X-ray Optics for the Microfocus Beamline L

G. Falkenberg, O. Clauss, and Th. Tschentscher

Hamburger Synchrotronstrahlungslabor (HASYLAB) at Deutsches Elektronen-Synchrotron (DESY),Notkestr. 85, 22607 Hamburg, Germany

1. IntroductionBeamline L is dedicated to fluorescence detection from micro-focus illuminated samples. Untilrecently, the microfocus set-up was employed mainly for the determination of elementconcentrations by microscopic synchrotron radiation x-ray fluorescence analysis (micro-SRXRF).All trace elements with 20 ≤ Z ≤ 84 are analysed simultaneously at detection levels of a few partsper million when white bending magnet radiation is used for excitation [1]. Glass capillaries areemployed to collimate the beam down to sizes 2-20 µm [2] and element distribution maps withmicrometer range spatial resolution can be recorded by scanning the sample across beam.Recently, beamline L was equipped with an X-ray optical system comprized of Si- and multilayermonochromators and X-ray mirrors in order to extend the range of scientific applications beyondwhite beam experiments [3]. The installation of a Si double crystal monochromator mademicroscopic X-ray absorption near edge structures (micro-XANES) experiments feasible atbeamline L. Information on the valence and binding configuration of the elements can be obtainedon micron sized particles. Broad-band monochromatization of the multilayer doublemonochromator combines high incident photon flux with high ratio of fluorescence signal tobackground scattering from the matrix. The multilayer monochromator is applied inmonochromatic SRXRF and total reflection X-ray fluorescence (TXRF) experiments. A pair of X-ray mirrors will serve both for high-energy cut-off and focussing purposes.The rebuilding of the beamline was started in the long winter shutdown 2000/2001. The vacuumsystem of beamline L inside the ring tunnel was almost completely restructured in order to fit in anew slit system and the first mirror chamber. Also, the vacuum system for the monochromator wasinstalled during the shutdown period. In service weeks alternating with white beam usage the Si-monochromator, the second mirror chamber, the multilayer monochromator and severalcomponents for beamline diagnostics were installed and commissioned. The Si- and multilayermonochromators are available for users since May 2001 and September 2001, respectively. Theinstallation of the first mirror is expected for the winter shutdown 2001/2002. In this contributionwe describe the re-design of the beamline and its optical components. Several reports onapplications of both monochromators (micro-XANES, TXRF, micro-SRXRF) are given in thesection „scientific user contribution“ (see contributions of Janssens et al., Wilke et al., Osan et al.,Wobrauschek et al.).

2. Beamline layoutThe existing beamline was modified considerably to fit the new vacuum chambers for mirrors andmonochromators. The beam position monitors, which are used to keep the vertical position of thebeam at the experiment constant within 50 µm, were moved to positions at distances of 4.3m and7.6m from the bending magnet source. A four-slit system with water-cooled tungsten jaws at 8.1mdefines the beam dimensions as indicated in Fig. 1.The first mirror chamber is positioned at 9.4m before the beam shutter inside the storage ring tunneland the second mirror chamber at 16.1m is located at the entrance of the experimental hutch. Themirrors operate at a fixed deflection angle of 2.67mrad with a vertical offset of 37mm. The first

mirror is made of Si and has water cooling. It has three different parts and by horizontal translationit can be used as Pt-coated planar or cylindrically shaped device or as uncoated plane Si mirror. Thecylindrical groove with a fixed radius of 35mm is optimised for horizontal focusing at the micro-beam experiment at 25m. The second mirror is made of Al and is equipped with a dynamical benderto parallelise or focus the beam in vertical direction. The second miror also has Pt and Si coatedparts laterally displaced. Moving the mirror horizontally one can choose between an energy cut-offof 30keV and 11keV, respectively. Both mirrors are 1000mm long and 110mm wide and accept2.6mm vertical beam height and 2.6mrad horizontal beam divergence. The second mirror chamberwas modified relative to HASYLAB standard in order to enable both the direct beam and themirrored beam to pass with fixed position of the second mirror. In this way, only the first mirror hasto be moved vertically for a change from mirrored beam to direct beam.

8.1m 9.4m 16.1m 17.7m 18.5m 24.5m

Exp.Multilayer MonochromatorSi-MonochromatorSlit 1.Mirror 2. Mirror

0mm

26mm

51mm

Figure 1: Layout of the new optical components of beamline L.

Figure 2: View into the open monochromator vessel. The Si monochromator is shown in the upper left(upstream) and the multilayer monochromator in the lower right (downstream).

