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World’s Highest-Performing Neutron Focusing Technology:
A mirror that can intensify a neutron beam 50-fold

Jul. 22, 2011

Japan Science and Technology Agency(JST)
Osaka University
J-PARC Center

Kazuya Yamamura (Associate Professor, Osaka University Graduate School of Engineering), Kazuhiko Soyama (Section Leader, The Japan Proton Accelerator Research Complex (J-PARC) *1 of the Japan Atomic Energy Agency (JAEA)), and colleagues were the first to successfully develop an elliptical supermirror*2 that can focus a neutron beam with extremely high efficiency, thereby boosting the neutron beam intensity per unit area by more than 50-fold. The present study was conducted in an effort to develop elemental technologies for the Technology Transfer Program “Development of Systems and Technology for Advanced Measurement and AnalysisEof the Japan Science and Technology Agency (JST).

Neutrons, which along with protons make up nuclei, are electrically neutral particles and have magnetic properties. Neutrons can be accelerated in a neutron beam to study light elements (such as hydrogen) and magnetic materials that are difficult to examine using X-rays. Powerful neutron beams can be generated at J-PARC, which has been operating since 2008; however, more powerful beams are needed to precisely study the surfaces/interfaces of materials as well as the structures of materials under extreme conditions. To this end, it is indispensable to create a high-performance focusing technology for neutron beams.

The team has successfully developed a neutron-beam focusing technology with the world’s best performance by stably fabricating a multilayer film consisting of nickel carbon and titanium on a quartz substrate.*3 This film is fabricated with nanometer-level precision (1 nm = 10-9 m). This focusing technology enables us to effectively focus neutron beams, allowing precision studies of magnetic structures on storage media at the nanometer level. It can potentially be the foundational technology for developing next-generation high-density storage media. Moreover, neutron beams with higher intensity can be utilized to precisely examine the atomic arrangement of water molecules contained in tiny particles under high-temperature, high-pressure conditions. This would be useful in research exploring deep internal structures of the Earth and other planets.

Results of the present study have been presented on July 22, 2011 at the 5th European Conference on Neutron Scattering (ECNS2011) held in Prague, Czech Republic.

The present study was supported by the following two projects.

Project Name: Development of Systems and Technology for Advanced Measurement and Analysis

Principal Investigator: Mitsuhiro Motokawa (Professor Emeritus, Tohoku University)

Project Name: Development of Neutron-Focusing Aspheric Supermirror Devices (2009E012)

Team Leader: Kazuya Yamamura (Associate Professor, The Research Center for Ultra-Precision Science & Technology, Osaka University)

JST aims to develop innovative novel elemental technologies that are expected to drastically improve the performance of measurement/analysis devices.

Background and History

Neutron beams have been used to measure physical quantities, such as the positions of the light-element atoms (e.g., hydrogen) and their magnetic-field distributions, which cannot be examined using X-rays. In the past, neutron beams have been produced mainly from neutron sources generated at nuclear reactors. However, the intensity of these neutron beams was not high enough to precisely observe such physical quantities.

In 2008, the Japan Proton Accelerator Research Complex (J-PARC), which is the world’s highest quality neutron source, was constructed in Tokai-mura, Ibaraki, Japan. A neutron beam produced at J-PARC has peak intensity a 100 times higher than conventional neutron beams available at nuclear reactors. This neutron beam is focused with a high-performance focusing mirror to further increase its intensity per unit area, which allows investigating material properties that could not be examined in the past.

Focusing a neutron beam can be achieved by using one of the following three methods: (1) reflection by a total reflection mirror*4 or a supermirror, (2) refraction at the material interface, and (3) a magnetic field. Method (2) has a drawback in that there are large losses due to the absorption and scattering of neutrons. Method (3) cannot be used to effectively focus the beam since neutrons have low magnetic dipole moment. Therefore, to effectively focus a neutron beam, supermirrors have been utilized around the world. However, there are many challenges to be overcome in increasing the performance of a supermirror, including the fabrication techniques of high-precision aspherical substrates and the exfoliation of a mirror.

To overcome these difficulties, in 2004 Dr. Yamamura of Osaka University and colleagues have developed a numerically controlled local wet etching (NC-LWE) technique*5 by which quartz is processed in a contactless manner. This new technique has lead to the fabrication of a quartz substrate of an arbitrary shape at nanometer-level precision. In addition, Dr. Soyama of the Japan Atomic Energy Agency (JAEA) and colleagues have developed the world’s largest ion beam sputtering*6 system for a neutron supermirror to form films on large substrates. They have succeeded in fabricating a high-reflectivity neutron supermirror with the largest critical angle*7 to date by preparing multilayer films consisting of several thousand nickel carbon and titanium (NiC/Ti) layers using this system.

Since 2009, these two groups have been working together on developing the world’s highest performance neutron focusing mirrors by integrating these two technologies developed by Osaka University and JAEA as part of element technology projects of the Development of Systems and Technology for Advanced Measurement and Analysis of JST.

Materials and Methods

A neutron beam is conventionally focused using a multilayer film consisting of nickel carbon and titanium formed on a flat substrate. To use such a substrate as a neutron-focusing mirror, it needs to be curved upon installation, so as to make a curved surface appropriate for focusing. Until now, it has been difficult, however, to make a curved surface with enough precision for effective focusing. To overcome this difficulty, we have developed a novel technology to shape a substrate for focusing and then a supermirror is fitted onto it. The resultant focusing mirror can be utilized to increase the intensity of a neutron beam more effectively compared to conventional focusing techniques by simply adjusting the angle and position of the mirror with respect to the direction of a neutron beam (Fig. 1).

