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Antimatter Discovered - New Fuel Source


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Antimatter, which annihilates matter upon contact, seems to be rare in the universe. Still, for decades, scientists had clues that a vast cloud of antimatter lurked in space, but they did not know where it came from.


"It was quite a surprise back then to discover part of the universe was made of antimatter," researcher Gerry Skinner, an astrophysicist at Goddard Space Flight Center in Greenbelt, Md., told SPACE.com.


Science Fiction Shapes the Future


One of the best attempts for the public to understand this concept was Star Trek's "Mirror Universe" was first recorded as visited by James T. Kirk and several officers from the USS Enterprise in 2267. This parallel universe coexists with our universe on another dimensional plane. The universe

was so named because many people and places seemed to be the exact opposites of their "normal" selves in "our" universe, but with numerous "good" aspects now "evil", and vice versa.


Mirror-universe Spock



While antimatter propulsion systems are so far the stuff of science fiction, antimatter is very real.


The mysterious source of this antimatter has now been discovered "stars getting ripped apart" by neutron stars and black holes.


CERN, the European Organization for Nuclear Research, is one of the world's largest and most respected centres for scientific research. Its business is fundamental physics, finding out what the Universe is made of and how it works. At CERN, the world's largest and most complex scientific instruments are used to study the basic constituents of matter - the fundamental particles. By studying what happens when these particles collide, physicists learn about the laws of Nature.




The Large Hadron Collider (LHC) is a gigantic scientific instrument near Geneva, where it spans the border between Switzerland and France about 100 m underground. It is a particle accelerator used by physicists to study the smallest known particles – the fundamental building blocks of all things. It will revolutionise our understanding, from the miniscule world deep within atoms to the vastness of the Universe.


Within the LHC, the world's largest particle collider, protons do not race around the 27 km ring at near the speed of light of their own accord. Instead they are guided by huge 1700 magnets have been connected together, implying a total of about 40 000 leak-tight welds (1 0km worth!) and 65 000 electrical 'splices' of superconducting cables through eight separate sectors.


Each of these sectors must be sealed in a vacuum and cooled to a freezing '271.3°C, just 1.9 degrees above absolute zero. In fact the whole ring consists of a wealth of electrical lines and piping forming an intricate system running the full 27 km of the LHC, which must connected before the beam can run.


Two beams of subatomic particles called 'hadrons' - either protons or lead ions - will travel in opposite directions inside the circular accelerator, gaining energy with every lap. Physicists will use the LHC to recreate the conditions just after the Big Bang, by colliding the two beams head-on at very high energy. Teams of physicists from around the world will analyse the particles created in the collisions using special detectors in a number of experiments dedicated to the LHC.


What Antimatter



All elementary particles, such as protons and electrons, have antimatter counterparts with the same mass but the opposite charge. For instance, the antimatter opposite of an electron, known as a positron, is positively charged.


Antimatter is the extension of the concept of the antiparticle to matter, whereby antimatter is composed of antiparticles in the same way that normal matter is composed of particles. For example an antielectron (a positron, an electron with a positive charge) and an antiproton (a proton with a negative charge) could form an antihydrogen atom in the same way that an electron and a proton form a normal matter hydrogen atom. Furthermore, mixing of matter and antimatter would lead to the annihilation of both in the same way that mixing of antiparticles and particles does, thus giving rise to high-energy photons (gamma rays) or other particle-antiparticle pairs. The particles resulting from matter-antimatter annihilation are endowed with energy equal to the difference between the rest mass of the products of the annihilation and the rest mass of the original matter-antimatter pair, which is often quite large.


There is considerable speculation both in science and science fiction as to why the observable universe is apparently almost entirely matter, whether other places are almost entirely antimatter instead, and what might be possible if antimatter could be harnessed, but at this time the apparent asymmetry of matter and antimatter in the visible universe is one of the greatest unsolved problems in physics.


When a particle meets its antiparticle, they destroy each other, releasing a burst of energy such as gamma rays. In 1978, gamma ray detectors flown on balloons detected a type of gamma ray emerging from space that is known to be emitted when electrons collide with positrons — meaning there was antimatter in space.


What exactly generated the antimatter was a mystery for the following decades. Suspects have included everything from exploding stars to dark matter.


The debut of antihydrogen


An atom of antihydrogen consists of an antiproton and a positron (an antielectron), which makes it the simplest antiatom. Unfortunately, this does not make it any easier to produce in the lab. Persuading the antiprotons and positrons to combine together was a challenge that no one had managed to solve until the PS210 at CERN created the first atoms of antihydrogen in 1995.


