The Story of Nuclear Energy, Volume 3 (of 3): Beyond Fusion - Antimatter

Is there anything that lies beyond fusion?

When hydrogen undergoes fusion and becomes helium, only 0.7% of the original mass of the hydrogen is converted to energy. Is it possible to take a quantity of mass and convert all of it, every bit, to energy? Surely that would be the ultimate energy source. Mass for mass, that would deliver 140 times as much energy as hydrogen fusion would; it would be as far beyond hydrogen fusion as hydrogen fusion is beyond uranium fission.

And, as a matter of fact, total annihilation of matter is conceivable under some circumstances.

In 1928 the English physicist Paul Adrien Maurice Dirac (1902- ) presented a treatment of the electron’s properties that made it appear as though there ought also to exist a particle exactly like the electron in every respect except that it would be opposite in charge. It would carry a positive electric charge exactly as large as the electron’s negative one.

If the electron is a particle, this suggested positively charged twin would be an “antiparticle”. (The prefix comes from a Greek word meaning “opposite”.)

The proton is not the electron’s antiparticle. Though a proton carries the necessary positive charge that is exactly as large as the negative charge of the electron, the proton has a much larger mass than the electron has. Dirac’s theory required that the antiparticle have the same mass as the particle to which it corresponded.

In 1932 C. D. Anderson was studying the impact of cosmic particles on lead. In the process, he discovered signs of a particle that left tracks exactly like those of an electron, but tracks that curved the wrong way in a magnetic field. This was a sure sign that it had an electric charge opposite to that of the electron. He had, in short, discovered the electron’s antiparticle and this came to be called the “positron”.

Positrons were soon detected elsewhere too. Some radioactive isotopes, formed in the laboratory by the Joliot-Curies and by others, were found to emit positive beta particles—positrons rather than electrons. When an ordinary beta particle, or electron, was emitted from a nucleus, a neutron within the nucleus was converted to a proton. When a positive beta particle, a positron, was emitted, the reverse happened—a proton was converted to a neutron.

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A positron, however, does not endure long after formation. All about it were atoms containing electrons. It could not move for more than a millionth of a second or so before it encountered one of those electrons. When it did, there was an attraction between the two, since they were of opposite electric charge. Briefly they might circle each other (to form a combination called “positronium”) but only very briefly. Then they collided and, since they were opposites, each cancelled the other.

The process whereby an electron and a positron met and cancelled is called “mutual annihilation”. Not everything was gone, though. The mass, in disappearing, was converted into the equivalent amount of energy, which made its appearance in the form of one or more gamma rays.

(It works the other way, too. A gamma ray of sufficient energy can be transformed into an electron and a positron. This phenomenon, called “pair production”, was observed as early as 1930 but was only properly understood after the discovery of the positron.)

Of course, the mass of electrons and positrons is very small and the amount of energy released per electron is not enormously high. Still, Dirac’s original theory of antiparticles was not confined to electrons. By his theory, any particle ought to have some corresponding antiparticle. Corresponding to the proton, for instance, there ought to be an “antiproton”. This would be just as massive as the proton and would carry a negative charge just as large as the proton’s positive charge.

An antiproton, however, is 1836 times as massive as a positron. It would take gamma rays or cosmic particles with 1836 times as much energy to form the proton-antiproton pair as would suffice for the electron-positron pair. Cosmic particles of the necessary energies existed but they were rare and the chance of someone being present with a particle detector just as a rare super-energetic cosmic particle happened to form a proton-antiproton pair was very small.

Physicists had to wait until they had succeeded in designing particle accelerators that would produce enough energy to allow the creation of proton-antiproton pairs. This came about in the early 1950s when a device called the “Cosmotron” was built at Brookhaven National Laboratory in Long Island in 1952 and another called the “Bevatron” at the University of California in Berkeley in 1954.

Using the Bevatron in 1956, Segrè (the discoverer of technetium who had, by that time, emigrated to the United States), the American physicist Owen Chamberlain (1920- ), and others succeeded in detecting the antiproton.

The antiproton was as unlikely to last as long as the positron was. It was surrounded by myriads of proton-containing nuclei and in a tiny fraction of a second it would encounter one. The antiproton and the proton also underwent mutual annihilation, but having 1836 times the mass, they produced 1836 times the energy that was produced in the case of an electron and a positron.

There was even an “antineutron”, a particle reported in 1956 by the Italian-American physicist Oreste Piccioni 162(1915- ) and his co-workers. Since the neutron has no charge, the antineutron has no charge either, and one might wonder how the antineutron would differ from the neutron then. Actually, both have a small magnetic field. In the neutron the magnetic field is pointed in one direction with reference to the neutron’s spin; in the antineutron it is pointed in the other.

In 1965 the American physicist Leon Max Lederman (1922- ) and his co-workers produced a combination of an antiproton and an antineutron that together formed an “antideuteron”, which is the nucleus of antihydrogen-2.

This is good enough to demonstrate that if antiparticles existed by themselves without the interfering presence of ordinary particles, they could form “antimatter”, which 163would be precisely identical with ordinary matter in every way except for the fact that electric charges and magnetic fields would be turned around.

If antimatter were available to us, and if we could control the manner in which it united with matter, we would have a source of energy much greater and, perhaps, simpler to produce than would be involved in hydrogen fusion.

To be sure, there is no antimatter on earth, except for the submicroscopic amounts that are formed by the input of tremendous energies. Nor does anyone know of any conceivable way of forming antimatter at less energy than that produced by mutual annihilation, so that we might say that mankind can never make an energy profit out of it—except that with the memory of Rutherford’s prediction that nuclear energy of any kind could never be tapped, one hesitates to be pessimistic about anything.