There were many fabrication issues to be addressed in order to create such a focusing mirror. First of all, conventional techniques of quartz processing degrade its surface and thus decrease the strength of the surface. Therefore, a multilayer film attached to the processed surface may become detached along with the deteriorated quartz. In contrast, when the surface of a quartz substrate is processed using the technique developed here, a multilayer film will not be detached from the surface since the surface is not degraded. Second, there is the issue of quality regarding a multilayer film formed on a substrate by conventional techniques. A multilayer film formed using conventional nickel and titanium has a rough interface between these two materials. This issue can be resolved by replacing nickel with nickel carbon, resulting in a smooth interface between nickel carbon and titanium. This technique can yield a significantly better multilayer film.

In the first step of developing a supermirror, a 400-mm quartz substrate is processed by NC-LWE and precision polishing technique to fabricate a high-precision elliptical substrate with the surface error of less than 0.5 micrometer (μm) and the surface roughness of less than 0.2 nm. The resultant quartz substrate is then processed by ion beam sputtering to form a multilayer film consisting of 1,200 atomic layers of nickel carbon and titanium (NiC/Ti) on the substrate. This completes a supermirror. By simply adjusting the angle and position of the supermirror with respect to the direction of a neutron beam, a tightly focused high-intensity neutron beam can be obtained.

The focusing performance of the developed supermirror was evaluated using a neutron beam line, NOBORU, at J-PARC (Fig. 2). A neutron beam that came out spreading from a source slit (0.1 mm width) was reflected with a mirror that can focus the beam at the same magnitude, and the beam width was measured at the narrowest point. As a result, the full-width half-maximum (FWHM) was 0.128 mm, which is equivalent with the width of the source slit (Fig. 3). In addition, the use of this focusing supermirror increased the beam intensity 52-fold. This focusing supermirror achieved the world’s best focusing performance and can increase the intensity of a neutron beam by an order of magnitude compared to conventional focusing mirrors (Fig .4).

Future Prospects

The neutron focusing technology that incorporates the supermirror developed in the present study has drastically improved focusing efficiency of a neutron beam. At the same time, this technology does not alter the wavelength of a neutron beam in the focusing process. For example, by utilizing this supermirror in a reflectometer that is under construction at J-PARC, more than 50-fold performance improvement can be expected. This improvement will enable us to study physical properties that have been difficult to observe, such as the growth process of the magnetic domain of storage elements and the structure of biological membranes. A neutron beam can be also applied to observe hydrogen atoms, which are almost impossible to observe with X-rays. In addition, integration of this supermirror into a power diffractometer will allow us to study the physical properties of materials, including water, more precisely. In particular, the physical properties of watery materials in high-temperature, high-pressure conditions are thought to resemble to the deep internal structures of the Earth and planets; therefore, the results of the present study are expected to significantly advance research on the deep interior of planets.

Our research team is now striving to develop two-dimensional focusing elements that can more effectively focus a neutron beam by incorporating multiple miniaturized mirror elements or ellipsoidal mirrors, while maintaining high focusing performance in order to apply the technology developed in the present study to various neutron measurement systems.

Reference Figure





Glossary

*1 Japan Proton Accelerator Research Complex (J-PARC)
A world-class accelerator facility constructed in Tokai-mura, Ibaraki in 2008. At the Materials and Life Science Experimental Facility in J-PARC, the world’s highest intensity neutron beam is produced using a pulsed proton beam, and is then utilized to conduct research on materials science and life science.
*2 Supermirror
A mirror on which total internal reflection can occur even at an angle exceeding the critical one by utilizing Bragg reflection from a multilayer film. Bragg reflection is a phenomenon whereby an incident beam (such as X-ray, electron, or neutron beam) is reflected from a surface that has a periodic structure (e.g., crystal) at a particular angle defined by the wavelength of an incident beam and the spacing of the periodic structure of the surface. A supermirror for a neutron beam is fabricated by alternately stacking up nickel (which has a large reflectance) and titanium (which has a small reflectance) films, while slightly changing the thickness of each film.
*3 Quartz substrate
A substrate that is made of quartz glass. A quartz glass is a silicon oxide that contains no impurities. Quartz glass has a periodic structure similar to a crystal or an amorphous structure that has no grain boundary, like a polycrystallite. Quartz glass has an advantage in that its surface will not deteriorate due to chemical etching.
*4 Total reflection mirror
A mirror from which a beam is totally reflected at an angle (with respect to its surface) that is smaller than the critical one.
*5 Numerically controlled local wet etching (NC-LWE)
An ultra-precision processing technique by which surface fabrication is conducted via a speed-controlled scanning of a local liquid-phase etching domain formed using a special nozzle. Since this is a non-contact, chemical processing technique, it is insensitive to external disturbances such as vibration, and fabrication can be precisely performed at the nanometer level by controlling processing duration.
*6 Ion beam sputtering
A film fabrication technique by which accelerated ions are brought into contact with a target material, and particles emitted from the target are deposited onto a substrate placed on the other side of the target. A film with a strong adhesion can thus be fabricated.
*7 Critical angle
An incident angle at which only reflection of an incident beam (such as light or neutron beam) occurs, without refraction.

Reference

“High-precision figured thin supermirror substrates for multiple neutron focusing deviceE/p>

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