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Everyone has heard of antimatter – it has long played a role in science fiction – you can imagine Dr McCoy in Star Trek probing his patients with antimatter beams, perhaps. Well, you might be surprised to know that antimatter is already routinely used in this way – possibly even in your local hospital. First, I will say a little about what antimatter is and how it was discovered.
Paul Dirac was an English theoretical physicist, famous for his absolutely logical approach to the world. He was renowned for never making an utterance unless it was strictly necessary. In 1928 Dirac took the first steps towards uniting the two great theories of the early twentieth century—relativity and quantum mechanics. He devised a wave equation that would describe the behaviour of electrons even when travelling at close to the speed of light. This equation is known as the Dirac equation.
The great theoretical physicist Paul Dirac. (There seems to be an uncharacteristic mistake on the blackboard.)
Dirac soon realised his equation had a very curious feature. It only seemed to work consistently if the electron had a mirror image counterpart, a particle with the same mass as an electron, but opposite electric charge. So, whereas the electron has a negative electric charge, this mirror particle would have a positive electric charge. At the time the only known fundamental particles were the electron, the proton and the photon—the fundamental constituent of light—so the prediction of a new particle was a really big deal.
Cosmic Rays
The first track of a positron published by Carl Anderson in 1932.
The puzzle was resolved just four years later in 1932. Carl Anderson was studying the tracks left by particles produced in collisions between high-energy cosmic rays arriving from deep space and atoms in the Earth’s atmosphere. Anderson spotted particles that looked similar to electrons, but in the magnetic field of his cloud chamber they spiralled in the opposite direction, so they had to be positively charged. We know these particles as positrons, which is short for positive electrons. The positron is the antiparticle of the electron.
Dirac’s prediction of the positron from the quirks of his equation is one of the most amazing insights in the history of theoretical physics, and his reasoning applies equally well to other matter particles. For each such particle there is a corresponding antiparticle whose existence is now well established. We have antiprotons, antineutrons, antiquarks, antimuons, antineutrinos and so on. When a particle meets its antiparticle they mutually annihilate.
It is believed that in the early universe almost equal quantities of matter and antimatter formed. Particles soon met up with their counterparts and disappeared in a blast of radiation, leaving behind the small surplus of matter particles from which the visible universe is composed. The reason for this small excess of matter over antimatter is still not fully understood. But without it we would not be here.
What Has Antimatter Ever Done for Us?
This is all rather abstruse, the everyday food for theoretical physicists musing on the meaning of it all. But, as Monty Python might have said, what has antimatter ever done for us?
Artist’s conception of an interstellar rocket with an antimatter propulsion system. Credit: NASA/MSFC.
Well, if we ever want to visit the stars antimatter propulsion may be the only feasible way of getting there, and the possibility of such propulsion systems has been considered by NASA. A space vehicle whose thrust was generated through the mutual annihilation of protons and antiprotons might achieve 40% of the speed of light. There are, however, a few challenges that remain to be overcome. The biggest being the supply of antiprotons. Antiparticles such as antiprotons are routinely created in particle physics laboratories such as CERN and Fermilab. But so far the total production may be measured in nanograms, which would be about enough to boil a kettle. Nonetheless, this hasn’t stopped the space artists from dreaming.
Back in the Real World
Back in the real world there is a very important technology that makes good use of antimatter. If you have ever had a brain scan you may well have benefited from it. It is known as PET (Positron Emission Tomography) and the way it works is quite remarkable.
PET scanning utilises a radioactive nucleus that undergoes positive beta decay, a process in which a proton transforms into a neutron with the emission of a positron and a neutrino. The neutrino disappears without a trace, but the positron proves to be very useful. The most suitable nucleus for this purpose is fluorine-18. It is composed of nine protons and nine neutrons, and converts into a stable oxygen-18 nucleus composed of eight protons and ten neutrons when it undergoes beta decay.
The positron emitted by a decaying fluorine-18 nucleus meets an electron almost immediately and they mutually annihilate to produce two gamma ray photons. This is electromagnetic radiation, just like visible light, but with much greater energy. Whereas the energy of a photon of blue light is about 3 eV (electron Volts), the gamma rays produced by electron-positron annihilation carry 511 keV, over 100,000 times as much.
A patient having a PET scan.
Before a PET scan fluorine-18 must be delivered to the target area of the patient’s body. This is usually achieved by injecting a solution of an appropriate fluorine-18 containing compound. The patient then lies within a scanner that detects the gamma rays emitted by electron-positron annihilations. It is like having an X-ray, but with the gamma rays arising from within the body.
The most widely used delivery compound has the fancy name fluorodeoxyglucose (FDG), which simply means that one of the oxygen atoms in a glucose molecule is replaced with a fluorine atom, and for a PET scan this fluorine atom must be the radioactive fluorine-18 rather than the stable fluorine-19. FDG is chemically similar to ordinary glucose so it is transported around the body to organs that require large amounts of energy such as the brain and kidneys, but also to rapidly multiplying cancerous cells. The FDG is not easily metabolized by the body, so it remains within those cells requiring glucose until the fluorine-18 decays, at which point the fluorine atom is converted into an oxygen atom and the FDG becomes normal glucose. In the mean time the gamma rays produced following the decay of fluorine-18 reveal the sites within the body where glucose is being consumed.
The half-life of fluorine-18 is 110 minutes, so during every 110 minute period half the remaining nuclei will decay. Within a day or two the patient will be free of fluorine-18, which is great as we don’t want people walking around emitting too many gamma rays. But this means that fluorine-18 must be made on site using a cyclotron or other particle accelerator. It is a rather surprising fact that almost all the particle accelerators in the world are now found in hospitals.
Whole-body PET scan using FDG labelled with fluorine-18. The brain and kidneys can be seen, and radioactive urine from the breakdown of FDG is visible in the bladder. The scan shows a large tumour mass in the liver. Credit: Jens Maus.
About an hour after being injected with the FDG solution, the patient lies within a scanner for about thirty minutes. The scanner detects the pairs of gamma rays emitted in the electron-positron annihilations and gradually builds up a picture of the inside of the patient’s body. The gamma rays reveal the sites within the body where glucose is being consumed. It is like having an X-ray, but with the gamma rays arising from within the body.
PET scans are complimentary to other types of medical scan, such as CAT scans which reveal structural or anatomical information about the body and its tissues. PET scans reveal functional information. They provide information about the consumption of glucose in various parts of the body. Because cancer cells consume more glucose than normal tissue they show up as bright spots in a PET scan. So it is possible to locate very small tumours before they would show up with other types of scan. PET scans can also reveal brain abnormalities such as Alzheimer’s disease and other forms of dementia.
What about the risks? The half-life of fluorine-18 is 110 minutes, which is long enough to perform the imaging, but not too long. Within a few hours the radioactive tracer has completely disappeared from the patient’s body. The radiation dose received by the patient is equivalent to two to three years of normal background radiation or about what you would receive due to cosmic rays on 25 long-distance flights. The benefits of the scan far outweigh the low risk due this radiation dose.
Further Information
There is more about Dirac and the discovery of the positron in my book Higgs Force: Cosmic Symmetry Shattered.
There is more about antimatter propulsion system at the following link: Antimatter Propulsion
