Particle Accelerators Used in the Production of Antimatter.
The energy stored in antimatter is liberated by annihilation with matter; the mass equivalent of energy is usually released in the form of other particles and radiation – generally gamma rays. Three main antimatter particles would be used for fuel, if a substantial amount could be created; these are positrons, antiprotons and antihydrogen –antihydrogen must be created from positrons and antiprotons.
NASA is particularly interested in antimatter as a fuel for space travel; according to NASA, for a mission to Mars, only 10 milligrams of positrons (about 1 x 1025 positrons) would be required to fuel the spaceship at a cost of about US$250 million using current technologies. Whereas using chemical rocket fuel, about 4.06 x 106 kg, would cost about US$90 billion. As well as the improved cost, mass would no longer be a constraint on the spacecraft which would allow for effective interplanetary exploration. The problem is being able to create such an amount of antimatter, as presently the yearly production is between 1 and 10 nanograms.
Currently antimatter is a component of some particle accelerator experiments, but none have the dedicated purpose of producing antimatter, there is great interest in modifying accelerators that are currently in use to produce vast quantities of antimatter.
Technanogy is presently working with Fermilab in the hope to increase the production and efficiency of antimatter using the Tevatron collider. Gabrielse is at present combining its efforts at CERN to produce antihydrogen. There are plans to build a FLAIR at the GSI accelerator in Germany which could produce up to 1012 low-energy antiprotons a year.
Using accelerators is the most successful method of producing antimatter, alternate methods include that by HiPAT, which is attempting to capture antimatter within the interstellar medium.
Accelerators create antimatter for use in reactions with ordinary matter; the main objective is to discover/study elementary particles from the resulting annihilation.
Accelerators are not needed to create positrons as they can be collected directly from specific radioactive isotopes, or a better yielding method is by firing a laser at a very small sample of gold, this process can produce hundreds of billions of positrons.
The main need for accelerators is in the production of antiprotons, the protons used to create antiprotons have to reach energies of up to 120GeV, and so are highly relativistic. The two requirements that the accelerator must adhere to are to reach the required energy, and produce a beam current that is as large as possible.
There are no current accelerator designs that satisfy both of these requirements that are realistic in size and expense, for example a linear accelerator that could reach these energies would consist of around 8,000 drift tubes. So an Alternating Gradient focusing synchrotron is used, which uses alternating focusing and defocusing fields that circulate the accelerator ring, this accelerator can reach the required energies, but does not provide the required continuous, high current beam of particles that would most effectively yield the largest amount of antiparticles.
There are only two accelerators currently in use that produce antiprotons; the Tevatron synchrotron at Fermilab and the Super Proton Synchrotron (SPS) at CERN. The antiprotons are created in a subsection of the main synchrotron: at CERN – the Proton Synchrotron (PS) is used to collide protons into an Iridium rod, and at Fermilab – the Antiproton Source (AS) is used to collide protons into a Nickel plate. The antiprotons created are then accelerated to an energy of 120GeV.
Unfortunately the production process is very inefficient with 1 in 1010 collisions producing an antiproton.
The storage volume for antiprotons is very small; the allowed number of particles per unit volume can be calculated using the following formula:
n0 = B2 / 2 μ0 mc2 (1)
For a 25T magnetic field, 2 x 1010 m-3 antiprotons can be contained, but the maximum achieved so far has only been about 104 m-3, so a neutral form of antimatter – antihydrogen must be used.
Antihydrogen as an antiparticle of the smallest element would yield more energy as a fuel than positrons or antiprotons due to its greater mass.
Antihydrogen is created at CERN by introducing positrons and antiprotons to each other at the right conditions; in 2002, the ATHENA project at CERN successfully created 100,000 low-energy antihydrogen atoms, so the use of antimatter as a fuel may soon be possible.
The PS is used to create the antiprotons and the positrons are collected from a decaying 22Na source. Following creation in the PS, the antiprotons enter the Antiproton Decelerator (AD) which uses electric fields to slow the particles down as they are very energetic (hot); for antihydrogen to be created both the antiprotons and positrons must be of low energy (cold).
The antiprotons and positrons, once cooled to ideally a fraction of a Kelvin, are then introduced to each other in a Penning trap, where, once they reach an equilibrium with each other and their surroundings, antihydrogen will begin to form.
A Penning trap uses an axial magnetic field to control the particles’ radial component, and a quadrupole electric field to control the particles’ axial component. A Penning trap is analogous to an atom, as the particles are arranged similar to an electron cloud around a central electric and magnetic field playing the role of the nucleus.
A Penning trap or storage ring cannot be used to control the position of antihydrogen as it has no charge, so a magnetic quadrupole surrounding the Penning trap could be used to manipulate the particles’ magnetic dipole.
When antihydrogen comes into contact with ordinary matter, the positron will annihilate with an electron, producing two 511keV back-to-back gamma rays.
The antiproton will annihilate with a nucleon to produce three charged pions, for example the annihilation of antihydrogen with a neutron produces two negatively charged pions and one positively charged pion:
As well as using positrons, antiprotons and antihydrogen as antimatter fuel, antimatter such as antideutrium, antitritium and antihelium could be used which are produced using a similar method for creating antihydrogen. This is very difficult though as they are created at such a high velocity that they instantly annihilate with the accelerator walls, only a few antideutrium nuclei have successfully been synthesised.
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Some information adapted from 2009 lecture notes by Dr. Nicky Davis