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''This article is about the exotic atom. For the hydrogen isotope, see Protium.'' Positronium (Ps) is a system consisting of an electron and its anti-particle, a positron, bound together into an ''exotic atom'', specifically an ''onium''. The system is unstable: the two particles annihilate each other to predominantly produce two or three gamma-rays, depending on the relative spin states. The orbit and energy levels of the two particles are similar to that of the hydrogen atom (which is a bound state of a proton and an electron). However, because of the reduced mass, the frequencies of the spectral lines are less than half of the corresponding hydrogen lines. ==States== The ground state of positronium, like that of hydrogen, has two possible configurations depending on the relative orientations of the spins of the electron and the positron. The ''singlet'' state, , with antiparallel spins (''S'' = 0, ''Ms'' = 0) is known as ''para-positronium'' (''p''-Ps). It has a mean lifetime of 125 picoseconds and decays preferentially into two gamma rays with energy of each (in the center-of-mass frame). Detection of these photons allows to reconstruct the vertex of the decay and is used in the positron-emission tomography. Para-positronium can decay into any even number of photons (2, 4, 6, ...), but the probability quickly decreases with the number: the branching ratio for decay into 4 photons is .〔 〕 Para-positronium lifetime in vacuum is approximately〔 :: The ''triplet'' state, 3S1, with parallel spins (''S'' = 1, ''Ms'' = −1, 0, 1) is known as ''ortho-positronium'' (''o''-Ps). It has a mean lifetime of ,〔 〕 and the leading decay is three gammas. Other modes of decay are negligible; for instance, the five-photons mode has branching ratio of ~.〔 〕 Ortho-positronium lifetime in vacuum can be calculated approximately as:〔 :: However more accurate calculations with corrections to order O(α²) yield a value of 7.04 μs⁻¹ for the decay rate, corresponding to a lifetime of s.〔 Positronium in the 2S state is metastable having a lifetime of against annihilation. The positronium created in such an excited state will quickly cascade down to the ground state, where annihilation will occur more quickly. 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「Positronium」の詳細全文を読む スポンサード リンク
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