Neutron Optical Devices Research Articles

High-Transmission Neutron Optical Devices Utilizing Micro-Machined Structures

Neutrons are a powerful probe in material science with unique penetrating abilities. A major challenge stems from the fact that neutron optical devices are limited to refractive indices on the order of n≈1±10−5. By exploiting advances in precision manufacturing, we designed and constructed micro-meter period triangular grating with a high-aspect ratio of 14.3. The manufacturing quality is demonstrated with white-light interferometric data and microscope imaging. Neutron-scattering experiment results are presented, showing agreement with refraction modelling. The capabilities of neutron Fresnel prisms and lenses based on this design are contrasted with existing neutron focusing techniques, and the path separation of a prism-based neutron interferometer is estimated.

Study of a nested neutron-focusing supermirror system for small-angle neutron scattering

Abstract Neutron current is crucial for neutron scattering measurements, especially for those at compact sources. Among various neutron optical devices , neutron-focusing mirrors have been drawing increasing attention due to their great potential in enhancing neutron current on sample for small-angle neutron scattering instruments. The state-of-the-art neutron-focusing mirrors, which can collect a large divergence of neutron beam , are required to be nestable and simultaneously be able to be coated with supermirrors. In this paper, we employ the nested conical optics to manufacture nested neutron-focusing mirrors with supermirror coatings. This technique enables such mirrors by assembling partial cylindrical glass segments into nested shells. A prototype system, containing two shell conical mirrors with m =2 Ni/Ti supermirror coatings, has been designed and manufactured. Ray-tracing simulations and neutron beam experiments have been carried out to evaluate its focusing performance and neutron reflectivity. The results show that neutron current collected by this prototype system is about 26 times higher than that by a pinhole design with the same Q min . Moreover, we discuss some possible further improvements on the fabrication process.

A Study of the Kinetics of Oxidation of Titanium Nanolayers and Their Application in Neutron Optical Devices

The kinetics of oxidation of 10–40-nm-thick titanium films in air is studied by combining X-ray and neutron reflectometry. It is found that the thickness of the oxide layer on the surface of Ti nanolayers does not increase after keeping the samples at room temperature for a year. At temperatures of 100–300°C the oxidation kinetic curves of Ti nanolayers are described by a logarithmic dependence. The obtained results indicate the applicability of the Cabrera–Mott and Evans model approximations to describe the oxidation process of Ti nanolayers in air. Examples of the application of Ti nanolayers in polarizing mirrors and a neutron mirror spin flipper are given.

Concept of Small-Angle Diffractometer in Classical Configuration at the Cold Moderator of the IBR-2 Reactor

The possible concepts whereby a time-of-flight small-angle diffractometer optimized for a neutron moderator operating in the cold (at 30 K) and thermal (at 300 K) modes can be implemented at the IBR-2 reactor are studied on the basis of numerical calculations. Under cold conditions, the peak of the neutronbeam energy spectrum is shifted toward low energies (long wavelengths). This extends the sensitivity range of the instrument with respect to the sizes of the objects under study (1–100 nm and higher). A classical scheme enabling the separation of thermal/cold neutrons (E ~10–3–10–2 eV) from the background (formed mainly by fast neutrons), which is based on bent neutron-optical devices, is discussed. Due to restrictions imposed by the geometry of the beamline within which the instrument is planned to be located, a configuration with a short multichannel mirror device for beam bending (beam bender) is preferable. Simulation and optimization of the proposed small-angle instrument is carried out taking into account the real beamline geometry and the available space in the experimental reactor hall. A comparison of the setup has been made with the facility based on the curved neutron guide and the facility with direct view of the moderator.

