Neutron Diffraction

Neutron atomic scattering lengths depending on the atomic number

Neutron atomic scattering lengths
depending on the atomic number

 

Using the particle-wave dualism, particulate radiation can be used for diffraction experiments. A particulate radiation widely used in crystallography is neutron radiation. This radiation is only accessible at central facilities as the fabrication of unbound neutrons is not feasible on a laboratory scale. Two different types of neutron sources are used: reactors and spallation sources. Reactors use the process of fission to produce a constant beam of neutrons, while spallation sources produce a pulsed beam. In both cases the produced neutrons have to be decelerated in a modulator fluid before they are useable for diffraction purposes. Depending on the chemical nature of the modulator, different velocities, i.e. wavelengths, of neutrons are obtained.

Due to their size and spin, neutrons interact with the atomic nucleus, not the electron shell, and thus give complementary information to X-ray diffraction. This characteristic facilitates the distinction of atoms neighbouring in the periodic table. Additionally, different isotopes generally have different scattering lengths (e.g. hydrogen and deuterium) and thus the exchange of isotopes introduces another dimension of experiments. Last, but not least, since the wavelength of the used neutron radiation is so much bigger than the size of the atomic nucleus, the scattering angle dependency seen for X-ray diffraction does not occur for neutrons, i.e. the scattering intensity does not decrease with increasing scattering angle. The major drawback of neutron radiation is the relatively weak interaction with matter, which results in long measurement times, which can affect the stability of some samples.

Figure 2 Theophylline monohydrate crystal
structure measured by neutron
diffraction

In molecular crystallography, neutron diffraction is mostly used to determine most accurate hydrogen positions. Since the neutron scattering length does not depend on the atomic number, hydrogen is a rather good neutron scatterer. Additionally, the neutron scattering length of hydrogen is negative, which makes distinction of hydrogen contribution to observed diffraction peaks easier. Finally, since neutrons are scattered by the nucleus, hydrogen positions can be detected most accurately. This is obviously advantageous for the determination of accurate hydrate structures, in which it is not always possible to determine the hydrogen bonding pattern by X-ray diffraction alone. Using neutron diffraction will give this information unambiguously and make the determination of OH bond lengths possible. Thus, the neutron structure of a molecular hydrate is seen as the gold standard at the moment.