Development of Ni/Ti supermirrors with large-m and a curved surface

Nickel/titanium supermirrors with critical reflection angles θc of m=3–5 times the critical angle of total reflection of natural Ni θcNi were fabricated on silicon substrates 3 in. in diameter and 3 mm in thickness by a DC magnetron sputtering system using the facing targets technique, and the neutron reflectivity of the supermirrors was evaluated for cold neutrons of 8.8 Å in wavelength. The supermirrors with 532 layers and with 468 layers achieved a reflectivity of 84% and 53% at their θc of m=2.9 and 3.7, respectively, while supermirrors with more than 630 layers had reduced reflectivity at angles smaller than θc. This reduction resulted from the formation of a non-multilayered pillar structure due to the growth of interface roughness between layers. Using supermirrors with m not more than 3, we designed a paraboloidal focusing mirror and proposed a prototype to obtain approximately 5 times the utilizable neutrons at the focal point for cold neutrons of 8 Å.