Publications by authors named "Kawal Sawhney"

48 Publications

Investigation of the stripe patterns from X-ray reflection optics.

Opt Express 2021 Feb;29(3):4270-4286

X-ray beams reflected from a single layer or multilayer coating are widely used for X-ray tomography, holography, and X-ray phase contrast imaging. However, the observed irregular stripe patterns from either unfocused or defocused beams often cause disturbing artifacts and seriously deteriorate the image quality. In this work, we investigate the origin of these irregular fine structures using the wave optics theory. The connection to similar results obtained by the geometric optics theory is also presented. The proposed relation between the second derivative of the wavefront and the irregular structures was then verified by conducting at-wavelength metrology with the speckle-based wavefront sensing technique. This work will not only help to understand the formation of these irregular structures but also provide the basis for manufacturing future 'stripe-free' refection optics.
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http://dx.doi.org/10.1364/OE.417030DOI Listing
February 2021

Hard X-ray omnidirectional differential phase and dark-field imaging.

Proc Natl Acad Sci U S A 2021 Mar;118(9)

Diamond Light Source Ltd., Harwell Science and Innovation Campus, Didcot, OX11 0DE, United Kingdom.

Ever since the discovery of X-rays, tremendous efforts have been made to develop new imaging techniques for unlocking the hidden secrets of our world and enriching our understanding of it. X-ray differential phase contrast imaging, which measures the gradient of a sample's phase shift, can reveal more detail in a weakly absorbing sample than conventional absorption contrast. However, normally only the gradient's component in two mutually orthogonal directions is measurable. In this article, omnidirectional differential phase images, which record the gradient of phase shifts in all directions of the imaging plane, are efficiently generated by scanning an easily obtainable, randomly structured modulator along a spiral path. The retrieved amplitude and main orientation images for differential phase yield more information than the existing imaging methods. Importantly, the omnidirectional dark-field images can be simultaneously extracted to study strongly ordered scattering structures. The proposed method can open up new possibilities for studying a wide range of complicated samples composed of both heavy, strongly scattering atoms and light, weakly scattering atoms.
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http://dx.doi.org/10.1073/pnas.2022319118DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7936267PMC
March 2021

Hard X-ray ptychography for optics characterization using a partially coherent synchrotron source.

J Synchrotron Radiat 2020 Nov 16;27(Pt 6):1688-1695. Epub 2020 Oct 16.

Department of Engineering Science, University of Oxford, Parks Road, Oxford OX1 3PJ, United Kingdom.

Ptychography is a scanning coherent diffraction imaging technique which provides high resolution imaging and complete spatial information of the complex electric field probe and sample transmission function. Its ability to accurately determine the illumination probe has led to its use at modern synchrotrons and free-electron lasers as a wavefront-sensing technique for optics alignment, monitoring and correction. Recent developments in the ptychography reconstruction process now incorporate a modal decomposition of the illuminating probe and relax the restriction of using sources with high spatial coherence. In this article a practical implementation of hard X-ray ptychography from a partially coherent X-ray source with a large number of modes is demonstrated experimentally. A strongly diffracting Siemens star test sample is imaged using the focused beam produced by either a Fresnel zone plate or beryllium compound refractive lens. The recovered probe from each optic is back propagated in order to plot the beam caustic and determine the precise focal size and position. The power distribution of the reconstructed probe modes also allows the quantification of the beams coherence and is compared with the values predicted by a Gaussian-Schell model and the optics exit intensity.
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http://dx.doi.org/10.1107/S1600577520012151DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7642961PMC
November 2020

Correction of the X-ray wavefront from compound refractive lenses using 3D printed refractive structures.

J Synchrotron Radiat 2020 Nov 19;27(Pt 6):1518-1527. Epub 2020 Oct 19.

Diamond Light Source, Harwell Science and Innovation Campus, Didcot, Oxon OX11 0DE, United Kingdom.

