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A review of its role in the and its applications THE DISCOVERY OF ’S

ntimatter plays an intrinsic role in our Aunderstanding of the subatomic THE UNIVERSE THROUGH THE LOOKING- C.D. ArchivesSegrè Visual Anderson,Emilio The beginning of the or vice versa, it absorbed or emitted saw a cascade of brilliant insights into quanta of electromagnetic the nature of and . The of definite energy, giving rise to a first was ’s realisation that characteristic of bright or energy (in the form of electromagnetic dark lines at specific . radiation i.e. ) had discrete values The Austrian , Erwin – it was quantised. The second was Schrödinger laid down a more precise that energy and were equivalent, mathematical formulation of this as described by Einstein’s special behaviour based on and and his iconic . The first image of a track found in cosmic rays , E = mc2, where c is the The Schrödinger wave equation could in a ; the theory predict the spectrum of the simplest or positron; when an also predicted that objects behave , , which consists of met a positron, they would annihilate somewhat differently when moving a single electron orbiting a positive according to Einstein’s equation, . However, the spectrum generating two gamma rays in the featured additional lines that were not process. The concept of antimatter explained. In 1928, the British physicist was born. realised that because the

electron was very light and moved very THE IMPORTANCE OF DIRAC’S IDEAS quickly, needed to Today, it is accepted that all subatomic Paul Dirac's equation of (right) be incorporated into the equation to can have a “mirror” or predicted the existence of antimatter explain this fine structure. antimatter counterpart with opposite with velocities close to the speed Dirac produced a more complicated and right- or left-handed . of light. The third was ’s wave equation that revealed the Furthermore, our understanding of ground-breaking description of matter electron’s true character. First, it particles and – the fundamental as composed of consisting predicted that had spin, building blocks of the universe – is built of lightweight, negatively charged which could be right- or left-handed. on these kinds of mirror symmetries, electrons orbiting a central, positively The second, more bizarre characteristic which are reflected in the powerful charged nucleus. The key feature of the was that the electron could have a mathematical framework of quantum model was that the electrons occupied range of negative as well as positive mechanics. The concept of is a of energy levels or quantum . This suggested that this now used throughout physics to explain states. When an electron jumped from concept predicted the existence of a and classify phenomena. a lower to a higher one positively charged electron – Another important idea to emerge

2 Antimatter | A review of its role in the universe and its applications

THE UK ROLE The UK has always been a world-leader in physics. It has participated in most of the major international involving antimatter, particularly through its membership The first detection in 1983 of a carrier of the weak , the Z particle, in CERN's ground-breaking proton– (the Super Proton ), which was later superseded by the Large of CERN. The UK has invested in Electron–Positron collider, LEP (top right) for future collider designs and experiments. It plans to become an Associate Member of FAIR. from Dirac’s equation is that the from its and negative vacuum is not empty but a charge, and show that it annihilated of virtual particles. According to in a characteristic way when it hit a quantum probability, particles can proton or in ordinary matter. pop in and out of existence, allowing particle– pairs to be THE ROLE OF ANTIMATTER IN generated from the vacuum according to Einstein’s equation. Experiments employing or generating antimatter particles have played a key role in formulating ideas about The high-energy FAIR facility in Germany generate THE DISCOVERY OF intense antiproton beams for many experiments The existence of was the fundamental particles and confirmed when they were discovered forces. Particle at very composed of the two lightest . in cosmic rays using a , high energies may generate particle– Each particle has an antimatter in 1932 by Carl Anderson at the antiparticle pairs, which provide partner. The four fundamental forces, California Institute of . about the characteristics the electromagnetic force, strong Then in 1955, a team led by Owen of the elementary units of which the and weak forces, and , are Chamberlain and Emilio Segrè at the universe is composed. also described as being mediated by University of California at Berkeley Europe’s main particle physics particles, known as (although uncovered the antiproton using a new research , CERN in Geneva, gravity is still not incorporated into the type of accelerator called the . as well as in the US, used ). They were able to identify the particle beams of antiparticles to confirm the In the 1980s, experiments at so-called Standard Model of Particle CERN colliding beams of and Physics. In this quantum-based confirmed the existence of picture, matter encompasses six types , which carry the weak

