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Positron emission or beta plus decay (β+ decay) is a particular type of radioactive decay and a subtype of beta decay, in which a proton inside a radionuclide nucleus is converted into a neutron while releasing a positron and an electron neutrino (''ν''e).〔(【引用サイトリンク】title=Nuclear Chemistry )〕 Positron emission is mediated by the weak force. The positron is a type of beta particle (β+), the other beta particle being the electron (β−) emitted from the β− decay of a nucleus. An example of positron emission (β+ decay) is shown with magnesium-23 decaying into sodium-23: : → + + Because positron emission decreases proton number relative to neutron number, positron decay happens typically in large "proton-rich" radionuclides. Positron decay results in nuclear transmutation, changing an atom of a chemical element into an atom of an element with an atomic number that is less by one unit. Positron emission should not be confused with electron emission or beta minus decay (β− decay), which occurs when a neutron turns into a proton and the nucleus emits an electron and an antineutrino. Electron capture (sometimes called inverse beta decay) is also occasionally classified as a type of beta decay. In some ways, electron capture can be regarded as an equivalent to positron emission, since capture of an electron results in the same transmutation as emission of a positron. Electron capture occurs when electrons are available and requires less energy difference between parent and daughter, so occurs much more often in smaller atoms than positron emission does. Electron capture always competes with positron emission where the latter is seen, and in addition, occurs as the only type of beta decay in proton-rich nuclei when there is not enough decay energy to support positron emission. ==Discovery of positron emission== In 1934 Frédéric and Irène Joliot-Curie bombarded aluminium with alpha particles to effect the nuclear reaction + → + , and observed that the product isotope emits a positron identical to those found in cosmic rays by Carl David Anderson in 1932.〔I. Curie and F. Joliot, ''C. R. Acad. Sci.'' 198, (254 ) (1934)〕 This was the first example of decay (positron emission). The Curies termed the phenomenon "artificial radioactivity," since is a short-lived nuclide which does not exist in nature. The discovery of artificial radioactivity would be cited when the husband and wife team won the Nobel Prize. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「positron emission」の詳細全文を読む スポンサード リンク
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