Considerations for a cold moderator on a pulsed neutron source

The purpose of a cooled moderator on a pulsed neutron source may be to extend to longer wavelengths the neutron slowing down range with its narrow pulse widths, or to enhance the long wavelength intensity. The intensity and pulse length as a function of wavelength are determined by the rate of energy exchange, and by leakage and capture of neutrons. Energy deposition and radiation damage present serious problems, and restrict the choice of moderators. Radiation damage, in particular, may limit the lifetime and stability of the moderator, giving rise to problems of corrosion and, in the case of solid moderators, stored energy release. The advantages of solid ammonia are its high hydrogen atom density, and its thermal neutron absorption cross-section which allows the moderator to act as its own homogeneous poison. On the other hand this impairs its use for cold neutron production in those instances where neutron intensity rather than burst width is important. In such cases solid methane is favourable, though the radiation damage problem for methane at high intensity sources is severe, and liquid hydrogen is indicated.