Antireflection foils with multi-layer converter for ultracold neutron detectors

For experiments at the ultracold neutron (UCN) source of the new high-luminosity neutron source, Forschungsreaktor München II (FRM II), highly efficient detectors are needed. The desired type of detectors utilizes ⁶Li as the neutron converter based on the huge cross-section reaction ⁶Li(n,α)t and detection of the 2.06MeV α-particles or the 2.73MeV t-particles. The high reflectance of UCN from ⁶Li with its positive optical potential must be compensated by a material with negative optical potential. Instead of the expensive ⁶²Ni material, natural Ti can be chosen. It was demonstrated that neither ⁶Li metal nor Ti metal can be deposited in a high-vacuum evaporation apparatus without creating a positive optical potential and thus increasing the reflectivity for the UCN due to oxygen impurities coming from the H₂O partial pressure in the high-vacuum system. To overcome these problems, a new UHV evaporation apparatus was developed and built which is capable of reaching a vacuum in the 10^-10 Pa range. Such a good vacuum can be obtained only when annealing the vacuum system up to 470K. At such a high annealing temperature the silicon detectors for the α- and t particles might suffer degradation of their energy resolution. Therefore, the multi-layer system of ⁶Li and ^natTi is not deposited directly on the silicon detector but a thin rolled Ti foil is applied as backing. A test deposition of 200 double layers of ⁶LiF/ ⁶²Ni on a thin Ti foil is described and a report is given about the setup of the new UHV evaporation apparatus.