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. 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.