. This forces operation at larger (~1 degree) grazing
angles to allow reasonable size optics, which in turn basically requires a multilayer optic. As ellipsoidal optics
have not yet progressed to a level sufficient for this problem, we use an elliptical KB mirror pair with multilayer
coatings.
The parameters for the mirror pairare given in table 1. Notably the verticalfocusing is stronger than the horizontal
as is unusual at a SR facility. However
the beamline has 7 vertically scatteringreflections upstream of the KB pair (2mirrors and 5 crystal reflections), so it
was known that the effective source sizein the vertical was degraded, so strong
focusing was chosen to compensate for
this.
The mirror substrates were
polished by J-Tec, and they successfully
achieved all critical specifications. The
multilayer was deposited by Rigaku
Innovative
Technologies(RIT,
previously Osmic) and consisted of 200
layers of B4C/Mo (approximately 2:1
thickness ratio) with a 95 Å SiC cap
layer. The multilayer on the verticalfocusing mirror was graded from d=2.1
to 2.8 nm and the horizontal from 2.2 to
2.7 nm. The slope error specification
was set to limit the nominal contribution
from this to less than 1µm FWHM. The
relatively good, 0.2 nm rms, roughness
specification was to keep goodreflectivity from the multilayer.

VerticalHorizontal
17793 (+-3) eV
0.2 m
0.4 m
109.8 m109.6 m
<5 µm
Desired Spot Size (FWHM)<5 µm FWHM
FWHM
Nominal Demagnification
~500
~270
Nominal Source Size
< 0.1 mm~0.6 mm
Ideal Focus< 1 µm~2.2 µm
Est. Multi-Source Contribution< 1 µm~1.5 µm
Est. Slope Error Contribution
<1 µm
<1 µm
~2 x 2.35 x rms x q
Operating Energy
Distance to Sample (q)Distance to Source (p)Substrate Material
Substrate Outer Dimensions(L x W x H)
Grazing Angle of Incidence
at Mirror Center
Mirror Active Length
Mirror Active Width
Acceptance
Roughness
Longitudinal Slope Error (rms)Transverse Slope Error (rms)
Reflectivity at operating energy

Single Crystal Silicon
80 x 40 x 15
250 x 40 xmm3
30 mm314.00 (+-0.02) mrad
>70 mm>240 mm> 10 mm> 6 mm
> 0.95 mm> 3.35 mm< 0.2 nm rms< 1 µrad<0.5 µrad< 20 µrad<10 µrad> 70 %TABLE 1. Design parameters for the KB mirror pair (partial list).

Boldface quantities are primary while others are secondary and are
Figure 1 shows the results from
included for convenience/reference.
measuring the KB pair.
The
reflectivity was found to be good overall, but in each case, there is a slight worsening of the reflectivity as the d-spacing becomes smaller (translation<0), consistent with calculation. But, in both cases the average reflectivity over the beam spot was > 75% and thethroughput of the pair was ~60% at 17.793 keV. The measured beam spot size can be seen in the lower panel offig. 1, and the size, 4.4 µm V x 4.1 µm H, is both within the desired specification and reasonable given the concerns
mentions above, namely, the perturbation of the vertical beam response by other optical elements, and the presence

FIGURE 1. KB Mirror Performance. The top panels give the measured reflectivity of each mirror as a function of
position and grazing angle when the mirror is scanned through a small beam. The main Bragg peak at 14 mrad and the usualsubsidiary maxima are easily visible. The lower panels show the beam profiles measured using a wire scan with the calculated
derivative and a pseudo-Voigt fit. The numbers give the FWHM from the fit.

of multiple source points. We note, for completeness and reference, that the beam sizes given by differentiation
using the PyMca code are about 10% larger than those from our fits, with a smoother looking derivative plot.

