Why Does Antimatter Matter?

by Nicholas Mee on October 17, 2020

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.

A patient having a PET scan.

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.

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.

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.

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

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