ANTIMATTER (part 5/5)

Antimatter for daily use

    Antimatter - a mirror image of matter - is an idea so revolutionary that even its discoverer initially feared its consequences. It annihilates with ordinary matter, disappearing in a puff of energy - the ultimate scientific experiment.

    This annihilation is a compelling scenario for science fiction. The first example was robots with brains having antimatter pathways.

    Transforming all its mass into pure energy, antimatter is the perfect fuel. Star Trek's faster-than-light science-fiction spaceships use antimatter power, but research projects have also investigated the use of antimatter fuel for real.

   

    Medical

    Now antimatter is used every day in medicine for brain scans.

    Antimatter-matter reactions have practical applications in medical imaging, such as positron emission tomography (PET). In positive beta decay, a nuclide loses surplus positive charge by emitting a positron (in the same event, a proton becomes a neutron, and neutrinos are also given off). Nuclides with surplus positive charge are easily made in a cyclotron and are widely generated for medical use. Particle physicists regularly use collisions between electrons and their antiparticles, positrons, to investigate matter and fundamental forces at high energies.

    When electron and positron meet, they annihilate, turning into energy which, at high energies, can rematerialise as new particles and antiparticles. This is what happens at machines such as the Large Electron Positron (LEP) collider at CERN.

    At low energies, however, the electron-positron annihilations can be put to different uses, for example to reveal the workings of the brain in the technique called Positron Emission Tomography (PET).

    In PET, the positrons come from the decay of radioactive nuclei incorporated in a special fluid injected into the patient. The positrons then annihilate with electrons in nearby atoms. As the electron and positron are almost at rest when they annihilate, there is not enough annihilation energy to make even the lightest particle and antiparticle (the electron and the positron), so the energy emerges as two gamma-rays which shoot off in opposite directions to conserve momentum.

   

    Fuel

    In antimatter-matter collisions resulting in photon emission, the entire rest mass of the particles is converted to kinetic energy. The energy per unit mass (9×1016 J/kg) is about 10 orders of magnitude greater than chemical energy (compared to TNT at 4.2×106 J/kg, and formation of water at 1.56×107 J/kg), about 4 orders of magnitude greater than nuclear energy that can be liberated today using nuclear fission (about 200 MeV per atomic nucleus that undergoes nuclear fission, or 8×1013 J/kg), and about 2 orders of magnitude greater than the best possible from fusion (about 6.3×1014 J/kg for the proton-proton chain). The reaction of 1 kg of antimatter with 1 kg of matter would produce 1.8×1017 J (180 petajoules) of energy (by the mass-energy equivalence formula E = mc²), or the rough equivalent of 43 megatons of TNT. For comparison, Tsar Bomba, the largest nuclear weapon ever detonated, reacted an estimated yield of 50 megatons, which required the use of hundreds of kilograms of fissile material (Uranium/Plutonium).

    Not all of that energy can be utilized by any realistic propulsion technology, because as much as 50% of energy produced in reactions between nucleons and antinucleons is carried away by neutrinos in these applications, so, for all intents and purposes, it can be considered lost.

    Antimatter rocketry ideas, such as the redshift rocket, propose the use of antimatter as fuel for interplanetary travel or possibly interstellar travel. Since the energy density of antimatter is vastly higher than conventional fuels, the thrust to weight equation for such craft would be very different from conventional spacecraft.

    The scarcity of antimatter means that it is not readily available to be used as fuel, although it could be used in antimatter catalyzed nuclear pulse propulsion for space applications. Generating a single antiproton is immensely difficult and requires particle accelerators and vast amounts of energy — millions of times more than is released after it is annihilated with ordinary matter due to inefficiencies in the process. Known methods of producing antimatter from energy also produce an equal amount of normal matter, so the theoretical limit is that half of the input energy is converted to antimatter. Counterbalancing this, when antimatter annihilates with ordinary matter, energy equal to twice the mass of the antimatter is liberated — so energy storage in the form of antimatter could (in theory) be 100% efficient.

    For more regular (earthly) applications however (e.g. regular transport, use in portable generators, powering of cities, ...), artificially created antimatter is not a suitable energy carrier, despite its high energy density, because the process of creating antimatter involves a large amount of wasted energy and is extremely inefficient. According to CERN, only one part in ten billion (10−10) of the energy invested in the production of antimatter particles can be subsequently retrieved.

    Antimatter production is currently very limited, but has been growing at a nearly geometric rate since the discovery of the first antiproton in 1955 by Segrè and Chamberlain. The current antimatter production rate is between 1 and 10 nanograms per year, and this is expected to increase to between 3 and 30 nanograms per year by 2015 or 2020 with new superconducting linear accelerator facilities at CERN and Fermilab.

    Some researchers claim that with current technology, it is possible to obtain antimatter for US$25 million per gram by optimizing the collision and collection parameters (given current electricity generation costs). Antimatter production costs, in mass production, are almost linearly tied in with electricity costs, so economical pure-antimatter thrust applications are unlikely to come online without the advent of such technologies as deuterium-tritium fusion power (assuming that such a power source actually would prove to be cheap).

    Many experts, however, dispute these claims as being far too optimistic by many orders of magnitude. They point out that in 2004, the annual production of antiprotons at CERN was several picograms at a cost of $20 million. This means to produce 1 gram of antimatter, CERN would need to spend 100 quadrillion dollars and run the antimatter factory for 100 billion years.

    Storage is another problem, as antiprotons are negatively charged and repel against each other, so that they cannot be concentrated in a small volume. Plasma oscillations in the charged cloud of antiprotons can cause instabilities that drive antiprotons out of the storage trap. For these reasons, to date only a few million antiprotons have been stored simultaneously in a magnetic trap, which corresponds to much less than a femtogram. Antihydrogen atoms or molecules are neutral so in principle they do not suffer the plasma problems of antiprotons described above. But cold antihydrogen is far more difficult to produce than antiprotons, and so far not a single antihydrogen atom has been trapped in a magnetic field.

    One researcher of the CERN laboratories, which produces antimatter regularly, said: If we could assemble all of the antimatter we've ever made at CERN and annihilate it with matter, we would have enough energy to light a single electric light bulb for a few minutes.