Thermal neutron standard field with a Maxwellian distribution using the KUR heavy water facility

A heavy water facility attached to the KUR (Kyoto University Reactor, swimming pool type. 5 MW) yeilds pure thermal neutrons with a Maxwellian distribution. The facility is placed next to the core of KUR and contains about 2t of heavy water. The width of the heavy water layer is about 140 cm. The neutron spectrum was measured with the time-of-flight technique using a fast chopper. The measured spectrum was in good agreement with a Maxwellian distribution in the whole energy region for thermal neutrons. The neutron temperature was slightly higher than the heavy water temperature. The contamination of epithermal and fast neutrons caused by photo-neutrons from the γ-n reaction in heavy water is very small. The maximum intensity of thermal neutrons is 3 X 1011 n/cm.s. When a bismuth scatterrer is attached, the gamma ray contamination is decreased to a ratio of 0.05 of gamma rays to neutrons in Rem. This standard neutron field has been used for such experiments as thermal neutron cross section measurement, diffusion length measurement, detector calibration, activation analysis and for biomedical purposes. (Auth.)