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Demonstration of Focusing Wolter Mirrors for Neutron Phase and Magnetic Imaging

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Demonstration of Focusing Wolter Mirrors for Neutron Phase and Magnetic Imaging Demonstration of Focusing Wolter Mirrors for Neutron Phase and Magnetic Imaging The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. Citation Hussey, Daniel et al. "Demonstration of Focusing Wolter Mirrors for Neutron Phase and Magnetic Imaging." Journal of Imaging 4, 3 (March 2018): 50 © 2018 The Authors As Published http://dx.doi.org/10.3390/jimaging4030050 Publisher MDPI AG Version Final published version Citable link http://hdl.handle.net/1721.1/114674 Terms of Use Creative Commons Attribution Detailed Terms http://creativecommons.org/licenses/by/4.0/ Journal of Imaging Article Demonstration of Focusing Wolter Mirrors for Neutron Phase and Magnetic Imaging Daniel S. Hussey 1,*, Han Wen 2, Huarui Wu 3, Thomas R. Gentile 1, Wangchun Chen 4,5, David L. Jacobson 1, Jacob M. LaManna 1 ID and Boris Khaykovich 6 ID 1 Physical Measurement Laboratory, NIST, Gaithersburg, MD 20899-8461, USA; [email protected] (T.R.G.); [email protected] (D.L.J.); [email protected] (J.M.L.) 2 National Heart Lung Blood Institute, National Institute of Health, Bethesda, MD 20892, USA; [email protected] 3 Key Laboratory of Particle & Radiation Imaging, Tsinghua University, Ministry of Education, Beijing 100084, China; [email protected] 4 NIST Center for Neutron Research, NIST, Gaithersburg, MD 20899, USA; [email protected] 5 Department of Materials Science Engineering, University of Maryland, College Park, MD 20742, USA 6 Nuclear Reactor Laboratory, Massachusetts Institute of Technology, Cambridge, MA 02139, USA; [email protected] * Correspondence: [email protected]; Tel.: +1-301-975-6465 Received: 1 November 2017; Accepted: 2 March 2018; Published: 6 March 2018 Abstract: Image-forming focusing mirrors were employed to demonstrate their applicability to two different modalities of neutron imaging, phase imaging with a far-field interferometer, and magnetic-field imaging through the manipulation of the neutron beam polarization. For the magnetic imaging, the rotation of the neutron polarization in the magnetic field was measured by placing a solenoid at the focus of the mirrors. The beam was polarized upstream of the solenoid, while the spin analyzer was situated between the solenoid and the mirrors. Such a polarized neutron microscope provides a path toward considerably improved spatial resolution in neutron imaging of magnetic materials. For the phase imaging, we show that the focusing mirrors preserve the beam coherence and the path-length differences that give rise to the far-field moiré pattern. We demonstrated that the visibility of the moiré pattern is modified by small angle scattering from a highly porous foam. This experiment demonstrates the feasibility of using Wolter optics to significantly improve the spatial resolution of the far-field interferometer. Keywords: neutron imaging; Wolter optics; polarized neutron imaging; far-field interferometer 1. Introduction Nearly all existing neutron-imaging instruments can be described as pinhole cameras that measure the neutron shadow of a sample. In such cameras, the geometric blur, which fundamentally limits spatial resolution, is given by λg = z D/(L − z), where z is the sample to detector distance, D is the beam defining aperture, and L is the aperture to detector distance. Due to the inherent low intensity of neutron sources, D is of order 1 cm. This size limitation precludes the use of geometric magnification to improve spatial resolution, and currently achievable spatial resolutions reach ~10 µm. The primary reason for the utilization of the pinhole geometry is the lack of a practical neutron-image forming lens. While refractive neutron lenses have been incorporated into small angle neutron scattering instruments, they are not suitable for imaging, since the refractive index for materials is n ~1–10−4 λ (nm)2. Therefore, neutron refractive lenses have very long focal lengths (~100s of meters) and are strongly chromatic [1]. The geometric blur limits the achievable spatial resolution especially for methods that require placing of sample environments or optical components between the sample and the detector. We J. Imaging 2018, 4, 50; doi:10.3390/jimaging4030050 www.mdpi.com/journal/jimaging J. Imaging 2018, 4, x FOR PEER REVIEW 2 of 9 The geometric blur limits the achievable spatial resolution especially for methods that require placing of sample environments or optical components between the sample and the detector. We consider below two examples of emerging techniques which require such equipment: polarized and grating-interferometry imaging. First, after the work of Kardjilov et al. [2], there is renewed interest inJ. Imaging exploiting2018, 