Roughness replication in neutron supermirrors

Neutron supermirrors (SMs), the major components of neutron optical devices, are depth-graded d-spacing multilayers of several hundreds to several thousands of bilayers. The interface roughness is a major factor in the reflectivity of multilayers. This influence is especially significant if the number of bilayers is large. In this work, the interface roughness and its correlations were studied in DC-sputtered Ni–Ti neutron supermirrors. Detector scans were carried out to observe off-specular neutron scattering in selected regions of the q space from (increasing bilayer thickness) normal- and (decreasing bilayer thickness) reverse-layer-sequence SMs. In-plane and out-of-plane roughness correlations are manifested in diffuse scatter plateaus and peaks which are interpreted in terms of resonant diffuse scattering. Distorted wave Born approximation simulations quantitatively reproduce the characteristic features of the measured detector scans with reasonable roughness correlation parameters, i.e. in-plane and out-of-plane correlation lengths, common interface roughness, and Hurst parameters. The different character of resonant diffuse scattering from normal- and reverse-layer-sequence SMs is qualitatively explained and systematized using quasi-kinematical considerations in terms of material and SM parameters. The total off-specular intensity of the supermirrors was found to be non-monotonic with respect to the specular reflectivity at the corresponding angle of incidence.

The ion beam sputtering facility at KURRI: Coatings for advanced neutron optical devices

We describe a film coating facility for the development of multilayer mirrors for use in neutron optical devices that handle slow neutron beams. Recently, we succeeded in fabricating a large neutron supermirror with high reflectivity using an ion beam sputtering system (KUR-IBS), as well as all neutron supermirrors in two neutron guide tubes at BL06 at J-PARC/MLF. We also realized a large flexible self-standing m=5 NiC/Ti supermirror and very small d-spacing (d=1.65nm) multilayer sheets. In this paper, we present an overview of the performance and utility of non-magnetic neutron multilayer mirrors fabricated with the KUR-IBS

Neutron spin manipulation optics: basic principles and possible applications

A number of basic elements for neutron spin manipulation optics (NSMO) based on Larmor and non-Larmor (quantum) precessions under reflection are considered. It is concluded that transition to 3D in neutron polarization optics may bring additional instrumental possibilities. New neutron optical devices will include spin turners (particularly, π/2-turners and π- turners, or flippers), spin precessors and antiprecessors, 3D-polarizers, 3D-analyzers, 3D- rotators, spin manipulators, hyper-polarizers. The innovative neutron optics is directly applicable to developing 3D polarization and polarimetry techniques, such as reflectometry with 3D- polarimetry, Neutron optical Spin Echo (NoSE), including compact NoSE and TOF NoSE schemes. A hyper-polarizer is a device which not only separates neutrons with the opposite spins, but also flips the 'wrong' spins. Thus, hyper-polarizers can double the intensity of polarized neutron beams, although a gain in the intensity can be achieved only with the increase either in the angular divergence or in the width of the beam, in full accordance with the Liouville theorem. The tasks to be solved for implementation of the NSMO concepts are discussed.

High Performance Supermirrors on Metallic Substrates

Recently, we have developed an optimized process for polishing Al substrates leading to an extraordinary low surface roughness comparable to float glass of high quality. Indeed, supermirror coatings with R = 91% at the critical angle of reflection of m = 2 were produced demonstrating the excellent quality of the Al surface. Supermirror coated Al substrates open new options for advanced neutron optical devices. As one example neutron guides can start very close from the moderator. We investigated this option by Monte-Carlo simulations for i) a conventional, curved guide and ii) an elliptic guide. In case of i) an increase of flux at long wavelengths is obtained because of complete illumination of the phase space accepted by the guide. The elliptic guide starting close to the moderator (ii) has significantly enhanced neutron transport properties since only useful neutrons are extracted and transported to the sample. As a result such a guide concept is superior in terms of flux when compared with a conventional guide system and in terms of signal to background and well defined beam focusing compared with an elliptic guide starting at a larger distance from the moderator. In particular, experiments on novel materials, which are often only available in small quantities or samples under extreme conditions will profit from neutron beams of such high quality.