- of lightweight particles: the electron, force. These new particles were later π the and , together studied throughout the 1990s using with their corresponding ; the Large Electron–Positron collider, η π + and also six heavy particles called LEP. A new electron-positron collider quarks: up, down, strange, charm, is being planned to probe the Higgs- π + bottom and top (in order of increasing -like particle that has now been π o - μ mass). Protons and are discovered in the Large Collider (LHC) at CERN. γ γ Another high-energy project at Darmstadt in Germany, FAIR (Facility for Antiproton and Research), will μ + e- υ υ e- use high- antiproton beams to investigate the strong force, and how matter first formed and evolved in

+ the universe. e e+

3 NATURE’S SYMMETRIES BROKEN?

e live in a universe made mostly Wof matter, so where did all the antimatter go? LOST OF ANTIMATTER now believe that the particles “right- or left-handedness”; Universe was born some 13.75 billion and the reversibility of their behaviour years ago in a unified high-energy state in (T) – the film of a particle that rapidly expanded and cooled, looks the same whether run coalescing into the matter and force forwards or backwards. Particles and carriers responsible for our existence. their antiparticles with opposite charge Matter and antimatter particles and handedness should behave in the should have been created in equal same way, that is – symmetrically. numbers and should have annihilated. To search for any , Somehow, some matter – but no researchers probe C, P and T, either antimatter – survived; observations separately or combined (CP or CPT), indicate that all the and by comparing how particles and their we see are made of the same version anti-versions behave. of matter as our own solar The CPLEAR at CERN investigated the and ourselves. CP mirror "breaking" of the CP mirror (CP-violation) in the Theoretical physicists think that early 1990s, using particles called an imbalance between particles and decays of exotic composite particles antiparticles must have developed known as neutral kaons (K0). These during the primordial expansion, and consist of a down and a strange that there must be some inherent quark (p3) but exist in two forms that asymmetrical difference in the are each “quantum mixtures” of both behaviour of matter and antimatter the matter and antimatter versions. that favoured the survival of matter. The two forms decay at different rates, However, no one is sure what those + - and are known as K-long and K-short. processes were and how they Tiny discrepancies in their decay worked. Identifying subtle effects indicated that the constituent associated with the matter–antimatter quarks and antiquarks were behaving asymmetry is now one of the main slightly differently in terms of their research areas in fundamental physics. Symmetry inherent in quantum mechanics shows , and were in fact “violating CP how the CP (charge–parity) "mirror" (a mathematical symmetry”. More precise experiments operation) reverses the basic attributes of SEARCHING FOR ASYMMETRY fundamental particles like electrons: the electric at CERN using the Low Energy As explained on p2, physicists classify charge transforms from negative to positive Antiproton Ring (LEAR) and at other the quantum properties of particles or vice-versa; and the direction of the particle's laboratories confirmed thatCP and T according to various symmetries: in reverses with respect to the sense of its spin symmetries were violated. so that its “handedness” changes particular, their electrical charge (C), In the 1990s, physicists turned which can be positive or negative; their The first hints of asymmetry were to the decays of a heavier particle “parity” (P), which describes what seen in 1964 when at the called the B-, which contains a happens when they are inverted in Brookhaven National Laboratory in the . A major experiment at space (as in a mirror) and gives the US observed a tiny difference in the the Stanford Linear Accelerator Center

