Basic properties of the neutron

With the advent of nuclear reactors, thermal neutrons have become a valuable tool for investigating many important features of matter – particularly condensed matter. The usefulness of thermal neutrons arises from the basic properties of the neutron. These are listed in Table 1.1.

The value of the mass of the neutron results in the de Broglie wavelength of thermal neutrons being of the order of interatomic distances in solids and liquids. Thus, interference effects occur which yield information on the structure of the scattering system.

Secondly, the fact that the neutron is uncharged means, not only that it can penetrate deeply into the target, but also that it comes close to the nuclei – there is no Coulomb barrier to be overcome. Neutrons are thus scattered by nuclear forces, and for certain nuclides the scattering is large. An important example is light hydrogen which is virtually transparent to X-rays but which scatters neutrons strongly.

Thirdly, the energy of thermal neutrons is of the same order as that of many excitations in condensed matter. So when the neutron is inelastically scattered by the creation or annihilation of an excitation, the change in the energy of the neutron is a large fraction of its initial energy. Measurement of the neutron energies thus provides accurate information on the energies of the excitations, and hence on the interatomic forces.