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

摘要

High efficiency detectors for ultracold neutrons (UCN) are needed at the new high flux neutron source, Forschungsreaktor Munchen II. In the development described, silicon PIN diodes were chosen to detect the a-particles or the tritons created in the reaction Li-6(n,alpha)t. The high reflectance of UCN on Li-6 with its positive optical potential must be compensated by a material with negative optical potential. The isotope Ni-62 was chosen for this. To avoid problems due to chemical reactions of Li with humidity, the compound (LiF)-Li-6 was chosen. One hundred and fifty double layers of (LiF)-Li-6/Ni-62 had to be deposited by physical vapor deposition on silicon PIN diodes which had already been coated with 88 nm approximate to 77 mug/cm(2) of Ni-58 for reflection of the UCN. The theoretical optimal thickness of the Ni-62 layers is 3 nm, and that of (LiF)-Li-6 is 6 mn. 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. (C) 2001 Published by Elsevier Science B.V. References: 7