It was a difficult task both for the physicists and for the operation team at CERN's Low Energy Antiproton Ring (LEAR) - the main machine used for the experiment. The researchers allowed antiprotons circulating inside LEAR to collide with atoms of a heavy element. Any antiprotons passing close enough to a heavy atomic nucleus could create an electron-positron pair; in a tiny fraction of cases, the antiproton would bind with the positron to make an atom of antihydrogen.


The process was complicated, time-consuming and required a lot of effort but it led to a ground-breaking achievement. When the announcement of the production of 9 antiatoms at CERN was made early in 1996, the news travelled around the world to be reported in newspapers, on radio and on television.


However, the fleeting existence of the antiatoms meant that they could not be used for further studies. Each one existed for only about 40 billionths of a second, travelling at nearly the speed of light over a path of 10 m before it annihilated with ordinary matter.



Artificial production


The artificial production of atoms of antimatter (specifically antihydrogen) first became a reality in the early 1990s. An atom of antihydrogen comprises a negatively-charged antiproton being orbited by a positively-charged positron. Stanley Brodsky, Ivan Schmidt and Charles Munger at SLAC realized that an antiproton, traveling at relativistic speeds and passing close to the nucleus of an atom, would have the potential to force the creation of an electron-positron pair. It was postulated that under this scenario the antiproton would have a small chance of pairing with the positron (ejecting the electron) to form an antihydrogen atom.


In 1995 CERN announced that it had successfully created nine antihydrogen atoms by implementing the SLAC/Fermilab concept during the PS210 experiment. The experiment was performed using the Low Energy Antiproton Ring (LEAR), and was led by Walter Oelert and Mario Macri. Fermilab soon confirmed the CERN findings by producing approximately 100 antihydrogen atoms at their facilities.


Now, an international research team looking over four years of data from the European Space Agency's International Gamma Ray Astrophysics Laboratory (INTEGRAL) satellite has pinpointed the apparent culprits. Their new findings suggest these positrons originate mainly from stars getting devoured by black holes and neutron stars.


As a black hole or neutron star destroys a star, tremendous amounts of radiation are released. Just as electrons and positrons emit the tell-tale gamma rays upon annihilation, so too can gamma rays combine to form electrons and positrons, providing the mechanism for the creation of the antimatter cloud, scientists think.




Integral stands for the International Gamma-Ray Astrophysics Laboratory.


Description Integral is the first space observatory that can simultaneously observe objects in gamma rays, X-rays, and visible light. Its principal targets are violent explosions known as gamma-ray bursts, powerful phenomena such as supernova explosions, and regions in the Universe thought to contain black holes.


Launch 17 October 2002 (Proton launcher from Baikonur, Kazakhstan).


Status: In operation.


Journey Integral circles the Earth in a highly elliptical orbit once every three days. It spends most of its time at an altitude higher than 60 000 kilometres - well outside the Earth's radiation belts. It does this to avoid the background radiation effects which would interfere with the measurement of gamma rays.


Notes Integral is the most sensitive, accurate, and advanced gamma-ray observatory ever launched.




The SPI instrument on board ESA's Integral has performed a search for 511 keV emission (resulting from positron-electron annihilation) all over the sky. The figure represents the results of this search: the all-sky map in galactic co-ordinates shows that 511 keV emission is - so far - only seen towards the center of our Galaxy. The SPI data are equally compatible with galactic bulge or halo distributions, the combination of a bulge and a disk component, or a combination of a number of point sources. Such distributions are expected if positrons originate either from low-mass X-ray binaries, novae, Type Ia supernovae, or possibly light 'dark matter'.


Billions and billions


The researchers calculate that a relatively ordinary star getting torn apart by a black hole or neutron star orbiting around it - a so-called "low mass X-ray binary" - could spew on the order of one hundred thousand billion billion billion billion positrons (a 1 followed by 41 zeroes) per second. These could account for a great deal of the antimatter that scientists have inferred, reducing or potentially eliminating the need for exotic explanations such as ones involving dark matter.


"Simple estimates suggest that about half and possibly all the antimatter is coming from X-ray binaries," said researcher Georg Weidenspointner of the Max Planck Institute for Extraterrestrial Physics in Germany.


The Gamma-ray Large Area Space Telescope (GLAST) will open this high-energy world to exploration and help us to answer these questions. With GLAST, astronomers will at long last have a superior tool to study how black holes, notorious for pulling matter in, can accelerate jets of gas outward at fantastic speeds. Physicists will be able to study subatomic particles at energies far greater than those seen in ground-based particle accelerators. And cosmologists will gain valuable information about the birth and early evolution of the Universe.