ANALZYER MASKS FOR SMALL MOMENTUM TRANSFERS
The BL43LXU high-resolution spectrometer presently operates, mostly, using a 6x4 array of sphericalanalyzers located 9.8 m from the sample on a large two-theta arm, as can be seen in fig. 2. The acceptance of
these analyzers is controlled by a set of motorized "venetian blind" [4] slits located about 9m from the sample thatcan be set to any rectangular size between 3x3 mm2 and 80x85 mm2, with, always, an identical size set for all
analyzers (e.g. one motor for a common vertical gap control and one for a common horizontal gap). This geometryis effective for measuring crystalline samples (see discussion in [2]). However, for disordered materials at low
momentum transfers one often desires an annular acceptance that corresponds to a constant momentum transfer:a rectangular slit is not optimized for fixed momentum transfer, |Q|, resolution at small Q. We therefore
implemented a series of analyzer masks specifically for low-Q measurements of disordered materials, with thegoal of improving rates without sacrificing momentum resolution.

FIGURE 2. Photo of the 10m Two-Theta arm at BL43LXU. The sample position is the center of the Huber 512 Eulerian
Cradle at the far right and the analyzers are kept inside the vacuum chamber at the far left. The air gap between the longflight path and the analyzer chamber (both built by Ayumi Industries) allows easy installation of the masks on the venetianblind slit (Kohzu) at 9m from the sample.

The masks were wire-cut in 3 mm aluminum (see fig. 3), and then covered by Pb tape and then Kapton tape,as seen in the photograph in fig. 3. Two sets were fabricated, one, mask set A, is designed to have the incident
beam between the first two analyzers and collect momentum transfers down to ~0.5 nm-1, simultaneously taking
data at two small momentum transfers. The other, set B, operates with the first analyzer at slightly higher
momentum transfers, >1 nm-1, with the beam just to one side of the entire array. Both mask sets are designed towork with the Soller slit described in the next section. The shapes were chosen so the momentum resolution (3.5times the rms deviation from the mean) at most momentum transfers was < 10% of the momentum transfer, witha worst case ~20% acceptance at the smallest |Q|. Calculations, such as those shown in fig. 3, were done to confirm
the shape and magnitude of the each mask's |Q| acceptance. Eventually we expect to improve IXS modeling to
explicitly include the calculated momentum resolution using continuous parameters (e.g. the slope of some mode
dispersion, not just the mode frequency at one Q) and fit spectra at several momentum transfers simultaneously to
a common parameter set.

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FIGURE 3. Analyzer mask set "A". The left panel shows the design drawing, the middle a photo of the masks
installed on the pins behind the venetian blind slits, and the right shows the calculated momentum acceptance profile of
all the masks - intensity vs momentum transfer - in one operational configuration with the numbers giving the central(centroid of) the momentum transfer of each analyzer in nm-1. These are mostly reasonably flat-topped. Note that thelast (right-hand) panel is plotted as seen from the sample (while the first two are as seen from the analyzers) so the plot
for the lowest momentum transfer is in the lower right. The scale of the photo can be understood by noting the spacing
between the mask centers is about 110 mm in each direction.
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     $SOLLER SLIT & SCREEN
Soller slits are a well-known and early [5], but still useful, method of reducing backgrounds in scatteringexperiments by limiting the detector (analyzer) field of view to a narrow region around the sample. Especially for
IXS with spherical analyzers, there is a discrete separation of beams to different analyzers (see above) so it isnatural to consider Soller slits matching the periodicity of the analyzers. However, Soller slits can be tricky toimplement as, to be most effective, one would like small spacing between the different channels of the Soller slit.
In the present case, the center-to-center spacing of the analyzer channels is 120 mm at 9.8m, corresponding to 12.2
mrad or 0.702 degrees. Here we discuss two geometries, one with a "conventional" Soller slit with an entrance
window 68 mm from the sample (so a center-to-center channel spacing of 0.830 mm) and then what we call a
Soller "screen" that starts 5 mm from the sample so the analyzer center-to-center spacing is 0.061 mm. In the firstcase, the conventional Soller slit is designed to operate with the masks mentioned above. Separately, though not
yet tested, this Soller slit is also expected to be useful for looking at crystals at larger momentum transfers, to e.g.,
reduce backgrounds from the beryllium cap used in many refrigerators. In the second, 5mm, case, due to limits infabrication technology, a pair of screens (without foils between channels), is used to collect every other* analyzer
channel, with, effectively, half the channels sacrificed to improve S/N on the other half.
The conventional Soller slit includes incident and outgoing beam masks that were wire-cut in 0.3 mm tungsten
pate (calculated transmission ~3x10-9 at 25.7 keV). The incident mask was designed to be 68 mm from the sample
position and had rectangular openings, 3.5 mm high by 0.32 mm wide on a ~0.830 mm pitch, while the outgoing

