Upgrade of the ultra-small-angle scattering (USAXS) beamline BW4

Transcription

1 Upgrade of the ultra-small-angle scattering (USAXS) beamline BW4 S.V. Roth, R. Döhrmann, M. Dommach, I. Kröger, T. Schubert, R. Gehrke Definition of the upgrade The wiggler beamline BW4 is dedicated to ultra-small-angle x-ray scattering (USAXS) in material science. After more than ten years of successful operation major refurbishment and upgrading has been performed, starting in 2003 [1]. This report describes the recent upgrade of the USAXScamera, the SAXS setups and the present status of BW4. New SAXS options The flight tube starting from the USAXS guard slit S2 located at the entrance of the beam into the experimental hutch has been reconstructed. It is now segmented into six modules to allow various distances between the sample position and the detector. Each module is mounted on a height adjustable support driven by stepping motors. By this means the flight tube between sample and detector can be linearly offset with respect to the incident beam to increase the accessible q-range or be tilted to adapt the flight path to the grazing incidence SAXS (GISAXS) geometry which combines SAXS and a reflection setup. The positioning is easily controlled via a Tcl/Tk-based perl script which can be run under the HASYLAB SPECTRA software [2]. A screen shot of the GUI of this script is shown in fig. 1a. To achieve a given angle of inclination determined by the angle of reflection α i or a given offset all or some of the supports can be moved simultaneously. The number and position x mot,j (j=1...n<7) of the supports depend on the sample to detector distance L SD and is determined in a parameter file. The inclination β i is done with respect to a defined point of rotation, usually a hinge at variable position. Fig. 1b shows the coordinate system used to rotate about the hinge. The automation and motorization reduces the time to switch from transmission geometry to SAXS significantly. The flight tube consists of two parts of different diameter. The first part with 9.5 m length is based on DN160 ISO-K tubes, whereas the final 2.0 m to the detector are formed by DN200 ISO-K tubes to increase the aperture and thereby facilitate (GI)USAXS experiments. A photograph of the refurbished beamline is shown in fig. 2. The use of six tube modules allows to install the sample environment at four basic distances 2 m<l SD <13 m. The length of the vertical traverse path of each support is 100 mm. Table 1 summarizes the different SAXS setups which can now be realized at BW4. Included are the different sample-to-detector distances (L SD ), the corresponding minimum and maximum detectable length scales (d min, d max ) and the beam size. The µ-focus option is described in greater detail in another contribution to this annual report dedicated to this topic. Here we concentrate on the standard beam size of 400x400 µm 2 as produced by the slit collimation system together with the two focusing mirrors of BW4. Using this beam size the highest resolution can be obtained with L SD =12.7 m. In transmission USAXS it is limited by the minimum size of the beam stop for the direct beam and then the maximum detectable length scale is d max =650 nm. The primary beam stop is a rectangular shaped lead block with a size of 6 mm in the horizontal and 3 mm in the vertical direction. A photo diode in its centre is used to monitor the intensity of the primary beam. The d max values for transmission SAXS, USAXS and µ SAXS given in table 1 are achieved with the beam stop centered symmetrically around the primary beam profile. The values in brackets are achieved in the vertical direction with an off-centre adjustment, where the primary beam profile is close to one edge of the beam stop to measure the scattered intensity as close as possible to the primary beam. For GISAXS, GIUSAXS and µ GISAXS only two separate point-like beam stops are used to shield the specularly reflected and primary beam and do not affect the resolution as in transmission geometry. Thus, the resolution is only limited by the divergence of the primary beam, the sampleto-detector distance L SD and the pixel size of the detector. The SAXS positions listed in table 1 require a guard slit mounted close to the sample position in order to optimize the achievable resolution. For this purpose a new piezo-driven slit system is used, see fig. 3a. This portable slit system is housed in a vacuum compatible enclosure equipped with DN160 ISO-K flanges (p<1x10-6 mbar possible) which can be attached to the flight tube modules at 73