A refractive phase corrector optics is proposed for the compensation of fabrication error of X-ray optical elements. Here, at-wavelength wavefront measurements of the focused X-ray beam by knife-edge imaging technique, the design of a three-dimensional corrector plate, its fabrication by 3D printing, and use of a corrector to compensate for X-ray lens figure errors are presented. A rotationally invariant corrector was manufactured in the polymer IP-S using additive manufacturing based on the two-photon polymerization technique. The fabricated corrector was characterized at the B16 Test beamline, Diamond Light Source, UK, showing a reduction in r.m.s. wavefront error of a Be compound refractive Lens (CRL) by a factor of six. The r.m.s. wavefront error is a figure of merit for the wavefront quality but, for X-ray lenses, with significant X-ray absorption, a form of the r.m.s. error with weighting proportional to the transmitted X-ray intensity has been proposed. The knife-edge imaging wavefront-sensing technique was adapted to measure rotationally variant wavefront errors from two different sets of Be CRL consisting of 98 and 24 lenses. The optical aberrations were then quantified using a Zernike polynomial expansion of the 2D wavefront error. The compensation by a rotationally invariant corrector plate was partial as the Be CRL wavefront error distribution was found to vary with polar angle indicating the presence of non-spherical aberration terms. A wavefront correction plate with rotationally anisotropic thickness is proposed to compensate for anisotropy in order to achieve good focusing by CRLs at beamlines operating at diffraction-limited storage rings.
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http://dx.doi.org/10.1107/S1600577520011765DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7642970PMC
November 2020

Room-temperature performance of 3 mm-thick cadmium-zinc-telluride pixel detectors with sub-millimetre pixelization.

J Synchrotron Radiat 2020 Sep 17;27(Pt 5):1180-1189. Epub 2020 Aug 17.

Department of Physics and Chemistry (DiFC) - Emilio Segrè, University of Palermo, Viale delle Scienze, Edificio 18, 90128 Palermo, Italy.

Cadmium-zinc-telluride (CZT) pixel detectors represent a consolidated choice for the development of room-temperature spectroscopic X-ray imagers, finding important applications in medical imaging, often as detection modules of a variety of new SPECT and CT systems. Detectors with 3-5 mm thicknesses are able to efficiently detect X-rays up to 140 keV giving reasonable room-temperature energy resolution. In this work, the room-temperature performance of 3 mm-thick CZT pixel detectors, recently developed at IMEM/CNR of Parma (Italy), is presented. Sub-millimetre detector arrays with pixel pitch less than 500 µm were fabricated. The detectors are characterized by good room-temperature performance even at high bias voltage operation (6000 V cm), with energy resolutions (FWHM) of 3% (1.8 keV) and 1.6% (2 keV) at 59.5 keV and 122.1 keV, respectively. Charge-sharing investigations were performed with both uncollimated and collimated synchrotron X-ray beams with particular attention to recovering the charge losses at the inter-pixel gap region. High rate measurements demonstrated the absence of high-flux radiation-induced polarization phenomena up to 25 × 10 photons mm s.
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http://dx.doi.org/10.1107/S1600577520008942DOI Listing
September 2020

Detection and imaging of gadolinium accumulation in human bone tissue by micro- and submicro-XRF.

Sci Rep 2020 04 14;10(1):6301. Epub 2020 Apr 14.

TU Wien, Atominstitut, Stadionallee 2, Vienna, 1020, Austria.

Gadolinium-based contrast agents (GBCAs) are frequently used in patients undergoing magnetic resonance imaging. In GBCAs gadolinium (Gd) is present in a bound chelated form. Gadolinium is a rare-earth element, which is normally not present in human body. Though the blood elimination half-life of contrast agents is about 90 minutes, recent studies demonstrated that some tissues retain gadolinium, which might further pose a health threat due to toxic effects of free gadolinium. It is known that the bone tissue can serve as a gadolinium depot, but so far only bulk measurements were performed. Here we present a summary of experiments in which for the first time we mapped gadolinium in bone biopsy from a male patient with idiopathic osteoporosis (without indication of renal impairment), who received MRI 8 months prior to biopsy. In our studies performed by means of synchrotron radiation induced micro- and submicro-X-ray fluorescence spectroscopy (SR-XRF), gadolinium was detected in human cortical bone tissue. The distribution of gadolinium displays a specific accumulation pattern. Correlation of elemental maps obtained at ANKA synchrotron with qBEI images (quantitative backscattered electron imaging) allowed assignment of Gd structures to the histological bone structures. Follow-up beamtimes at ESRF and Diamond Light Source using submicro-SR-XRF allowed resolving thin Gd structures in cortical bone, as well as correlating them with calcium and zinc.
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http://dx.doi.org/10.1038/s41598-020-63325-9DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7156386PMC
April 2020

Room-temperature X-ray response of cadmium-zinc-telluride pixel detectors grown by the vertical Bridgman technique.

J Synchrotron Radiat 2020 Mar 27;27(Pt 2):319-328. Epub 2020 Jan 27.

IMEM-CNR, Parco Area delle Scienze 37/A, 43100 Parma, Italy.

