The main natural source of free neutrons is secondary radiation from cosmic radiation (creation of particles by interactions of cosmic radiation particles with atoms of the earth’s atmosphere). The main sources of neutrons generated artificially are nuclear reactors. Free neutrons have an average lifetime of about 888 seconds (e.g., Mampe et al. 1989a,b: 887.6 ± 3 s) and decay according to \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \[n\ {\rightarrow}\ p\ +\ e^{{-}}\ +\ {\bar{v}}\] \end{document}(1) into a proton p, an electron e− and an anti-neutrino v. Hence, free neutrons must be produced shortly before their use. Besides reactors, where neutrons from nuclear fission reactions after moderation induce further nuclear fissions in a chain reaction, neutrons for research may be generated by a process called spallation. Neutron sources based on these two concepts as well as some special cases of neutron generation are described in more detail below. Figure 1⇓ shows the historical evolution of the performance of neutron sources. In all cases it should be noted that the neutron flux (number of neutrons per unit area and unit time) at the sample is much lower than the peak flux provided by the source. For instance, the spallation neutron source at LANSCE (Los Alamos Neutron Science Center, Los Alamos, U.S.A.) produces 1016 neutrons cm−2 s−1, but at the HIPPO instrument, with a moderator to sample distance of 9 m, only a flux of 2.4 × 107 neutrons cm−2 s−1 is available for neutron energies in the “thermal” (< ~0.4 eV) range (see below) suitable for diffraction. Other facilities provide thermal neutron fluxes of similar magnitude. From these low intensities, as compared to the number of atoms in a cubic centimeter of solid material, it is apparent that the radiation damage from thermal neutrons for most materials is negligible, even for days of …