Reflectivity improvements for neutron mirrors and supermirrors

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Abstract
In order to study multilayers for neutrons optics, Ni/Ti multilayers were first realized by magnetron sputtering technique on super-polished Si wafer and Borkron glasses. Including carbon in the nickel target in order to improve neutron reflectivity, nickel carbide/titanium multilayers were deposited under the same conditions. The neutron optical quality of NiC/Ti multilayers are better than Ni/Ti stacks. The grazing angle neutron reflectivity results were subsequently confirmed by other techniques. X-ray diffraction measurements show a nickel layer crystalline (111) whereas the NiC layers are quasi-amorphous. In consequence, the a- NiC/Ti interfaces are sharper than the Ni/Ti ones. Reactive hydrogenation of titanium layers during the deposition process allows us to decrease the neutron refractive index. In this case, the contrast between the high and low refractive index increase and the Ni/Ti-H multilayers have a better reflectivity than those obtained with pure Ni/Ti materials. Moreover, strong affinity of the titanium for hydrogen reduce the Getter effect corresponding to a stabilization of the titanium layers. Accordingly, better contrast and better interfaces are obtained for NiC/TiH multilayers. Some results of super-mirrors with less than 50 layers using these types of materials are given. In these cases, we obtained more than two times the critical angle of pure nickel with a value close to one for the reflectance coefficient.

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