4 Antimatter | A review of its role in the universe and its applications

An display in the LHCb detector at which may just provide some answers. the LHC, which is looking for rare particle Neutrinos are electrically neutral and decays that signal CP-violation have hardly any mass, but reveal themselves in the of some unstable nuclei. All three types, the electron-neutrino, muon-neutrino and tau-neutrino, appear to oscillate from one type to another. Recent experiments on neutrinos produced in nuclear reactors, which measure the rate of one type of predicted to involve CP-violating processes, indicate that the effect could be measured. The huge Sudbury Neutrino Observatory (SNO) in a Even more significant is that the mine in Canada is being upgraded to search for a rare spin of lightweight neutrinos is always particle event (neutrinoless double ) that left-handed. Theorists postulate that a may uncover CP-violation in neutrinos right-handed, much heavier neutrino in California, called BaBar, together could have existed in the high-energy THE UK ROLE with a Japanese experiment, Belle, primordial universe, and could have UK researchers participated in extensively measured various aspects generated the necessary imbalance CP-violation experiments, CPLEAR and of CP-violation in Bs and anti-Bs. in particle–antiparticle numbers BaBar, and are now collaborating in These measurements have continued through CP-violation. However, for such LHCb. In addition, they are carrying out at the LHC with the LHCb experiment CP-violation to exist, the neutrinos subtle experiments on neutrons at the (upgrade planned for 2018), which and antineutrinos (which have no Institute Laue-Langevin in Grenoble, is currently measuring very rare charge) would have to be identical, as France that may also uncover C, P and T B-meson decays. predicted by the Italian physicist Ettore . The UK also has a strong Majorana in 1932. presence in neutrino research: in two NEUTRINOS TO THE RESCUE? This attribute of neutrinos projects searching for neutrinoless So far, none of the CP-violating may reveal itself in a rare type of double beta decay: SNO+ at the Sudbury phenomena seen is large enough radioactivity called neutrinoless Neutrino Laboratory in Canada, and to explain the disappearance of double beta decay. In normal beta SuperNEMO, located in the Modane antimatter. Physicists are now turning decay, an electron and an antineutrino Underground Laboratory in the French to another particle, the neutrino, are emitted. However, double beta Alps; and in the search for neutrino decay can happen in some , CP-violation at the T2K experiment and it is possible that the two (JPARC facility) in Japan. The UK is also DETECTING ANTIMATTER IN SPACE neutrinos generated could just cancel hosting new technology needed for a No antimatter galaxies have been each other out – if they behave both neutrino factory, called MICE (Muon detected in the universe, but single as particles and antiparticles. A Ionisation Cooling Experiment). antiparticles do rain down on the number of high-precision experiments as cosmic rays, probably produced are searching for this, as yet unseen in high-energy processes such as . those occurring around black holes, in Neutrino CP-violation will also supernovae or the of dark be studied in a future international matter. Several satellites including the neutrino factory producing intense European mission, Pamela (Payload neutrino beams. for Antimatter Exploration and Light- nuclei ) launched in 2006, and the AMS ( Magnetic ) installed on the International Space Station in 2011, Part of the international SuperNEMO experiment have been sifting through cosmic rays to detect neutrinoless double beta decay, being developed at the University of Manchester for antimatter. If any antimatter nuclei, such as nuclei, were detected, it might indicate the existence of bulk antimatter somewhere in the .

5 HARNESSING ANTIMATTER

ntimatter is extremely difficult A to make and store CREATING ANTIMATTER IN THE LABORATORY Top and left: atoms are held in magnetic COLD ANTIHYDROGEN traps at CERN; untrapped atoms are seen here While subatomic antiparticles can be annihilating on the inner of the ALPHA trap generated in high-energy experiments, synthesising even the simplest form of everyday atomic antimatter is a atoms. Using a slightly different trap major challenge. To make antihydrogen configuration and method of detecting (an antiproton bound to a positron) antihydrogen, ATRAP reported equally requires supplies of positrons and impressive results. Soon after, both antiprotons that are moving slowly experiments were making millions of enough to combine into atoms without antihydrogen atoms. annihilating. The first few antihydrogen To study the antihydrogen atoms were not produced until 1995. produced, the atoms needed to be Using LEAR, CERN physicists passed stored. ATHENA was replaced by decelerated antiprotons through a jet ALPHA (Antihydrogen Physics Apparatus) which had a magnetic trap specially designed to confine HYDROGEN ANTIHYDROGEN the neutral atoms at a close to . By 2010, 38 antihydrogen atoms had been stored for one-fifth of a second, with proton antiproton the number increasing to hundreds of atoms for many minutes in the following year. electron positron

