Slowing Down of Neutrons to Very Low Temperature by Cold Solid Hydrogenous Moderators

The slowing down of neutrons to very low energy has been examined with particular reference to cold neutron production. The neutron spectrum formed in very cold light water ice has been measured with time-of-flight technique. It is observed that at extremely low temperature the neutron temperature is much higher than the moderator temperature, whereas in the intermediate range of temperature the neutron temperature does not differ much from the moderator temperature. It is suggested that there is a limiting neutron temperature below which it does not fall, even when the moderator temperature drops further. This observation can lead to information on the nature of the mechanism for removing small amounts of energy from slow neutrons in cold solid hydrogenous moderators. By using a simplified model for the cold solid hydrogenous moderators, it is shown that low frequency lattice vibrations play an important role in producing cold neutrons. Also, quantitative analysis indicates that while the cold neutron temperature depends only slightly on the neutron absorption, the gain in cold neutron flux is strongly affected by the neutron absorption.