The ultra-high vacuum of the storage ring is terminated from the monochromator vacuum by agraphite window of 25µm thickness. The transmission of the window is satisfactory down to 2keV

photon energy. A single vacuum vessel houses both the Si-monochromator (at 17.7m) and themultilayer monochromator (at 18.5m). The monochromators itself are described in detail in section3. The vessel has almost cylindrical shape and is 2.6m long and 1.2m wide. The vessel can beopened completely towards the top, which allows easy access to all monochromator components asshown in Fig. 2. A combination of turbomolecular pump and scroll pump enables the evacuation ofthe vessel within 3h and maintains a base pressure of 2×10-6mbar. A beryllium window of 400•mthickness separates the vacuum in the vessel from the helium atmosphere in the intermediate tube,which is connected to the micro-beam experiment at 24.5m. The tube is 3m long and can easily beremoved in favor of other (temporarily installed) experiments like TXRF. A rigid table is available,which serves as support for these setups. Windowless connection of high-vacuum experiments tothe monochromator vessel is possible in order to use the lower part of the photon energy range (2-6keV).

3. HYMO monochromator: design parameterWide energy range in fixed exit geometry, both narrow and broad energy band width, highharmonics suppression and high stability were basic demands, which determined the layout of themonochromator. These requirements translated into a monochromator combination, where a Sidouble monochromator operating in +/- geometry and a multilayer double monochromatoroperating in -/+ geometry are placed one behind the other on a single optical bench. The possibilityto apply both double monochromators in combination (hybrid monochromator mode) inspired thenaming HYMO for the monochromator system. A modular mount was chosen for the fourreflectors. Each reflector has its own tower with independend height translation, rotation for θ- andρ-angles and crystal changer with three positions, as shown in Fig. 3 and Fig. 4. The θ-movement isrealized by goniometers for the Si monochromator and as circle segments for the multilayermonochromator. A piezo-driven tilt table mounted on the first Si monochromator unit enables ultra-fine θ-angle adjustment and monochromator stabilization by feedback control (MOSTAB). Forfixed exit operation the second towers of Si- and multilayer monochromators are mounted onlongitudinal translations with ranges of (-17mm) – 290mm and 75mm – 700mm. A heighttranslation of 80mm for each reflector is needed to adjust with respect to the incident beam at thatparticular position. Taking the white beam as a reference, the pink beam from the upstream doublereflecting mirror has a 37mm offset (as indicated in Fig. 1). High resolution and multilayermonochromators deflect upwards and downwards, respectively, with a variable offset. In general,an offset of 15mm is chosen for the Si monochromator and an offset of 10 mm is chosen for themultilayer monochromator. The Si monochromator units are equipped with a second set of heigthtranslations for a precise adjustment of the crystal surfaces with respect to the axis of θ-rotation inorder to obtain fixed exit operation within the desired precision of +/- 20µm.

The monochromator crystal holders of all four reflectors are made of copper and the first holders ofSi-crystal and multilayer monochromators are water-cooled. Whereas the approximately 100W totalpower of the bending magnet radiation in the horizontally accepted angle of maximum 2mradwould lead for the Si monochromator to distortion only, for multilayers an irreversibletransformation of the layer structure occurs. Absorber foils and x-ray mirrors, cutting the lowenergy part and high energy part of the spectral distribution, respectively, further reduce the powerload on the optical elements. A closed-cycle scheme for the cooling water is used which operates atroom temperature and at low pressure. Thus, a laminar flow of cooling water is enabled in order tosuppress vibrations from the water cooling.

Figure 3: The Silicon double monochromator. a) Complete monochromator consisting of two independentunits. b) Water cooled crystal holder equipped with Si(111) and Si(113) crystals. c) Schematical drawing of

the first Si-monochromator unit

Figure 4: The Multilayer double monochromator. a) Complete monochromator consisting of twoindependent units. b) second multilayer holder equipped with Ni/C and W/C multilayers.

Each holder of both monochromators has three crystal positions. The maximum sizes are 50×20mm2 and 300×30 mm2 for Si crystals and multilayers, respectively. At present, the high energy-resolution monochromator is equipped with a Si(111) crystal pair (50×20 mm2) and a Si(113)crystal pair (30×20 mm2). The crystals are 10mm thick and have grooves at their sides whichenables strain-free mounting. Energy ranges of 3.5 – 75 keV and 3.8-100keV are accessable forSi(111) and Si(311) crystals, respectively, at a monochromator offset of 15mm. The energy rangeof the Si(111) monochromator can be extended to 2.2 keV (100 keV) by changing the offsetparameter to 25mm (11mm). The first pair of synthetic crystals applied in the broad bandmonochromator are Ni/C multilayers deposited on Si wafer material. The lattice spacing of 40 Ådetermines the usable energy range of 2.4 – 20 keV at a monochromator offset of 10mm. Using anoffset of only 3mm monochromatization at 60 keV is possible. The specification and performanceof this multilayer pair is described in section 4. Because of inhomogeneities of the monochromaticbeam profile, it is forseen to replace the multilayers with 10mm thick substrates and grooves at theside for strain-free mounting.

