Secrets being revealed at atomic level
8 april 2022
With its six radiating beam lines, this machine looks a bit like a giant steel hydra. At around 50 metres long, it occupies most of the space in the 750-square-metre underground hall in which it is housed. Inside its steel arms, particles are accelerated with the aim of capturing or even altering what we cannot see. Let's take a tour down to the Pelletron accelerator in the Tandem Laboratory and get a closer look.
It’s difficult to imagine anything more abstract than the processes that take place in this lab in the basement of the Ångström Laboratory. The fact that this machine with its steel tubes is able to contain electric potentials of several million volts, and that particles accelerated through these tubes can interact with materials within a few femtoseconds – a femtosecond is one quadrillionth of a second – is hard enough to get one’s head around. But it's even more difficult to imagine how the researchers are able to control and draw conclusions from the immense forces that are set in motion here.
The considerable length of the tubes is due to the speed and the energy levels the researchers want the particles to attain. The difficulties of controlling particles that have so much energy is why you need such a lot of space according to Daniel Primetzhofer, professor of applied nuclear physics and director of the Tandem Laboratory.
“The more energy we want the particles to attain, the bigger the machine has to be. And it is these high energy levels that can give us the maximum information, based on the interaction between the particle beam and the sample material we want to analyse.”
The starting point for the Pelletron accelerator can just be seen behind a wall constructed from what look like large concrete blocks. Before going behind this barrier, Daniel explains that they have to shut off the high-voltage power so that no one gets exposed to radiation. After completing this standard shut-down procedure, we are cleared to pass in among the beam lines.
Daniel’s collaborator on this tour of the facility is Sascha Ott, professor of synthetic molecular chemistry. In their most recent project, published last autumn in the prestigious Journal of the American Chemical Society, these researchers successfully measured a group of highly porous substances, known as metal–organic frameworks without destroying them. These compounds could become part of systems that can generate energy-rich molecules from carbon dioxide and water – in short, imitating photosynthesis.
“To investigate Sascha’s micrometre-sized crystals, we created a particle beam by means of a glow discharge in an ion source. Then we fed the beam into the accelerator, where the particles were accelerated along the line,” says Daniel.
Daniel describes what happens in the measurement stations at the end of the various lines as a kind of billiards with atomic nuclei. If the accelerated ions targeted at Sascha’s crystal sample bounce off a heavier atom – which are only found at specific active sites – the ions will return faster. If the ions bounce off lighter atoms, they will return more slowly.
“The ions also bounce off the electrons in an atom. Since electrons are relatively light, what happens to the ions is similar to what happens in a children’s ball pit – they decelerate only slowly,” Daniel explains.
The beam line's detectors measure the velocity of the ions and this reveals which elements are present in the sample and where they are located in the crystal. Active sites are of particular interest to the researchers.
“This is where chemical reactions occur,” says Sascha. “The goal is to be able to introduce special catalytic units at different sites in the crystals so they can then be used in, for example, an electrolysis apparatus for water splitting. If we know exactly where in the crystal our active sites are, we can control these chemical reactions optimally.”
Daniel nods in agreement.
“In being able to show that we can measure the distribution of these units within Sascha’s crystals, we have taken a decisive step towards applications with a sustainability and energy focus such as artificial photosynthesis and/or the production of hydrogen using sunlight.”
Why doesn’t the accelerator destroy the materials?
“Because the data we get from the particles bouncing back is so clear that we don’t need to use very many particles. We only accelerate a few particles to hit a sample that consists of very many atoms. Even if the crystals are only micrometre-size, there are many billions of atoms in such a crystal,” says Daniel.
All of the equipment in the Tandem Laboratory makes it the largest facility of its kind in the Nordic region and its total of four accelerators is valued at more than SEK 250 million. Although there are accelerator laboratories in other cities in the Nordic region, the Tandem Laboratory’s broad range of beam lines and other measurement and analysis equipment is unique. The six beam lines that make up the Pelletron accelerator also each have their own specialities.
Daniel points to one of the lines with a large round window.
“In this chamber we’ve introduced the element yttrium, which is vapor-deposited in a controlled way while we let hydrogen in and, in a further step, oxygen. If we simultaneously run a particle beam, we can follow exactly how the growth process and oxidation occur. We can then tweak a parameter that will give the material different properties and thus customise the material to have the properties we want.”
A company that came out of research at the Ångström Laboratory and has regularly used the Tandem Laboratory is ChromoGenics. They produce what is called electrochromic glass for dynamic or smart windows that consists of multiple layers of material. This kind of glass uses an electric potential through the layers that can control how light and heat flow through the window.
In his own research in collaboration with materials physicists, Daniel has focused on a variant of this technology in the form of thermochromic glass windows where the glass – based on a compound containing yttrium – darkens when illuminated .
“We look at the entire life cycle from synthesis to destruction due to exposure to excessively high temperatures. Thanks to specific detectors in the various chambers along the lines, we can conduct measurements with extremely high precision and see how the samples change when heated, for example,” says Daniel.
“What also makes the work here so interesting is that we are constantly combining things as diverse as heavy infrastructure with advanced molecular chemistry and catalysis. In other words, we are pretty much always working in a multidisciplinary way. And that's needed if we are going to increase our understanding of how we can reuse materials, dispose of environmentally hazardous substances safely, and create energy efficient solutions for our society for example.”
FACTS ABOUT THE TANDEM LABORATORY
- The Tandem Laboratory conducts world-leading research based on advanced material analysis and modification with ion beams in a wide range of fields, from basic research to practical applications for industry and society.
- The laboratory is a national infrastructure facility supported by the Swedish Research Council and the Swedish Foundation for Strategic Research.
- It is also a node in the EUROfusion research programme, which is funded by the European Commission and focused on the development of fusion reactors for future energy production. The European Commission also supports transnational access to the Tandem Laboratory’s equipment for international research teams.