Neutron Optical Beam Splitter from Holographically Structured Nanoparticle-Polymer Composites

We report a breakthrough in the search for versatile diffractive elements for cold neutrons. Nanoparticles are spatially arranged by holographical means in a photopolymer. These grating structures show remarkably efficient diffraction of cold neutrons up to about 50% for effective thicknesses of only 200 μm. They open up a profound perspective for next generation neutron-optical devices with the capability to tune or modulate the neutron diffraction efficiency.

The Origins of Neutron Reflectometry

The optical properties of slow neutrons, which includes refraction, reflection and interference, were recognized in the very early days of neutron physics. Fermi and co-workers [1] reported the total (or critical) reflection of slow neutrons. Goldberger and Seitz showed that the intensity of reflected and transmitted neutrons follow the same laws as electromagnetic radiation. The early applications of those neutron optical phenomena included the determination of neutron scattering lengths, and the production of neutron guides and neutron spin polarizers. The development of graded multi-layer mirrors, “supermirrors” [2] quickly followed. From such developments and the pioneering neutron guide installations at the Institute Laue Langevin, ILL, in Grenoble, sophisticated neutron optical devices are now commonplace on all neutron sources. The potential to use neutron reflectivity to probe surface structure arose out of those early applications. As such neutron reflectometry (NR) has evolved [3–6] over the past quarter century to become an established probe of the structure of thin films and periodic multilayers of both soft and hard condensed matter. From the beginning, specular neutron reflectivity measurements have been used to determine scattering length density depth profiles related to chemical composition and, with polarized beams, the vectorial magnetization depth profile in magnetic thin film systems. The success of NR can be attributed not only to instrumentation advances and the formulation of quantitatively accurate theoretical methods for the analysis of reflectivity data, but to the advances in various thin film deposition techniques required to prepare satisfactory sample systems for study. In this article, a partial history of neutron reflectometry development is briefly outlined in the form of complementary perspectives from the two authors. Length limitations, however, make it impossible to acknowledge here, let alone adequately describe, all of the important work that so many people have contributed to the implementation of neutron reflectometry in many fields of scientific investigation.

Implementation of Neutron Mirror Modeling Capability into MCNPX and Its Demonstration in First Applications

Reflection of thermal and cold neutrons by polished surfaces and so-called supermirrors effect radiation fields in and around neutron beamlines. To allow the prediction of these radiation fields with MCNPX 2.5.0, two new input cards were implemented for defining mirror properties of surfaces. Mirror properties can be linked to any type of surface, in contrast to other neutron optics codes, where the mirror properties are part of component descriptions, allowing the simulation of very complex neutron optical devices. First calculations are under way to verify the new capability against combinations of MCNPX and MCSTAS (neutron optics code) simulations. Also, simulations are under way to compare the predicted neutron beam characteristics against measurements conducted at Paul Scherrer Institut.

Trevor Hicks and solid state neutron optics

Following an idea of Trevor Hicks we built neutron polarizing benders with silicon wafersforming the bender channels. In the last decade a variety of neutron optical devices basedon this idea were realized such as collimators with absorbing as well as reflectingwalls, radial benders and collimators and focusing devices which are all muchshorter than their classical counterparts. Their development will be reviewedhere.

Highly collimating neutron optical devices

Two Soller type highly collimating neutron optical devices with a length of 200 mm are presented. They provide a triangular shaped beam divergence distribution with a FWHM of about 0 . 11 ∘ (2 mrad) and a peak transmission of up to 80% for neutrons with a wavelength λ 0 of 5 Å. This corresponds to an improvement of more than a factor three compared to state of the art collimators. Measurements are presented which demonstrate the usefulness of these devices also for neutron radiography.

Slow Neutron Optics

We survey the current status of reflective neutron optical devices, compound refractive and magnetic ones for enhancing the effectiveness of the beam utilization to extract physics information of samples.

