Development of antireflection coatings with a ⁶LiF/⁶²Ni multilayer converter for ultracold neutron detectors

High efficiency detectors for ultracold neutrons (UCN) are needed at the new high flux neutron source, Forschungsreaktor München II. In the development described, silicon PIN diodes were chosen to detect the α-particles or the tritons created in the reaction ⁶Li(n,α)t. The high reflectance of UCN on ⁶Li with its positive optical potential must be compensated by a material with negative optical potential. The isotope ⁶²Ni was chosen for this. To avoid problems due to chemical reactions of Li with humidity, the compound ⁶LiF was chosen. One hundred and fifty double layers of ⁶LiF/⁶²Ni had to be deposited by physical vapor deposition on silicon PIN diodes which had already been coated with 88 nm ≈ 77 μg/cm² of ⁵⁸Ni for reflection of the UCN. The theoretical optimal thickness of the ⁶²Ni layers is 3 nm, and that of ⁶LiF is 6 nm. Since expensive isotopes were involved, a small source-to-substrate distance had to be used, but with the requirement of excellent uniformity. The complicated setup designed for deposition in the 10^-6 Pa range and the evaporation procedures are described.