4,the 50 spin of the neutron to image magnetic fields, for instance the trapped flux2 ofin 9 superconductors [3]. In order to measure the magnetic field, one must measure the neutron spin rotation angle by the sample with the help of spin analyzers such as compact polarizing supermirrors orconsider 3He neutron below spin two filters. examples These of devices emerging require techniques a minimum which of requirez ~ 20 cm such of separation equipment: between polarized the sampleand grating-interferometry and the detector, limiting imaging. achievable First, spatial after the resolution. work of The Kardjilov state-of-t ethe-art al. [2], polarized there is renewed neutron imaginginterest ininstrument exploiting PONTO the spin [4] of has the neutronbeen able to to image achieve magnetic only 120 fields, µm resolution, for instance almost the trapped a factor flux of tenin superconductors more coarse than [3 what]. In ordercan be to routinely measure theachi magneticeved using field, conventional one must measureneutron-imaging the neutron layouts. spin Second,rotation the angle recently by the introduced sample with broadband the help of far spin fiel analyzersd interferometer such as compactis a potentially polarizing groundbreaking supermirrors 3 methodor He neutron of obtaining spin filters. phase Thesegradient devices and requiredark-field a minimum images [5,6]. of z ~20However, cm of separation in order to between probe the longestsample autocorrelation and the detector, lengths, limiting one achievable places the spatial sample resolution. in the middle The state-of-the-art of the flight path polarized for a geometric neutron magnificationimaging instrument of 2, producing PONTO strongly [4] has been blurred able images. to achieve While only a small 120 µ aperturem resolution, could almost be used a to factor obtain of resolutionten more coarse of order than 0.5 what mm, can the bestrong routinely reduction achieved in intensity using conventional would obviate neutron-imaging the advantages layouts. of the method.Second, the recently introduced broadband far field interferometer is a potentially groundbreaking methodIt was of obtainingrecently demonstrated phase gradient that and Wolter dark-field mirrors, images which [5 are,6]. widely However, used in in order X-ray to telescopes, probe the couldlongest be autocorrelation used as neutron-image lengths, forming one places optics the sample[7,8]. Briefly, in the Wolter middle mirrors of the flight are axisymmetric path for a geometric mirrors composedmagnification of oftwo 2, producingconic sections, strongly see blurredFigure images.1. With Whilesuch optics, a small one aperture could could realize be usedseveral to improvementsobtain resolution in ofneutron order 0.5imaging mm, theover strong the pinhole reduction optics in intensity geometry. would First, obviate there is the a large advantages distance, of ofthe order method. meters, between the sample and optics (and optics and detector) enabling the introduction of bulkyIt was sample recently environment demonstrated or beam that conditioning Wolter mirrors, devices which without are widely the loss used of in spatial X-ray resolution. telescopes, Second,could be the used image as neutron-image resolution is determined forming optics by the [7,8 optics]. Briefly, and Wolter not the mirrors beam collimation are axisymmetric (or L/D mirrors ratio). (Thiscomposed is generally of two conic true sections,for any seeoptical Figure system1. With which such optics, employs one image-forming could realize several mirrors improvements or lenses.) Therefore,in neutron the imaging aperture over size the D pinhole could be optics increased geometry. so that First, the theresample is ais large illuminated distance, by of the order full meters,power ofbetween the divergent the sample polychromatic and optics neutron (and optics beam. and Third, detector) many enabling mirrors can the introductionbe nested to increase of bulky the sample solid angleenvironment portion orof beamthe beam conditioning collected devicesby the withoutoptics, resulting the loss ofin spatialorders resolution.of magnitude Second, increase the imagein the neutronresolution fluence is determined rate for by high the opticsspatial and resolution not the beamimaging. collimation Fourth, (or optical L/D ratio). magnification (This is generally can be achievedtrue for any in opticalorder to system improve which the employsspatial resolution image-forming. As in mirrors geometrical or lenses.) optics, Therefore, the magnification the aperture is givensize D by could the ratio be increasedof focal lengths so that (fi theand samplefo in Figure is illuminated 1). by the full power of the divergent polychromaticIn this article, neutron we beam. demonstrate Third, many how mirrorsto exploi cant bethe nested advantages
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