Research Reactor
In order to maximize the neutron flux research reactors are equipped with a rather small core. The water, which surrounds the reactor core cools the fuel elements and is moderating (slowing down) the emitted neutrons. This is achieved in the following way: The outgoing neutrons undergo numerous collisions with the hydrogen nuclei (protons) of water (H2O) and become strongly decelerated; these neutrons are called thermal neutrons (wavelength of 0.1 to 0.3 nm). Only thermal neutrons can participate in nuclear fission and induce the chain reaction and be used in experiments. The reactor core is surrounded with beryllium, which acts as a reflector for neutrons and enhances by backward reflection of neutrons the efficiency of the reactor.
To produce especially slow neutrons one uses a so-called cold source, which is situated in the maximum of the thermal neutron flux inside the reflector. The cold source moderates neutrons by collisions with cold hydrogen gas at a temperature of -245 °C and a pressure of 13 bar. In a volume of approximately 1 liter of hydrogen, a new source of very slow neutrons is obtained, which extends the available range of neutron wavelengths up to 0.5 to 2 nm (10-9 m). Neutrons with these "long" wavelengths can be transported through reflecting "neutron guides" to distances up to 40 meters to further experiment sites.
Texts: Th. Wilpert , Ch. Schulz
