In 2014, the UK joined several other major European countries in its commitment to building the European Spallation Source (ESS) in Lund, Sweden. The UK government earmarked £165m for the project. The pulsed neutron source, which is expected to break ground later in 2014, should be up and running by 2030, when it will be the most powerful neutron source in the world and the biggest super microscope.
There are currently over 30 neutron sources around the world, used by researchers to do fundamental research into the structure and properties of materials, as well as the discovery of new particles. William Stirling, director of the Institut Laue-Langevin (ILL) in Grenoble, home to the world’s most powerful reactor, comments that neutrons can also be ‘good for business’.
While industry, in general, tends to use onsite X-ray facilities for its R&D work, Stirling says, ‘Neutrons, along with synchrotron X-rays, are unique experimental probes in that they give information at the atomic (nm or Å) level on the structure and the dynamics of molecules, and provide insight into how materials perform and are affected by processing, ageing and machining, for example.’ Neutrons have the added advantage that they can penetrate deep into samples without damaging them, and can provide information in situ and in operando, he says. While one of the major current uses of neutrons in industry and engineering is to understand the deformations and stresses inside structural materials, big growth areas are in understanding energy storage systems, such as fuel cells and battery technology, and in biological research.
The ILL is run by three major partners – France, Germany, and the UK – each responsible for a quarter of the overall budget, with the remaining quarter from various groups of countries mainly within Europe and, more recently, India. Around 2000 users, predominantly from universities in ca 40 countries, visit ILL every year, notching up some 850 experiments. There are 38 different scientific instruments, including state-of-the art spectrophotometers and diffractometers, clustered around a fission reactor. The fuel element in the reactor is a compact core of enriched uranium; the heat generated in the ongoing chain reaction is removed via a cooling system.
Stirling explains that the high energy neutrons produced, moving at speeds around 20,000km/s, are slowed down before they are guided to the scientific instruments via neutron guide tubes. This is done by making them collide with heavy water molecules surrounding the core, resulting in ‘thermal neutrons’ with a speed of 2.2km/s. ‘The ILL’s reactor produces the most intense thermal neutron flux in the world – 1.5 x 1015 neutrons/s/cm2,‘ he says. Three other components located close to the core – a hot source and two cold sources – make it possible to produce hot neutrons (10km/s) as well as cold (700m/s) and ultra-cold (10m/s) neutrons. Scientists thus have access to a wide spectrum of neutron energies, from µeV to eV, and wavelengths of 30nm (300Å) 0.03nm (0.3Å).
In a typical experiment, neutrons are scattered as they pass through a sample; the angle of scattering is a function of nuclear density. The usefulness of neutrons is due to their energy and the fact they have spin and can penetrate materials without damaging them. ‘Their wavelength, in Å, is comparable to the spacing between atoms in materials, so diffraction studies can give us a lot of information about atomic structures,’ Stirling says.
ILL associate director Charles Simon adds that one of the main reasons for using neutrons is to locate the precise position of atoms, in particular light atoms such as H, O and C, which are not easily visible with X-rays.
This is partly because neutrons interact directly with the nucleus, whereas X-rays interact with the electron cloud surrounding the atom. Neutrons are also sensitive to different isotopes, which can be useful for studying complex proteins, for example. Unlike X-rays and electrons, they are non-invasive and can be used to study delicate biological samples.
Crucially, thermal neutrons can be used for studying the movement of atoms in materials because they have incredibly high resolution, compared with X-rays – of the order of 1m times better, Simon points out. By measuring the motion of an atom relative to others in a material, neutrons add crucial information on mechanisms such as friction, as well as drug interactions in the body. And because neutrons have spin, scientists can explore the magnetic behaviour of materials at the atomic level, important in designing magnetic storage and superconducting materials.
The ILL researchers are hopeful that the building of even more powerful super microscopes means that scientists will one day be able to see even more detail, faster.