Proceedings Volume 4509

Neutron Optics

James L. Wood, Ian S. Anderson
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Proceedings Volume 4509

Neutron Optics

James L. Wood, Ian S. Anderson
View the digital version of this volume at SPIE Digital Libarary.

Volume Details

Date Published: 8 November 2001
Contents: 4 Sessions, 18 Papers, 0 Presentations
Conference: International Symposium on Optical Science and Technology 2001
Volume Number: 4509

Table of Contents

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Table of Contents

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  • Crystal Optics
  • Multilayers
  • Novel Devices
  • Simulations
Crystal Optics
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Neutron imaging with Bragg mirrors
Alexandru Dan Stoica, Xun-Li Wang, Mihai Popovici, et al.
Phase-space analysis of neutron optics has revealed that neutron imaging by Bragg reflection from thick bent perfect crystals can be non-dispersive (independent of the neutron wavelength), like with an optical mirror. The corresponding devices, called Bragg mirrors (BM), can be used for neutron imaging at pulsed neutron sources. Using a position sensitive detector (PSD) and time-of-flight analysis (TOF), a BM imaging system will make it possible to collect both real space mapping data and scattering space data simultaneously. Each pixel of PSD will correspond to a point in the sample and will contain a segment of the diffraction pattern (useful for strain, texture or phase analysis), or of an inelastic spectrum. In this paper the resolution and efficiency of BM in TOF diffraction experiments are calculated and compared with the usual sequential method of mapping. Experimental tests performed at steady state neutron sources showed sub-millimeter spatial resolution in the one-dimensional case.
Neutron optics of the ILL high-flux polarized neutron three-axis spectrometer IN20B
Jiri Kulda, Pierre Courtois, Jan Saroun, et al.
The three-axis spectrometer IN20 has been upgraded to enhance significantly the data collection rate in experiments using polarized neutrons to study magnetic excitations in the (higher) thermal energy range. To increase the monochromatic polarized neutron flux, a new geometry of the primary spectrometer, optimized by detailed ray-tracing simulations, has been adopted. The main ingredients are a neutron source of a diameter increased from 100 mm to 170 mm and a large double focusing monochromator, illuminated through a heavy input slit (virtual source) of adjustable width. This geometry permits to keep the background at a possibly low level while maximizing the solid angle available for monochromatic focusing. The real challenge of the project has been the new Heusler monochromator. With its active surface of 230 x 150 mm2, consisting of 75 crystal plates mounted in 15 columns, it is the largest polarizing crystal assembly ever built. In combination with the horizontally focusing analyzer of a similar design, implemented in spring 2000, the data collection rate in the polarization analysis mode has increased by a factor 30 - 50 in April 2001 as compared to the original IN20, which up to now has provided world's highest polarized neutron flux in the thermal energy range.
Multiwafer focusing neutron monochromators and applications
Mihai Popovici, Alexandru Dan Stoica, Camden R. Hubbard, et al.
Multi-wafer silicon monochromators for neutron focusing instruments have been developed at MURR. A first unit, made from commercial thin silicon [100] wafers with manual control of horizontal curvature was designed and fabricated for the MURR stress machine. It was tested on the stress machine at the HFIR reactor at ORNL. A second similar unit but with stepper motor control of curvature was installed on the NIST stress machine. Both confirmed expectations, with significant intensity gains at equal or better resolution in comparison with the monochromators they replaced. A third unit with two back-to-back assemblies of non-standard silicon wafers custom sliced obliquely from big [100] ingots has been fabricated for an upgrade of the ORNL stress machine. The phase space analysis of the neutron optics of multi-wafer assemblies has revealed exciting new possibilities for applications. The correlation between the coordinates of real space and wavevector space allows a new type of focusing, the thickness focusing. The many wafers in a packet can be made to look as a single wafer when seen from a given point of a position sensitive detector (PSD). This allows high resolutions in scattering, corresponding to a bent thin single wafer, at intensities given by the whole packet, that is comparable with pyrolytic graphite crystals. One can thus have the best of two worlds - but only in PSD instruments. A whole array of new applications becomes possible, including dispersive and non-dispersive neutron imaging at the spatial resolution of a single thin wafer. Some of these applications are discussed and demo experiments are presented.
