2024 marks 40 years since ISIS, the UK’s neutron user facility, first became operational. Since its origins in the 1940s, neutron scattering has emerged as a powerful means for understanding everything from human cells to quantum spins. Rosie de Laune and colleagues from ISIS examine the complex science and history behind the technique, and highlight some of the key research that has been performed at the facility over the last 40 years
When British physicist James Chadwick discovered the neutron in 1932, he supposedly said, “I am afraid neutrons will not be of any use to anyone.” The UK’s neutron user facility – the ISIS Neutron and Muon Source, now operated by the Science and Technology Facilities Council (STFC) – was opened 40 years ago. In that time, the facility has welcomed more than 60,000 scientists from around the world. ISIS supports a global community of neutron-scattering researchers, and the work that has been done there shows that Chadwick couldn’t have been more wrong.
By the time of Chadwick’s discovery, scientists knew that the atom was mostly empty space, and that it contained electrons and protons. However, there were some observations they couldn’t explain, such as the disparity between the mass and charge numbers of the helium nucleus.
The neutron was the missing piece of this puzzle. Chadwick’s work was fundamental to our understanding of the atom, but it also set the stage for a powerful new field of condensed-matter physics. Like other subatomic particles, neutrons have wave-like properties, and their wavelengths are comparable to the spacings between atoms. This means that when neutrons scatter off materials, they create characteristic interference patterns. In addition, because they are electrically neutral, neutrons can probe deeper into materials than X-rays or electrons.
Today, facilities like ISIS use neutron scattering to probe everything from spacecraft components and solar cells to studying how cosmic ray neutrons interact with electronics to ensure the resilience of technology for driverless cars and aircraft.
The origins of neutron scattering
On 2 December 1942 a group of scientists at the University of Chicago in the US, led by Enrico Fermi, watched the world’s first self-sustaining nuclear chain reaction, an event that would reshape world history and usher in a new era of atomic science.
One of those in attendance was Ernest O Wollan, a physicist with a background in X-ray scattering. The neutron’s wave-like properties had been established in 1936 and Wollan recognized that he could use neutrons produced by a nuclear reactor like the one in Chicago to determine the positions of atoms in a crystal. Wollan later moved to Oak Ridge National Laboratory (ORNL) in Tennessee, where a second reactor was being built, and at the end of 1944 his team was able to observe Bragg diffraction of neutrons in sodium chloride and gypsum salts.
A few years later Wollan was joined by Clifford Schull, with whom he refined the technique and constructed the world’s first purpose-built neutron-scattering instrument. Schull won the Nobel Prize for Physics in 1994 for his work (with Bertram Brockhouse, who had pioneered the use of neutron scattering to measure excitations), but Wollan was ineligible because he had died 10 years previously.
The early reactors used for neutron scattering were multipurpose, the first to be designed specifically to produce neutron beams was the High Flux Beam Reactor (HFBR) at Brookhaven National Laboratory in the US in 1965. This was closely followed in 1972 by the Institut Laue–Langevin (ILL) in France, a facility that is still running today.
Rather than using a reactor, ISIS is based on an alternative technology called “spallation” that first emerged in the 1970s. In spallation, neutrons are produced by accelerating protons at a heavy metal target. The protons collide like bullets with the nuclei in the target, absorb the proton and then discharge high-energy particles, including neutrons.
The first such sources specifically designed for neutron scattering were the KENS source at the Institute of Materials Structure Science (IMSS) in Japan, which started operation in 1980, and the Intense Pulsed Neutron Source at the Argonne National Laboratory in the US, which started operation in 1981.
The pioneering development work on these sources and in other institutions was of great benefit during the design and development of what was to become ISIS. The facility was approved in 1977 and the first beam was produced on 16 December 1984. In October 1985 the source was formally named ISIS and opened by then UK prime minister Margaret Thatcher. Today around 20 reactor and spallation neutron sources are operational around the world and one – the European Spallation Source (ESS) – is under construction in Sweden.
The name ISIS was inspired by both the river that flows through Oxford and the Egyptian goddess of reincarnation. The relevance of the latter relates to the fact that ISIS was built on the site of the NIMROD proton synchrotron that operated between 1964 and 1978, reusing much of its infrastructure and components.
Producing neutrons and muons
At the heart of ISIS is an 800 MeV accelerator that produces intense pulses of protons 50 times a second. These pulses are then fired at two tungsten targets. Spallation of the tungsten by the proton beam produces neutrons that fly off in all directions.
Before the neutrons can be used, they must be slowed down, which is achieved by passing them through a material called a “moderator”. ISIS uses various moderators which operate at different temperatures, producing neutrons with varying wavelengths. This enables scientists to probe materials on length scales from fractions of an angstrom to hundreds of nanometres.
