A study on reflectivity limit of neutron supermirror

We have fabricated m=2.9 NiC/Ti supermirror on a silicon wafer in which surface roughness is about 0.4 nm by ion beam sputtering technique. The reflectivity of supermirror at m=2.8 and 2.9 were 0.94 and 0.90, respectively. The reflectivity of 0.94 at m=2.8 is in the vicinity of theoretical limit including the effects of absorption and incoherent scattering. We show the experimental results and theoretical expectation based on a simple Debye-Waller roughness model, and discuss realistic reflectivity limit of supermirror deposited on the ordinary silicon substrate.