In this work, the spectroscopic performances of new cadmium-zinc-telluride (CZT) pixel detectors recently developed at IMEM-CNR of Parma (Italy) are presented. Sub-millimetre arrays with pixel pitch less than 500 µm, based on boron oxide encapsulated vertical Bridgman grown CZT crystals, were fabricated. Excellent room-temperature performance characterizes the detectors even at high-bias-voltage operation (9000 V cm), with energy resolutions (FWHM) of 4% (0.9 keV), 1.7% (1 keV) and 1.3% (1.6 keV) at 22.1, 59.5 and 122.1 keV, respectively. Charge-sharing investigations were performed with both uncollimated and collimated synchrotron X-ray beams with particular attention to the mitigation of the charge losses at the inter-pixel gap region. High-rate measurements demonstrated the absence of high-flux radiation-induced polarization phenomena up to 2 × 10 photons mm s. These activities are in the framework of an international collaboration on the development of energy-resolved photon-counting systems for high-flux energy-resolved X-ray imaging.
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http://dx.doi.org/10.1107/S1600577519015996DOI Listing
March 2020

Absolute metrology method of the x-ray mirror with speckle scanning technique.

Appl Opt 2019 Nov;58(31):8658-8664

As an important characterization method for beamline optics, at-wavelength metrology technology based on wavefront measurements has been developed for many years. However, the previous studies on at-wavelength metrology of reflective mirrors is limited to the indirect method. So, the accurate surface information of the mirror under test would normally be inaccessible because of lack of experimental deconvolution between the mirror and any backgrounds from upstream optics. In this study, an absolute metrology method is developed based on the speckle scanning technique. Using this method, the surface profile of the mirror can be extracted exactly from the mixed information of the entire upstream beamline. At the same time, data acquisition time can also be significantly reduced by the processing algorithm introduced in this study without sacrificing the angular sensitivity.
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http://dx.doi.org/10.1364/AO.58.008658DOI Listing
November 2019

High-energy, high-resolution, fly-scan X-ray phase tomography.

Sci Rep 2019 06 20;9(1):8913. Epub 2019 Jun 20.

Diamond Light Source Ltd, Harwell Science and Innovation Campus, Didcot, OX11 0DE, UK.

High energy X-ray phase contrast tomography is tremendously beneficial to the study of thick and dense materials with poor attenuation contrast. Recently, the X-ray speckle-based imaging technique has attracted widespread interest because multimodal contrast images can now be retrieved simultaneously using an inexpensive wavefront modulator and a less stringent experimental setup. However, it is time-consuming to perform high resolution phase tomography with the conventional step-scan mode because the accumulated time overhead severely limits the speed of data acquisition for each projection. Although phase information can be extracted from a single speckle image, the spatial resolution is deteriorated due to the use of a large correlation window to track the speckle displacement. Here we report a fast data acquisition strategy utilising a fly-scan mode for near field X-ray speckle-based phase tomography. Compared to the existing step-scan scheme, the data acquisition time can be significantly reduced by more than one order of magnitude without compromising spatial resolution. Furthermore, we have extended the proposed speckle-based fly-scan phase tomography into the previously challenging high X-ray energy region (120 keV). This development opens up opportunities for a wide range of applications where exposure time and radiation dose are critical.
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http://dx.doi.org/10.1038/s41598-019-45561-wDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6586786PMC
June 2019

High-speed adaptive optics using bimorph deformable x-ray mirrors.

Rev Sci Instrum 2019 Feb;90(2):021712

Diamond Light Source Ltd, Harwell Science and Innovation Campus, Didcot, Oxfordshire OX11 0DE, United Kingdom.

Recently, the dynamic performance of piezo-electric deformable "bimorph" mirrors for synchrotron radiation and X-ray free electron laser sources has been characterized and significantly improved. This innovation enables high intensity X-ray beams to be rapidly focused or defocused to either match to the size of the sample under test or to select different sized regions of interest in larger samples. In this paper, we extend these results by monitoring a bimorph mirror using a combination of ex situ metrology instruments. Comparison between results from the Diamond-NOM (Nanometre Optical Metrology) slope profiler, a Fizeau interferometer, and Zygo ZPS distance measuring probes shows that bimorph X-ray mirrors can reliably and accurately be driven at 1 Hz using advanced features recently added to the high voltage (HV), bipolar "HV-Adaptos" power supply from CAEN.
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http://dx.doi.org/10.1063/1.5060737DOI Listing
February 2019

A water-cooled monochromator for the B16 Test beamline at the Diamond Light Source: capabilities and performance characterization.