2 the SAXS positions listed in table 1. Each slit bracket is mounted on a piezo driven vacuum stage with a nominal position resolution of x=±3 µm and can be moved independently. This leads to a nominal slit width error of 2 x=6 µm. A maximum gap of 30x30 mm can be achieved. Special care has been taken for the blade design. Fig. 3b shows a SEM image of the blade surface [3]. The design has been optimized with respect to minimum background, maximum absorption and easy production. It is manufactured from sintered tungsten powder. The major sources of background are total reflection of the direct beam on the blade surface or scattering from edges. To avoid these effects the part of the blade touching the x-ray beam has been chamfered by mechanical polishing to an entrance radius of R=400 µm. Furthermore, to maximize absorption at the (rounded) edges the curved surface steadily passes over to a plain one which is inclined under an angle of φ=4 with respect to the beam direction (see fig. 3b). This guarantees minimum parasitic scattering in the SAXS range (L SD <8m). Besides the formerly used two dimensional gas detector (Gabriel Type) a CCD detector (MAR Research) went into operation at BW4 very successfully. The detector has an active area of 160 mm diameter, demagnified via a glass fibre taper to the CCD area with 2048 x 2048 pixels with 79.1µm pixel size. The quantum efficiency for 10 kev photons is around 0.8. The nominal gain is about 6e - per photon at the used photon energy and the full well capacity is e - /pixel which corresponds to photons/pixel. The CCD chips are cooled down to -70 o C thus featuring a very low dark current of less than 0.01 e - /pixel/sec and a readout noise of less than 10 e - /pixel at 3.5 sec readout time. Fig. 4 shows two scattering patterns obtained with the CCD detector. The sample was a highdensity polyethylene (trade-name Lupolen from BASF) which is used as a calibration standard at BW4. The USAXS data were obtained at L SD =12.4 m, the SAXS data at LSD=3.8 m. Clearly up to three diffraction orders are visible in the SAXS pattern. One-dimensional (1D) pie integrated data are also shown in a plot combining both measurements. The data sets fit excellently to each other and show a large overlap in q. Upgrade of auxiliary equipment Monitoring the intensity of both incoming and transmitted beam is essential for data evaluation in small-angle scattering experiments. To minimize background scattering caused by air-filled gaps along the flight path new beam monitors have been designed and constructed. Fig. 5a shows the design of the standard multiflange ionization chamber. The width of the kapton window is 25 mm, the height 15 mm. The length of the air path is 20 mm. Fig. 5b shows a photograph of this routinely used ionization chamber. It can be mounted at the end of chosen segment to ensure intensity monitoring behind all optical elements. The monitor can be mounted directly to KF160 or KF40 flanges due to its multiflange adapters. For in-vacuum applications special care has to be taken to ensure a continuous vacuum in the flight tube. This design features four holes in the material between the active monitor area and the actual flange area to ensure a vacuum connection between adjacent segments. It has been successfully tested at BW4. Recently a latest version (fig. 5c) of these monitors has been introduced and tested at BW4. It is incorporated in a housing with KF40 flanges at both sides allowing a small sample to monitor distance (minimum air gap, <3 cm possible) limited only by the sample environment. A novel generic heating controller has been designed and implemented. It is now in use for user experiments and can control heating devices for temperatures up to T max =350 C. The controller can be programmed in a very flexible manner by a perl-script running under spectra. Various heating procedures can be set up an started under program control. Sample Stretching Device Various in-situ applications at BW4 include deformation experiments. In close collaboration with the users community we are currently developing and designing a new stretching device, which can also be used at other SAXS beamlines of HASYLAB. The design is shown in Fig. 6. A first prototype will be completed in the beginning of The main design goals are as follows: 74

3 Synchronous stretching operation Maximum elongation L=640 mm, observation via TV camera Determination of the actual stretching distance via absolute position encoders Horizontal positioning of the whole stretching device via separate motor driven stage Maximum load F max = 500 N, load cells to monitor the force within various ranges (maximum error 0.2%) In-situ temperature control Operation entirely controlled via PC Maximum weight W max = 60kg Acknowledgement We would like to thank A. Frömsdorf and A. Kornowski for producing the SEM image of the collimation slit blades. L SD [m] d min [nm] d max [nm] beam size [µm 2 ] SAXS x x400 GISAXS > x400 USAXS x (12.7*) (1 µm*) 400x400 GIUSAXS > x400 µ SAXS (150) 65x35 µ GISAXS >400 65x35 Table 1: Overview of the different SAXS configurations at BW4. * The highest d max for transmission USAXS was measured at L SD =12.7 m. 75

4 Figure 1a: Screenshot of the perl-based GUI for controlling the movement of the flight tube supports. The graphics in the lower part give a sketch of the actual and projected flight tube / support positions together with the position of sample, hinge (rotation point) and detector. b: coordinate system for inclining the flight tube in a GiSAXS experiment. The beam is coming from the left side. The reflection angle of the specular peak is 2α i, the inclination angle of the flight tube with respect to a rotation point is 2β i. The positions of the supports are x motj, j=1...n<7 [2]. 76

5 Figure 2: Photograph of BW4 as seen from detector. One of the motorized supports is indicated by ST. CCD denotes the MAR CCD detector, S the sample position at L SD =3.8 m. Figure 3 a: Photograph of the piezo-driven slit. b. Scanning electron microscope (SEM) image of the edge of the tugsten blade [3]. 77

6 Figure 4: USAXS (LSD=13 m) and SAXS (LSD=3.8 m) of Lupolen. Upper panel: 2D Images taken with the CCD detector. The horizontal lines illustrate the resolution limits. Lower panel: Combined pie-integrated 1D USAXS and SAXS data in a double-logarithmic plot. The overlap region on the q-axis is large, and both curves can be perfectly combined. Figure 5 a: Sketch of the new beam monitor.in: high voltage input, KF160, KF40: flange adapter s, KW: Kapton window (thickness t KW =150 µm), OUT: Signal output to amplifier. b: Photograph of the installed device. c: Sketch of the latest (small) KF40 version to reduce sample to monitor distance. 78

7 Figure 6: Sketch of the stretching cell. The beam position is inidicated References [1] R. Gehrke, HASYLAB Annual report 2004 [2] I. Kröger, Trainee Report, HASYLAB 2005 [3] SEM-image taken at University of Hamburg by A. Frömsdorf and A. Kornowski 79