New ultracold neutron detectors with 6Li compounds

We have developed a new type of ultracold neutron detector. It consists of a 6Li-compound converter and a silicon-solid-state detector. The particles, t and α, from the reaction in the converter, 6Li+n → α(2.05 MeV)+t(2.73 MeV), are detected by the silicon-solid-state detector. Since we first designed it for counting ultracold neutrons in a liquid helium cryostat, it was asked not to be a gas counter and to be compact. One unit of detector can be kept in a shield case of 38x38x8 mm3. Two types of converter, Ti-LiF and LiOH-PET were developed and tested. The ultracold neutron measurements were made at the turbine-type Ultracold-Neutron Source Facility of the KUR. The measurements revealed the various practical advantages of new detectors as compactness, simplicity without complex schemes, high sensitivity for ultracold neutrons as 0.001 c/s based on their good S/N ratios and very low effective limiting velocities. It was found that the micro-structure of the converter film allows the counting of ultracold neutrons with velocity below the nominal limiting value. (author)