Electrons have been one of the shining lights in studies of atomic and molecular science, but the electron’s antiparticle, the positron, is also beginning to share centre stage.
Professor Stephen Buckman and his team from the Atomic and Molecular Physics Laboratories in the Research School of Physical Sciences and Engineering have been delving into the world of electron collision physics for years. Professor Buckman is also the Research Director of the new ARC Centre of Excellence in Antimatter-Matter Studies (CAMS), which draws together partners from around Australia to exploit the power of positron and electron interactions. He spoke to ANU Reporter about the potential for antimatter applications in medical science, materials science, and for our understanding of the universe.
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Professor Stephen Buckman is excited about the research possibilities arising from positron studies.
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Reporter: What is antimatter?
Stephen Buckman: Well, all of the particles that make up our matter world – like protons, neutrons, electrons – have antiparticles. These antiparticles have many of the same properties as their particle but they have an opposite charge. Electrons are negatively charged subatomic particles that make up a large amount of matter on earth. They impact our daily life in many ways through things like lights, lasers, and communication technologies. Positrons are the electron’s antiparticle: they are positively charged. When the two come together they annihilate each other – a typical signature of particle-antiparticle interactions – and their mass is turned into energy in the form of gamma rays. Although they’re the most common form of antimatter found on earth, positrons are still very rare. The most typical way that positrons are produced is as the result of radioactive decay, so we have to go to some rather interesting lengths to produce, control and use them.
R: What is the ratio of positrons to electrons in the universe?
SB: That’s actually a really big question, one of the big questions in physics, and a lot of scientists are turning their minds to it. It’s thought that at the creation of the universe the laws of physics as we understand them meant that matter and antimatter were produced in equal amounts. Most of it annihilated in the early universe but it is still interesting to pose the question: Why do we exist now? Why isn’t there an anti-Reporter around, which means that when one meets the other, both are annihilated? Why in our part of the universe is antimatter so very rare? There are scientists who are looking throughout the universe for signatures of antimatter, and while there are hot spots in the universe where it has been found the general consensus is that matter far outweighs antimatter. The big question is why that happens. It’s thought that there may be some – at a very, very small and fundamental level – violation of the symmetry laws in physics, which led us to believe matter and antimatter would’ve been created in equal amounts. If there was a small violation of those symmetry laws, that might explain why there’s more matter than antimatter.
R: When were positrons discovered?
SB: Their existence was theoretically predicted by Paul Dirac in 1928 during the golden era of quantum mechanics. His calculations for a problem involving electrons using a relativistic version of Schrodinger’s equation showed negative energy solutions, and he interpreted that result as indicating the existence of a particle with the opposite charge to the electron. This prediction came a few years before they were actually discovered experimentally by Carl Anderson in 1932. Dirac got the Nobel Prize for Physics for his work in quantum mechanics in 1933.
R: Why do we study positrons?
SB: Positrons have an infinite lifetime as long as you don’t bring them close to their antiparticle, the electron. If you bring them close to an electron, or to materials that contain electrons, then they both annihilate and all of their energy is converted to the production of photons, two gamma rays in this case. The interaction of electrons and positrons and the process of annihilation is central to the research activities in the ARC Centre of Excellence for Antimatter-Matter Studies. A common theme to nearly all of our activities is the formation of positrons and electrons into an ‘atom’ called positronium. Positronium is the state where the positron has ‘picked up’ an electron and the two are orbiting one another: they are doing this mutual little dance around one another. Positronium is most readily formed when the positron interacts at a low energy with a material and it picks up an electron from this material, which could be an atom or molecule. Positronium comes in two forms and the longest lived form only exists for about 140 nanoseconds – which sounds short but is in fact relatively long compared to other interactions between electrons, atoms and molecules. When the positronium decays, it generally annihilates into two gamma rays which are emitted in opposite directions. Positronium formation and its subsequent annihilation really underpin all of the uses of positrons in bioscience and in materials science.
R: Can you provide an example?
SB: I think a perfect example is the use of positrons in Positron Emission Tomography (PET). This is a diagnostic tool used in cancer detection, and for monitoring brain function. As part of the PET scan process, a positron emitter, such as radioactive fluorine, is attached to a biomolecule, usually glucose. The biomolecule carrier is introduced into the body and being a sugar it naturally seeks out those areas of high metabolic activity. Cancers are such areas, as are the brain and liver. The radioisotope is delivered to the cancer and it is continually emitting high-energy positrons, which are quickly thermalised, within tens of picoseconds, in the body down to an energy low enough to enable them to form positronium. The positronium subsequently annihilates and gives off two gamma rays, which are detected by the PET scanner. In this way tumour sites can be determined with a resolution of two to five millimetres. One of the aims of our centre is to attempt to quantify the atomic and molecular interactions that take place both in the positron slowing down, or thermalising, in the body and when it forms positronium.