Propagation of neutron de Broglie waves inside the slot cut in a single Si crystal
Eugene M. Iolin, Samuel A. Werner, Michael M. Agamalian
It was recently proposed (by Dombeck et al) to search for a Neutron Electric Dipole Moment (EDM) by means of the neutron multiple Bragg back-scattering. The dynamical diffraction analysis of the proposed experiment is the subject of this paper. The neutron wave modes were calculated for the case of the infinitely long slot cut inside of a thick Si crystal parallel to the crystallographic planes and placed in a steady magnetic field. The calculated neutron modes have a discrete spectrum of a momenta along the direction of the slot axis. The external magnetic field causes some particular discrete modes to become degenerate. However, the Schwinger and EDM interactions of neutrons with the slot walls break this degeneracy, which in turn leads to the complicated motion of the neutron polarization vector along the slot axis. The spin deviation from the starting direction is accumulated during neutron motion in slot. The energy spectrum of neutrons transmitted through the slot contains several peaks instead of one existing for the case of the ultra back-scattering regime.
Multilayers
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Neutron supermirrors: an accurate theory for layer thickness computation
We present a new theory for the computation of Super-Mirror stacks, using accurate formulas derived from the classical optics field. Approximations are introduced into the computation, but at a later stage than existing theories, providing a more rigorous treatment of the problem. The final result is a continuous thickness stack, whose properties can be determined at the outset of the design. We find that the well-known fourth power dependence of number of layers versus maximum angle is (of course) asymptotically correct. We find a formula giving directly the relation between desired reflectance, maximum angle, and number of layers (for a given pair of materials). Note: The author of this article, a classical opticist, has limited knowledge of the Neutron world, and begs forgiveness for any shortcomings, erroneous assumptions and/or misinterpretation of previous authors' work on the subject.
Flux gain for next-generation neutron-scattering instruments resulting from improved supermirror performance
Christine Rehm, Michael M. Agamalian
Next-generation spallation neutron source facilities will offer instruments with unprecedented capabilities through simultaneous enhancement of source power and usage of advanced optical components. The Spallation Neutron Source (SNS), already under construction at Oak Ridge National Laboratory and scheduled to be completed by 2006, will provide greater than an order of magnitude more effective source flux than current state-of-the-art facilities, including the most advanced research reactors. An additional order of magnitude gain is expected through the use of new optical devices and instrumentation concepts. Many instrument designs require supermirror (SM) neutron guides with very high critical angles for total reflection. In this contribution, we will discuss how the performance of modern neutron scattering instruments depends on the efficiency of these supermirrors. We outline ideas for enhancing the performance of the SM coatings, particularly for improving the reflectivity at the position of the critical wave vector transfer. A simulation program has been developed which allows different approaches for SM designs to be studied. Possible instrument performance gains are calculated for the example of the SNS reflectometer.
Device for cutting the time tail from spallation neutron pulses
Paul Langan, Benno P. Schoenborn, Luke L. Daemen
The presence of long time tails in the neutron pulse produced by spallation sources is sometimes a problem in that it leads to background and can diminish the resolving power of the spectrometer. This problem is potentially severe with coupled moderators that are capable of providing increased intensity but at the cost of longer tails. The simple device we propose here is based on rocking a stack of multi-layers in the incident beam to attenuate these tails in a given band of incident wavelengths. The device and its operation are described and we present the results of Monte Carlo simulations.
Depth-graded multilayers as neutron Doppler converters at pulsed neutron source
Alexandru Dan Stoica, Morris Lowell Crow, Jack Lee Robertson
A moving diffractor changes the energy of elastically diffracted neutrons by the Doppler effect. Depth-graded multilayers can diffract neutrons over a large band of energy. Using a pulsed neutron source, such a depth-graded multilayer, decelerating synchronously with the incident neutron pulse, can shift the reflected neutrons into a compressed energy window. This focusing in energy is associated with a broadening of the pulse in time, but the process does not involve a significant decrease in the neutron phase-space density. The proposed method can be used to design long pulse or quasi-continuous sources of cold, very cold or ultra cold neutrons (UCN). The analysis concentrates on enhanced production of UCN at pulsed neutron sources.
