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Tayloring neutron optical performance of Fe/Si multilayers

Reflected neutron beam originating from a crystal monochromator contains higher order wavelength contribution. Creating multilayer mirror structures with custom reflectivity curves including a monochromatic polarized neutron beam is a challenge in modern neutron optics. In this work we present the study of three types of magnetron-sputtered aperiodic Fe/Si layer structures with the purpose of higher harmonic suppression. First, an approximate sinusoidal profile was achieved directly by carefully controlling the evaporation parameters during sputtering process. Second, we implemented a random distribution of the layer thicknesses in which the layer structure of the sample was derived from a prescribed simulated spectrum. Third, a quasi-sinusoidal rounded bilayer scattering length profile was attempted to achieve by 0, 5, 10, 27 and 270x1e15 /cm2 fluence of 350 keV neon irradiation starting from a sharp bilayer profile. Besides the highest fluence, which considerably decreased the overall bilayer contrast, we found an improvement of the monochromatisation with increasing irradiation fluence by the decrease of the total intensity of higher order reflections from 11.1% to 2.2% while the intensity of the first order Bragg peak decreased from 80% to 70%. The polarizing efficiency also increased with increasing fluence of Ne+ irradiation from 78.8% to 90.7%. We were able to achieve 92% and 82% reflectivity of the random multilayer structure and sinusoidal profile sample with 87.6% and 97.1% polarizing efficiency, respectively.