Several components for beam diagnostics and radiation protection complete the monochromatorsetup. They are mounted on height translations with 100mm range, which are placed on the sameoptical bench as the monochromator units. The first translation is located upstream of the firstmonocromator unit. It carries an electrically isolated copper cube for the characterisation of thewhite and mirrored beam and several Al filters of various thicknesses ranging from 0.01 – 2 mm.Beamstops are placed behind each second monochromator unit. At the end of the monochromatorvacuum vessel a fourth translation carries a ZnS fluorescence screen, a copper net and a tungstenwire, which are applied for measurements of the intensity and profile of the monochromatic beam.A two jaw slit system in front of the exit window of the monochromator tank defines themonochromatic beam vertically and reduces stray light contributions.

The optical bench is made of cast steele with a polished surface of high flatness. A precision alongall optical elements of better 20 µm is optained. Special care has been taken during alignment of theoptical elements as deviations will strongly affect the position of the beam spot when changing theenergy of the monochromatic beam (fixed exit condition). The optical bench has a dynamicalmount and can be adjusted horizontally and vertically. Vibrational decoupling of the optical benchincluding all monochromator components from the monochromator vessel is achieved by bellowsand mounting on separate concrete blocks, which are fixed on the floor of the experimental hall.

4. HYMO Monochromator: performanceThe performance of the Si- and multiplayer double monochromators was characterized with respectto energy resolution, divergence and, in the case of the Si monochromator, energy scanningcapability. For the sake of simplicity the permanent standard setup of the micro-fluorescenceexperiment depicted in Fig. 8 was applied for the measurements. An estimate of the energyresolution of the Si(111) double monochromator was obtained by scanning the incident angle θ ofthe second Si(111) crystal. The FWHM value of the resulting rocking curve is 8.6 arcsec at 12.7°and meets the requirements of a Si(111) monochromator (see Fig. 5 a). The vertical profile for themonochromatic beam was measured for various energies in the range 5-70keV. No manualreadjustment of monochromator units is neccessary after changing the energy within the completerange. The intensity of the monochromatic beam was measured by ionisation chambers (monitor)and the scattering signal from a teflon sample was used to determine the degree of linearpolarisation. The measured beam profiles for 11keV and 40keV are shown in Fig. 5 b-c. The height

of the beam at 11keV is determined by the opening of the slit system upstream of themonochromator. With increasing energy the vertical divergence of the beam becomes smaller andthe 50mm long Si(111) crystals are still capable to accept the full beam height. The ratio ofscattering to monitor signal indicates the degree of linear polarisation of the beam, which is animportant parameter for background suppression in XRF measurements. At both energies itscharacteristic minimum at the position of maximum intensity is preserved.

Figure 5: The Si(111) double monochromator. a) Rocking curve of the second crystal indicating the energyresolution. b-c) Monitor signal, scattering intensity and ratio of scattering to monitor as a function of thevertical position of the experiment table indicating the vertical beam profile and degree of polarization at

11keV and 40keV, respectively.

Figure 6: The NiC double multilayer monochromator. Rocking curve of the second multilayer. Thecoppernet inside the monochromator vessel is applied and the high background is due to straylight b-c)

Monitor signal, scattering intensity and ratio of scattering to monitor as a function of the vertical position ofthe experiment table indicating the vertical beam profile and degree of polarization at 11keV and 21keV,

respectively.

A pair of Ni/C multilayers, prepared at the IWS, Dresden by pulsed laser deposition of 100 periodsof 2nm thick Ni and 2nm thick C layers on Si wafer material, are first applied in the doublemultilayer monochromator. Energy resolution and profile of the monochromatized beam weredetermined in the same way as for the Si(111) monochromator. The monochromatic flux on thesample is 50 times higher for the Ni/C double monochromator compared to the Si(111) doublemonochromator and the rocking curve of the second Ni/C multilayer shows the expected energyresolution (see Fig. 6a). Though, the vertical beam profile is disturbed. Figure 6b and 6c showprofiles, which were measured at 11keV and 21keV, respectively. The profiles are wider than usingthe Si(111) monochromator, especially at higher photon energies, and the characteristic minimumin the ratio of scattering to monitor signal is not observed. It is assumed that slope errors of thewafer material – either intrinsic or introduced by the multilayer deposition or mounting of the

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a) b) c)

a) b) c)

multilayers on the monochromator – causes the irregular divergence of the beam. Therefore, thedeposition of the multilayers on compact Si mirrors of 10mm thickness is foreseen to improve themultilayer performance.

