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A nuclear isomer is a metastable state of an atomic nucleus caused by the excitation of one or more of its nucleons (protons or neutrons). "Metastable" refers to the fact that these excited states have half-lives more than 100 to 1000 times the half-lives of the excited nuclear states that decay with a "prompt" half life (ordinarily on the order of 10−12 seconds). As a result, the term "metastable" is usually restricted to refer to isomers with half-lives of 10−9 seconds or longer. Some sources recommend 5 × 10−9 s to distinguish the metastable half life from the normal "prompt" gamma emission half life.〔(Nuclear isomers )〕 Occasionally the half-lives are far longer than this, and can last minutes, hours, years, or in one case , so long that it has never been observed to decay (at least 1015 years). Sometimes, the gamma decay from a metastable state is given the special name of an isomeric transition, but save for the long-lived nature of the meta-stable parent nuclear isomer, this process resembles shorter-lived gamma decays in all external aspects. The longer lives of nuclear isomers (metastable states) are often due to the larger degree of nuclear spin change which must be involved in their gamma emission to reach the ground state. This high spin change causes these decays to be so-called forbidden transitions, and thus delayed. Other reasons for delay in emission, such as low or high available decay energy, also have effects on decay half life. The first nuclear isomer and decay-daughter system (uranium X2/uranium Z, now known as /) was discovered by Otto Hahn in 1921. == Nucleus == The nucleus of a nuclear isomer occupies a higher energy state than the corresponding non-excited nucleus, which exists in the lowest energy state, called the ground state. In an excited state, one or more of the protons or neutrons in a nucleus occupy a nuclear orbital of higher energy than an available nuclear orbital of lower energy. These states are analogous to excited states of electrons in atoms. Excited atomic states decay by fluorescence which usually involves emission of light near the visible range. Because of the much higher binding energies involved in nuclear processes, most nuclear excited states decay instead by gamma ray emission. For example, a well-known nuclear isomer used in various medical procedures is , which decays with a half-life of about 6 hours, by emitting a gamma ray of 140 kiloelectron-volts of energy (this is close to the energy of medical diagnostic X-rays). Nuclear isomers owe their long half lives to the fact that their gamma decay is "forbidden" due to a large change in nuclear spin needed to emit a gamma. For example, has a spin of −9 and must gamma decay to with a spin of +1. Similarly, has a spin of -1/2 and must gamma decay to with a spin of +9/2. 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「Nuclear isomer」の詳細全文を読む スポンサード リンク
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