ABSTRACT
This chapter discusses cold and ultracold neutrons, particularly since these are gaining increasing popularity as a tool for basic and applied studies of the matter. The nuclear reactor is one of the most intense and widely used sources of neutrons, at the same time providing an attractive energy requirement. The maximum of the fission cross section of even–even heavy nuclides by neutrons lies in the thermal region and is hundreds of times larger than that of the fast neutrons. The relative content of resonance neutrons is characterized by the cadmium ratio. Low-energy neutrons which leak from the target are potentially useful because suitable materials can reduce their speed to produce pulsed neutron beams useful for the research in materials science and nuclear physics. The number of low-energy neutrons which are produced will depend on the size of the spallation target.