J Synchrotron Radiat 2019 Jan 1;26(Pt 1):253-262. Epub 2019 Jan 1.

Instrument Design Technology Ltd, Unit 2 Turnstone Business Park, Mulberry Avenue, Widnes WA8 0WN, UK.

Systematic studies of the performance of a water-cooled X-ray monochromator, designed and built for the B16 Test beamline at the Diamond Light Source, UK, are presented. A technical description of the monochromator is given and the results of commissioning measurements are discussed. Overall, the monochromator satisfies the original specifications well and meets all the major requirements of the versatile beamline. Following its successful implementation on B16, the basic monochromator design has been reproduced and adapted on other Diamond Light Source beamlines, including B18 and B21.
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http://dx.doi.org/10.1107/S1600577518014662DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6337885PMC
January 2019

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 Radiat 2019 Jan 1;26(Pt 1):45-51. Epub 2019 Jan 1.

Diamond Light Source, Harwell Science and Innovation Campus, Didcot, Oxfordshire OX11 0DE, UK.

The tangential curvature of actively bent X-ray mirrors at synchrotron radiation and X-ray free-electron laser (XFEL) facilities is typically only changed every few hours or even days. This operation can take tens of minutes for active optics with multiple bending actuators and often requires expert guidance using in situ monitoring devices. Hence, the dynamic performance of active X-ray optics for synchrotron beamlines has historically not been exploited. This is in stark contrast to many other scientific fields. However, many areas of synchrotron radiation and XFEL science, including macromolecular crystallography, could greatly benefit from the ability to change the size and shape of the X-ray beam rapidly and continuously. The advantages of this innovative approach are twofold: a large reduction in the dead time required to change the size of the X-ray beam for different-sized samples and the possibility of making multiple changes to the beam during the measurement of a single sample. In the preceding paper [Part I; Alcock, Nistea, Signorato & Sawhney (2019), J. Synchrotron Rad. 26, 36-44], which accompanies this article, high-speed visible-light Fizeau interferometry was used to identify the factors which influence the dynamic bending behaviour of piezoelectric bimorph deformable X-ray mirrors. Building upon this ex situ metrology study, provided here is the first synchrotron radiation beamline implementation of high-speed adaptive X-ray optics using two bimorphs operating as a Kirkpatrick-Baez pair. With optimized substrates, novel opto-mechanical holders and a next-generation high-voltage power supply, the size of an X-ray beam was rapidly and repeatedly switched in <10 s. Of equal importance, it is also shown that compensation of piezoelectric creep ensures that the X-ray beam size remains stable for more than 1 h after making a major change. The era of high-speed adaptive X-ray optics for synchrotron radiation and XFEL beamlines has begun.
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http://dx.doi.org/10.1107/S1600577518015965DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6337883PMC
January 2019

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 Radiat 2019 Jan 1;26(Pt 1):36-44. Epub 2019 Jan 1.

Diamond Light Source, Harwell Science and Innovation Campus, Didcot, Oxfordshire OX11 0DE, UK.

Piezoelectric bimorph deformable mirrors (`bimorphs') are routinely used on many synchrotron and free-electron laser beamlines to provide active variation in the size and shape of the X-ray beam. However, the time-domain potential of such optics has never been fully exploited. For the first time, the fast dynamic bending response of bimorphs is investigated here using Fizeau interferometry. Automated scripts for acquisition and analysis were developed to collect Fizeau data at a rate of 0.1 Hz to record dynamic changes in the optical surface as voltages were applied to the electrodes of the piezoelectric actuators. It is demonstrated that residual drift in the tangential radius of curvature of a bimorph can be significantly reduced using enhanced opto-mechanical holders and a fast programmable high-voltage power supply. Further improvements are achieved by applying small opposing voltages to compensate for piezoelectric creep. The present study shows that bimorphs can truly be used as high-speed adaptive optics for the X-ray domain, even without closed-loop feedback correction. This opens the possibility for relatively simple real-time tuning of the profile of X-ray bimorphs. Part II of this study [Alcock, Nistea, Signorato, Owen, Axford, Sutter, Foster & Sawhney (2019), J. Synchrotron Rad. 26, 45-51] builds upon these results and demonstrates how bimorphs can rapidly provide customisable sizes and shapes of synchrotron X-ray beams, specifically tailored to suit the experimental samples being investigated.
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http://dx.doi.org/10.1107/S1600577518015953DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6337880PMC
January 2019

Auto-alignment of X-ray focusing mirrors with speckle-based at-wavelength metrology.

