Papers by Author: Mikinori Nagano

Abstract: Focusing neutron beam with wide wavelength range is an indispensable technique used to compensate for weak signals from tiny samples in various experiments using pulsed neutron beam generated from high intensity proton accelerator facilities, such as J-PARC. Aspherical supermirror device is one of the most effective optical devices for focusing neutron beam with wide wavelength range since it has no chromatic aberration. Stack of aspherical supermirror enables us to focus neutron beams with wide divergence. Thin mirrors with a millimeter thickness are required to minimize the absorption loss of incident neutron beams since the thickness of a mirror shadows the reflective area of the other mirrors. Previously, we developed a fabrication process of a precise millimeter-thick elliptical supermirror. This process consists of noncontact figuring by the numerically controlled local wet etching technique, the finishing of surface without degrading mirror shape by low-pressure polishing, and the ion beam sputter deposition of NiC/Ti multilayers on both sides of the mirror substrate to compensate for film stress. In this paper, we report fabrication of elliptical supermirror with a thickness of 1 mm and development of multiply-arranged neutron focusing mirror device using stacked 4 fabricated elliptical supermirror with a thickness of 1mm.

Abstract: Neutron beam generated by high intensity proton accelerator facility is powerful tool to investigate characteristics of soft and hard materials. However, neutron beam is not major tool for material science since intensity of neutron beam is very weak compared to that of X-rays. Neutron focusing device is required to increase in intensity of neutron beam. Aspherical supermirror is effective for neutron focusing with wide wavelength range without chromatic aberration. In this research, we proposed a fabrication process for large and cost-effective aspherical mirror substrate made of aluminum alloy because metal can be figured coarsely at low cost by using conventional machining. The mirror fabrication process proposed by us consists of grinding for coarse figuring, numerically controlled electrochemical machining (NC-ECM) to correct objective shape with form accuracy of sub-micrometer level and low-pressure polishing to decrease in surface roughness to sub-nanometer level. In the case of figure correction of the mirror substrate by NC-ECM, deterministic correction is realized because NC-ECM is a non-contact electrochemical removal process for metal materials, without workpiece deformation. In this paper, we report fundamental machining characteristics of ECM, which uses electrode with a diameter of 10 mm and NaNO3 electrolyte.

Abstract: Numerically controlled local wet etching (NC-LWE) is very promising technique for deterministic figuring of ultraprecision optical devices, such as aspherical lens, photo mask substrate and X-ray or neutron focusing mirror. NC-LWE technique is non-contact removal process using chemical reaction between etchant and surface of workpiece, so this technique enables us to figure the objective shape without introduction both substrate deformation and sub-surface damage. It is essential to measure temperature and concentration of the etchant to maintain the material removal rate constant over a processing time, since the etching rate of NC-LWE strongly depends on these parameters. Hydrofluoric (HF) acid solution is used as an etchant for synthesized quartz glass. We aim to develop an in situ monitoring system of etchant concentration using Raman spectroscopy and electric conductivity measurement. Raman spectroscopy measurement result indicates that there is a good linear relationship between HF concentration and intensity ratio of two specific Raman bands.

Abstract: We propose the application of open-air type numerically controlled plasma chemical vaporization machining (NC-PCVM) to fabricate a doubly curved crystal (DCC) substrate. Since PCVM utilizes only a chemical reaction to remove the work piece surface, there is no degradation of the crystallographical properties of the work piece material. In our previous study, we succeeded in fabricating a curved Si (111) crystal substrate with a curvature radius error of 0.08 %. Rocking curve measurement results revealed that there was no lattice strain on the processed surface. However, surface roughness degraded after PCVM figuring. To reduce the surface roughness, we modified the structure of the electrode unit, which generates plasma, to be able to supply a shielding gas. By supplying helium with a flow rate of 0.5 L/min as the shielding gas, rms surface roughness of the silicon was reduced from 0.73 nm to 0.42 nm. Excessive supply of helium (> 1 L/min) and/or supply of argon caused deterioration of the surface roughness. These results indicate that appropriate supply of the shielding gas is effective in reducing roughness in the open-air type PCVM process.