FIGURE 4. Conventional Soller slit separating analyzer channels in the scattering plane. The left panel is a close-up of the slit
(approximately 115 mm long) while the right panel shows a photo of the sample area, including the Huber 512 cradle with aclosed cycle cryostat mounted. The Soller Slit location is shown by the light rectangle just left of center.

beam mask was designed to be 177 mm from the sample, with the slit sizes enlarged proportionately. Foils of 0.1
mm thick Mo were used to separate each channel, and the location of these foils was determined by adjusting,under a microscope, the position of 6 (3 above, 3 below) stainless steel supports with wire-cut grooves to accept
the foil (see fig. 4). Mo was chosen as it is both absorbing and relatively stiff. The entire assembly was thenmounted on the XZθ assembly mounted on the spectrometer two-theta arm. This provides sufficient degrees of
freedom to allow the Soller slit to be aligned with both the analyzers and the sample position. (Note that thevertical acceptance of the Soller slit, perpendicular to the scattering plane, was set generously so it did not have tobe precisely aligned.) The collimator has been used to reduce backgrounds from windows and gas around liquidsamples.
Experiments on samples in diamond anvil cells (DACs) also could benefit from a Soller slit. However, here
the constraints are more severe as one wants to reduce background either from the diamonds or, if used, a beryllium
gasket, which are directly adjacent to the sample - typical dimensions along the x-ray beam path are two ~1.5 mm
thick diamonds, or a ~6 mm diameter beryllium gasket. Meanwhile typical sample sizes are a few 10's of microns.
Then the goal is then to separate the scattering from the central 10s of microns from that of the adjacent few mm.
This*

Sometimes people also say “every second”.

FIGURE 5. Left: Soller screen mounted near the DAC. Right: Spectra measured from a sample of Re metal at~180 GPa pressure with and without the Soller screen. The reduction of the of the background from diamondand the improved contrast for the Re LA signal at 24 meV is clear.

requires that the entrance to the Soller slit be very close to the sample position and, consequently, that the Soller
slit apertures have a very fine pitch - nominally 61 microns at 5 mm distance, as mentioned above. We were notsuccessful in finding an appropriate fabrication technology that would allow the correct periodicity with therequired tolerances for an acceptable price. However, we implemented a pair of screens (one 5 mm from thesample and one 10 mm further from the sample) on a common rigid support with apertures spaced by double theanalyzer spacing, so 2x12.2 = 24.4 mrad, and without foils between the channels. By setting apertures of 0.048
mm H x 0.24 mm V on a 0.122 mm H pitch (laser cut into 0.1 mm tungsten, 4x10-8 transmission at 17.79 keV)and a scaled aperture at 10 mm further from the sample (so apertures at 5 and 15 mm from the sample position),
one can strongly cut the scattering from the diamonds and gasket, even without the foils - see fig. 5. However,this blocks the beam from every other analyzer, so comes at a cost, but, in some cases, cleaner data can be worth
sacrificing half of the momentum transfers. The Soller screen
assembly is mounted on the same XZθ assembly as used for theconventional Soller slit, and again, with a generous size in the vertical(transverse to the horizontal scattering plane) this is sufficient freedom
to align the system with the analyzers and the sample position.