GLAST will carry two instruments, the Large Area Telescope (LAT) and the GLAST Burst Monitor (GBM), to study the extreme universe, where nature harnesses energies far beyond anything scientists can achieve in their most elaborate experiments on Earth.



Roland Diehl, Garching/Germany, March 2001


"Radiation" in common language often describes an effect which pervades our environment on straight paths with high penetration power; often exposure to radiation carries a conotation of "danger", be it "cosmic radiation", solar UV radiation, or mobilephone radio waves. We know of two types of radiation: corpuscular and electromagnetic radiation.


"Cosmic rays" as the example of the former are predominantly very energetic small particles pervading the Galaxy in the empty space between stars.


"Electromagnetic radiation" on the other hand consists of propagating fluctuations inelectrical and magnetic fields. Gamma-rays are electromagnetic radiation. The electromagnetic spectrum spans more than 20 decades of frequency.


When we study the interaction of material bodies with electromagnetic radiation, the relative dimensions of the two interacting partners play a leading role: For an electromagnetic wave, the characteristic dimension is its wavelength; for material bodies, it may be their geometrical size. Thin films of oil have thicknesses comparable to the wavelength of visible light, hence small variations in thickness of the oil layer strongly prefer reflection or absorption of light with correspondingly small wavelength differences, or slightly different colors. Therefore we can see nice colors of the oil film, caused by the differences in interference patterns of light waves of different wavelengths with this oil film.


Now for X-rays or gamma-rays, the wavelength of the radiation is short compared to the spacing of the atoms in the material, hence the

radiation mainly sees the atom's components, the compact nucleus and the electrons orbiting the nucleus far out, like planets orbit the sun. The atom thus mostly is "empty space" for an X or gamma-ray. An iteraction of a gamma-ray with matter will not occur as an ensemble of photons with the ensemble of matter, but rather in an individual and specific interaction with matter small-scale components, the electrons and nuclei.


This resembles a collisional process, it becomes more appropriate to view X and gammarays as energetic particles, their characteristic size being their wavelength. So we can understand (see below) that materials which should act as mirrors for X and gammarays need very special geometries of their fundamental building blocks to operate at all, and will nevertheless cease to function below some critical wavelength.


In such a particle picture, it becomes more appropriate to use the energy of the photon as its characteristic quantity, rather than its wavelength or frequency (reasons will become clear below, from the interaction of photons with matter and fields). Energy is measured

n voltage units for electromagnetic particles (photons), one million electron volts (1 MeV) being the energy an electron would have if accelerated by a voltage of a million volts. The gamma-ray domain spans the energy regime above about 0.1 MeV, extending at least over five orders of magnitude in frequency / wavelength / energy. (For comparison, our eyes see the optical regime from about 0.4-0.7 micrometers in

wavelength, from colors blue to red - a fairly modest dynamical range.)


Electromagnetic radiation is distinguished among astrophysicists as either "thermal" radiation, or due to other special processes that are summarized as "non-thermal". Thermal radiation is characterized by a temperature, and the spectrum of radiation intensity (or relative number of photons) versus frequency follows the "black-body" distribution. This distribution results from the population of the states of the radiation field, as derived by Max Planck and Ludwig Boltzmann early this century: the interaction of the radiating material and the radiation is so intense that the energy density of both

are identical, equilibrium is obtained, and "temperature" is a unique and characteristic parameter.


For increasing temperature, the black body distribution shifts its median towards more energetic radiation, so that each of the photons carries more energy, again balancing the increased energy of a hotter radiator. This shift is called Wien's law, the product of temperature and the wavelength of the peak of the radiation spectrum is a constant.


Thermal gamma-rays of 1 MeV correspond to a temperature of above two billion degrees, a truly hot fireball! Note that nuclear fusion inside the sun occurs at 15 million degrees, so gamma-ray fireballs must be even much hotter. Clearly we cannot expect such energetic fireballs to withstand explosion. Therefore we expect to study violent explosions of fireballs through thermal gamma-rays.


Atomic nuclei: The nuclear (or strong) force holds together protons and neutrons in a compact structure called the atomic nucleus. The strong force

outweighs the electric repulsion of all those positively charged protons at small distance, but the compact assembly of nucleons requires very specific,

quantized, states of energy for the nucleons.


Now we just need something which disturbs the state of a nucleus: An energetic collision at kinetic energies of MeV or more will just do that. This is the case when cosmic rays hit interstellar gas in our Galaxy.


