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''Yrast'' (; (:ˈyːɾast)) is a technical term in nuclear physics that refers to a state of a nucleus with a minimum of energy (when it is least excited) for a given angular momentum. ''Yr'' is a Swedish adjective sharing the same root as the English ''weary''. ''Yrast'' is the superlative of ''yr'' and can be translated ''whirlingest'', although it literally means "dizziest" or "most bewildered". The yrast levels are vital to understanding reactions, such as off-center heavy ion collisions, that result in high-spin states. ''Yrare'' is the comparative of ''yr'' and is used to refer to the second-least energetic state of a given angular momentum. ==Background== An unstable nucleus may decay in several different ways: it can eject a neutron, proton, alpha particle, or other fragment; it can emit a gamma ray; it can undergo beta decay. Because of the relative strengths of the fundamental interactions associated with those processes (the strong interaction, electromagnetism, and the weak interaction respectively), they usually occur with frequencies in that order. Theoretically, a nucleus has a very small probability of emitting a gamma ray even if it could eject a neutron, and beta decay rarely occurs unless both of the other two pathways are highly unlikely. In some instances, however, predictions based on this model underestimate the total amount of energy released in the form of gamma rays; that is, nuclei appear to have more than enough energy to eject neutrons, but decay by gamma emission instead. This discrepancy is found by the energy of a nuclear angular momentum, and documentation and calculation of ''yrast'' levels for a given system may be used for analyzing such a situation. The energy stored in the angular momentum of an atomic nucleus can also be responsible for the emission of larger-than-expected particles, such as alpha particles over single nucleons, because they can carry away angular momentum more effectively. This is not the only reason alpha particles are preferentially emitted, though; another reason is simply that alpha particles (He-4 nuclei) are energetically very stable in and of themselves. 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「Yrast」の詳細全文を読む スポンサード リンク
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