Arrayed around the two neutron sources and the moderators are more than 25 beamlines that direct neutrons to one of ISIS’s specialized experiments. Many of these perform neutron diffraction, which is used to study the structure of crystalline and amorphous solids, as well as liquids.
When neutrons scatter, they also transfer a small amount of energy to the material and can excite vibrational modes in atoms and molecules. ISIS has seven beamlines dedicated to measuring this energy transfer, a technique called neutron spectroscopy. This can tell us about atomic and molecular bonds and is also used to study properties like specific heat and resistivity, as well as magnetic interactions.
Neutrons have spin so they are also sensitive to the magnetic properties of materials. Neutron diffraction is used to investigate magnetic ordering such as ferrimagnetism whereas spectroscopy is suited to the study of collective magnetic excitations.
Neutrons can sense short and long-ranged magnetic ordering, but to understand localized effects with small magnetic moments, an alternative probe is needed. Since 1987, ISIS has also produced muon beams, which are used for this purpose, as well as other applications. In front of one of the neutron targets is a carbon foil and when the proton beam passes through this it produces pions, which rapidly decay into muons. Rather than scattering, muons become implanted in the material, where they rapidly decay into positrons. By analysing the decay positrons, scientists can study very weak and fluctuating magnetic fields in materials that may be inaccessible with neutrons. For this reason, muon and neutron techniques are often used together.
“The ISIS instrument suite now provides capability across a broad range of neutron and muon science,” says Roger Eccleston, ISIS director. “We’re constantly engaging our user community, providing feedback and consulting them on plans to develop ISIS. This continues as we begin our ‘Endeavour’ programme: the construction of four new instruments and five significant upgrades to deliver even more performance enhancements.
“ISIS has been a part of my career since I arrived as a placement student shortly before the inauguration. Although I have worked elsewhere, ISIS has always been part of my working life. I have seen many important scientific and technical developments and innovations that kept me inspired to keep coming back.”
Over the last 40 years, the samples studied at ISIS have become smaller and more complex, and measurements have become quicker. The kinetics of chemical reactions can be imaged in real-time, and extreme temperatures and pressures can be achieved. Early work from ISIS focused on physics and chemistry questions such as the properties of high-temperature superconductors, the structure of chemicals and the phase behaviour of water. More recent work includes “seeing” catalysis in real-time, studying biological systems such as bacterial membranes, and enhancing the reliability of circuits for driverless cars.
Understanding the building blocks of life
Unlike X-rays and electrons, neutrons scatter strongly from light nuclei including hydrogen, which means they can be used to study water and organic materials.
Water is the most ubiquitous liquid on the planet, but its molecular structure gives it complex chemical and physical properties. Significant work on the phase behaviour of water was performed at ISIS in the early 2000s by scientists from the UK and Italy, who showed that liquid water under pressure transitions between two distinct structures, one low density and one high density (Phys. Rev. Lett. 84 2881).
Water is the molecule of life, and as the technical capabilities of ISIS have advanced, it has become possible to study it inside cells, where it underpins vital functions from protein folding to chemical reactions. In 2023 a team from Portugal used the facilities at ISIS to investigate whether the water inside cells can be used as a biomarker for cancer.
Because it’s confined at the nanoscale, water in a cell will behave quite differently to bulk water. At these scales, water’s properties are highly sensitive to its environment, which changes when a cell becomes cancerous. The team showed that this can be measured with neutron spectroscopy, manifesting as an increased flexibility in the cancerous cells (Scientific Reports 13 21079).
If light is incident on an interface between two materials with different refractive indices it may, if the angle is just right, be perfectly reflected. A similar effect is exhibited by neutrons that are directed at the surface of a material, and neutron reflectometry instruments at ISIS use this to measure the thickness, surface roughness, and chemical composition of thin films.
One recent application of this technique at ISIS was a 2018 project where a team from the UK studied the effect of a powerful “last resort” antibiotic on the outer membrane of a bacterium. This antibiotic is only effective at body temperature, and the researchers show that this is because the thermal motion of molecules in the outer membrane makes it easier for the antibiotic to slip in and disrupt the bacterium’s structure (PNAS 115 E7587).
Exploring the quantum world
A year after ISIS became operational, physicists Georg Bednorz and Karl Alexander Müller, working at the IBM research laboratory in Switzerland, discovered superconductivity in a material at 35 K, 12 K higher than any other known superconductor at the time. This discovery would later win them the 1987 Nobel Prize for Physics.
High-temperature superconductivity was one of the most significant discoveries of the 1980s, and it was a focus of early work at ISIS. Another landmark came in 1987, when yttrium barium copper oxide (YBCO) was found to exhibit superconductivity above 77 K, meaning that instead of liquid helium, it can be cooled to a superconducting state with the much cheaper liquid nitrogen. The structure of this material was first fully characterized at ISIS by a team from the US and UK (Nature 327 310).