of . Some of the antiprotons’ designed for antiatom studies. energy was converted into electron– Two international collaborations, positron pairs – and, occasionally, a ATHENA and ATRAP, employed novel positron formed a transient bond with technology to slow the antiprotons an antiproton. further and capture them in a trap Such a -up was not suitable using magnetic and electric fields. for studying antihydrogen, and in The antiprotons were then carefully 1999, CERN built a new facility, the introduced into a second trap (AD), to containing chilled positrons so that decelerate antiprotons down to a both sets of antiparticles could speed one-tenth that of light. The combine into atoms. In 2002, the low-energy antiprotons could then be ATHENA team announced it had transferred to experimental set-ups detected 50,000 antihydrogen The ALPHA antihydrogen facility

6 Antimatter | A review of its role in the universe and its applications

HOW DO YOU MAKE of experiments (p11). Irradiating a ANTIMATTER PARTICLES? target with a powerful, ultra-intense laser, such as the new European Positrons facility being planned, the Extreme Positrons are produced naturally Light Infrastructure (ELI), offers a in the radioactive decay of some potential route to generating billions of elements. -22 is the electron–positron pairs for antimatter most used to create positron beams. experiments. However, a shortage of sodium-22 has meant that new methods of Antiprotons production are being studied. For The positron beam facility at FRMII, Technical Antiprotons are produced via pair example, bombarding a target with an University of Munich production by firing protons at a target electron beam, or gamma rays from a FRMII research reactor at the Technical made of a heavy metal such as nickel reactor, generates electron–positron University of Munich provides an or . They are then collected pairs that can then be separated. The intense positron beam for a variety using a magnetic .

A device that will transport antiprotons in a new low-energy decelerator, ELENA, to replace the existing AD set-up that serves the ALPHA and THE UK ROLE ASACUSA experiments Mike Charlton a team from Swansea University collaborating in the ALPHA experiment. His team and QUARK ANTIMATTER researchers from Queen’s University High-energy experiments in the US (the Belfast have also been involved in Relativistic Heavy Ion Collider, RHIC, at ASACUSA. the Brookhaven National Laboratory) Researchers at the University and in Europe (the ALICE experiment at of Birmingham are participating in the LHC) are studying collisions of heavy ALICE, and, with STFC Daresbury nuclei in order to simulate conditions in Laboratory in Cheshire, in the the universe when ordinary matter was WHY STUDY ANTIHYDROGEN? experiment at RHIC. first crystallising out of the primordial Comparing the spectra of antihydrogen quark–antiquark soup. Detecting and hydrogen offers a unique probe of particles consisting of quark–antiquark the combined CPT symmetry described pairs that would have been initially on p4. Any violation would mean a generated throws light on the processes drastic revision of fundamental physics ideas. In 2011, the ALPHA team made its first spectroscopic measurements, which are being continued using a laser to excite the atoms. Another CERN experiment called AEGIS is being built to whether the effect of gravity on antihydrogen and hydrogen The ASACUSA facility at CERN is the same, which again will provide insight into describing the fundamental forces and the universe. Part of the STAR detector at the RHIC facility, which The of the AD experiments A further CERN experiment, ASACUSA has detected the generation of antimatter nuclei has spurred on CERN to develop (Atomic and Collisions involved, and how matter came to form a new antiproton source to be Using Slow Antiprotons), makes rather than antimatter. Atomic nuclei ready in 2017. ELENA (Extra Low precision measurements on helium made of antiprotons and Energy Antiproton Ring), will slow atoms in which one of the two orbiting such as antideuterium, antihelium-3 antiprotons to under a one-fiftieth of electrons has been replaced with and antihelium-4, as well as exotic the current energy of the AD, which an antiproton. A small amount of nuclei containing heavy antiquarks, will increase the trapping efficiency antiprotonic helium is created when have also been observed by the STAR by up to 100 . The FAIR facility antiprotons are mixed with helium collaboration at RHIC. Their study probes (p3), which will provide much more gas at very low . Laser the nature of the forces that made our intense antiprotons beams, also plans spectroscopy again is used to provide universe what it is today. to carry out antihydrogen experiments. another test of CPT symmetry.