Another possibility for nuclear state disturbance is radioactive decay of an atomic nucleus. In such events, one of the particles of the nucleus transforms into one of another kind as a consequence of the 'weak force'; neutrons decay into protons ('β--decay'), or protons can transform into neutrons ('β+-decay'). This modifies the quantum state in this tight arrangement of nucleons, and new, stable arrangements

have to be sought, through emission of the energy difference, often as a gamma-ray photon. Reversing this insight, the observation of characteristic

gamma-ray lines thus tell us about nuclear transitions in a source region, hence about collisions or radioactivity.


Decay of pions is a similar process. The pion ('Pi-meson') is an elementary particle which participates in the strong nuclear interaction, similar to protons or neutrons inside an atomic nucleus (we view pions as the carriers of strong interactions, in analogy to the photon carrying electromagnetic interactions). Pions are released during strong-interaction events like collisions of energetic protons with nuclei.


Observation of a pion-decay peak in a gamma-ray spectrum thus testifies energetic collisions between protons, used in the study of cosmic proton accelerators.


Pairs of particle and anti-particle: According to the equivalence of mass and energy discovered by Einstein early this century, energy may be converted into pairs of particle and anti-particle in the presence of a strong field. (Actually, one does not need an additional field for this conversion to occur, but the inverse process occurs so rapidly that there is no observable net particle production in the absence of fields). The lightest particle-antiparticle pair that can be generated is the electron and its antiparticle, the positron: it requires an avaliable energy of 1.022 MeV.


The inverse process, when a particle encounters its anti-particle, is called 'annihilation': The mass of both particles is transformed into radiation-field energy. Momentum conservation demands that an electron-positron encounter will produce two photons, and each of those photons obtains an energy of 0.511 MeV in the rest mass system of the annihilation process.


Annihilation photons thus are commonly produced in environments where anti-electrons are generated. These can be regions of high energy density and strong fields, as described above, but also radioactive decay processes involving 'beta-(β)-decays' from weak interaction.


Charged particles caught in strong magnetic fields: Similar to the electrical (or Coulomb) force which holds electrons close to a nucleus in the quantized

system of an atom, strong magnetic fields can force electrons into orbits around their field lines. Magnetic fields close to neutron stars can be

sufficiently strong such that the quantized energy levels involve steps between such allowed electron-orbit levels in the range of tens of keV, the 'low gammaray regime' (or 'hard X-ray regime').


Most of us are not familiar with these processes which produce gamma-rays; they even are not common in the universe, and go along with rather exotic conditions. In most cases, violent forces are at play. Observation of gamma-rays enables us to study such exceptional places in nature, which we cannot mimic in our laboratories on Earth.


Thermal radiation from a cosmic 'gamma-ray fireball' constitutes probably the most violent site one can imagine. Explosions of stars through supernovae in principle come close to this extreme. In those events, huge amounts of energy are released within short times (fractions of seconds), and thus may generate such extreme heat. For core collapse supernovae, the energy originates from the collapse of a star when nuclear

burning in its core starved from fuel exhaustion, and no internal energy source can counterbalance the gravitational pressure of the overlying mass of the stellar gas. In the case of thermonuclear supernovae, accumulated nuclear fuel ignites on the surface of aperfectly heat-conducting compact remnant of a star (a 'white dwarf'), and causes the entire remnant to incinerate nuclear burning, consuming all the fuel in an instant due to

the high compactness and heat conduction.


Temperatures in those supernova conditions exceed several billion degrees, and cause atomic nuclei to melt and rearrange upon cooling down, with

radioactivities as by products. Less extreme nuclear burning occurs in nova events, when the igniton of accumulated hydrogen fuel proceeds more slowly into a nuclear surface flame on the compact remnant.


Temperatures in this case are below billion degrees, yet sufficient to generate radioactivities among the light elements which can undergo nuclear burning. The cores of these explosive events cannot be studied in gamma-rays however, because the hot inner regions of the event is hidden

behind large amounts of overlying envelopes, thick enough to even occult gamma-rays.


This occultation is the reason why the nuclear burning inside stars like our sun proceeds without associated gamma-ray luminosity of the star. For very massive stars 20 or more times as massive as the sun this is not necessarily so: Their atmosphere is more violently mixed, and strong stellar winds blow of large parts of the envelopes. Radioactive products generated in all those objects have in some cases sufficiently long decay times to produce their characteristic gamma-rays only after the explosion of the event or the stellar wind has sufficiently diluted the material.