Another example of the quantum systems studied at ISIS is quantum spin liquids (QSLs). Most magnetic materials form an ordered phase like a ferromagnet when cooled, but a QSL is an interacting system of electron spins that is, in theory, disordered even when cooled to absolute zero.
QSLs are of great interest today because they are theorized to exhibit long-range entanglement, which could be applied to quantum computing and communications. QSLs have proven challenging to identify experimentally, but evidence from neutron scattering and muon spectroscopy at ISIS has characterized spin-liquid states in a number of materials (Nature 471 612).
Developing sustainable solutions and new materials
Over the years, experimental set-ups at ISIS have evolved to handle increasingly extreme and complex conditions. Almost 20 years ago, high-pressure neutron experiments performed by a UK team at ISIS showed that surfactants could be designed to enhance the solubility of liquid carbon dioxide, potentially unlocking a vast array of applications in the food and pharmaceutical industries as an environmentally friendly alternative to traditional petrochemical solvents (Langmuir 22 9832).
Today, further developments in sample environment, detector technology and data analysis software enable us to observe chemical processes in real time, with materials kept under conditions that closely mimic their actual use. Recently, neutron imaging was used by a team from the UK and Germany to monitor a catalyst used widely in the chemical industry to improve the efficiency of reactions (Chem. Commun. 59 12767). Few methods can observe what is happening during a reaction, but neutron imaging was able to visualize it in real time.
Another discovery made just after ISIS became operational was the chemical buckminsterfullerene or “buckyball”. Buckyballs are a molecular form of carbon that consists of 60 carbon atoms arranged in a spherical structure, resembling a football. The scientists who first synthesized this molecule were awarded the Nobel Prize for Chemistry in 1996, and in the years following this discovery, researchers have studied this form of carbon using a range of techniques, including neutron scattering.
Ensembles of buckyballs can form a crystalline solid, and in the early 1990s studies of crystalline buckminsterfullerene at ISIS revealed that, while adjacent molecules are oriented randomly at room temperature, they transition to an ordered structure below 249 K to minimize their energy (Nature 353 147).
Four decades on, fullerenes (the family of materials that includes buckyballs) continue to present many research opportunities. Through a process known as “molecular surgery”, synthetic chemists can create an opening in the fullerene cage, enabling them to insert an atom, ion or molecular cluster. Neutron-scattering studies at ISIS were recently used to characterize helium atoms trapped inside buckyballs (Phys. Chem. Chem. Phys. 25 20295). These endofullerenes are helping to improve our understanding of the quantum mechanics associated with confined particles and have potential applications ranging from photovoltaics to drug delivery.
Just as they shed light on materials of the future, neutrons and muons also offer a unique glimpse into the materials, methods and cultures of the past. At ISIS, the penetrative and non-destructive nature of neutrons and muons has been used to study many invaluable cultural heritage objects from ancient Egyptian lizard coffins (Sci. Rep. 13 4582) to Samurai helmets (Archaeol. Anthropol. Sci. 13 96), deepening our understanding of the past without damaging any of these precious artifacts.
Looking within, and to the future
If you want to understand how things structurally fail, you must get right inside and look, and the neutron’s ability to penetrate deep into materials allows engineers to do just that. ISIS’s Engin-X beamline measures the strain within a crystalline material by measuring the spacing between atomic lattice planes. This has been used by sectors including aerospace, oil and gas exploration, automotive, and renewable power.
Recently, ISIS has also been attracting electronics companies looking to use the facility to irradiate their chips with neutrons. This can mimic the high-energy neutrons generated in the atmosphere by cosmic rays, which can cause reliability problems in electronics. So, when you next fly, drive or surf the web, ISIS may just have had a hand in it.
With its many discoveries and developments, ISIS has succeeded in proving Chadwick wrong over the past 40 years, and the facility is now setting its sights on the upcoming decades of neutron-scattering research. “While predicting the future of scientific research is challenging, we can anchor our activities around a couple of trends,” explains ISIS associate director Sean Langridge. “Our community will continue to pursue fundamental research for its intrinsic societal value by discovering, synthesizing and processing new materials. Furthermore, we will use the capabilities of neutrons to engineer and optimize a material’s functionality, for example, to increase operational lifetime and minimize environmental impact.”
The capability requirements will continue to become more complex and, as they do so, the amount of data produced will also increase. The extensive datasets produced at ISIS are well suited for machine-learning techniques. These can identify new phenomena that conventional methods might overlook, leading to the discovery of novel materials.
As ISIS celebrates its 40th anniversary of neutron production, the use of neutrons continues to provide huge value to the physics community. A feasibility and design study for a next-generation neutron and muon source is now under way. Despite four decades of neutrons proving their worth, there is still much to discover over the coming decades of UK neutron and muon science.