Neutron waveguides: a new neutron optical device for the production of submicrometer neutron beams
Franz Pfeiffer, Peter Hoghoj, Ian S. Anderson, et al.
Planar neutron waveguide structures can be used as resonant beam coupling devices to produce a coherent and divergent neutron line source with cross-sections in the sub-micrometer range. This article reviews our recent work on the theoretical model, the fabrication and the characterization of these devices, demonstrating clearly the good agreement between the theoretical model and the feasability of using neutron waveguides for various applications. As one major result,the farfield-pattern of the first three modes of a neutron waveguide was measured, yielding a 17 times enhanced flux throughput compared to a pair of slits corresponding to the thickness of the guiding layer. Additionally we have generalized the principle of neutron resonant beam coupling to waveguides containing multiple guiding layers, where several beams with a width of the order of 10 to 100 nm can be extracted at the end leading to an typical farfield interference pattern.
Novel Devices
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Solid state neutron polarizers and collimators
Thomas Krist, Ferenc Mezei
In most cases, neutron optical elements like polarisers or collimators use coated surfaces which define the neutron flight path in air or vacuum. To reduce the size of such elements silicon single crystals can be used as the medium in which the neutrons travel. We have built and tested a neutron polarising bender which consists of a stack of thin silicon wafers. The neutrons enter at the front side. Inside the wafers the spin up component is reflected from the supermirror coated side and can leave the wafers while the spin down component passes the supermirror and is absorbed in the Gd layer of the adjacent wafer. Other neutron optical element, we tested for the first time are several collimators made from silicon wafers coated with either Gd or reflecting coatings below an absorbing Gd layer. These collimators produce a beam with quasi-rectangular distribution of angles, which represents an intensity gain at equal resolution compared to the triangular distribution in conventional Soller collimators. Finally, we report on the first test of a solid state radial collimator. Some general aspects of sold state neutron optical elements are also discussed.
Development of a magnetic lens and prism for cold neutrons and their application to neutron scattering experiments
Takayuki Oku, Tomohiro Adachi, Hiromi Sato, et al.
We have been developing a neutron lens and prism based on neutron refractive optics. As a neutron has a magnetic dipole moment, it is accelerated in a magnetic field gradient. Thus, we can control a neutron beam free from beam attenuation using the magnetic field gradient. Moreover, its spin dependence of the acceleration is profitable in the case of using the polarized neutron beam. The sextupole magnetic field functions as a focusing or defocusing lens for neutrons depending on the neutron spin states. The focusing and defocusing effects of a prototype sextupole magnet was experimentally studied. By combining focusing and defocusing functions of the sextupole magnet, we can control the neutron beam shape and divergence more flexibly. Adiabatic and nonadiabatic field connections make it possible to realize the magnetic doublet system. A quadrupole magnetic field functions as a neutron prism, which were experimentally confirmed. The neutron spin and energy dependence of the refracting power is applicable to an analysis of the neutron spin and energy. In this paper, the details of the experimental results of the magnetic devices are described and their applications in the neutron scattering experiment are discussed.
Development of neuron compound lens and prism based on neutron refractive optics
Tomohiro Adachi, Takayuki Oku, Shin-ya Morita, et al.
We have developed compound refractive prism for cold neutrons. To prevent an increase in neutron absorption, we have developed prism array like a Fresnel lens. The prism characteristics were investigated with experimental and numerical simulation studies. We achieved transmission of 0.75 and refractive angle of 7.5 mrad for 15 neutrons with 49 layered prism array.
Requirements for the optical elements of a spin-resonance energy filter for neutrons at reactor and pulsed source instruments
Andre de Azevedo Parizzi, Wai-Tung Lee, Frank Klose
