At the Heart of the Atom


What is antimatter?

In 1932 Carl Anderson (1905-1991) was studying particles produced when cosmic rays struck lead sheets in a cloud chamber that was in a magnetic field. He found low-mass particles that curved the opposite direction from electrons, showing that they had positive charge. He later confirmed the existence of these particles by using a laboratory source of high-energy gamma rays. This positive electron was named the positron and was the first form of antimatter found. Anderson shared the 1936 Nobel Prize for his discovery.

When a gamma ray with sufficient energy strikes matter it can produce an electron-positron pair. Energy is converted into particles with mass. The minimal amount of gamma-ray energy needed is given by Einstein’s famous equation, Egamma = melectronc2 mpositronc2. The uncharged gamma produces a negatively charged electron and a positively charged positron, so electric charge is conserved.

Positrons are also emitted in radioactive decay of isotopes that have a deficit of neutrons. For example, stable carbon exists as 12C or 13C; six protons and either six or seven neutrons. As was discussed above, 14C decays by emitting an electron. One of the neutrons changes to a proton with the emission of the electron and anti-neutrino. On the other hand, 11C, with only five neutrons, is a positron emitter. One of the protons changes to a neutron with the emission of a positron and a neutrino.

When a positron strikes matter the positron and an electron annihilate each other, producing two or three gammas. Particles with mass are converted to energy. Again, Einstein’s equation can be used to find the total energy of the gammas: Egamma = melectronc2 mpositronc2

Antiprotons are created in accelerators, where the accelerated particle is slammed into a metal target, emitting gammas that create, in this case, proton-antiproton pairs.


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