Supermirrors for Neutrons on Atomically Smooth Metal Substrates

Similarly as in visible light optics where fibers are used to transport photons over large distances, at modern neutron sources the neutrons are transported by internal reflection within rectangular channels made of glass being coated with nickel, the material with the highest angle of total reflection θ_c that is given by θ_c(deg) = 0.1 m λ (Å). For Ni, m corresponds to 1.  The neutron flux can be increased significantly by using artificial multilayers called “supermirror” composed of bilayers Ni/Ti with depth graded thickness. These bilayers provide a “continuous” regime of Bragg diffraction resulting in a high reflectivity up to m timesθ_c. The number of necessary layers increases quickly with m, namely with N 4m4 while the interface roughness must remain on an atomic level to achieve high reflectivity. In the early nineties, a research group at Paul Scherrer Institute succeeded for the first time to produce supermirrors with m = 2 on a large scale for the Swiss spallation source SINQ. During the funding period of MaNEP, SwissNeutronics succeeded to optimize the coating process further such that it became possible to manufacture supermirrors with m–values exceeding m = 7. Fig. 1 shows reflectivity data taken from the production of such mirrors for the spallation source SNS (USA). These results are most impressive, because it became possible to deposit more than 8300 layers Ni and Ti with atomically smooth interfaces. The bilayer thickness varies between 41 Å and 500 Å. Moreover, the layers exhibit no stress to guarantee a long life time of the guides under high irradiation.

Reflectivity data for supermirror with m = 4, 5, 6, and 7, establishing a new benchmark in the history of supermirror.

In addition, during the course of the MaNEP program, SwissNeutronics made major breakthroughs in polishing Al and steel surfaces leading to extraordinarily smooth surfaces with an RMS roughness of approximately 2 Å. When coated with large-m supermirrors, atomically smooth metallic substrates allow the fabrication of advanced neutron optical devices for new applications. In contrast to glass guides that start typically 2 m away from the moderator, metallic guides may approach the moderator as close as a few cm thus making the extraction of the neutrons more efficient.

* SwissNeutronics, Neutron Optical Components, Brühlstrasse 28, CH-5313 Klingnau

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