Opt Express 2018 Oct;26(21):26961-26970

Significant improvements have been made in the fabrication of diffraction-limited X-ray optics used to pursue an aberration-free wavefront. Alignment of these optics plays a crucial role in the resultant beam quality. Here, we present a simple and fast alignment method based on imaging X-ray near-field speckle patterns, with experimental demonstration using a pair of Kirkpatrick-Baez mirrors. The proposed technique has the potential to be an alternative to conventional methods. It loosens the stringent demand for high-resolution scanning stages compared to conventional knife-edge scan and, hence, can be applied to nano-focusing optics. The flexibility and straightforward implementation of the method allow it to be applied to a wide range of experiments at synchrotron facilities and laboratory-based sources.
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http://dx.doi.org/10.1364/OE.26.026961DOI Listing
October 2018

X-ray phase-contrast imaging with engineered porous materials over 50 keV.

J Synchrotron Radiat 2018 Jul 12;25(Pt 4):1182-1188. Epub 2018 Jun 12.

Diamond Light Source, Harwell Science & Innovation Campus, Didcot OX11 0DE, UK.

X-ray phase-contrast imaging can substantially enhance image contrast for weakly absorbing samples. The fabrication of dedicated optics remains a major barrier, especially in high-energy regions (i.e. over 50 keV). Here, the authors perform X-ray phase-contrast imaging by using engineered porous materials as random absorption masks, which provides an alternative solution to extend X-ray phase-contrast imaging into previously challenging higher energy regions. The authors have measured various samples to demonstrate the feasibility of the proposed engineering materials. This technique could potentially be useful for studying samples across a wide range of applications and disciplines.
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http://dx.doi.org/10.1107/S1600577518005623DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6038599PMC
July 2018

Dual-polarity pulse processing and analysis for charge-loss correction in cadmium-zinc-telluride pixel detectors.

J Synchrotron Radiat 2018 Jul 26;25(Pt 4):1078-1092. Epub 2018 Jun 26.

IMEM/CNR, Parco Area delle Scienze 37/A, Parma 43100, Italy.

Charge losses at the inter-pixel gap are typical drawbacks in cadmium-zinc-telluride (CZT) pixel detectors. In this work, an original technique able to correct charge losses occurring after the application of charge-sharing addition (CSA) is presented. The method, exploiting the strong relation between the energy after CSA and the beam position at the inter-pixel gap, allows the recovery of charge losses and improvements in energy resolution. Sub-millimetre CZT pixel detectors were investigated with both uncollimated radiation sources and collimated synchrotron X-rays, at energies below and above the K-shell absorption energy of the CZT material. The detectors are DC coupled to fast and low-noise charge-sensitive preamplifiers (PIXIE ASIC) and followed by a 16-channel digital readout electronics, performing multi-parameter analysis (event arrival time, pulse shape, pulse height). Induced-charge pulses with negative polarity were also observed in the waveforms from the charge-sensitive preamplifiers (CSPs) at energies >60 keV. The shape and the height of these pulses were analysed, and their role in the mitigation of charge losses in CZT pixel detectors. These activities are in the framework of an international collaboration on the development of energy-resolved photon-counting systems for spectroscopic X-ray imaging (5-140 keV).
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http://dx.doi.org/10.1107/S1600577518006422DOI Listing
July 2018

Pulse picker for synchrotron radiation driven by a surface acoustic wave: erratum.

Opt Lett 2018 06;43(11):2490

An erratum is presented to correct the typographical errors concerning the composition of the multilayer used in the experiment in Opt. Lett. 42, 1915.
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http://dx.doi.org/10.1364/OL.43.002490DOI Listing
June 2018

Development of an X-ray imaging system to prevent scintillator degradation for white synchrotron radiation.

J Synchrotron Radiat 2018 May 24;25(Pt 3):801-807. Epub 2018 Apr 24.

Diamond Light Source Ltd, Harwell Science and Innovation Campus, Didcot OX11 0DE, UK.

The high flux of the white X-ray beams from third-generation synchrotron light sources can significantly benefit the development of high-speed X-ray imaging, but can also bring technical challenges to existing X-ray imaging systems. One prevalent problem is that the image quality deteriorates because of dust particles accumulating on the scintillator screen during exposure to intense X-ray radiation. Here, this problem has been solved by embedding the scintillator in a flowing inert-gas environment. It is also shown that the detector maintains the quality of the captured images even after days of X-ray exposure. This modification is cost-efficient and easy to implement. Representative examples of applications using the X-ray imaging system are also provided, including fast tomography and multimodal phase-contrast imaging for biomedical and geological samples.
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http://dx.doi.org/10.1107/S1600577518003193DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5929358PMC
May 2018

Quantitative investigation of linear arbitrary polarization in an APPLE-II undulator.

