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Linné on line arrow Physics and the Cosmos arrow Physics and the Cosmos arrow The atomic nucleus – a new world with new forces arrow Radioactivity


Natural radioactivity was discovered more than a 100 years ago, or more precisely in 1896, by the Frenchman Henri Becquerel. He noted that a piece of mineral containing uranium blackened a photographic plate which had not been exposed to light. From this he concluded that the mineral must emit some invisible radiation which made the silver bromide in the plate react. Later one realized that are three different kinds of radioactive radiation: "alpha", "beta" and "gamma" radiation. In 1903 Becquerel shared the Nobel prize in physics with Marie and Pierre Curie for his studies of radioactivity.

The three different kinds of radiation correspond to the emission of three different kinds of particles: alpha particles are the atomic nuclei of helium, beta particles are electrons and gamma particles are high energy photons. The three different types of decay depend on separate forces: alpha radiation takes place through the strong force, gamma radiation through the electromagnetic force and beta radiation through the weak force. The weak force is only of importance at the subatomic level, in the same way as the strong force, but it is much weaker.

When beta decay was studied in more detail it first appeared that the energy was not conserved in the decay. One saw that the electrons did not have a given energy but that it varied between zero and the highest one available. In 1930 Wolfgang Pauli suggested that the missing energy was due to the emission of one more particle and that this particle carried the remaining energy. The reason that the neutrino, as the particle is called, was not detected was simply that the probability for it to interact is about 100 million times smaller than that of the electron. In fact the probability for a neutrino to interact is so small that it most of the time passes through the earth without interacting.

The difference in lifetime for radioactive isotopes can be used for dating archaeological findings.