Abstract: Numerical controlled local wet etching is a novel non-contact deterministic figuring method in ultra precision optics fabricating and functional material manufacturing fields, and the cross-sectional shape of the traditional removal spot is a simple cylinder, so the removal function has no adjustability. In order to create more practical and regular removal function, an eccentric rotation system is introduced to improve the LWE system. By controlling the eccentricity, it can achieve varied shapes removal function. When the rotary axis is controlled to a proper eccentricity, the removal function can be close to the Gaussian function. Moreover, the theoretical calculation and experimental validation are coincident and can give the research a steady foundation. The improvement not only can increase the adjustability of the removal function in LWE, but also can expand its applied field and provide reference for other ultra precision machining methods whose removal function does not have circular symmetry.

Abstract: Aspherical supermirrors are some of the most useful neutron-focusing optics. We aim to develop multiple aspherical supermirror devices using high-precision figured aspherical focusing supermirrors to focus neutron beams with high intensities, because multiple mirrors collect a very large beam divergence. Thin mirrors with millimetre thickness are required to minimize the absorption loss of incident neutron beams since the thickness of a mirror shadows the reflective area of other mirrors. However, it is difficult to fabricate thin mirror substrates with a form accuracy at the sub-micrometre level by conventional machining. Conventional machining deforms a substrate by machining force and spring back after machining causes figure error. Furthermore the deposition of supermirrors deforms the mirror substrate by film stress. Thus, we developed a new process of fabricating a precise millimetre-thick elliptical supermirror. This process consists of non-contact figuring by the numerically controlled local wet etching technique, the minimization of surface roughness without degrading form accuracy by low-pressure polishing with a polishing pressure less than about 7 kPa (1psi), and the ion beam sputter deposition of NiC/Ti multilayers on both sides of the mirror substrate to compensate for film stress. In this paper, we report on the fabrication results of aplano-elliptical mirror substrate with a thickness of 1 mm.

Abstract: The aspherical supermirror is among the most useful optics for focusing a neutron beam with a wide wavelength range. The improvement in surface roughness is essential for increasing the focusing gain. A highly efficient and high-precision fabrication process for the substrate of the aspherical mirror combining conventional precision grinding, numerically controlled local wet etching (NC-LWE) figuring, and low-pressure polishing was developed. Using this new fabrication process, plano-elliptical neutron-focusing mirror substrates were successfully fabricated with a figure error of submicrometer order and an rms surface roughness of less than 0.3 nm. In this report, the surface roughness of a quartz glass substrate for a neutron focusing supermirror was evaluated.

Abstract: In the X-ray fluorescence analysis on sub-micron particle, application of the doubly curved crystal (DCC) spectrometer with Johansson-type geometry is effective to improve the lowest limit of detection because DCC makes it possible to focus and monochromatize an X-ray beam simultaneously. A strain-free crystal is essential for the high-performance focusing crystal spectrometer. We propose the application of the open-air type numerically controlled plasma chemical vaporization machining (NC-PCVM), which utilizes neutral reactive species generated by atmospheric pressure plasma, to fabricate the DCC substrate. By applying NC-PCVM technique, a curvature radius error of 0.08% was obtained, and there was no degradation of the crystallinity of the Si (111) substrate.

Abstract: Numerically controlled local wet etching (NC-LWE) is a novel technique to fabricate the ultraprecision optical components and/or finishing the functional materials. In this technique, a figuring is performed by controlling the dwelling time of the combination nozzle, which consists of a supply and a suction part of an etchant, on the workpiece. In this paper, we proposed fabrication process of millimeter-thick elliptical neutron focusing mirror substrate by applying NC-LWE figuring involving CeO2 slurry polishing. We fabricated a millimeter-thick elliptical neutron focusing mirror substrate with a figure error of less than 0.2 μm and obtained a surface roughness of less than 0.15 nm rms.

Abstract: Numerically controlled local wet etching is a novel figuring method for fabricating the ultraprecision optical components and/or finishing the functional materials. In this method, localized wet etching area is formed by applying the combination nozzle which consists of a supply part and a suction part of an etchant, and the removal volume at any point on the workpiece surface is determined by the dwelling time of the nozzle. In this paper, we proposed the two-step figuring process, which consists of a rough figuring process by applying a one-dimensional numerically controlled scanning using a large rectangular nozzle and a finishing process by applying two-dimensional numerically controlled scanning using a small circular nozzle, for figuring the plano-aspherical mirror. By applying the two-step figuring process, we fabricated the plano-elliptical neutron focusing mirror with the figure accuracy of less than 0.5 µm and succeeded in achieving the focusing gain of 6.