Design and advanced preparation of multilayer converters for ultracold-neutron detectors

Abstract

For experiments at the planned ultracold neutron (UCN) source of the new Munich research reactor (FRM II) highly efficient UCN detectors must be developed. The principle of this type of detector is the conversion of UCN in 6Li layers into 2.06 MeV α-particles and 2.73 MeV t-particles created in the reaction 6Li(n,α)t. Since the α- and t-particles are emitted in opposite direction, each reaction of a neutron with a 6Li atom could be identified, e.g. in a silicon PIN diode. The design of an UCN converter must take in account the high reflectance of UCN at 6Li with its positive optical potential. This must be compensated by a material with negative optical potential. Candidates for this are either 62Ni or 48Ti and as cheap compromise natTi. Since Ti and 6Li-metal can only be processed in an UHV plant without introducing disturbing oxygen impurities, the new multilayer stacks have been prepared all with 62Ni and 6LiF. One UCN converter with a stack of 125 double layers of 6LiF/ 62Ni was deposited on a 450 μg/cm 2 rolled natTi backing foil. This was investigated by means of ERD analysis for its atomic concentration. These results are presented. A proposal is discussed where the backing foil is made from the neutron reflector material 58Ni. The required self-supporting 58Ni foil of 200–400 μg/cm 2 thickness and a square area of 6.2 cm 2 can be prepared by rolling. A UHV vacuum system for electron beam evaporation of Ti and 6Li is being assembled. Some annealing tests at another UHV system led to a modification of the planned pumping system. One of the three refrigerator cryopumps foreseen in the design has been replaced for a turbomolecular pump. Its gate valve is closed when annealing to 470 K is finished.