Neutron diffraction has several major advantages over x-rays, except that neutrons are much more difficult to generate and, even at the world’s most powerful sources, have much lower fluxes than available routinely from x-rays. Thus, the number one goal when designing neutron instrumentation is to preserve as much of the neutron beam as can be done. This typically means that neutron beams are big – on the scale of centimeters, though samples are sometimes sub-millimeter. Shielding for neutron instruments is also massive. Typical shielding contains hydrogen-containing materials, which moderate high-energy neutrons into thermal neutrons and neutron-absorbing materials, such as boron, cadmium, or gadolinium, which will absorb thermal neutrons effectively. Such shielding is typically massive with many tons of shielding blocks. Measurement times traditionally have been long, hours to a day, but with some modern instruments and larger samples, measurements may be completed in minutes.
Note that neutrons readily penetrate most metals, which allows furnaces, cryostats and sample refrigerators to be readily used at neutron diffractometers. It is common to collect data at temperatures in the range of 4 K to 10 K as these are easy to achieve and provide for minimal loss of high Q intensity from the Debye-Waller factor, but use of neutrons can be useful for high-temperature studies as well. Many forms of operando and in situ experiments benefit from the rapid measurements available from synchrotrons, but where possible neutrons should be considered. GSAS-II allows neutron diffraction analysis to be combined with x-ray diffraction. Even x-ray single-crystal diffraction studies can benefit from a combined refinement with a neutron powder diffraction dataset. For complex parametric studies it may well make sense to obtain neutron powder diffraction data under selected conditions.
There are two very different types of neutron diffraction instrumentation: Those based on pulsed spallation sources where neutrons are energy-separated by the neutron’s velocity. These are called time-of-flight (TOF) diffractometers and the pattern is recorded as intensity as a function of arrival time for the neutron, which can be converted to Q. Nuclear reactors create a broad spectrum neutron source where a specific wavelength is selected and used for a conventional, angular dispersive instrument where intensity is recorded as a function of angle. These are known as constant wavelength (CW) neutron diffractometers. Hybrids between the two technologies are possible.
Neutrons are scattered from atom nuclei and the strength of this scattering is independent of Q. The scattering strength is dependent of on isotope of the element, though the cost of isotopically separated materials often prevents this from being exploited. Nonetheless, use of deuterium (\({}^2\rm {H}\) is common for neutron experiments as the \({}^1\rm {H}\) component in natural abundance hydrogen contributes very large amounts of incoherent and inelastic scatter, which appears as background and can require use of a correction for absorption. Another example is that boron is a very strong absorber of neutrons and natural-abundance boron-containing samples are usually two absorbing for diffraction measurements. However, \({}^{10}\rm {B}\) is not a neutron absorber and can be used in neutron diffraction without concerns, other than for cost.
For magnetic materials, neutrons offer one additional benefit. Neutrons are scattered from unpaired electrons, though with a scattering factor that falls-off with Q faster than the form-factor for x-ray scattering. When a material undergoes a magnetic transition (usually at lowered temperatures), additional intensity will be added to the pattern, often at new peak locations from the magentically scattering atoms.
The processes that produce neutrons, usually create high energy neutrons (MeV) but neutrons needed for diffraction must be of much lower energy ( circa \(10^{-2}\) eV or equivalently \(\approx \)10 meV ). The energy of neutrons is lowered by allowing them to scatter inelastically, typically from hydrogen-containing materials. This process is called moderation. These neutrons, with lowered energy, are typically called thermal neutrons, as a 25 meV neutron has a temperature of \(\approx 290\) K.
The facilities that host neutron instrumentation usually require that you write a proposal to be granted time at the facility. Before submitting a proposal, it is worthwhile to contact a instrument scientist at the facility to discuss your intended project. This will help you verify that your intended measurement is feasible and that you have selected the optimum instrument for the work.