J Synchrotron Radiat 2018 Mar 22;25(Pt 2):378-384. Epub 2018 Feb 22.

Diamond Light Source, Harwell Science and Innovation Campus, Didcot OX11 0DE, UK.

Insertion devices are utilized at synchrotron radiation facilities around the world for their capability to provide a high-brilliance X-ray beam. APPLE-II type undulators are especially important for their capacity to switch between a variety of photon beam polarization states. A high-precision soft X-ray polarimeter has been used to investigate the polarization calibration of an APPLE-II undulator (period length λ = 64 mm) installed on beamline I06 at Diamond Light Source. Systematic measurement of the beam polarization state at a range of linear arbitrary angles has been compared with the expected result for a given set of undulator gap and row phase parameters calculated from theory. Determination of the corresponding Stokes-Poincaré parameters from the measured data reveals a discrepancy between the two. The limited number of energy/polarization combinations included in the undulator calibration tables necessitates the use of interpolated values for the missing points which is expected to contribute to the discrepancy. However, by modifying the orbit of the electron beam through the undulator by at least 160 µm it has been found that for certain linear polarizations the discrepancies can be corrected. Overall, it is suggested that complete correction of the Stokes-Poincaré parameters for all linear angles would require alteration of both these aspects.
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http://dx.doi.org/10.1107/S1600577518001960DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5829679PMC
March 2018

In situ beamline analysis and correction of active optics. Erratum.

J Synchrotron Radiat 2017 09 1;24(Pt 5):1112. Epub 2017 Sep 1.

Diamond Light Source Ltd, Harwell Science and Innovation Campus, Chilton, Didcot, Oxfordshire OX11 0DE, UK.

A correction to one of the equations in the paper by Sutter et al. (2012). [J. Synchrotron Rad. 19, 960-968] is made.
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http://dx.doi.org/10.1107/S1600577517012322DOI Listing
September 2017

Analytic theory of alternate multilayer gratings operating in single-order regime.

Opt Express 2017 Jul;25(14):15987-16001

Using the coupled wave approach (CWA), we introduce the analytical theory for alternate multilayer grating (AMG) operating in the single-order regime, in which only one diffraction order is excited. Differing from previous study analogizing AMG to crystals, we conclude that symmetrical structure, or equal thickness of the two multilayer materials, is not the optimal design for AMG and may result in significant reduction in diffraction efficiency. The peculiarities of AMG compared with other multilayer gratings are analyzed. An influence of multilayer structure materials on diffraction efficiency is considered. The validity conditions of analytical theory are also discussed.
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http://dx.doi.org/10.1364/OE.25.015987DOI Listing
July 2017

Using refractive optics to broaden the focus of an X-ray mirror.

J Synchrotron Radiat 2017 07 30;24(Pt 4):744-749. Epub 2017 May 30.

Indus Synchrotrons Utilisation Division, Raja Ramanna Centre for Advanced Technology, Indore 452012, India.

X-ray mirrors are widely used at synchrotron radiation sources for focusing X-rays into focal spots of size less than 1 µm. The ability of the beamline optics to change the size of this spot over a range up to tens of micrometres can be an advantage for many experiments such as X-ray microprobe and X-ray diffraction from micrometre-scale crystals. It is a requirement that the beam size change should be reproducible and it is often essential that the change should be rapid, for example taking less than 1 s, in order to allow high data collection rates at modern X-ray sources. In order to provide a controlled broadening of the focused spot of an X-ray mirror, a series of refractive optical elements have been fabricated and installed immediately before the mirror. By translation, a new refractive element is moved into the X-ray beam allowing a variation in the size of the focal spot in the focusing direction. Measurements using a set of prefabricated refractive structures with a test mirror showed that the focused beam size could be varied from less than 1 µm to over 10 µm for X-rays in the energy range 10-20 keV. As the optics is in-line with the X-ray beam, there is no effect on the centroid position of the focus. Accurate positioning of the refractive optics ensures reproducibility in the focused beam profile and no additional re-alignment of the optics is required.
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http://dx.doi.org/10.1107/S1600577517006038DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5493024PMC
July 2017

Pulse picker for synchrotron radiation driven by a surface acoustic wave.