QUANDRANT BEAM POSITION MONITOR (BPM)
A diamond beam-position monitor is installed 1.6 m upstream of
the IXS sample location (see fig. 6). This uses a RIGI diamondquadrant sensor from Dectris held in rough (few Pa) vacuum, placedon an XZ stage with single bellows feed-through (conceptual design
by Baron & Kohzu, fabrication by Kohzu), The electronics are readthrough a d-type connector coupled to a CAENels TetrAMM 4channel current amplifier and bias unit read out over Ethernet. The
CF135 cross is held on a steel support bolted to the experimental floor.
Temperature monitoring of the support shows it is stable at the level of+-0.1 K (diurnal peak to valley) corresponding to motion of about +-3
microns for a coefficient of thermal expansion of ~15 ppm/K.
The x-ray beam position is calculated, in a linear approximation in
the usual way from current differences divided by sums, with the scale
factor determined from a small translation of the BPM, and a hard zerofrom the center of the quadrants. Up to issues of beam shape, thisallows the beam position to be determined relative to the quadrant
center over a range of about half of the FWHM of the beam. We use
the BPM either in a partly focused beam (~0.3 mm) in our standard
setup or in a large beam (1x3 mm2) when the KB mirrors describedabove are used. The main purpose is to keep the beam position fixedduring energy (temperature) scans of the backscattering

FIGURE 6. Diamond quadrant BPM
mounted in a CF135 cross. X and Z
stages are visible, as is the bellows for the
motion feed-through. The current
amplifier is at the bottom

monochromator (see [2]) which are often accompanied by concomitant drifts in the angle of the backscattering
crystal: typically we set a threshold of 10 to 20 microns of beam motion at the BPM before it is corrected byadjusting the backscattering crystal stages. This fixes the position beam at the sample with a similar level ofprecision in the normal (~50 micron beam size) setup. When the KB is used, the placement of the BPM before
the KB and the severe demagnification of the KB means the precision is better than 1 micron at the sample given
the nearest optical element that changes the beam angle is ~18 m upstream of the BPM. The BPM then works
nicely, even easily, with, perhaps, the most notable issues being proper grounding of the BPM and some software
updates for the current amplifier. In general, the BPM provides ~micron resolution at the sensor location, evenwith the relatively weak, narrow bandwidth, x-ray beams used for IXS.

CONCLUDING COMMENTS
We have discussed several "auxiliary" optical components that are useful for meV IXS with sphericalanalyzers. The multilayer KB performs well at 17.793 keV (2.8 meV resolution) and has become a standard partof the high-pressure beamline operation. A set optimized for 21.747 keV (1.3 meV resolution) should arrive within
the year. The BPM setup is now in routine operation in all work. The other components have proven useful for
specific experiments, with the masks and Soller slit used for most liquid measurements at low Q and the Soller
screen used for some high pressure work, especially when beryllium gaskets are used.

REFERENCES
[1]
[2]
[3]
[4]

[5]

A. Q. R. Baron, SPring-8 Inf. Newsl. 15, 14 (2010).
A. Q. R. Baron, in Synchrotron Light Sources Free. Lasers Accel. Physics, Instrum. Sci., edited by E.
Jaeschke, S. Khan, J. R. Schneider, and J. B. Hastings (Springer International Publishing, Cham, 2016),
p. 1643–1757. See also arXiv 1504.01098v4.
D. Ishikawa, H. Uchiyama, S. Tsutsui, H. Fukui, and A. Q. R. Baron, in Proc. SPIE - Int. Soc. Opt. Eng.
(2013).
We use the term “venetian blind” slits to describe a set of slats (or flat plates) arranged in a geometry
similar to venetian blind widow shades. The slits operate by rotating the slats about an axis centered in
the middle of the slat to change the slit aperture. The axis locations are chosen so that when fully open
the slats lie in the ~ 2mrad dead region between the analyzers and so do not occlude the beam, and when
fully closed, the distance between the slat edges is ~2 mm. Arguably, this arrnagment could also becalled persian blind slits, or slat-type slits, but here we use venetian as being a common/familiar choice.
W. Soller, Phys. Rev. 24, 158 (1924).