Diamond Light Source: shining a light
When Diamond Light Source, the UK’s national synchrotron, opened in 2007 it was the brightest object in Oxfordshire, the UK, Europe, the world and – 10 billion times brighter than the sun – the entire solar system. Created by accelerating electrons to near light speed, this implausibly powerful light source feeds a series of beamlines that focus the light so that scientists can examine the structure and composition of matter in exquisite detail.
At a recent showcase in London, a cross-section of scientists using Diamond explained how they were employing these beamlines to pursue a range of advances from faster, smaller electronics to safe long-term storage of nuclear waste and more biocompatible medical implants.
Professor Andrew Harrison, CEO of Diamond Light Source, put the capabilities of the facility into perspective in his introductory talk, with reference to Rosalind Franklin and her student Raymond Gosling’s famous 1952 X-ray diffraction image of DNA, ‘Photo 51’.
‘It would have taken her 24 hours to five days to take that photograph. We can take photographs far more detailed than that in less than a second. What that means is that when it comes to looking at the structure of matter – be that a very complex virus or looking deep inside alloys for next-generation turbine blades – we can look at these materials very quickly, we can look at them under operating conditions and at incredible detail, down to where the individual atoms are.’ It means that researchers are gathering enough data in beamline sessions of a few days to keep them busy for months.
At the University of Manchester, UK, Dr Sam Shaw has been examining data gathered on Diamond’s X-ray absorption spectroscopy beamline B18 to understand the fundamental behaviour of radionuclides at the atomic scale, under conditions relevant to the geological disposal of radioactive waste.
Shaw described his research team’s study of technetium-99 (Tc), a radionuclide with a half-life of 210,000 years that is produced during nuclear fission, and its interaction with magnetite, an iron oxide mineral that is naturally abundant and predicted to form within and around deep geological disposal facilities (GDFs) via the corrosion of canisters and the breakdown of minerals in the host rock by alkaline groundwater.
The aim of Shaw’s research was to ascertain if Tc is incorporated into magnetite particles, and whether this could offer long-term sequestration and immobilisation of Tc. To do this, the team had to mimic the chemical conditions of a GDF, such as the complete absence of oxygen and the alkaline nature of groundwater produced in the presence of cement – the equivalent pH to that of household bleach – which forms part of the multi-layered GDF.
Through use of X-ray absorption spectroscopy on the B18 beam, the team were able to observe at the atomic scale that Tc atoms directly substituted iron atoms in the mineral, locking the Tc inside the mineral’s crystal structure. ‘What you can see here is essentially the fate of this radionuclide determined at the atomic scale,’ said Shaw. ‘This is just one small brick in the wall of the overall research being conducted in this area, which will be needed to underpin the safety case for future geodisposal facilities in the UK.’
Metal to the bone
Another chemical relationship that Diamond Light Source has recently helped to explain is that between human bone and metal ions. Dr Karan Shah and colleagues, University of Sheffield, UK, used the microfocus spectroscopy beamline I18, which focuses the beam to a micron-sized dot, to study the elemental distribution of cobalt and chromium ions in human bone, following joint replacement.
Osteoclasts, the digestive cells of human bone, and osteoblasts, the cells that form bone, are known to be affected by metal ions that are released due to corrosion and friction between moving surfaces following joint replacement, often causing failure due to toxicity and, ultimately, the need for a new replacement.
Shah explained that metal-on-metal implants have a higher failure rate than metal-on-polyethylene and ceramic-on-ceramic replacements, with 71% of failures in metal joint replacements being bone related. The aim of the research was to identify how metal ions enter the bone cells and their location and behaviour within them.
Using micro-focus X-ray fluorescence on beamline I18, Shah examined how different metal ions behaved when combined with bone cells. Cobalt concentration was higher at osteoblast nuclei, while in osteoclasts the cobalt was attracted to discrete sites and the cell’s membrane. Chromium 6+ was distributed throughout osteoblasts and osteoclasts, and a change from chromium 6+ to chromium 3+ inside the cells suggested a redox change, which could lead to the creation of reactive oxygen species that are detrimental to the cells’ survival.
Examining how these ions enter the bone cells, Shah found a route via the protein P2X7R, concluding that blocking this protein could prevent damage following replacement surgery.
Electronics are another area of research for which Diamond Light Source offers the highest specification tools for study. Jon Riley, a joint PhD student at the University of St Andrews, UK, and Diamond carried out his work on the IO5 beamline, which is dedicated to the study of electronic structures by angle-resolved photoemission spectroscopy (ARPES).
‘The processor in the iPhone 6 has 2bln transistors, and each are about the size of a virus – around 500 times smaller than a transistor available in 1970. So, they’ve got a lot smaller, and we’re getting loads of them packed in, but we’re quickly approaching a point where we can’t get smaller. They become really inefficient when they get really small,’ he says.
One potential solution is the use of spintronics – rather than using the charge of an electron to transfer data, using its spin. ‘What we want to look at is controlling the orientation of an electron’s spin as it moves through a material, and to do that we have to understand how the electron moves.’
ARPES is founded on the photoelectric effect – the very simple idea that when you shine a light on a material, if it has the right energy you can eject electrons from it. This is what earned Einstein the Nobel Prize in Physics in 1921. ‘The electrons that are ejected retain a lot of their information about how they behave inside the material, so using ARPES we can understand how they move, and for that we need places like Diamond Light Source,’ said Riley.
Using ARPES, Riley and his co-researchers built 3D maps, ‘essentially a roadmap’, to show how electrons moved through tungsten diselenide. ‘By noticing asymmetries, we can actually measure if there is a preferred direction of the electron’s orientation – its spin polarisation.’ By combining spin measurements with ARPES, the researchers were able to observe an intricate pattern of spin polarisation, previously thought impossible for this class of materials. ‘By observing asymmetries, we can actually measure if there is a preferred direction of electronic spin – a spin polarisation.’
Being able to predict and control the orientation of an electron’s spin would mean that a binary system could be achieved much more efficiently using materials of a single atomic layer.
‘This material, the way the layers of it are stacked, says that we shouldn't have these preferred directions. When we look at it and we see the spin polarisation, we find that the individual layers themselves allow this to happen – it's not the whole. The important thing is that these materials had been completely disregarded years ago as possible candidates for spintronic devices. This research couldn't have been done without the help of Diamond, and we've opened up a whole new class of materials that we now need to go back and look at, that could lead to the next generation of electronics.’