CSEM’s Materials Monthly May 2006 Making materials matter New light on the dark side of matter CAMS and working with positrons Mention antimatter in public and it’ll are negative where normal protons be assumed you’re either a sci ﬁ fan are positive. Indeed, it’s believed or a theoretical bofﬁn. That’s because there’s an antimatter equivalent most people see antimatter as an for each of the known subatomic abstract concept with little place in particles. the real world. And yet the truth “If an antimatter particle comes in is a bit different – antimatter is a contact with a normal matter particle commonly used substance with many they annihilate and are converted important applications. PET scans, into energy. The amount of energy for example, are a common form of released is predicted by Einstein’s diagnostic imaging used to dectect equation: E = mc2; where m is the tumours and it works by releasing mass of the particles and c equals the To make sure that Australia antimatter in our bodies. Firing speed of light. That energy is released captures the beneﬁts of antimatter at the surface of materials, in the form of gamma rays.” antimatter research an ARC as another example, is a technique for characterising those surfaces. And therein lies the reason why Centre of Excellence devoted Indeed, antimatter plays a number of antimatter is considered so exotic, so to antimatter-matter studies roles in the real world and its value ‘other wordly’. If it comes into contact has been established. Known in materials science, medicine and with normal matter it dissappears in as CAMS – the Centre for biology is only set to increase. a burst of gamma rays so how can it Antimatter-Matter Studies serve any useful function in a world – it’s hosted by the ANU and So what is antimatter? made from normal matter? includes other universities and government laboratories, and “As the name implies, and as most “There’s no question that antimatter promises to raise the status people would have heard, antimatter is an exotic substance and it’s not is the same as normal matter but with easy to work with but there’s a lot of antimatter from something an opposite electrical charge,” says more to it than abstract physics,” says obscure and abstract to Professor Stephen Buckman, Research Professor Buckman. “When antimatter something real and now. Director of CAMS. “Antimatter meets normal matter it takes only a electrons are identical to normal split second before its gone but in that electrons but carry a positive charge split second of interaction it’s possible where normal electrons carry a negative charge. Antimatter protons continued on next page Inside this MM 2-3 CAMS & positrons (cont) 4 Slivers away 6 CAMS & positrons (cont) Volume VII, Issue IV Materials Monthly is produced by the ANU Centre for Science and Engineering of Materials Professor Stephen Buckman (right), Director of the Centre for Antimatter-matter Studies (CAMS) with Dr James Sullivan. The two scientists were responsible for the design and THE AUSTRALIAN NATIONAL UNIVERSITY construction of the positron beamline. (Photo by Tim Wetherell) Antimatter-matter research to be mapped, usually down to a (continued from previous page) resolution of 2-3mm. Thinking up antimatter “PET scans are a well-developed Antimatter is a good example of to learn a lot about the physical diagnostic tool and yet little science predicting the existence environment around the antimatter is known about how positrons of something before it’s actually particle. interact with biomolecules despite observed (or even suspected). the sophistication and cost of the “On top of this, when antimatter In 1928 the British physicist Paul technology,” says Professor Buckman. annihilates with a normal matter Dirac combined quantum theory “One of the aims of CAMS is to study particle, gamma rays are released. the interaction of positrons with bio- and special relativity in one Gamma rays are easily detected molecules and try and shed light equation in an effort to better allowing researchers to pinpoint on what happens between positron model the behaviour of electrons. where the antimatter-matter particle emission and positron annihilation. His solution, known as the Dirac pair was when it disappears. These In particular, we want to look at ways equation, worked exceptionally particles are too small to see with any in which the efﬁciency of the process well describing many attributes microscope but when they annihilate might be improved. In so doing we of electron motion that previous the gamma rays they give off are a think we can improve the resolution of equations could not. However, it crystal-clear signal that is easy to pick PET scans.” also suggested the possibility of up. These aspects of the interaction the existence of electrons with between antimatter and matter lend Positrons are also a useful tool in the