Optics and Metrology Group

Kawal Sawhney, Optics Group Leader

The Optics and Metrology Group continues to provide expert guidance that serves Diamond’s diverse beamlines and other synchrotrons^1-13. For Diamond-II, by simulating the effects of the source’s increased brightness on the beamline optics, we are ensuring that the beamlines will receive the greatest possible benefit. In metrology, we are developing new ex situ capabilities in micro-stitching interferometry and continuously improving our current techniques. Moreover, we are strengthening our at-wavelength characterisation of optics by using X-ray speckle tracking, ptychography, and crystal topography. Our work on novel optics and detectors, described below, has already been applied successfully to Diamond’s beamlines.

Optics for variable beam sizes

: Figure 1: (Left) Schematic of the multilane mirror – the schematic shows only three lanes
whereas the mirror installed on VMXm has seven lanes. (Right) Measured beam-size (fwhm) at
sample position on VMXm beamline as the mirror is translated across lanes 2 to 7 and inset, the
measured beam profile for mirror lane 1.

VMXm is a new beamline at Diamond, which is designed to produce a variable size micro-focused beam for rapid data collection on large molecule biological samples in crystalline form such as proteins. The requirements for the focused beam at the sample are that the focused intensity should be high and the beam size should be switchable in a range of sizes from under 0.5 μm to over 5 μm on a time scale of less than one second to enable high-speed data collection and to minimise radiation damage of the samples. This required use of a high aperture double mirror system (Kirkpatrick-Baez) to focus the X-ray beam from the undulator source to the sample position.

The vertically focusing mirror was a novel design developed by the Optics Group to provide a variable focused beam size. The mirror has a basic elliptical shape with a glancing incidence angle of 3 mrad and provides a high demagnification to give sub-0.5 μm focal spot. The mirror was, however, split into seven lanes, running the length of the mirror. The profile of the reflecting surface in each lane was a modification of the basic elliptical shape and calculated to generate a smooth symmetrical intensity distribution at the sample position, with full width at half maximum varying in sequence from 0.4 μm (lane 1) to 8 μm (lane 7). The change in beam size was achieved by translating the mirror to illuminate a different lane with the X-ray beam. Initial testing of the multilane mirror was carried out on the Diamond Test beamline (B16)6. Following first beam, the mirror was installed on the VMXm beamline and alignment was performed using wavefront sensing. The initial measurements showed a focused beam size of 0.41 μm using lane 1 of the mirror and beam-sizes in sequence 0.97, 1.42, 2.5, 4.7, 6.7, 8.7 μm for the remaining lanes (Fig. 1).

Multilayer-coated bimorph mirror for focusing high-energy X-rays

: Figure 2: Schematic of the Diamond I15-1 multilayer-coated bimorph mirror.

Diamond’s X-ray Pair Distribution Function (XPDF) beamline I15-1 presented the Optics and Metrology Group with a unique challenge. A wiggler X-ray beam with a large cross-section (11.0 mm H × 4.4 mm V) and photon energy ≥ 40 keV had to be focused into a spot 680 μm H × 20 μm, or smaller, at a distance continuously variable over nearly 1 m. The energy selection and horizontal focusing are performed together by a set of three bent Laue crystals, one for each energy 40.0 keV, 65.4 keV and 76.6 keV. However, the tight vertical focus demanded a precisely deformable optic of large aperture that would introduce extremely low phase errors throughout its operating range of focal distances. Neither lenses nor conventional mirrors could fulfil these needs with the high-energy X-rays used on I15-1. Instead, a multilayer-coated bimorph mirror of 1 m active length (Fig. 2), the first on a synchrotron beamline, was installed. Three parallel, laterally graded, multilayer stripes were deposited: Ni/B₄C for 40.0 keV, W/B₄C for 65.4 keV and Pt/B₄C for 76.6 keV. Each has a Bragg angle of 4.2 mrad, permitting a sufficient aperture. The bimorph has 16 actuators, allowing not only almost ideal elliptical shaping of the reflecting surface, but also correction of waviness and gravitational sag. Vertical focal sizes of 13 μm FWHM (full width at half maximum) have been achieved and waviness has been reduced to 0.5 μrad rms (root-mean-square). The relative rms error of the multilayer spacing over the mirror length is below 0.17%. Peak reflectivity at all points exceeds 70%.

