Development of Ni/Ti multilayer supermirrors for neutron optics

Abstract

Artificial multilayer neutron supermirrors are composed of discrete sets of bilayer structures based on Ni and Ti layers with graded thicknesses and with high scattering contrast between individual layers. Various samples consisting of NiX x TiY y (X = C, N, dry air, Y = H, V) periodic multilayers and supermirrors, as well as NiX x and TiY y monolayers were deposited on normal float-glass, borkron glass and on silicon substrates using d.c. magnetron sputtering. The structure and stability of these thin films is the object of a detailed study mainly by means of X-ray diffraction, TEM, AFM and neutron reflectometry. The addition of N to the Ni layers yields a Ni lattice dilatation, a reorientation of the Ni grains from {111} to {200} parallel to the layer surface, a reduction of the grain size, a smoothing of the layer surfaces ( R r.m.s.∼0.3 nm for d = 10 nm), a sharpening of the multilayer interfaces and an enhancement of the scattering length density ( nb=9.53×10 −4nm −2 for Ni− N). Similar results are obtained with C and dry air, which in addition suppress the diffusion of Ni into Ti. The addition of V to the Ti layers shows a smoothing of the layer surfaces and helps to suppress interdiffusion in multilayers, leading also to sharp and stable interfaces. Large neutron supermirrors made of 40 bilayers of NiNO/TiV with a reflectivity in the range of 90% to 94% at glancing angles up to more than twice the critical angle of total reflection from an ordinary Ni surface have been achieved and are fabricated in mass production.