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) dete ctor which works at liquid helium temperature has been improved. The n eutron converter was a Li-6/Ti multilayer evaporated onto the surface of a commercial silicon-surface-barrier detector for the charged parti cles. The advantage of a Li-6/Ti multilayer is that the positive optic al potential of Li-6 is compensated by the negative potential of Ti. U CNs can therefore penetrate into the neutron converter which transform s neutrons to charged particles. The surface of the neutron converter was coated by a 1800 Angstrom thick Ti layer which prevents lithium fr om reacting with oxygen in the atmosphere. Ti easily absorbs oxygen du ring evaporation so that the effect of the Ti negative potential was d iminished in our previous detector due to the positive potential of ox ygen. The main improvement now is that the Li-6/Ti multilayer and a pr otective Ti layer were evaporated in one order of magnitude higher vac uum than in the previous case. As a result, the neutron reflectivity o f a Li-6/Ti multilayer mirror, which was made in the same evaporation as the solid state UCN detector, has been decreased at wavelengths lon ger than 800 Angstrom compared with the previous case and is 0.6 at 15 00 Angstrom. This implies that even UCN of 1500 Angstrom can penetrate into the Li-6/Ti multilayer and could be detected with a high detecti on efficiency by the solid-state UCN detector with a Li-6/Ti multilaye r neutron converter.