NEWS April 2010

Supermirror coatings on super-polished Steel

The processes to refine the surface of metallic substrates were first developed on aluminum. Recently, we succeeded to adapt the refining process to steel substrates. 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. Floated glass has been proven to be the best substrate for supermirror coatings when prepared properly. The super-polished steel substrate was then coated with a Ni/Ti supermirror. The measurement of the reflectivity revealed an excellent result of R ≈ 77% @ m = 3.7 as plotted below.

Metallic substrates are envisaged for neutron optics in order to overcome limitations implied by glass. Benefits are:

• increased durability against large changes in temperature and intense irradiation,
• enhanced possibilities to deform substrates mechanically in order to impose more complicated geometrical designs of guides, e.g. for focusing,
• robustness against mechanical damages and implosions,
• steel substrates act as shielding and absorb a considerable proportion of the energy of fast neutrons and gamma rays escaping from the moderator of the neutron source.

Dedicated applications of neutron guides with metallic substrates are initial guide sections to approach the moderator very closely and to reduce streaming of background radiation.