“m=1” coatings

For neutrons the critical angle of Ni is defined as m=1. If one needs a coating with reflectivity above m=1, people have traditionally used Ni58 or Ni-based multilayers. The
reason to use Ni is the high neutron scattering density and the fact that it is easy to coat Ni using magnetron sputtering.
For a neutron guide the cost of shielding around the guide is a substantial part of the total cost of the guide. We are therefore looking at other materials than Ni for m=1
coatings. Both Be and diamond have the same or higher neutron scattering density than Ni, and have a much smaller absorption cross section. Because of the lower absorption
cross section, and because of fewer emitted gamma ray photons when a neutron is absorbed, these coatings are producing much less gamma radiation and therefore reduce the
shielding costs.
Be is frequently used in a wide range of science and technology applications. The only chemical hazard after manufacture is Be dust if the sample is destroyed. We have a
sample of 276 nm Be coated on a Si wafer for these tests.
Diamond Like Carbon (DLC) coatings with 99% sp3 bindings (meaning it is very close to diamond) are made commercially using CVD techniques. In the coating there will also be
about 2% H which will reduce the neutron scattering density of the coating. But the total scattering density of these coatings should still be higher than for Ni. In the very
near future we will present experimental results on these mirrors.