Shin Takeda

New fabrication method for an ellipsoidal neutron focusing mirror with a metal substrate

We propose an ellipsoidal neutron focusing mirror using a metal substrate made with electroless nickel-phosphorus (NiP) plated material for the first time. Electroless NiP has great advantages for realizing an ellipsoidal neutron mirror because of its amorphous structure, good machinability and relatively large critical angle of total reflection for neutrons. We manufactured the mirror by combining ultrahigh precision cutting and fine polishing to generate high form accuracy and low surface roughness. The form accuracy of the mirror was estimated to be 5.3 μm P-V and 0.8 μm P-V for the minor-axis and major-axis direction respectively, while the surface roughness was reduced to 0.2 nm rms. The effect of form error on focusing spot size was evaluated by using a laser beam and the focusing performance of the mirror was verified by neutron experiments.

Development of highly-mechanically polished metal-substrate for neutron supermirrors

A small-angle neutron scattering instrument using an ellipsoidal focusing mirror has been developed at Hokkaido University and a prototype machine (mfSANS) has been installed at the JRR-3 research reactor at Japan Atomic Energy Agency (JAEA). They are based on an ellipsoidal focusing mirror on a borosilicate glass substrate. It turned out that borosilicate glass was very brittle and difficult to machine or polish it to the required surface-finishing process. In order to improve this situation, we decided to develop a method to create a metal substrate based ellipsoidal focusing mirror. As the first step, test pieces of flat and ellipsoidal surface neutron supermirrors using metal substrates were fabricated. They were first roughly shaped by a conventional numerically controlled (NC) cutting machine and amorphous NiP was plated. They were then machined using an ultrahigh precision cutting (UPC) method and finished by NC polishing techniques. Surface roughness of Ra ≈ 0.78 nm and surface figure error of submicrometer were realized so far.

Development of a large plano-elliptical neutron-focusing supermirror with metallic substrates.

Results of this study demonstrated that electroless nickel-phosphorus (NiP) plated metal substrate is an excellent material for producing large aspherical neutron-focusing supermirrors. A large plano-elliptical neutron-focusing supermirror comprising two metallic segments was fabricated using single-point diamond cutting, precision polishing and supermirror coating. The average surface roughness of the metallic substrates was approximately 0.3 nm rms. For evaluation, the focusing supermirror was installed at the SOFIA neutron reflectometer, showing high neutron reflectivity and giving minimal beam width of 0.34 mm in FWHM. Because of the large beam divergence accepted by the mirror, the count rate with the focusing mirror was 3.3 times higher than that obtained using conventional two-slit collimation.

Figure correction of a metallic ellipsoidal neutron focusing mirror

An increasing number of neutron focusing mirrors is being adopted in neutron scattering experiments in order to provide high fluxes at sample positions, reduce measurement time, and/or increase statistical reliability. To realize a small focusing spot and high beam intensity, mirrors with both high form accuracy and low surface roughness are required. To achieve this, we propose a new figure correction technique to fabricate a two-dimensional neutron focusing mirror made with electroless nickel-phosphorus (NiP) by effectively combining ultraprecision shaper cutting and fine polishing. An arc envelope shaper cutting method is introduced to generate high form accuracy, while a fine polishing method, in which the material is removed effectively without losing profile accuracy, is developed to reduce the surface roughness of the mirror. High form accuracy in the minor-axis and the major-axis is obtained through tool profile error compensation and corrective polishing, respectively, and low surface roughness is acquired under a low polishing load. As a result, an ellipsoidal neutron focusing mirror is successfully fabricated with high form accuracy of 0.5 μm peak-to-valley and low surface roughness of 0.2 nm root-mean-square.

Profile measurement of a bent neutron mirror using an ultrahigh precision non-contact measurement system with an auto focus laser probe

A bent neutron mirror has been considered as one of the best solutions for focusing neutron beams from the viewpoint of cost-benefit performance. Although the form deviation of the bent profile is expected because of the large spot size, it is difficult to measure due to its geometric limitation. Here, we propose a non-contact measurement system using an auto focus (AF) laser probe on an ultrahigh precision machine tool to precisely evaluate the form deviation of the bent mirror. The AF laser probe is composed of a diode laser, a position sensitive sensor, a charge-coupled device (CCD) camera and a microscope objective lens which is actuated by an electromagnetic motor with 1 nm resolution for position sensing and control. The sensor enables a non-contact profile measurement of a high precision surface without any surface damage in contrast with contact-type ultrahigh precision coordinate measurement machines with ruby styli. In the on-machine measurement system, a personal computer simultaneously acquires a displacement signal from the AF laser probe and 3-axis positional coordinates of the ultrahigh machine tool branched between the linear laser scales and the numerical controller. The acquisition rate of the 4-axis positional data in 1 nm resolution is more than 10 Hz and the simultaneity between the axes is negligible. The profile of a neutron bent mirror was measured from a transparent side using the developed system, and the result proves that the form deviation of the mirror enlarged the the spot size of focused neuron beam.

Development of precision elliptic neutron-focusing supermirror

This paper details methods for the precision design and fabrication of neutron-focusing supermirrors, based on electroless nickel plating. We fabricated an elliptic mirror for neutron reflectometry, which is our second mirror improved from the first. The mirror is a 550-millimeter-long segmented mirror assembled using kinematic couplings, with each segment figured by diamond cutting, polished using colloidal silica, and supermirror coated through ion-beam sputtering. The mirror was evaluated with neutron beams, and the reflectivity was found to be 68-90% at a critical angle. The focusing width was 0.17 mm at the full width at half maximum.

Preliminary Investigation on Design and Fabrication of a Metallic-Ellipsoidal Neutron Focusing Mirror

Neutron beam is a very useful probe for material characterization and non-destructive testing. There are a number of plans to construct neutron instrumens using focusing mirror to improve beam quality with higher intensity and smaller spot size. In this paper, an ellipsoidal mirror made by a metal substrate is proposed. As the metal substrate, electroless nickel is adopted because of its amorphous structure, machinability and relatively large critical angle of neutron reflection. The design of ellipsoidal neutron focusing mirror is made by considering the neutron reflectivity. A mirror is fabricated by ultrahigh precision cutting and 1st polishing process, and the form accuracy of 5 μm P-V with surface roughness of 0.7 nm Ra is obtained. Through this experimental fabrication, the feasibility of the proposed method is verified.