positive charges – anti-electrons. themselves to some very useful analysis of materials. When positrons But where would you look to ﬁnd applications in the areas of medical are ﬁred into a material they tend such a strange beast? scanning, materials characterisation to drift towards any open volumes and theoretical particle physics.” — very small holes. This is because The existence of anti-electrons they like to be away from the positive created a real problem for If antimatter can’t coexist with normal charge of the ﬁxed nuclei in the the growing ﬁeld of quantum matter for more than an instant and material — like charges repel. When it physics. Everyone accepted the our world is made of normal matter, ﬁnds a hole in the material, there are mathematics of Dirac’s equation where does it come from and how do no electrons to annihilate with, so the but anti-electrons did not you use it? size of the hole determines how long correspond to anything known “There are two basic sources of the positron lives. in the physical world. Werner antimatter on Earth,” says Professor “By looking at the lifetime of the Heisenberg, one of the world’s Buckman. “It’s produced when positrons in the material, we can leading quantum physicists at subatomic particles collide, as occurs get information about the size and the time, initially called this in particle colliders, and from the distribution of holes, or defects, that “the saddest chapter of modern radioactive decay of some isotopes. are as small as a nanometre in size,” physics”. However, as events were Antimatter used in diagnostic scanning Says Professor Buckman. “Holes of soon to demonstrate, it was to end and materials characterisation uses this size are related to important in intellectual triumph. antimatter produced by radioactive properties in some materials, such as isotopes. Four years later when studying porosity and conductivity. They can the particle tracks left by collisions “The commonest form of antimatter also be an early indicator of material with cosmic rays a particle was used is the positron – the antimatter degradation. In CAMS we plan to identiﬁed that had the same mass equivalent of the electron. Indeed, the use one of the positron beamlines as an electron but the opposite positron is identical to the electron for the study of materials for various charge – the anti-electron or in all aspects except that it carries a applications, from new generation plastics to silicon wafers. positron had been found. positive charge.” Since then it’s been determined Putting positrons to work Crossing to the dark side that an antiparticle exists for each One of the most common applications Professor Buckman’s interest in of the known sub atomic particles of positrons is in medical imaging positrons and antimatter arose from and that these are naturally and PET scans. While many people his earlier research on electrons. He generated by cosmic rays have heard of these, few would says it’s not an enormous jump to impacting with normal matter. connect them with antimatter, and switch from electrons to positrons. Amidst the cascade of resulting yet PET stands for Positron Emission particles antiparticles will often “Several years ago I spent some time Tomography. The procedure involves over at the University of California, result, existing for nanoseconds injecting a patient with glucose San Diego, where they have a before combining with normal containing a radioactive isotope, beamline set up speciﬁcally for the matter and annihilating. usually ﬂuorine-18, which emits study of positrons,” he explains. positrons. The body directs the In 1933 Dirac (pictured below) “When I saw what they were achieving was awarded a Nobel Prize for glucose to areas of high metabolic over there I realised that antimatter activity, often indicating the presence his modelling of the electron and science was a real opportunity for us of a tumour or some change causing prediction of in Australia because our strengths increased blood ﬂow or immune in electron atomic physics meant we antimatter. He system activity. already possessed the core expertise was just 31 to undertake these studies. You might years of age. The ﬂuorine-18 has a half life of a few hours. As it breaks down it say my exposure to this positron emits a steady stream of positrons. research convinced me it was worth These combine with electrons in the crossing over ‘to the dark side’. surrounding tissue, they annihilate “When I returned to Australia I got in and give off energy in the form of contact with several research groups gamma rays. The gamma rays are around the country with an interest easily detected and allow the source in antimatter and we were successful of the increased metabolic activity in obtaining funds in 2004 from the page 2 - May06 Australian Research Council to build a These positrons are then conﬁned low energy