super-polished metallic substrates

Present neutron guides are usually assembled from flat glass substrates. The natural surface of floated glass provides an extremely small roughness enabling for high reflectivity even for very large angles of reflection, i.e. up to m = 7 times the critical angle of Ni.

Unfortunately, glass has limited stability against irradiation, temperature, and mechanical stress, effects which are particularly severe close to the moderator. In contrast to neutron guides made from glass, metallic guides can approach the moderator as close as a few centimeters or even touch the vessel of the moderator. As a result, complete illumination of the guides and an improved transport for advanced guides, e.g. elliptic guides, can be realized. For the latter, the transport of only the useful neutrons is strongly pronounced leading to a “clean” beam with a very well defined and compact phase space.

With the motivation to improve the neutron transport performance of neutron guides/optics further, SwissNeutronics started a research project with the aim to prepare metallic substrates with an atomically smooth surface enabling supermirror coating with an excellent reflectivtiy, even at high m-values. The processes to refine the surface were first developed on aluminum but could be adapted to refine also steel substrates to the same quality. Measurements of the surface roughness of the substrates using atomic force microscopy (AFM) gave a RMS roughness of ≈2 Ǻ. This is comparable to the results known from floated glass, measured with the same AFM. Fig. 1 shows typical AFM images of the surface of metallic and float glass substrates.

Such atomically smooth surfaces allow supermirror coating with high reflectivity. Fig. 2 depicts typical reflectivity profiles of supermirror on aluminum substrates. The data are from the first customer project, an in-pile guide assembly for the initial section of the new neutron guide system at the NCNR at the National Institute for Standards and Technology (NIST).