Development of fabrication process for aspherical neutron focusing mirror using numerically controlled local wet etching with low-pressure polishing M. Nagano a , F. Yamaga a , N. Zettsu a , D. Yamazaki b , R. Maruyama b , K. Soyama b , K. Yamamura a,n a Research Center for Ultra-Precision Science and Technology, Graduate School of Engineering, Osaka University, Yamadaoka 2-1, Suita, Osaka 565-0871, Japan b J-PARC Center, Japan Atomic Energy Agency, Japan article info Available online 30 June 2010 Keywords: Neutron supermirror Local wet etching Ion beam sputtering Elliptical mirror Focusing abstract The aspherical supermirror is among the most useful optics to focus a neutron beam with a wide wavelength range. The improvement of surface roughness and enlargement of mirror size are essential for increasing the focusing gain. A highly efficient and high-precision fabrication process for the substrate of a large 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, a 400 mm-long plano-elliptical neutron focusing mirror substrate was successfully fabricated with a figure error of 0.39 mm p–v and an rms surface roughness of less than 0.2 nm. & 2010 Elsevier B.V. All rights reserved. 1. Introduction Various measurements using neutron beams have attracted much attention because of their ability to identify individual hydrogen atoms and water molecules, their sensitivity to magnetism and their high penetrating power. Moreover, high- intensity proton accelerator facilities have been constructed for neutron beam research such as J-PARC in Japan, ISIS-TS2 in UK and SNS in USA. However, the neutron intensity is very weak compared with that of X-rays in synchrotron sources, even in the most recent facilities. Therefore, high-performance optical devices for focusing of neutron beam within a small sample ( o1 mm range) are required. Reflective optics are among the most useful devices for focusing a neutron beam with a wide wavelength range since there is no chromatic aberration [1]. Neutrons can be focused within a small area of less than 1 mm 2 using a high-performance elliptical supermirror with high form accuracy and low roughness of both the substrate surface and the multilayer interface. Therefore, reflective optics using an elliptical supermirror can be applied to a wide range of neutron experiments, such as the diffraction-based measurement of small biomolecules and posi- tion sensitive prompt gamma analysis, at neutron reactors and pulsed neutron sources. Because only very small incident angles can be used to obtain total reflection of neutrons (  11), long mirrors are required in order to be able to analyze neutron beams of usable sizes (a few mm wide at least). A supermirror enables the reflection of a neutron beam with a large incident angle in accordance with Bragg’s law, and an ion beam sputter deposition technique for fabricating a NiC/Ti supermirror with a high m-value and high reflectivity has been developed [2–4]. To take advantage of this high-performance supermirror, sub-micrometer form accuracy, sub-nanometer surface roughness and a large effective reflective area with a strongly curved shape are required. For aspherical mirror optics, bending mirror systems have been developed and a focusing spot size less than 0.1 mm has been obtained [5,6]. On the other hand, figured aspherical focusing mirror substrates, which do not require a bending mechanism, can be fabricated by conventional fabrication processes such as single-point diamond turning [7] and electro- lytic in-process dressing (ELID) grinding [8]. However, in these conventional machining processes, form accuracy of the figured mirror strongly depends on stiffness of the machining equipment. It is also affected by external disturbances such as vibration and thermal deformation, because these machining processes utilize contact removal mechanism. Therefore, it is very difficult to fabricate an aspherical mirror substrate with nanometer-level form accuracy and high reproducibility. Bent polishing has also been reported as another mechanical fabrication process [9]. However, it is difficult to figure a steep shape using bent polishing because of the elastic limit of the mirror material. In contrast, as unconventional noncontact techniques for the fabrication of precise optics with nanometer-level form accuracy, elastic emission machining (EEM) [10], ion beam figuring (IBF) [11] and figure correction by differential deposition [12] have been reported. However, since the removal rate or deposition rate of these figuring techniques is very low, an efficient figuring technique is needed for fabricating mirror shapes requiring the removal of a large volume of material. Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/nima Nuclear Instruments and Methods in Physics Research A 0168-9002/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2010.06.253 n Corresponding author. Tel.: + 81 6 6879 7293. E-mail address: yamamura@upst.eng.osaka-u.ac.jp (F. Yamaga). Nuclear Instruments and Methods in Physics Research A 634 (2011) S112–S116