positron beamline here at RSPSE.” in an electrostatic potential well where they cool to room temperature The stuff of sci ﬁ through collisions with nitrogen One of the reasons antimatter The funding, which amounted to gas molecules. The positrons don’t has such as an exotic aura is its around $1million, was part of the ARC combine with electrons of the nitrogen frequent use in science ﬁction as National Facilities program. The other atoms because their energies are too a source of seemingly unlimited partners in the project were Grifﬁth low. This technique allows for the energy or as a weapon. Mix University, Flinders University, Charles production of a high resolution beam. a little antimatter and normal Darwin University and CSIRO. When the trapped positrons have matter and their combined mass “In the couple of years that it has is converted to massive amounts cooled sufﬁciently the beam is formed taken us to build the low energy of pure energy. It sounds pretty by carefully raising the conﬁning beamline we’ve discovered there’s potential and allowing the trapped appealing, and has been the a lot more interest in antimatter- positrons to spill out. This produces driving force behind the Starship matter studies around the country a very high resolution beam (with Enterprise since the inception of than we ﬁrst realised,” explains an energy width of around 25 meV) Star Trek. However, it overlooks Professor Buckman. “To capitalise on which can then be used for various the fact that antimatter is very, this interest and the diverse range of experiments. The beam is conﬁned in very expensive to create. To skills available around the country the the radial direction by magnetic ﬁelds, produce a single gram of sodium original concept has been expanded while electrostatic potentials allow 22, the radioactive isotope used with an ARC Centre of Excellence the positrons to be directed along the grant worth $7 million over ﬁve years. by CAMS to produce positrons, apparatus. In addition to this the institutions you’d need about twice the involved are chipping in an extra $3 million. This funding will enable us Positrons & materials GDP of the United States (as measured in 2004). to build an additional high energy The materials side of the CAMS beamline which will greatly enhance experimental program will be our capacity for positron studies on undertaken by ANU, ANSTO and with a positive charge, with electrons materials.” the University of Western Australia. orbiting around it. In the case of Each institution brings a range of positronium, the positive nucleus is And the beamline is the key to experience in positron handling and replaced by the positive positron. studying positrons as it allows you to materials characterisation. Positronium is really an exotic type of control the energy of the positrons atom. It is very light and only lives for and precisely measure its interactions “The goal of the materials research 120 picoseconds or 142 nanoseconds, with matter. is to provide new tools for Australian depending on the conﬁguration of scientists to investigate the properties the electron and positron. The key A beam of positrons of novel materials, such as polymers, to these studies is understanding thin ﬁlms and semi-conductors,” the formation and annihilation of The source of the positrons in the says Professor Buckman. “It’s also positronium.” CAMS beamlines is a tiny speck of hoped that our research and that of radioactive sodium 22. It’s a only a Positronium can be formed in two our collaborators, will be useful in few nanograms in size, so tiny that conﬁgurations: para- and ortho- the investigation of such properties you can’t see it with the naked eye. positronium (the singlet and triplet as surface effects, fatigue in some It was produced by a nuclear reactor form of the positronium ground metals and even for some medical in South Africa, the only place in the state, respectively). Para-positronium applications.” world capable of creating this isotope, decays via 2-gamma emission (with and costs around US$25 thousand. Three types of experiments in both gamma rays having an energy materials analysis are possible within of 511 keV) with a lifetime of 120 ps. “That would make it the most CAMS: Positron Annihilation Lifetime Ortho-positronium, on the other hand, expensive material I’ve ever worked Spectroscopy (PALS), Doppler decays via 3-gamma emission with with,” observes Professor Buckman. measurements and surface studies. a lifetime of 142 ns. However, inside And, if you do the sums, he’s not a solid material the lifetime of ortho- PALS relys on the detection of 511 joking. A gram of this material, about positronium depends on how easily it keV emitted gamma rays as a function the size of pill, would cost billions of can annihilate via ‘pickoff’ annihilation, of time elapsed from the entry of the dollars. where