Mikinori Nagano

Figuring of plano-elliptical neutron focusing mirror by local wet etching

Local wet etching technique was proposed to fabricate high-performance aspherical mirrors. In this process, only the limited area facing to the small nozzle is removed by etching on objective surface. The desired objective shape is deterministically fabricated by performing the numerically controlled scanning of the nozzle head. Using the technique, a plano-elliptical mirror to focus the neutron beam was successfully fabricated with the figure accuracy of less than 0.5 microm and the focusing gain of 6. The strong and thin focused neutron beam is expected to be a useful tool for the analyses of various material properties.

One-dimensional neutron focusing with large beam divergence by 400mm-long elliptical supermirror

Reflective optics is one of the most useful techniques for focusing a neutron beam with a wide wavelength range since there is no chromatic aberration. Neutrons can be focused within a small area of less than 1 mm2 by high-performance aspherical supermirrors with high figure accuracy and a low smooth substrate surface and a multilayer interface. Increasing the mirror size is essential for increasing the focusing gain. We have developed a fabrication process that combines conventional precision grinding, HF dip etching, numerically controlled local wet etching (NC-LWE) figuring, low-pressure polishing and ion beam sputtering deposition of the supermirror coating to fabricate a large aspherical supermirror. We designed and fabricated an piano-elliptical mirror with large clear aperture size using the developed fabrication process. We obtained a figure error of 0.43 μm p-v and an rms roughness of less than 0.2 nm within an effective reflective length of 370 mm. A NiC/Ti supermirror with m = 4 was deposited on the substrate using ion beam sputtering equipment. The results of focusing experiments show that a focusing gain of 52 at the peak intensity was achieved compared with the case without focusing. Furthermore, the result of imaging plate measurements indicated that the FWHM focusing width of the fabricated mirror is 0.128 mm.

High-precision figured thin supermirror substrates for multiple neutron focusing device

An aspherical supermirror is one 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 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. However, it is difficult to fabricate thin mirror substrates with a form accuracy of sub-micrometer 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 millimeter-thick elliptical supermirror. This process consists of noncontact figuring by the numerically controlled local wet etching technique 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 and focusing performance of elliptical supermirrors with a thickness of 1.5 mm.

Effect of Si interlayers on the magnetic and mechanical properties of Fe/Ge neutron polarizing multilayer mirrors

The neutron polarizing supermirror is one of the most important optical devices for polarizing neutron beams. To meet a variety of research demands, neutron polarizing supermirrors need to display high polarization efficiencies at low external magnetic fields. Fe/Si and Fe/Ge multilayers are typically used in neutron polarizing supermirrors because the contrast in scattering length densities almost vanishes for spin-down neutrons. The Fe/Si/Ge/Si multilayer, obtained by adding thin interlayers of Si to an Fe/Ge multilayer, is effective in reducing the external field strength necessary to achieve efficient neutron polarization. To gain insight into the mechanism that controls the required external field strength for a neutron polarizing supermirror, we investigated the magnetic and mechanical properties of Fe/Si, Fe/Ge, and Fe/Si/Ge/Si multilayers. The external field strength required to achieve efficient neutron polarization was found to be proportional to the compressive film stress. The compressive stre...

Highly efficient damage-free correction of thickness distribution of quartz crystal wafers by atmospheric pressure plasma etching

A new finishing method was developed to correct the thickness distribution of a quartz crystal wafer by the numerically controlled scanning of a localized atmospheric pressure plasma. The thickness uniformity level of a commercially available AT-cut quartz crystal wafer was improved to less than 50 nm without any subsurface damage by applying one correction process. Furthermore, applying a pulse-modulated plasma markedly decreased the correction time of the thickness distribution without breaking the quartz crystal wafer by thermal stress.

Figuring of Aspherical Metal Mirror Substrate for Neutron Focusing by Numerically Controlled Electrochemical Machining

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.

Neutron beam focusing using large-m supermirrors coated on precisely-figured aspheric surfaces

We have developed a 1-dimensional elliptic mirror combining a supermirror coated with ion-beam sputtering and precise elliptic surface figured with the numerically-controlled local wet etching process. In this study, NiC/Ti supermirror (m = 4) was deposited on a precisely figured surface of synthesized quartz glass over 90 mm × 40 mm. Wideband neutrons of λ > 3.64A were focused with focal spot size down to 0.25 mm, peak intensity gain up to 6 without significant diffuse scattering. Time-of-flight measurements suggest that wideband neutrons are effectively focused to the focal point.

Fabrication of plano-elliptical neutron focusing supermirror by numerically controlled local wet etching with ion beam sputter deposition

High-performance optical devices with a figure accuracy of sub-micrometer level and a surface roughness of atomic level are required to collect and/or to focus a neutron on a sample without scattering loss. To fabricate a high-performance neutron focusing supermirror, a two-stage numerically controlled local wet etching technique combined ion beam sputter deposition was developed. By applying this technique, a plano-elliptical substrate made of synthesized quartz glass with a figure accuracy of less than 0.5 μm was fabricated, and its surface was deposited with NiC/Ti multilayers by ion beam sputtering to produce an m=4 supermirror. The focusing performance of the supermirror was evaluated at the SUIREN of JRR-3M, and a focusing gain of 6 in peak intensity was achieved compared with a nonfocused direct beam.

A Study on Surface Roughness of Curved Silicon Crystal Spectrometer Fabricated by Plasma Chemical Vaporization Machining

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.

Study on In Situ Etching Rate Monitoring in Numerically Controlled Local Wet Etching

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.