7 APPLICATIONS

ntimatter provides a A powerful biomedical tool ANTIMATTER PET scans provide effective of the brain and other organs IN THE CLINIC

THE POWER OF IMAGING A positron may combine with an yyPositrons also combine with electrons Antimatter annihilation provides one electron to form positronium. in organic to form of the main imaging techniques used Positronium comes in two forms: positronium states before annihilation. in hospitals today. parapositronium with the positron This is important in influencing the tomography (PET) relies on short-lived and electron having oppositely aligned effectiveness of PET imaging: when positron-emitting isotopes (of , spins, which has a lifetime of 125 the positrons interact with biological , nitrogen and fluorine) which picoseconds; and orthopostronium tissue, the initial formation of act as tracers when incorporated into with spins parallel, which survives positronium increases the positron selected pharmaceuticals introduced for 142 nanoseconds in a vacuum lifetime. into the body. The pairs of gamma (though only about a nanosecond in a yyPositrons are produced in many rays emitted, when the positrons molecular ). astrophysical processes such as annihilate as they come into contact Positronium is interesting for supernovae explosions, and may well with surrounding tissues, are detected several reasons: play a role in mediating interstellar by a position-sensitive detector yyVery cold positronium temporarily via positronium formation. array (largely developed as a spin- forms when positrons are fired at a How positrons and positronium interact off from high-energy physics and silicon surface so that they combine with matter is also of fundamental research). The signals are with electrons in the . interest and is revealed by studying used to construct a series of visual This offers a way of generating the their collisions with single atoms “slices” through the body that are resulting low-energy positrons in and molecules. Recently, researchers then combined into a 3-D image, in a a controlled way for antihydrogen at University College London led by similar fashion to a computerised -ray experiments (p6). Gaetana Laricchia found, somewhat tomography (CT) scan. surprisingly, that positronium scatters The ACE experiment investigating the effectiveness off various atoms and molecules in of antiproton therapy at CERN a similar way to an electron moving at the same speed. As described by theorist, James Walters, at Queen’s University Belfast, the positron appears to be “cloaked”. Although the reason is not fully understood, it does mean that the large library of electron data available can be used in understanding how positronium interacts with matter – ranging from the body to interstellar clouds.

Part of the equipment used at University College London to make and study the behaviour of positronium