the positron annihilates with positrons into the sample. The lifetime Fortunately, for CAMS’ purposes, a few of the positrons in the sample changes one of the other electrons in the nanograms is all they need. Sodium as a function of various properties continued on page 6 22 has a half life of around 2.6 years of the material, speciﬁcally on the meaning this solitary speck can supply presence of voids their beamline with a serviceable and defects. On number of positrons for around three entering the sample, to four years. positrons thermalise and either annihilate The sodium sits in a lead lined or form positronium chamber at one end of the beamline. with one of the It emits around a billion positrons target electrons. every second, ﬁring them off in all directions and with a wide range of “Positronium is the energies. Most of the positrons have substance formed energies that are too high to be of when a positron practical value so before they enter and an electron are the beamline they pass through a thin bound together,” ﬁlm of solid (frozen) neon. Around 1% explains Professor of the positrons make it through this Buckman. “In a step. normal atom, there Professor Buckman with the positron beamline. The cylinder on the is a heavy nucleus right houses the radioactive sodium 22, the source of the positrons. page 3 - May06 Slivers away Thinking outside the square = thinking inside the wafer Few industries can boast a sustained that simply cutting the silicon growth rate of 23% but that’s wafers thinner would only produce just what’s been happening in the incremental improvements and we photovoltaic (PV) industry over the were looking at more than merely last 15 years. And that growth is improving what we already had. So, set to continue as an energy hungry we began to think outside of the world searches for alternatives square or, for this problem, inside sources of electricity (23% was of the wafer. It occurred to us that an average growth rate; the PV maybe we should be working with industry grew by 65% in 2004!). the volume of the wafer rather than However, the PV industry is facing with its surface.” a major challenge in the form of a critical shortage of hyper-pure What they did was cut narrow silicon at an affordable price. grooves into the silicon wafer using an alkaline etch. The alkaline etch Solar cells are traditionally made attacks most of the silicon quickly from wafers of hyper-pure, but the (111) crystal plane slowly. crystalline silicon (see box on how A resistant mask is placed on the The output of 72 conventional cells is the this is produced). These wafers surface of the silicon wafer. The same as that from two wafers when the – typically 15 cm in diameter and wafers are manufactured into Sliver® cells. mask has long narrow slots cut into less than half a millimetre thick it. The silicon below the mask is was most efﬁcient using wafers that - are used in 95% of photovoltaic oriented such that a (111) plane is were 1-2mm thick. modules. The shortage of hyper- perpendicular to the wafer surface. pure silicon is proving a major Etching commences at the surface One important aspect is that after barrier to reducing the price of PV- of these narrow slots and continues creating the grooves the slivers based energy, and it’s a shortage down through the entire thickness are still attached to the wafer. This that only promises to grow over the of the wafer. The result is a large allows the slivers to undergo further next decade. number of thin silicon strips in the steps in the fabrication process centre of the wafer, held together before they are cut from the wafer. There have been many attempts to by the unetched surrounds of the Each sliver is now an individual solar get around this bottle neck. Some wafer. cell, and these are then assembled have involved using non-silicon into modules. semiconductors based on materials In effect the wafer has been cut into such as gallium or indium, while a series of slivers, with each sliver Cells can be laid out with no gaps others are based on using lower approximately 50-100mm long and between them or with any spacing grades of silicon. However, none 40-60μm thick. Each sliver is as between slivers up to about 1.5 of these efforts have produced a wide as the wafer is thick and the times the width of each cell. With a solution that can compete with researchers found that the process scattering reﬂector attached to the cells based on wafers of hyper-pure rear of each module, most of the silicon in terms of cost per watt. There’s also been work on using thinner wafers of silicon but only incremental savings can be achieved here – you can only cut a traditional wafer so thin and there are signiﬁcant losses of silicon in the process of slicing up the original ingot of silicon into wafers. But researchers at the ANU Centre for Sustainable Energy Systems (CSES) have found