Positron Physics

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"Physics is one of the oldest and most basic sciences and one which, through its discoveries and their applications, has created many other sciences and technologies. Much of physics today remains concerned with most fundamental and philosophical questions. More often, however, it takes real world problems from other areas of contemporary science and technology and attacks them with every technology available." (Prof. Roy N. West)

Positron Annihilation

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The positron is the antimatter particle of the electron. The two particles have identical properties except that they have opposite electric charges. When a positron encounters an electron, the particles are annihilated, and their mass is converted into pure energy in the form of gamma rays. The process is a direct demonstration of several fundamental conservation laws of modern physics. Certain characteristics of the process, however, can be influenced if the particles meet in an atomic environment. As a consequence positrons can be used as a probe for exploring the nature of matter. It turns out that the probe is unusually sensitive and revealing when it is applied to the regular arrays of atoms characteristic of most solids. It is also beginning to have its uses in the study of living matter.

When a positron and an electron annihilate each other in condensed matter (a solid or a liquid), they always give rise to two gamma ray photons. The conversion of the particles' mass into energy exactly follows Einstein's equation E = mc^2, in which E is the energy liberated, m is the mass of the particles and c is the speed of light; mass and energy are thus conserved. The sum of the positron's positive charged (+1) and the electron's negative charge (-1) is zero. The gamma ray photons that result from the annihilation carry no charge; charge is thus conserved. In the annihilation events that are of interest here the spins of the particles are antiparallel and add up to zero. The gamma ray photons have no spin; spin is thus conserved. The two photons each have an energy of .511 million electron volts (MeV), and they leave the site of the annihilation in exactly opposite directions. Their net momentum is zero; momentum is thus conserved. The annihilation process therefore conserves energy, charge, spin and momentum. All the same, electrons could annihilate the protons in the nuclei of atoms without violating these laws, and if they did so, there would be no atoms. Protons, however, are 2,000 times as massive as electrons. Electrons can be annihilated only by antielectrons, that is, positrons. The fact that our world exists therefore proves yet another universal law, a law that might be called the conservation of light and heavy particles. In short, the positron-electron system exemplifies the most basic conservation laws found in nature. (Prof. Werner Brandt)