Opt Lett 2017 May;42(10):1915-1918

A functional test for a pulse picker for synchrotron radiation was performed at Diamond Light Source. The purpose of a pulse picker is to select which pulse from the synchrotron hybrid-mode bunch pattern reaches the experiment. In the present work, the Bragg reflection on a Si/B4C multilayer was modified using surface acoustic wave (SAW) trains. Diffraction on the SAW alters the direction of the x rays and it can be used to modulate the intensity of the x rays that reach the experimental chamber. Using electronic modulation of the SAW amplitude, it is possible to obtain different scattering conditions for different x-ray pulses. To isolate the single bunch, the state of the SAW must be changed in the short time gap between the pulses. To achieve the necessary time resolution, the measurements have been performed in conical diffraction geometry. The achieved time resolution was 120 ns.
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http://dx.doi.org/10.1364/OL.42.001915DOI Listing
May 2017

Design of a multilayer-based collimated plane-grating monochromator for tender X-ray range.

J Synchrotron Radiat 2017 01 1;24(Pt 1):168-174. Epub 2017 Jan 1.

MOE Key Laboratory of Advanced Micro-Structured Materials, Institute of Precision Optical Engineering, School of Physics Science and Engineering, Tongji University, Shanghai 200092, People's Republic of China.

Collimated plane-grating monochromators (cPGMs), consisting of a plane mirror and plane diffraction grating, are essential optics in synchrotron radiation sources for their remarkable flexibility and good optical characteristics in the soft X-ray region. However, the poor energy transport efficiency of a conventional cPGM (single-layer-coated) degrades the source intensity and leaves reduced flux at the sample, especially for the tender X-ray range (1-4 keV) that covers a large number of K- and L-edges of medium-Z elements, and M-edges of high-Z elements. To overcome this limitation, the use of a multilayer-based cPGM is proposed, combining a multilayer-coated plane mirror with blazed multilayer gratings. With this combination, the effective efficiency of cPGMs can be increased by an order of magnitude compared with the conventional single-layer cPGMs. In addition, higher resolving power can be achieved with improved efficiency by increasing the blaze angle and working at higher diffraction order.
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http://dx.doi.org/10.1107/S1600577516017884DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5182023PMC
January 2017

Creating flat-top X-ray beams by applying surface profiles of alternating curvature to deformable piezo bimorph mirrors.

J Synchrotron Radiat 2016 11 12;23(Pt 6):1333-1347. Epub 2016 Oct 12.

Diamond Light Source Ltd, Harwell Science and Innovation Campus, Chilton, Didcot, Oxfordshire OX11 0DE, UK.

Beam shaping is becoming increasingly important for synchrotron X-ray applications. Although routine for visible light lasers, this is challenging for X-rays due to the limited source coherence and extreme optical tolerances required for the shaping mirrors. In deliberate defocusing, even surface errors <5 nm r.m.s. introduce damagingly large striations into the reflected beam. To counteract such problems, surface modifications with alternating concave and convex curvature on equal segments were polished onto the surface of non-active mirrors of fixed curvature. Such optics are useful for providing a fixed size of X-ray beam, but do not provide the adaptability required by many experiments. In contrast, deformable piezo bimorph mirrors permit a continuous range of X-ray beam sizes and shapes. A new theory is developed for applying non-periodic modifications of alternating curvature to optical surfaces. The position and length of the segments may be freely chosen. For the first time, surface modifications of alternating curvature are applied to bimorph mirrors to generate non-Gaussian X-ray beam profiles of specified width. The new theory's freedom is exploited to choose the segments to match the polishing errors of medium wavelength (>10 mm) and the piezos' influence on the mirror's figure. Five- and seven-segment modifications of alternating curvature are calculated and verified by visible light and X-ray metrology. The latter yields beam profiles with less striation than those made by defocusing. Remaining beam striations are explained by applying geometrical optics to the deviations from the ideal surface modifications of alternating curvature.
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http://dx.doi.org/10.1107/S1600577516013308DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5082462PMC
November 2016

Development of a speckle-based portable device for in situ metrology of synchrotron X-ray mirrors.

J Synchrotron Radiat 2016 09 16;23(Pt 5):1131-6. Epub 2016 Aug 16.

Diamond Light Source, Harwell Science and Innovation Campus, Didcot, Oxfordshire OX11 0DE, UK.