a way around the problem and their solution lies in a little lateral thinking and a lot of clever materials engineering. The lateral thinking involves cutting thin layers of silicon out of the wafer itself. The clever materials engineering is how they achieved it. “The main issue we needed to tackle was how to reduce the amount of high grade silicon being used in solar cells,” says Dr Klaus Weber from the CSES. “We knew page 4 - May06 Six strengths of Silicon for solar cells the Sliver As materials go, surely there is no element as versatile and useful as silicon. After oxygen it’s the most abundant element in the Earth’s 1. More bang for your buck: crust - it makes up a quarter of the planet’s crust by weight. How Sliver® cells use approximately is it then that hyper-pure silicon is in critically short supply for the one tenth the amount of manufacture of solar cells? expensive silicon compared with The silicon in solar cells is hyper-pure silicon, and making it is neither conventional cells. easy or cheap. It involves loading a crucible with high-purity silicon along with small amounts of either boron, phosphorus, arsenic 2. Designer transparency: any or antimony (these impurities or dopants give the ﬁnal product degree of module transparency different electrical properties). The silicon is then melted at a process can be easily achieved by temperature of 1400°C in an atmosphere of pure argon. Once the adjusting the Sliver® cell spacing. proper ‘melt’ is achieved a ‘seed’ of single crystal silicon is lowered into it. 3. Flexible ﬁtting: The thinness The temperature is then adjusted as the seed is rotated and slowly of the slivers makes them pulled out of the molten silicon. The surface tension between the seed ﬂexible meaning modules can and the molten silicon causes a small amount to rise with the seed. be designed to ﬂex and bend As the growing seed is pulled it cools to form a perfect single-crystal ingot with the same crystal orientation as the original seed. This ingot (opening up countless new is then sliced up into individual wafers which are used as the basis of architectural possibilities). solar cells or integrated circuits. 4. High voltage: Many Sliver® cells in series still take up very little area, so high voltages can be light incident in the space between as that from as little as two wafers obtained in very small modules. the cells can still be captured. when the wafers are manufactured This makes Sliver® cells ideal for Sliver® cells, as they are known, into Sliver® cells. powering small consumer items. are truly bifacial as they respond equally well to light on either As part of a research program 5. Bifacial response: The perfect surface. They can be connected in substantially funded by Origin Energy, the Sliver process concept bifacial response of a Sliver® any series and parallel conﬁguration was invented by Dr Weber with cell means that Sliver modules to deliver the output voltage required for any application. Professor Andrew Blakers, Director respond equally well to light of CSES, in 2000. Since then ANU’s falling on either surface. This So, how much hyper-pure silicon is commercial collaborator Origin saved using the Sliver approach? Energy has constructed a factory in allows for novel applications of Consider this: A typical silicon wafer Adelaide to produce Sliver® cells Sliver® modules. For example, conﬁgured as a conventional solar commercially. First sales occurred highway round barriers can utilise cell will require about 10kg of silicon in July 2005 and full commercial Sliver® modules that are mounted for each kilowatt of peak output production is expected to get vertically facing east-west. power. However, a wafer, when underway later this year. processed to produce Sliver® cells, 6. Energy payback: The energy can achieve the same output with In the meantime, research at CSES is further improving Sliver payback time of a Sliver® module 1kg or less of silicon, which is 10 times better than for conventional technology. Recent results indicate is short because the quantity of technology. Moreover, far fewer that with careful engineering energy-intensive silicon is sharply using well-known and established wafers need to be processed to techniques, Sliver technology could reduced. The energy payback give the same output compared to time is 1.5 years, two thirds of conventional cells. The output of reduce the costs of PV technology 72 conventional cells is the same to the point where it will be which is due to standard module competitive with wind energy and components (glass, aluminium ‘zero emission’ coal. Given the frame etc.) compared with 4 years vast worldwide solar energy resource, this has major for a conventional module. implications for climate change policy. More info: http://solar.anu.edu.au/ or email email@example.com MM page 5 - May06 Antimatter-matter research Two