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Radioactivity studies the spontanuous transformations of unstable nuclei; that is their spontanuous nuclear reactions. In 1896, Becquerel, Antoine-Henri underlined the natural radioactivity of the Uranium. He observed that photographic plates have been fogged when exposed to radiation from a uranium salt. Radiaoactive nuclei decay into another nucleus more stable, plus one or more light particles. In general, nuclei heavier than lead, and many isotopes of lighter nuclei, have a finite probability of decaying spontaneously into another nucleus plus one or more lighter particles. The decay products may be:

1. alpha-particle

An alpha-particle (α) that contains two protons and two neutrons. It is the stable nucleus of a helium atom. The related process is written as folows: (A,Z)X → (A-4, Z-2)Y + (4,2)He Example: (238,92)U → (234,90)Th + (4,2)He

2. Beta(electron)-particle

A nucleus with more neutrons than it can maintain in stability may decay by emission of an electron from the nucleus. This is the beta-decay (β-). Along this process, a neutron is converted to a proton: n → p + e- These electrons may emerge with a kinetic energy of up to several MeV. The process of β- is: (A,Z)X → (A, Z+1)Y + e- Example: (14,6)C → (14,7)N + e-

3. Beta(positron)-particle

The third process β+ is encountred only for the artificial nuclei, where a proton is converted to a neutron plus a positron: p → n + e+ The process of β+ is: (A,Z)X → (A, Z-1)Y + e+ Example: (74,33)As → (74,32)Ge + e+

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