Two ways of seeing atoms

What they have in common

Both probes work on the same basic principle. A beam hits a sample, and the way it is scattered, bent off in different directions, depends on how the atoms inside are arranged. To “see” atoms this way, the beam’s wavelength has to be similar to the distance between them. The ESRF’s X-rays have a wavelength of about 0.1 nanometre; a thermal neutron’s wavelength, according to the ILL, is of the order of an ångström, which is the same 0.1 nanometre. So both are the right size for the job.

Electrons versus nuclei

An ILL brochure puts the difference simply: X-rays locate the atomic electrons, while neutrons pinpoint the position of nuclei. Heavy atoms have many electrons and show up strongly in X-rays. Light atoms have few, so X-rays struggle to see them.

Finding hydrogen

Hydrogen is the clearest example. It scatters neutrons strongly but is almost invisible to X-rays. Biological materials are rich in hydrogen and other light elements, which makes them ideal samples for neutrons. Neutrons also tell apart isotopes, atoms of the same element with different nuclei, which is why the Grenoble campus has laboratories that replace ordinary hydrogen with deuterium in biological samples.

This matters in practice. A review of structural biology on the Grenoble campus notes that neutron crystallography can locate hydrogen and deuterium atoms in large biological molecules at a resolution of 1.5 to 2.5 ångströms, at room temperature and without radiation damage. That helps to explain how enzymes work and how drug molecules bind to their targets.

The same applies to batteries: the hydrogen and lithium ions trapped in electrodes scatter neutrons well, so they can be located inside a working material.

Looking deep and gently

Neutrons penetrate deep into materials, further than X-rays. Engineers use this to measure residual stresses deep inside metal parts without cutting them open, because the measurement does not destroy the sample. The ILL gives railway rails as an example, and lists the hardening and corrosion of concrete among the subjects studied with neutrons.

Tiny magnets

Neutrons also behave like microscopic magnets. That makes them a unique tool for finding the magnetic structure of materials, a field in which Grenoble has a long tradition: Louis Néel, one of the ILL’s founders, won the 1970 Nobel Prize for discoveries in magnetism. The ILL lists magnetic materials for future information technologies among the subjects studied with neutrons.

Watching atoms move

Neutrons can do more than locate atoms. When they scatter off a sample they can lose or gain a little energy, and measuring that change, a technique called inelastic scattering, reveals the dynamics of what is happening inside. X-rays have their own way of following motion: synchrotron light is extremely bright and arrives in short flashes, which the ESRF says lets it “film” the position and motion of atoms.

The trade-offs

Neither probe does everything. The ESRF stresses that its very intense X-rays suit small samples and fast processes. Neutrons have advantages of their own: the review of the Grenoble campus notes that neutron crystallography collects data without radiation damage. Neutron beam time, on the other hand, is scarce: the ILL says it can accept only about half of the nearly 1,500 requests for beam time it receives each year.

Why use both

The ILL describes neutron techniques as complementary to X-ray ones: neutron imaging alongside X-ray imaging, small-angle neutron scattering alongside its X-ray counterpart. One of its instruments, D22, combines both kinds of small-angle scattering. In Grenoble, a sample can be measured with X-rays and neutrons on the same campus. The two machines are described on The European Synchrotron and Europe’s neutron source; the campus that links them is on One campus, four institutes.