beamlines (continued from page 4) Partners in “The high energy beamline being built at CAMS will be dedicated to antimatter-matter research target (not the one it is bound to). materials studies, ”says Professor CAMS is hosted by ANU and has a The lifetime against direct annihilation Buckman. “This beamline will allow us total of 15 collaborating partners. is typically 100’s of picoseconds. to control the energy of the positrons being injected into the sample to be Australian members are Flinders, If annihilation gamma-rays are tuned up to 20 keV. This will allow the Murdoch and Grifﬁth universities, detected as a function of time after samples to be probed as a function of the University of WA, and ANSTO. entering the sample (usually the two depth, and we can go into the sample Overseas partners are The Open 511 keV are detected in coincidence to approximately 1 micron. University, The Universities of to improve timing resolution), then California in San Diego and Davis, a series of lifetimes can be seen in “Our existing low energy beamline The Lawrence Berkeley National has a peak energy of 100 eV. This is the spectrum. These lifetimes give Laboratory, Drake University, the information about the structural suitable for investigating biological University of Nebraska, Tohoku makeup of the sample, and the ortho- systems and fundamental atomic and University, and the University of positronium lifetime is particularly molecular interactions with positrons. Munster. sensitive to the size and distribution “The fundemental investigations of voids and defects. PALS, for involve understanding the interactions of positrons with understanding, we expect to greatly atoms and molecules increase our understanding of the in a way that gives interactions of positrons and matter at us insight into their a fundamental level.” behaviour at a The design of the two beamlines quantum mechanical are based on a positron beamline level. Despite that operated by the University of fact that quantum California, San Diego, though many mechanics has been aspects of these new facilities are around for a long unique to Australia. time the interactions of positrons with “Our two beamlines incorporate a lot single atoms is of second generation improvements notoriously difﬁcult to the technology,” says Professor to model correctly. Buckman. “We will have a more Instead, numerous intense beam with a broader program approximations of study. have to be made to describe the “This Australian facility will place us interactions and a at the global forefront of positron lot of computing physics. There is no other centre in power is needed to the world with such an adventurous solve the equations focus or such a breadth of activities numerically. involving positrons.” The facilities of The high energy beamline is scheduled Dr James Sullivan, an ARC Research Fellow, and PhD student Ms CAMS will allow to be operating by the end of this Violaine Vizcaino by the positron trap. (Photo by Tim Wetherell.) the experimental year. investigation of these example, has been used extensively interactions at an unprecedented More info: www.positron.edu.au by scientists at CSIRO to relate the level of accuracy. In addition, CAMS or email Professor Buckman average pore size in polymers to their includes some of the best theorists in <Stephen.Buckman@anu.edu.au oxygen porosity. the world in this area. By combining Doppler measurements provide experiments with a new theoretical MM additional information on the site of annihilation. Positrons that annihilate with a fast moving electron (ie, a core electron) will emit Doppler-shifted Contacting CSEM gamma rays. Measuring this shift can Materials Monthly comes out 10 then provide information about the Director Dr Zbigniew Stachurski times a year (February to November). chemistry at the annihilation site. We welcome your feedback and As positrons tend to be attracted to Phone: (02) 6125 5681 contributions. Please send them to defects in the materials, this then Email: Zbigniew.Stachurski@anu.edu.au David Salt, Editor, Materials Monthly, gives us the ability to determine the chemical makeup at defect sites, such Editor, Materials Monthly care of CSEM. as voids, which may be sites for solute David Salt Please let us know if you wish to atoms forming clusters or precipitates. Phone: (02) 6125 3525 be added to our electronic or postal Surfaces can also be studied Email: David.Salt@anu.edu.au mailing lists. using antimatter. Variable-energy, thermalised positrons will be scattered CSEM Ofﬁce Electronic copies of Materials from thin ﬁlms, surfaces and CSEM Monthly, useful links and additional interfaces for metals, semiconductors Department of Engineering information about CSEM can be found and insulators, thereby yielding a Bld #32, ANU ACT 0200 at our website: range of information on the nature of www.anu.edu.au/CSEM the surface.