An abnormal ultra-cold-neutron absorption in solid UCN-detectors

Solid ultra-cold neutron detectors consisting of a neutron converter layer and a silicon pin-diode were tested at Institute Laue Langevin. Two kinds of converter, LiF and LiOH, were used, and their critical velocities covered the range from 0.8 to 5.0 m/s by adjusting the isotopic fraction of 6Li in Li. All tested detectors measured the UCN below their critical velocities down to 1.5 m/s without a clear cutoff feature at their critical velocities. It is an abnormal absorption of UCN. It was also observed that the detector efficiencies of all tested detectors approach together in the UCN region, 4–6 m/s, in spite of the variety of critical velocity and thickness of converters.