8 Antimatter | A review of its role in the universe and its applications

PET scans can follow biological tracers, together with higher-resolution THE UK ROLE processes such as blood flow, detectors, have allowed physiological Several universities are working with metabolism, neuronal transmission, changes to be followed such as those hospitals to improve PET equipment tumour growth and are employed associated with Alzheimer’s disease. and extend applications. These by the pharmaceutical industry to PET is increasingly used to provide include Imperial College London monitor the behaviour of drugs in the rapid and accurate 3-D images of (Hammersmith Hospital), the body. It is particularly successful at tumours for radiotherapy planning. University of Manchester, Cardiff imaging structure and activity in the University and King's College London. brain. Although the images are not as ANTIPROTON CANCER THERAPY Both University College detailed as those from CT or magnetic Targeted proton beams are already London and Swansea University resonance imaging (MRI) scans, employed as a therapeutic tool to have extensive programmes they have the potential to measure kill tumours without damaging the studying positron and biomolecular changes in a quantitative surrounding tissue. They are particularly positronium, while theoretical way. The recent development of new effective for difficult-to-treat head and on positron– scattering and neck tumours. The beam energies positronium formation is also carried of protons and antiprotons can be out at the University of Nottingham adjusted so that they deposit most of and Queen’s University Belfast. their energy over a very short range that matches the of a living cell. However, antiprotons in the cellular matter, so deliver much also annihilate when in more energy over the same range. contact with protons Antiproton therapy is currently being investigated at CERN in the ACE experiment, which has shown that the rate of destruction of cancer cells is four times higher with antiprotons than protons. This means that the radiation dose can be much smaller, and therefore safer and more effective. Antiprotons are, of course, more expensive to produce, but researchers in the US are already working on the development of a portable trap capable of holding and transporting antiprotons, which might make the procedure more economic.

ENERGY FROM ANTIMATTER? Although matter–antimatter The intense gamma-ray beam from annihilation generates large amounts a positronium laser could also initiate of energy, it is unlikely that it would nuclear reactions such as fusion. ever be a practical source of power Theoretically, this could be achieved if (or used for making bombs), simply a cloud of positronium atoms held at because antiprotons, or positrons, room temperature could form what is are so difficult and expensive to known as a Bose–Einstein condensate make (p8). With today’s technology, (BEC), in which all the atoms fall into it is impossible to synthesise large the same quantum energy state and amounts of antimatter: the CERN behave as a single entity. The positrons accelerator would have to run for would then collectively annihilate to 100 billion years to produce a single emit a coherent beam of gamma rays. gramme. Nevertheless, an antiproton One way to achieve this is to trap beam could, in theory, start a nuclear the positrons in a of porous reaction that is useful for energy silica, as explained on p8. Researchers generation. Such antiproton drivers in the US have shown that double have been suggested for inertial positronium “molecules” can form, confinement fusion. which is the first step towards a BEC.

9 APPLICATIONS

ntiparticles interact with matter in unique A ways to provide a useful analytical tool for materials and technology ANTIMATTER PARTICLES AS PROBES Details of the physical and chemical nature of the of materials and thin films are revealed through precision measurements of ANALYSIS WITH POSITRONS the gamma-ray pairs emitted when Positron emission tomography is positrons annihilate with electrons in employed to image materials and samples, sometimes first binding with processes of industrial importance, them to form positronium (p8) – this such as the flow of oil or water is positron annihilation spectroscopy through rocks, lubricants in engines, (PAS). Using low-energy positron and radioactive waste in nuclear beams, the technique is ideal for Positron emission can be used as a tracer technique storage materials. A new variant of studying surfaces, near-surface layers, to track fast moving particles such as detergent the technique developed by David and thin films. PAS encompasses powders in washing and dishwashers Parker at the , positron emission particle tracking Paul Coleman/University of Bath (PEPT), can track tracer particles accurately in fast-flowing granular materials as they mix. His team has positronium decays via developed a portable PEPT gamma three gamma rays camera that enables them to carry out studies on working commercial such as fluidised beds (where positron pressurised are forced through a decays via vessel containing particulate material) two gamma rays used in coal gasification, chemical processing and food manufacture. three types of measurement. The most The lifetimes of positrons and positronium trapped used method is the measurement in nanoporous materials reveal information about their structure of the Doppler broadening of the gamma-ray energy spectrum, which and follow changes with temperature reflects the change in momentum and over time. Paul Coleman at the of the electrons when bound to University of Bath is currently using this positrons. This in turn depends upon method to study the behaviour of ice the immediate atomic – films, which is important for a range indicating, for example, whether there of astrophysical processes, as well as are structural defects. The method can the of biological tissues. He is probe the presence of missing atoms also developing a variant of PAS using and pores in the structure of polarised positrons (with spins all Researchers at the University of Bath working and other , pointing in the same direction) to study with positron beams

10 Antimatter | A review of its role in the universe and its applications Ashraf Alam/University of Bristol of designing tailor-made for drugs and nutrients. They also utilise the lifetime technique to relate the physical characteristics of –nano- composites to these open volumes.

