![]() ) This principle is used in medicine, in Positron-Electron Tomography (PET) scans. (Both the electron and the positron have a mass of 511 keV c 2. When a positron meets an electron, they annihilate to produce a pair of gamma ray photons, each of energy 511 keV. Take a look at PET scans and how they are made. The six leptons each have a lepton number of +1, while the six anti-leptons each have a lepton number of -1. There are now a total of 12 leptons: the electron, the muon, and a super-heavy version called the tau (t) a neutrino for each of these three and six antiparticles for these six particles. Furthermore, there are different neutrinos associated with the electron and the muon.īecause these light particles do not experience the strong force of hadrons, they form a different category of particle and given the name leptons. Like the electron and the muon, neutrinos have antiparticles. (compare with the electron, m e = 0.511 MeV c 2 ). ![]() ![]() Current (2005) thought is that the mass of the electron neutrino is in the range Enrico Fermi developed the theory of this new particle, which he called a neutrino, but it wasn’t until 1951 that Reines and Cowan discovered it at the Savannah River nuclear reactor. He suggested that the lost energy was carried away by a new particle, which must be chargeless and have virtually no mass. In 1930 Wolfgang Pauli suggested, in a famous letter to fellow physicists starting Dear Radioactive Ladies and Gentlemen, in which he wrote Ive done something terrible: I have predicted an undetectable particle’. This does not happen in beta decay sometimes a lot of the energy seems to be missing. Reactions such as carbon-14 → nitrogen-14 + β - were expected to produce beta particles with identical kinetic energy: this is what happens in alpha decay. The problems with beta decay are worth describing in detail. Like the electron, it has an anti-particle (the anti-muon, μ + ) which is positively charged. It turned out to be a heavy type of electron. The muon was a problem because it had exactly the mass predicted for Yukawa’s meson, but it didn’t undergo strong nuclear interactions at all, which the meson had to do (that was its job, after all!). The muon quote ( Who ordered that?) was from physicist Isadore Rabi – it’s whimsically supposed to be the sort of thing you say in a Chinese restaurant when you get some strange dish you don’t recognize. The positron was the first anti-particle discovered: since then it has been found that every particle has its antiparticle. At this stage, it’s enough to say that the particle is curved more when it is slower because the particle spends longer in the magnetic field.Īnderson could deduce, from the direction and magnitude of the curvature and the length of the particle track, that the particle was positive and had a mass not more than twice that of an electron. It has been slowed down by passing through the lead plate across the centre, and the curvature of the path is caused by a magnetic field. The particle is moving up the photograph. ![]() Episode 534-1: Anderson’s positron photograph (Word, 63 KB) ![]()
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