Urban legends of thermal moderator design

Neutron scattering has been an important tool for many areas of science for more than 50 years. Even though the theory of neutron scattering was developed in large part in the 1950s and 1960s, it is still to this day a challenge to design neutron sources. This is largely due to the fact that in order to achieve the necessary fluxes, it is necessary to construct either a fission reactor or a spallation source. These facilities are so complex that designers frequently rely on the experiences gained from previously designed facilities. Over the years these rules were often passed on more in form of an art than as science. The goal of this paper is to re-evaluate these rules and to establish if they are still valid today or if they have become urban legends.