The Value of Neutrons

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The Value of Neutrons

Generally speaking neutrons are less available and more expensive than X-ray photons. However there are many reasons why specific in-situ studies ought rather to be carried out using neutrons than X-rays, or perhaps using both neutrons and X-rays (see Combined Techniques). An introduction to neutrons for powder diffractometry has already been given in Neutrons Sources & Methods. Here we will just concentrate on those properties of neutrons that can be exploited in dynamic in-situ studies.

• Penetration: Although energetic X-rays are very penetrating, the penetrating power of neutrons can be tailored to some degree by the choice of element. For example aluminium is relatively transparent to neutrons and can often be a convenient metal from which to make specimen cells and enclosures; on the other hand vanadium is a negligible incoherent neutron scatterer and can be a useful material for making resistive heaters. These features can be exploited in the design of complex environmental sample cells for difficult experiments.
• Increased atomic contrast: The scattering of neutrons by different atoms appears to be unpredictable by comparison to X-rays where simply the heaviest elements with the most electrons scatter the most. If one element in the material scatters much more than the other elements then that element will dominate the behaviour of the powder diffraction pattern. One example is oxygen which for example scatters neutrons more strongly than many heavier elements, such as V, Co, Cs, and comparably to others such as Mo, Ag and Au.
• Deuteration: This is a very special case of the above, often applied to water. Hydrogen, ¹H, scatters neutrons only about half as strongly as Deuterium, ²D, its isotope, and with a different phase direction. But in addition ¹H scatters neutrons much more incoherently than ²D; this means that materials containing hydrogen will produce neutron diffraction patterns with a high background (due to the ¹H's incoherent scattering); the deuterated equivalent material will have a much lower background. Again many elegant experiments, exchanging partially or completely between ¹H and ²D, have been carried out to reveal features of the water or OH content of a material.
• Magnetic materials: This is outside the scope of the present course, but it should be noted that powder diffraction has been a useful tool in the study of magnetic materials due to the additional scattering of the neutron by the magnetic dipoles within magnetic material.
• Inelastic scattering: Whereas conventional diffraction by X-rays is said to be elastic (meaning the diffracted X-ray photon has the same energy/wavelength as the incident photon), neutrons generally have a measurable inelastic scattering due to the neutron's energy being similar to the thermal energy of the atoms within the material. This can provide more information on atomic motion and vibration. However this aspect is outside the scope of this course
• Enhanced atomic contrast (isotopic substitution): This is a clever trick whereby the contrast of a chosen element is increased by using its isotope. For example whereas ordinary iron, ⁵⁶Fe, has a neutron scattering power of 10 fm, its isotopes, ⁵⁴Fe, ⁵⁷Fe have quite different scattering powers of 4 and 2 fm respectively. By changing the isotope, or comparing between patterns using different isotopes (the "isotopic difference method"), some very elegant results have been achieved but these have been far more widely applied to the field of liquids and amorphous materials than to crystalline powders.

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