In November of 2007 Scientists of the Pierre Auger Collaboration announced that active galactic nuclei are the most likely candidate for the source of the highest-energy cosmic rays that hit Earth. Using the Pierre Auger Observatory in Argentina, the largest cosmic-ray observatory in the world, a team of scientists from 17 countries found that the sources of the highest-energy particles are not distributed uniformly across the sky. Instead, the Auger results link the origins of these mysterious particles to the locations of nearby galaxies that have active nuclei in their centers. The results appearred in the Nov. 9 issue of the journal Science.


Active Galactic Nuclei (AGN) are thought to be powered by supermassive black holes that are devouring large amounts of matter. They have long been considered sites where high-energy particle production might take place. They swallow gas, dust and other matter from their host galaxies and spew out particles and energy. While most galaxies have black holes at their center, only a fraction of all galaxies have an AGN.


There have been conjectures about its possible link with the production of high energy particles. Scientists think that most of the galaxies present black holes in the centre, with a mass of between one million and thousand million times the solar mass. The one of the Milky Way, our galaxy, has about 3 million solar masses. Galaxies with an active nucleus seem to be those which have suffered any collision with another galaxy or any important disturbance in the last hundred million years. The AGN capture the mass that falls in their gravity field releasing prodigious amounts of energy in particle jets. Auger’s result shows that AGN can produce the most energetic particles in the Universe.


Antimatter Cloud Discovered


Data from the European Space Agency's "Integral" satellite indicated that the cloud's distribution is similar to that of a group of binary star systems containing black holes or neutron stars. From NASA's article:


"The cloud itself is roughly 10,000 light-years across, and generates the energy of about 10,000 Suns. The cloud shines brightly in gamma rays due to a reaction governed by Einstein's famous equation E=mc^2. Integral found that the cloud extends farther on the western side of the galactic center than it does on the eastern side. Integral found certain types of binary systems near the galactic center are also skewed to the west. Because the two "pictures" of antimatter and hard low-mass X-ray binaries line up strongly suggests the binaries are producing significant amounts of positrons."


Georg Weidenspointner at the Max Planck Institute for Extraterrestrial Physics and an international team of astronomers made the discovery using four-years-worth of data from Integral. The cloud shows up because of the gamma rays it emits when individual particles of antimatter, in this case positrons, encounter electrons, their normal matter counterpart, and annihilate one another.


One signature of positron-electron annihilation is gamma rays carrying 511 thousand electron-volts (keV) of energy. There has been a vigorous debate about the origin of these positrons ever since the discovery of the 511 keV emission from the centre of the galaxy by gamma-ray detectors flown on balloons during the 1970s.


Some astronomers have suggested that exploding stars could produce the positrons. This is because radioactive nuclear elements are formed in the giant outbursts of energy, and some of these decay by releasing positrons. However, it is unclear whether these positrons can escape from the stellar debris in sufficient quantity to explain the size of the observed cloud.


Integral is currently the only mission that can see both the 511 keV radiation and the hard LMXBs. Weidenspointner and colleagues will be watching keenly to see whether it discovers more LMXBs and, if so, where they are located.


“The link between LMXBs and the antimatter is not yet proven but it is a consistent story,” says Weidenspointner. It has real astrophysical importance because it decreases the need for dark matter at the centre of our galaxy.


Notes for editors:


‘An asymmetric distribution of positrons in the galactic disk revealed by gamma rays’ by Georg Weidenspointner et al. is being published on 10 January, in the journal Nature.


For more information:


Georg Weidenspointner, Max Planck Institute for Extraterrestrial Physics

Email: Georg.Weidenspointner @ hll.mpg.de



The catch?


Right now it would cost about One-Hundred-Billion dollars to create one milligram of antimatter. One milligram is way beyond what is needed for research purposes, but that amount would be needed for large scale applications. To be commercially viable, this price would have to drop by about a factor of Ten-Thousand.


And what about using antimatter for power generation? - not promising.


It costs far more energy to create antimatter than the energy one could get back from an antimatter reaction. Right now standard nuclear reactors, which take advantage of the decay of radioactive substances, are far more promising as power generating technology than antimatter. Something to keep in mind, too, is that antimatter reactions - where antimatter and normal matter collide and release energy, require the same safety precautions as needed with nuclear reactions.



Now that they have witnessed the death of antimatter, the scientists hope to see its birth.



"It would be interesting if black holes produced more matter than neutron stars, or vice versa, although it's too early to say one way or the other right now," Skinner explained. "It can be surprisingly hard to tell the difference between an X-ray binaries that hold black holes and neutron stars."



Weidenspointner, Skinner and their colleagues, detailed their findings in the Jan. 10 issue of the journal Nature.

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