Figure 7: Performance of the double Si(111) monochromator for XAFS. a) XAFS spectrum of a pure copperfoil of 5µm thickness (4s sample time, 0.4×0.1 mm2 sample area, transmission mode). b) Derivatives of the

Cu preedge feature of 8 successive XAFS scans. The position is reproducible within the stepsize of thegoniometer of 0.2eV. The first scan is shifted by 0.5 eV. c) XANES spectrum around the Cu K edge on a40ppm multielement glass standard (NIST SRM 612) of 0.1mm thickness measured in fluorescence mode

(30s sample time, 1×1 mm2 sample area).

The main application of the Si-monochromator will be XANES or, in general, X-ray absorption finestructures (XAFS) experiments, for which the stability of the monochromatic beam afterincremental changes of the energy is crucial. The feasibility of the Si(111) monochromator for thiskind of experiments is demonstrated in Fig. 7. The XAFS scan shown in Fig. 7a reproduces thecharacteristic fine structure around a Cu-K absorbtion edge. The digital monochromatorstabilization system is capable to keep the intensity of the monochromatized beam stable within0.2%. In general, successive scans are shifted in energy by less than 0.2 eV (at 9keV), whichcorresponds to the step width of the monochromator goniometer (0.9 arcsec). Thus, Fig. 7b showsthat occasionally larger shifts were observed. Using the fluorescence signal instead of the

a)

b) c)

transmitted intensity, XANES measurements of trace elements are feasible down to the ppm levelas demonstrated in Fig 7c.

5. Microfocussing at the ExperimentOnly few changes and additional components were necessary in order to adapt the existing setup ofthe microfocus experiment to the new modes of mirrored and monochromatized beam and to enablemicro-XANES experiments (see Fig. 8). The vertical translation range of the experimental tablewas extended to 100mm and ionisation chambers and low current amplifiers were added. Thecentral optical element for the formation of the microbeam in monochromatic mode arepolycapillary half-lenses. These capillaries have the advantage of collecting radiation in a fairlylarge solid angle (collection area 2mm2) and can focus it down to sizes of a few ten micrometer.Moreover, they are non-dispersive and non-imaging optical elements. The size and position of themicrobeam is largely defined by the lens and does not depend strongly on size and position of thesource.

Figure 8:Schematic drawing of the µ-XANES and µ-SRXRF setup. A polycapillary lens focuses themonochromatic beam to a few ten micrometer.

At present, two different polycapillaries are available at beamline L. The characteristics of thepolycapillary supplied form X-ray Optics Laboratory, Bejing Normal University, was described byVincze et al. [4]. A second capillary made by X-ray Optical Systems (XOS) is applied forexperiments, which demand highest spatial resolution. Its basic features in terms of focal size,shape and gain in photon flux density are summarized in Fig. 9 relative to collimation by a pinhole.Large collection area along with high transmission (30-60%) enable to achieve a photon flux usedfor the XAFS experiments shown in Fig. 7 in a 15µm diameter focal spot (5×109 Photons/s/mm2 @10keV at the position of the experiment using the Si(111) double monochromator). In this wayspeciation of individual micron-sized samples and chemical species mapping with micrometerrange spatial resolution is possible at beamline L. Polycapillary lenses are further employed formonochromatic micro-SRXRF measurement.

Figure 9: Characteristics of XOS polycapillary lens. a-b) horizontal beam profile determined by wire scanand displayed in linear and logarithmic scale. c) gain factors. d) horizontal focus sizes (FWHM).

References

[1] F. Lechtenberg, S. Garbe, J. Bauch, D.B. Dingwell, J. Freitag, M. Haller, T.H. Hansteen, P. Ippach,A. Knöchel, M. Radtke, C. Romano, P.M. Sachs, H.-U. Schmincke, H.-J. Ullrich, J. Trace Micro.Anal. Tech. 14, 561 (1996)

[2] G. Falkenberg, O. Clauss, A. Swiderski, and Th. Tschentscher, X-ray Spectrom. 30, 170 (2001)[3] G. Falkenberg, O. Clauss, A. Swiderski, and Th. Tschentscher, Nuclear Instrum. Meth. A, 737

(2001)[4] L. Vincze, K. Janssens, F. Wei, C. Proost, B. Vekemmans, G. Vittiglio, Y. Yan, and G. Falkenberg,

HASYLAB 1999 annual report, URL: www-hasylab.desy.de/science/annual_report/1999_reports/index.htm

focus size

linear scalegain factor

logrithmicscale

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c)

d)