White beam X-ray imaging system

: Figure 3: In-house developed white beam camera for B16
beamline.

The high flux of the white X-ray beam from third generation synchrotron light sources can significantly benefit the fast developing field of high-speed X-ray imaging, while the degradation of the scintillator screen due to high flux brings technical challenges to the existing X-ray imaging systems. To solve this issue, we have designed and established a novel white beam X-ray imaging system at beamline B16 by embedding the scintillator in a flowing inert-gas environment. The X-ray imaging system (Fig. 3) has demonstrated excellent performance under the exposure of white beam over days. The spatial resolution and field of view can be adjusted continuously by varying the positions of the lens and the camera using the motorised stages.

The new X-ray imaging system has been combined with a fly-scan tomography stage with a ZEBRA system. It enabled the data acquisition time for one tomography to decrease from hours down to less than 30 seconds by taking advantage of the intense white beam. The fast tomography setup has been routinely used at B16 for various applications such as biological tissues, functioning metals and geological samples. Recently, combined with the X-ray speckle imaging technique, we have employed the fast fly-scan tomography system for investigating Moon rocks recovered during the Apollo Missions. It has allowed us to swiftly and effectively collect 3D information of olivine inside the lunar rock in much more detail than ever before.

References:

Sawhney K. et al. Development of pseudo-perfect x-ray optics using refractive compensators. AIP Conf. Proc. 2054, 060003 (2019). DOI: 10.1063/1.5084634
Alcock S.G. et al. Dynamic adaptive X-ray optics. Part I. Time-resolved optical metrology investigation of the bending behaviour of piezoelectric bimorph deformable X-ray mirrors. J. Synchrotron Rad. 26, 36-44 (2019). DOI: 10.1107/S1600577518015953
Alcock S.G. et al. Dynamic adaptive X-ray optics. Part II. High-speed piezoelectric bimorph deformable Kirkpatrick-Baez mirrors for rapid variation of the 2D size and shape of X-ray beams. J. Synchrotron Rad. 26, 45-51 (2019). DOI: 10.1107/S1600577518015965
Zhou T. et al. Optimized alignment of X-ray mirrors with an automated speckle-based metrology tool. Rev. Sci. Instrum. 90, 021706 (2019). DOI: 10.1063/1.5057712
Sutter J.P. et al. A novel, 1 m long multilayer-coated piezo deformable bimorph mirror for focusing high-energy X-rays. AIP Conf. Proc. 2054, 030005 (2019). DOI: 10.1063/1.5084568
Laundy D. et al. Optical elements for dynamically broadening the focus of micro-focus optics at synchrotron x-ray sources. AIP Conf. Proc. 2054, 060006 (2019). DOI: 10.1063/1.5084637
Hand M. et al. Compensation of x-ray mirror distortion by cooling temperature control. AIP Conf. Proc. 2054, 060044 (2019). DOI: 10.1063/1.5084675
Sutter J.P. et al. Ideal Cartesian oval lens shape for refocusing an already convergent beam. AIP Conf. Proc. 2054, 030007 (2019). DOI: 10.1063/1.5084570
Alcock S.G. et al. High-speed adaptive optics using bimorph deformable x-ray mirrors. Rev. Sci. Instrum. 90, 021712 (2019). DOI: 10.1063/1.5060737
Wang H. et al. X-ray phase-contrast imaging with engineered porous materials over 50 keV. J. Synchrotron Rad. 25, 1182-1188 (2018). DOI: 10.1107/S1600577518005623
Zhou T. et al. Development of an X-ray imaging system to prevent scintillator degradation for white synchrotron radiation. J. Synchrotron Rad. 25, 801-807 (2018). DOI: 10.1107/S1600577518003193
Zhou T. et al. Auto-alignment of X-ray focusing mirrors with speckle-based at-wavelength metrology. Optics Express 26, 26961-26970 (2018). DOI: 10.1364/OE.26.026961
Zhou T. et al. Single-shot X-ray dark-field imaging with omnidirectional sensitivity using random-pattern wavefront modulator. Appl. Phys. Lett. 113, 091102 (2018). DOI: 10.1063/1.5047400

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Optics and Metrology Group

Kawal Sawhney, Optics Group Leader

Optics for variable beam sizes

Multilayer-coated bimorph mirror for focusing high-energy X-rays

White beam X-ray imaging system