Particles and materials

The Diamond Light Source: This beautiful, gleaming structure houses a particle accelerator which uses fast-moving electrons to create intense beams of x-rays to examine samples from the chemical to the biological to the archaeological. Image © Diamond Light Source

Most particle accelerators used in science aren’t used for particle physics: we’ve got most of the easy particle physics that you can study with medium-sized machines under control, so the cutting-edge stuff all has to be done at enormous accelerators like the LHC, of which there are only a handful in the world.
Smaller accelerators are primarily used to examine materials and biological samples with beams of various kinds, using X-rays, neutrons and muons to examine matter on tiny scales.

X-rays

X-rays can be used to study the structure of crystals—any material made up of a repeating pattern of atoms or molecules. These range from new kinds of magnets, superconductors or semiconductors studied by physicists and chemists, to incredibly complex proteins which power biological processes.
The reason X-rays are suitable is because their wavelength is comparable to the gaps between the atoms being studied: somewhere around 0.1 to 1 nm. This similarity means that, when the X-rays are ‘diffracted’ off the atoms in a material, we can use the interference pattern which results to work out the distances between those atoms and how they are arranged. By contrast, visible light has a wavelength of hundreds of nanometres, and consequently the detail of how atoms are laid out in materials would be smudged out and thus invisible.

X-rays are used in the lab, but the intense X-ray sources used for precision experiments are currently mostly ‘synchrotron’ sources, where charged particles such as electrons are forced to turn around tight corners. As they screech around these corners, they spray out ‘synchrotron radiation’ in extremely intense, tight beams, which are shone onto the samples being examined.

Neutrons

Neutrons are also used to examine crystal structures and, as well as observing where atoms are, it’s possible to see how they’re moving around. Neutrons are also suitable for this task because of their wavelength but, in difference to X-rays which always travel at the speed of light, we can vary that wavelength by varying their speed.

The wavelength, λ, of a neutron is given by λ = h/p, where h is Planck’s constant and p is its momentum: so faster-moving neutrons have a shorter wavelength. Neutrons are produced by slamming a high-energy beam of protons into a target containing heavy nuclei, such as tungsten. This creates a shower of various daughter particles, including neutrons, moving at very high speed. Their high speed means that their wavelength would actually be somewhat too short, and so they’re slowed down using a substance known as a ‘moderator’, which is usually something containing a lot of hydrogen. They usually decelerate to ‘thermal’ speeds defined by the temperature of the moderator so, for example, very chilly liquid hydrogen could be used to make very slow neutrons, liquid methane is somewhat warmer, and water can be used for neutrons with energies corresponding to room temperature or so.

This method of producing neutrons by smashing accelerated protons into a target is called ‘spallation’. Other neutron sources use a nuclear reactor optimised to produce a large number of neutrons and less heat than a conventional electricity-producing nuclear plant would.

Muons

Often found tagged onto the spallation neutron sources are beamlines allowing muons to be implanted in a sample. Muons are heavy electrons and, unlike X-rays or neutrons, they are embedded in a sample rather than bounced off or passed through it. A muon is a tiny quantum bar magnet, so when inside the sample they react to the local magnetic field. Muons are radioactive with a mean lifetime of 2.2 μs and so, shortly after implantation, the muon will decay into an electron and two invisible neutrinos. The decay electrons are spat out in a specific direction based on the magnetic field in which the muon played out its short existence, which means that following that direction can tell you with incredible precision about the magnetism in a sample.