An improvement of the detection efficiency of a solid state UCN detector with a Li-6/Ti multilayer neutron converter working at liquid helium temperature

The detection efficiency of a solid state ultracold neutron (UCN) detector which works at liquid helium temperature has been improved. The neutron converter was a Li-6/Ti multilayer evaporated onto the surface of a commercial silicon-surface-barrier detector for the charged particles. The advantage of a Li-6/Ti multilayer is that the positive optical potential of Li-6 is compensated by the negative potential of Ti. UCNs can therefore penetrate into the neutron converter which transforms neutrons to charged particles. The surface of the neutron converter was coated by a 1800 Angstrom thick Ti layer which prevents lithium from reacting with oxygen in the atmosphere. Ti easily absorbs oxygen during evaporation so that the effect of the Ti negative potential was diminished in our previous detector due to the positive potential of oxygen. The main improvement now is that the Li-6/Ti multilayer and a protective Ti layer were evaporated in one order of magnitude higher vacuum than in the previous case. As a result, the neutron reflectivity of a Li-6/Ti multilayer mirror, which was made in the same evaporation as the solid state UCN detector, has been decreased at wavelengths longer than 800 Angstrom compared with the previous case and is 0.6 at 1500 Angstrom. This implies that even UCN of 1500 Angstrom can penetrate into the Li-6/Ti multilayer and could be detected with a high detection efficiency by the solid-state UCN detector with a Li-6/Ti multilayer neutron converter.