Like X-rays, neutrons can scatter off the arrays of atoms in materials to produce a characteristic diffraction pattern. However, rather than interacting with the electron clouds of the atoms in a sample (as in the case of X-rays) neutrons are scattered by the atomic nuclei.

The neutrons may penetrate quite deeply into a material before actually hitting any nuclei, so relatively thick samples can be studied – neutrons can ‘look’ inside the material. This is particularly useful for studying ‘real’ objects of commercial interest such as plastic bottles.

Another characteristic relevant to soft-matter systems, which can be used to great advantage, is that isotopes of the same atom may have very different scattering strengths. In particular, hydrogen (with a nucleus containing just a proton) scatters quite differently from its isotope containing an additional neutron, deuterium. In fact, the difference is the largest between any two isotopes of an element in the Periodic Table.

The elaborate structures that typify soft matter usually include a large number of hydrogen atoms, as do water or the organic solvent hosting them. Judiciously substituting deuterium for some or all of the hydrogen atoms (which does not alter the chemical properties) increases the scattering ‘contrast’ for selected components in a soft-matter system, so they are more ‘visible’ to neutrons.
For example, ordinary water can be partly replaced with deuterated water so its scattering matches that of one of the components, rendering it ‘invisible’. Another component then stands out. This is called ‘contrast variation’.

A third advantage is that neutron beams can be prepared with a wavelength that matches the lengthscales associated with nano-sized architectures, so making them a highly effective probe of soft matter. The scattering angles are very small (the angle of diffraction decreases as the distance between nuclei increases), and to measure them, detectors must be placed far away from the sample.

The ILL has two dedicated small angle neutron scattering (SANS) instruments, D11 and D22, used for soft matter. These instruments can be combined with special equipment to study, for example, the flow of material under a force, which is important in many practical applications. Another technologically important method is reflectometry (carried out with the reflectometer D17) whereby neutrons are reflected off a surface or interface to give information about thin films or layered structures.

Finally, the motions associated with soft matter structures – movements of various molecular components or vibrations of layers, for example – can also be studied with neutrons, by measuring the small exchanges of energy resulting from interacting with moving atoms – inelastic scattering.

A remarkable technique called neutron spin-echo, pioneered at the ILL by a Hungarian physicist, Ferenc Mezei, offers a highly sensitive method for following very slow dynamic processes in soft matter. It takes advantage of another important property of neutrons – their spin. The neutrons are first polarised so that their spins are all in the same direction and then passed through a magnetic field. This causes the spins to wobble, or precess (like a spinning top). The number of precessions depends on the time each neutron takes to traverse the field, which in turn depends on its energy. After being scattered by the sample, the neutrons pass through a second magnetic field, which winds back the spins through the same number of precessions (a sort of ‘echo’) over the same period of time so that the spins achieve their original orientations. If, however, some of the neutrons have lost or gained energy by interacting with the sample molecules, they will not all end up with the same spin polarisation. This deficiency then provides a neat way of measuring tiny energy changes associated with slow movements such as that of a polymer chain sliding through the tangled chains in a melt.
The ILL has several instruments for measuring inelastic scattering including IN5, IN6, IN11, IN15 and IN16 used in soft-matter studies.

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