‘Venetian blinds’-type stacked neutron mirrors

The properties of neutron optical devices consisting of a repetition of macroscopic substrates with a top reflecting layer (‘venetian blinds’) are discussed. The reflection and transmission of such a system are calculated analytically taking into account multiple scattering events. It is shown that these devices exhibit total neutron reflection at certain conditions of incoming beam angle and wavelength. Possible applications are discussed.

Fine ELID Grinding on the Symmetric Paraboloidal Mirror of Quartz

Quartz has excellent optical properties and thus it is often used as the material of mirror and lens. However, it is almost impossible to be machined by cutting due to its brittle and high hardness. Grinding is a common method for machining quartz. One of the authors designed a new type of a paraboloidal mirror of quartz for the neutron optical devices. The fabrication process of this mirror was investigated in this experiment. A jig with two grooves of 90 degrees was made and two workpieces were stuck on the jig using the wax melt at about 60 degrees centigrade. The two workpieces were first ground applying ELID (electrolytic in-process dressing) grinding method with #325 and #1200 cast iron bonding diamond abrasive wheels. Then the ground surface was polished with CeO2 slurry. The finished surface roughness was Ra2.0nm and rms2.4nm and its form error about 2μm. After coating process, its properties of focusing neutron beam were measured. The results were that the reflecting rate was 42%, gain 1.9 and 2.3mm×10.1mm beam focused to 1.6mm×2.1mm.

Colossal Light-Induced Refractive-Index Modulation for Neutrons in Holographic Polymer-Dispersed Liquid Crystals

We report strong diffraction of cold neutrons from an only 30 micro m thick holographic polymer-dispersed liquid crystal (H-PDLC) transmission grating. The light-induced refractive-index modulation for neutrons is about 10(-6), i.e., nearly 2 orders of magnitude larger than in the best photo-neutron-refractive materials probed up to now. This makes H-PDLCs a promising candidate for fabricating neutron-optical devices.

Applications of neutron refractive optical devices

Refractive optics has relatively large effective apertures compared with reflective optics and they are suitable for two-dimensional beam focusing. There are two types of refractive optics, which are compound refractive optics and magnetic optics. Both of them are applied to extend the accessible q-range in small-angle scattering of cold neutron. We discuss possibilities to apply magnetic optics at pulsed neutron sources. We also discuss the possibility to obtain an extremely high neutron polarization using the spin-selectivity of magnetic optics.

Possibility of using a PMN–PT single crystal as a neutron optical device

Variations of the Bragg diffraction angle induced by electric fields on piezoelectric single crystal were investigated with the aim of exploiting such crystals for novel neutron devices. A (1− x)(Pb(Mg 1/3Nb 2/3)O 3)– x(PbTiO 3) (PMN–PT) single crystal with strong piezoelectric characteristics was used in the experiment. The sample was poled along 〈0 0 1〉direction before the experiment. In the course of diffraction measurements, a 0.3° change was observed in the diffraction angle at the maximum induced by the electric field. The diffraction angle variation shows time-dependent behavior, which can be well described using a single exponential decay limited to a saturated value. This effect is considered a result of a change of the domain structure brought about through long-term application of an electric field for several hours. The obtained preliminary results confirmed the possibility of using a ferroelectric crystal as a neutron optical device.

Construction and Application of T2-3 Thermal Neutron Beam Port in JRR-3

We present the reconstruction of T2-3 beam port in the guide hall of JRR-3. A new T2-3 is a multi-purpose thermal neutron beam port, named “MUSASI” with two monochromatic beam used for extreme condition experiments under high fields or at ultra-low temperatures, development and performance tests of neutron detectors and optical devices for J-PARC, and industrial use such as residual stress analysis, etc. A sufficient beam time in a unit of 25 days is provided for internal and outside users. We request neutron community to join this project and suggestion/criticism aimed to effective use of this beam line, oriented to complementary use of JRR-3 with J-PARC.