A portable device for in situ metrology of synchrotron X-ray mirrors based on the near-field speckle scanning technique has been developed. Ultra-high angular sensitivity is achieved by scanning a piece of abrasive paper or filter membrane in the X-ray beam. In addition to the compact setup and ease of implementation, a user-friendly graphical user interface has been developed to ensure that optimizing active X-ray mirrors is simple and fast. The functionality and feasibility of this device have been demonstrated by characterizing and optimizing X-ray mirrors.
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http://dx.doi.org/10.1107/S1600577516012509DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5006653PMC
September 2016

Experimental comparison between speckle and grating-based imaging technique using synchrotron radiation X-rays.

Opt Express 2016 Aug;24(16):18664-73

X-ray phase contrast and dark-field imaging techniques provide important and complementary information that is inaccessible to the conventional absorption contrast imaging. Both grating-based imaging (GBI) and speckle-based imaging (SBI) are able to retrieve multi-modal images using synchrotron as well as lab-based sources. However, no systematic comparison has been made between the two techniques so far. We present an experimental comparison between GBI and SBI techniques with synchrotron radiation X-ray source. Apart from the simple experimental setup, we find SBI does not suffer from the issue of phase unwrapping, which can often be problematic for GBI. In addition, SBI is also superior to GBI since two orthogonal differential phase gradients can be simultaneously extracted by one dimensional scan. The GBI has less stringent requirements for detector pixel size and transverse coherence length when a second or third grating can be used. This study provides the reference for choosing the most suitable technique for diverse imaging applications at synchrotron facility.
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http://dx.doi.org/10.1364/OE.24.018664DOI Listing
August 2016

High energy X-ray phase and dark-field imaging using a random absorption mask.

Sci Rep 2016 07 28;6:30581. Epub 2016 Jul 28.

Diamond Light Source Ltd, Harwell Science and Innovation Campus, Didcot, OX11 0DE, UK.

High energy X-ray imaging has unique advantage over conventional X-ray imaging, since it enables higher penetration into materials with significantly reduced radiation damage. However, the absorption contrast in high energy region is considerably low due to the reduced X-ray absorption cross section for most materials. Even though the X-ray phase and dark-field imaging techniques can provide substantially increased contrast and complementary information, fabricating dedicated optics for high energies still remain a challenge. To address this issue, we present an alternative X-ray imaging approach to produce transmission, phase and scattering signals at high X-ray energies by using a random absorption mask. Importantly, in addition to the synchrotron radiation source, this approach has been demonstrated for practical imaging application with a laboratory-based microfocus X-ray source. This new imaging method could be potentially useful for studying thick samples or heavy materials for advanced research in materials science.
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http://dx.doi.org/10.1038/srep30581DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4964655PMC
July 2016

Wideband multilayer gratings for the 17-25 nm spectral region.

Opt Express 2016 Jun;24(13):15079-92

An approach to designing wideband blazed multilayer gratings is introduced and applied to gratings operating at 17-25 nm. We demonstrate single-order operation of broadband multilayer gratings, despite their very wide spectral and angular bandpass, when only one diffraction wave is excited and the diffraction efficiency reaches the reflectivity of a conventional depth-graded multilayer mirror, eliminating overlapping of different-order diffraction waves. The selection principles for the geometrical parameters of gratings are discussed. We formulate a "law of similarity" for wideband gratings that allows us to design gratings with different geometrical parameters but practically the same spectral dependence of the diffraction efficiency.
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http://dx.doi.org/10.1364/OE.24.015079DOI Listing
June 2016

Speckle-based at-wavelength metrology of X-ray mirrors with super accuracy.

Rev Sci Instrum 2016 May;87(5):052001

Diamond Light Source, Harwell Science and Innovation Campus, Didcot OX11 0DE, United Kingdom.

X-ray active mirrors, such as bimorph and mechanically bendable mirrors, are increasingly being used on beamlines at modern synchrotron source facilities to generate either focused or "tophat" beams. As well as optical tests in the metrology lab, it is becoming increasingly important to optimise and characterise active optics under actual beamline operating conditions. Recently developed X-ray speckle-based at-wavelength metrology technique has shown great potential. The technique has been established and further developed at the Diamond Light Source and is increasingly being used to optimise active mirrors. Details of the X-ray speckle-based at-wavelength metrology technique and an example of its applicability in characterising and optimising a micro-focusing bimorph X-ray mirror are presented. Importantly, an unprecedented angular sensitivity in the range of two nanoradians for measuring the slope error of an optical surface has been demonstrated. Such a super precision metrology technique will be beneficial to the manufacturers of polished mirrors and also in optimization of beam shaping during experiments.
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http://dx.doi.org/10.1063/1.4949004DOI Listing
May 2016