In this contribution, we evaluate novel applications of an energy band-pass filter for thermal and cold neutron beams. Our proposal is based on the original concept of the Drabkin spin-resonance flipper, which had been considered as a tunable neutron energy filter at reactor sources. The device takes advantage of the fact that the neutron has a magnetic moment, and consists of a wavelength-selective electromagnetic resonator and a supermirror polarizer/analyzer system. We are proposing improvements to the use of the device, making it suitable for time-of-flight experiments at spallation neutron sources. The modifications include utilizing time-dependence in the flipping process and revised magnetic field profiles. Calculations and preliminary results are presented, demonstrating the sensitivity of the device to the efficiency of the polarizing supermirror optics and to the angular divergence of the beam. We will compare the performance of the Drabkin energy filter used in static mode at reactor instruments and in dynamic mode at a pulsed source, and evaluate the potentially achievable wavelength resolution for the case of a reflectometry experiment.
Features of neutral particles channeling in capillary arrays
In this report some peculiarities of X-ray and neutron beam passage through the arrays of capillaries are considered. The analysis of coherent and incoherent scattering at surface beam channeling will be presented. Dependence of the transmission characteristics on internal channel structure and on transverse channel size will be discussed.
Simulations
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Models for ray-tracing simulations of thermal neutrons monochromators
In recent years there has been a continuous interest in applying ray-tracing techniques for simulating the performance of neutron instruments. This technique is well known for other applications dealing with photon beams (visible, IR, UV and X-rays). Several codes have been developed by different groups to either calculate, with high accuracy, some particular optical elements of a neutron instrument, or to give rough estimations of the whole instrument including simple models of the individual elements. Our goal is to create an optimised code for neutron optics using accurate descriptions for each optical element. In this paper we will analyse the existing models for treating mosaic crystals monochromators. We will report on the calculated and measured diffraction properties of mosaic copper and pyrolytic graphite crystals, which are two of the most commonly used neutron monochromators.
SNS magnetism reflectometer: basic design and neutron guide optimization using Monte Carlo simulations
Wai-Tung Lee, Frank Klose
In this contribution, we summarize the optimization of the neutron guide optics of the Magnetism Reflectometer, which is currently under construction at the Spallation Neutron Source. The guide system consists of a straight source tube, a polygonal curved multi-channel bender, and a converging guide section. The bender will be essential for high-energy neutron and g-ray background suppression, while the converging guide will focus the neutron beam onto the sample. Monte Carlo (MC) numerical methods were used to optimize the guide performance. The flux on sample and detector count rates were systematically determined for various guide configurations. The impact of guide imperfections like surface waviness and misalignment of sections are also studied.
Loss mechanisms in supermirror neutron guides
Peter M. Allenspach, Peter Boeni, Kim Lefmann
Supermirror neutron guides have become a standard solution to transport neutrons from the source to the instruments. The performance of the guides depends strongly on the reflectivity of the coating, the waviness of the substrate, and the alignment of the guide elements. We have performed Monte Carlo simulations in order to quantify these influences for non-perfect, real neutron guides and to compare the importance of the different loss mechanisms. The importance of a good reflectivity was demonstrated in previous publications and is supported by our investigations. We show that the waviness of the substrate can easily be compensated by increasing the critical angle of reflection without degrading the divergence of the outgoing neutron beam. For geometrical imperfections such as off-sets of the individual guide elements or deviations from an exact rectangular guide cross section, the degradation of the neutron flux at the end of a neutron guide corresponds roughly to the proportion of the cross section affected by these imperfections to the total guide cross section.
Characteristics of neutron multilayer devices from ray-tracing calculations
Anand M. Saxena
Characteristics of a neutron focusing device based on five thin-film multilayers that are (1) oriented at slightly different angles with respect to the mean ray to converge the neutron beam reflected by each individual section, (2) have slightly different d-spacings such that the neutron beam reflected by each section has the same mean wavelength, were evaluated using a Monte Carlo program for a number of geometries. The influence of changing the distance between the focusing device and the detector was studied,and the effect of small deviations from the desired values was also explored.