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A detail from the beamline at ANU used to study positrons.
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R: How are positrons produced for this technology and for research?
SB: The positrons we use are produced in the radioactive decay of an isotope – Sodium-22. We then manipulate the positrons in a ‘positron beamline’ in which we can trap, store and change their energy. The beamline here at ANU is custom built and based on a beamline at the University of California San Diego, though at ANU we’ve taken the principles of that apparatus and added in additional capacities and capabilities. It’s really a second generation beamline. In the beamline we can accelerate the positrons, change their energy and manipulate them to allow us to study them under different conditions and interactions. The beamline allows us to experiment on positrons from an energy very close to zero electron volts up to maybe 100 electron volts. This is the important regime for research on positron interactions with atoms and molecules, and particularly bio-molecules, such as the one used in PET. As part of CAMS, we’re developing a second, higher energy beamline where we will accelerate the positrons up to energies as high as 20,000 electron volts. This will be mainly used for materials sciences activities.
R: How are positrons used in materials science?
SB: It turns out that positrons are useful as a probe at the nanoscale level in materials because they love to find empty space. When a positron is fired into a material it slows down and picks up an electron very quickly to form positronium. Instead of having the normal lifetime of 140 nanoseconds as it does in free space, the positronium lifetime is greatly reduced to between one to five nanoseconds due to the large number of electrons available for annihilation. The positronium is drawn to free space; it seeks it out. If it finds a nice little void, a free space which might actually be a crack or a flaw in the material, then it will live longer. Essentially, the positronium lifetime is strongly mediated by the environment it finds itself in, and its lifetime in the material – based on whether it finds free space or not – can tell us something about the nature of the material. How long it takes for annihilation to take place informs us of the amount of free space in a material and the way in which this free space is connected.
R: Biomedical research and materials science are two areas which are really dynamic at the moment and you’re indicating positrons seem to be part of the advances. Can you guess at the potential impact of positrons when we know as much about them as electrons?
SB: That’s hard to say. Electrons were the vehicle for our communications last century, which was really the century of the electron. I don’t think that this century is necessarily going to be the century of the positron, mainly because they don’t naturally exist in our everyday lives. But there are clearly indications I think, across a range of disciplines, that they’re going to become more important than they are now.
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A beamline detail.
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R: What are Australia’s strengths in this area?
SB: Well, if I start at ANU, we’re strong in low energy electron physics, where we use electrons to probe atomic and molecular structure and materials. Within the Atomic and Molecular Physics Laboratories here we have history in electron-based investigations, which gives us significant background expertise to develop a world-class facility using positrons. Several years ago we became interested in investigations using the electron’s antiparticle after some quite serendipitous contacts we had with overseas laboratories, particularly the University of California, San Diego. This was about the time that we really got enthused about using positrons, and applying our expertise on electron interactions in this new area.In 2004 we received a national facility grant from the Australian Research Council with a number of partners – CSIRO, Griffith University, Flinders University and Charles Darwin University – and this is the origin of the positron beamline we now have. It drew a lot of interest from other scientists around the country for using positrons as probes of atoms, molecules and materials. It quickly dawned on me that there was a capacity to build this area up and to make it even stronger, and that’s really the genesis of the application for CAMS. The centre has a number of facilities and nodes around the country – Griffith University, Flinders University, the University of Western Australia (UWA), Murdoch University and the Australian Nuclear Science and Technology Organisation (ANSTO). These partners really bring in a great breadth to the research activities of the centre. For example, ANSTO are interested in developing positron-based radiopharmaceutical tracers, particularly for medical and biomedical research and for medical diagnostics. The UWA people give us an avenue in surface science; they will use positrons to probe the surfaces of materials. The Murdoch group is one of the leading atomic and molecular theoretical groups in the world and the experimentalists at Flinders and Griffith add an extra dimension to the expertise at ANU in experimental atomic and molecular physics.
R: Does it surprise you that it has taken this long to grasp the potential of positrons to science in Australia?
SB: The realisation of its potential came very quickly to me; I hadn’t been thinking about it for very long. But to know that there were other people around the country, in completely different areas, that had a use for, and an interest in a beamline such as the one that we were developing – that was a big surprise to all of us. We have some catching up to do in the sense that this is a new tool for us, but we already have quite exceptional expertise in the area of low energy electron-atomic physics, so the core expertise is already in existence. But there is a lot more that we have to learn about positrons and their applications in our world. It’s an exciting prospect.
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ANU Reporter
Winter 2006
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