Antimatter. For many, the very word conjures up images of Star Trek and other science fiction works. The majority of people think that antimatter is merely the pretentious conjuring of science fiction authours. But now let us take a look at what antimatter really is-a reality.
What Is Antimatter?
In 1928, British physicist Paul Dirac proposed the existence of Antimatter. It was not until 1932 that the existence of antimatter was proved with the experimental creation of an antielectron. Since then, all antiparticles have either been identified or evidence of their existence has been found.
Antimatter is often described as being a mirror image of matter. And to understand antimatter, one must first have a little knowledge of matter and its atoms. Most people know that all matter is made up of atoms, which, in turn, are made up of electrons, neutrons and protons. For the sake of simplicity, we will say that the electrons orbit a nucleus consisting of protons and neutrons. It is believed that the electron is not made of anything smaller, therefore, it is referred to as a fundamental particle. The electron is one of six types of particles classified as Leptons. Some important information about Leptons is as follows.
NAME
Electron
Electron Neutrino
Muon
Muon Neutrino
Tau
Tau Neutrino
LIFETIME
Stable
Stable
2.2 X 10-6 seconds
Stable
<2.3 X 10-12 seconds
Stable
(Table taken from Mirror Matter: Pioneering Antimatter Physics.)
Neutrons and protons are in a category of particles called Hadrons and are not fundamental particles. They are therefore made of even smaller things, called quarks. There are six types or flavours of quarks and each flavour has its own set of characteristics.
FLAVOUR
Down
Up
Strange
Charmed
Bottom
Top
CHARGE
-1/3
+2/3
-1/3
+2/3
-1/3
+2/3
All particles that are not Leptons are made of two or three quarks.
For example, the algebra for a proton and a neutron would look like:
Up quark +2/3 Up quark +2/3
Up quark +2/3 Down quark -1/3
+ Down quark -1/3 + Down quark -1/3
Proton +1 Neutron No Charge
It should be pointed out that electrons, protons, neutrons and their antiparticles all have a spin. When particles with a charge spin, they create a magnetic field. So, all these particles create a magnetic field around them.
With this basic knowledge, we are now ready to look at each of the more common antimatter particles.
The Antiproton
An antiproton has the same mass as a proton and the same magnetic field. The proton has a positive charge whereas the antiproton has a negative charge. To maintain the same magnetic field with two particles of opposite charges, the particles must have opposite spins-which the antiproton does. An antiproton is stable in a vacuum. When an antiproton collides with a proton or neutron, both particles are converted into mesons.
Physicists had predicted the existence of the antiproton in the 1930's. They finally made an antiproton and detected its existence in 1955 at the University of California Lawrence Berkeley National Laboratory.
The Positron (or Antielectron)
Positrons have the same mass as electrons. (An electron's mass is approximately 1/1836 the mass of a proton.) The rotation of both particles is also the same. The key difference is that the antielectron has a positive charge-thus the name positron.
The positron was the first antiparticle to be discovered. In the 1920's and 1930's cosmic rays were the only source of energy great enough to create positrons. Most cosmic rays in nature are protons from hydrogen atoms. These rays constantly bombard the upper atmosphere from outer space, causing atoms to break apart and give off gamma rays and X rays. The rays given off break other atoms apart. This "fallout" reaches the Earth and can be detected. In 1932, science caught up with nature in experimentally creating a positron. At the California Institute of Technology, Robert Millikan and Carl Anderson detected a positron in the cloud chamber they built. The air inside a cloud chamber is saturated, so when a particle is sent through the chamber, it leaves a visible trail. From the trail, the size and charge of the particle can be ascertained. They noted a particle that behaved much like an electron but its path curved in the opposite direction under the presence of magnets, thus indicating an electron with a positive charge. When electron-positron annihilation occurs, gamma rays or particles with a very short lifetime are produced.
In speaking with Jim Murphy, Facility Manager at the linear accelerator at the University of Saskatchewan, I was informed that they make positrons all the time-and not necessarily on purpose. He said basically that when you fire electrons having sufficient energy into a target (which is what the linear accelerator is designed to do) some positrons will be formed. Physicists' technology has come a long way in the past 70 years. Seventy years ago, Physicists did not have the machinery powerful enough to make antimatter; now, they make it by accident.