ANALYSIS WITH ANTIMUONS The heavier version of the positron, the positively charged antimuon, is also extensively employed as an analytical probe. Positive readily implant themselves in solids and , and combine with electrons to form , which behaves like a lightweight . Like protons, muons have a spin, so behave like tiny .

THE UK ROLE uses positron lifetime spectroscopy Analysis using positrons is carried Left and right: antimuons implanted into C60 form a hydrogen-atom-like state in the cage-like (using the positron beam at the FRMII out at the Universities of Birmingham, structure of the molecules, as revealed in muon spin Munich Research Reactor, p7) to study Bath, Bristol and Dundee, and experiments at the UK facility, ISIS complex oxides with technologically Imperial College London. thin magnetic films relevant to the next important properties such as The ISIS pulsed muon facility generation of magnetic devices. ferroelectricity, at the STFC Rutherford Appleton Measuring the variation in angles and magnetoresistance, as well as Laboratory near Oxford is the UK between the emitted gamma rays semiconductor materials used in solar centre for μSR experiments. Muon provides even more precise information cells and other electronic​ devices. beams are generated from an about the electronic structure of a The Bristol employs lifetime energetic proton beam when it material. Ashraf Alam, Stephen Dugdale measurements to probe subnanometre- collides with a target to generate and colleagues at the University scale open volumes in biopolymers, intermediate particles called . of Bristol employ this approach to with the aim – in conjunction with the These quickly decay to muons. explore the fundamental properties of pharmaceutical and food industries – electronic and magnetic compounds of technological interest such as high- Once implanted in a sample, the Ashraf Alam/University of Bristol temperature superconductors and magnetic environment around the shape-memory alloys. muon affects its spin direction in a Finally, the lifetime of the way that provides information about implanted positrons/positronium the local chemical and magnetic atoms depends on the density of behaviour. The technique, muon spin electrons at the annihilation site and rotation (μSR), relies on polarised gives information about the size and muons and is analogous to the well- nature of open defects in materials. In known analytical method of nuclear condensed matter, atomic vacancies magnetic resonance (NMR). Muons normally trap positrons, but there is not survive for just over 2 microseconds enough space for positronium formation, before they decay into positrons that and the positrons annihilate directly are then detected. The technique is with the electrons in the material. The ideal for studying the role of hydrogen lifetimes are therefore short, typically The electronic structures of technologically important in semiconductors, exotic materials can be constructed with the aid of positron less than 500 picoseconds. David annihilation spectroscopy molecular magnets, superconductors, Keeble at the University of Dundee and many other electronic materials.

11 The IOP thanks the researchers who helped with the preparation of this booklet.

Not all UK research on or using antimatter could be mentioned.

Further information on antimatter 1. Antimatter, Frank Close, OUP, Oxford, 2009 2. Antimatter: Mirror of the Universe – LIVEfromCERN http://livefromcern.web.cern.ch/livefromcern/antimatter 3. In London, King's College, Imperial College, University College and the Medical Research Council (MRC) have formed a PET imaging consortium, Imanova Ltd (www.imanova.co.uk), based at the Hammersmith Hospital where most PET research is now concentrated. 4. http://en.wikipedia.org/wiki/Antimatter

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This document is also available to download as a PDF from our website. h2o-creative The RNIB clear print guidelines have been considered in the production of Front cover image this document. Clear print is a design approach that considers the needs The distribution of galaxies across the of people with sight problems. For more information, visit www.rnib.org.uk. universe from the Sloan Digital Sky Survey