The Antineutron
The antineutron has a magnetic field the same as the neutron. It may seem strange for a neutral particle to create a magnetic field; but, one must remember that the neutron is made of smaller, charged particles. Neutrons are made up of three quarks, two with a -1/3 charge and the third having a +2/3 charge. Antineutrons consist of three antiquarks, two with a +1/3 charge and one with a -2/3 charge. As far as charges are concerned the negative quarks cancel the positive quarks and the negative antiquarks cancel the positive antiquarks. The same does not hold true for the magnetic field however. In a neutron the magnetic field created by the two -1/3-charge quarks is stronger than the field from the +2/3 quark. The same logic can be applied to the antineutron. As always, the mass of the particle and its antiparticle are the same.
In 1955 the existence of the antineutron was confirmed. This was the last of the three major antiparticles to be found. Its existence before that time was assumed and its discovery was awaited.
Making Antimatter
Making antimatter can be made to sound easy or difficult. If you hear a summary of what is done, it sounds easy. If you probe deeper into the process you will be amazed that it has ever been done. (For example, the aforementioned way of creating positrons.)
To make antiprotons, a proton accelerator accelerates protons and shoots them into a metal target. This is relatively inefficient. Of all the protons shot at the target only between .4% and 4.7% form proton-antiproton pairs. The more powerful the accelerator is, the more efficient the process. After being formed, the antiprotons fly away from the target in many directions. A magnetic lens is used to help focus the antiprotons into the antiproton collector and cooler. From there, medium energy antiprotons are sent to the antiproton decelerator, after which low energy antiprotons are sent to an antiproton accumulator (which will be described later).
In a more recent experiment, antihydrogen antiatoms have been made by shooting an antiproton beam through a stream of gas. This creates a few electron-positron pairs and sometimes the positron will bond with the antiproton to form antihydrogen.
How Far Have We Gone?
The Progress Of Antimatter
Much of the progress of antimatter has been mentioned above. However, recently, antihelium has been made by sending a beam of antiprotons into liquid helium. Physicists hope to learn much more about antimatter by studying anitatoms, so the search for better ways to make antimatter and the quest to make more continue. Antimatter was used to determine the existence of the different flavours of quarks. It has also been useful to help understand matter better and to create a better model for the atom. Until more efficient ways of handling and storing antimatter are found, the antimatter field will continue to plod along. Hopefully some day soon antimatter physics will find the technology boost that it needs in order to expedite discoveries.
Storing Antimatter
At the end of the antiproton creation process, a problem lies. If the antiprotons come in contact with matter, they are destroyed. The solution is what is called a bottle (it is actually an antiproton accumulator). This sort of bottle is usually a large ring made of piping. The bottle is a vacuum and the antiprotons are forced to go around the ring by magnets. Usually one percent or less of antiprotons that enter the ring are useful because the rest often react with matter.
It is hoped that a new 55 kilogram device currently being developed by Penn State's Gerald Smith will be successful in making antiprotons portable. The device should be able to hold about ten billion antiprotons. This should make antimatter available to more people and thus greatly increase its uses.
Possible Uses Of Antimatter
Many of antimatter's uses are limited right now because of the few places that produce it worldwide. If antimatter becomes more readily available the uses for it will increase incredibly. For example, Oxygen 15 is used in Positron Emission Tomography (PET) scanners in hospitals. Oxygen 15 can be made by firing antiprotons into water. It is believed that soon we will have Antiprotonic Radiography scans instead of CAT scans.
Antiprotons can be moved and controlled by magnets and electrical forces. This can be used to carve out pathways for so-called nanotechnologies. Think of the possibilities in miniaturization. Once again, we come back to Star Trek. The neural pathways for the android Data's brain were carved with positrons.
A beam of antimatter could be used to probe the interior of solid objects. It could also be used for X ray analysis of unknown substances with great accuracy.
Imagine the potential energy source. The annihilation of one billionth of a gram of antimatter produces 180 kilojoules of energy. Annihilating one millionth of a gram gives off 180 megajoules-which is equal to about 20 kilograms of chemical fuel or 5 gallons of gas. One thousandth of a gram is the equivalent of 20 tons of rocket fuel. The annihilation of one gram would release the energy of a 20 kiloton nuclear bomb. (This, however, would be too expensive to produce with current technologies.) Think of the potential for space travel!
Yes, antimatter is a most interesting field. It is amazing to look at the complexities of our universe. One can not help but think that there was some master plan behind all this. We look at our linear accelerators and synchrotrons and think we know a lot. The truth is we have not even scratched the surface of our universe! And who knows, there may be a whole other universe-an antimatter universe.