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Positronium (Ps) is a quasi-stable system consisting of an electron and its anti-particle, a positron, bound together into an "exotic atom". The orbit of the two particles and the set of energy levels is similar to that of the hydrogen atom (electron and proton). However, because of the different reduced mass, the frequencies associated with the spectral lines are less than half of those of the corresponding hydrogen lines.

Positronium is unstable, with a half-life of at most about 10-7 seconds (100 nanoseconds). The particles "spiral" closer to each other (although this actually takes place in quantized steps of decreasing radius), until their existence is terminated by electron-positron annihilation. At annihilation, gamma rays are produced. Any number of gamma rays can be produced with a total energy of 1022 keV (since both particles have mass of 511 keV), though natural reactions usually produce two or three rays, depending on the spin of the particles. Up to six have been created in the lab.

Positronium, like hydrogen, can have different configurations; the ground state can be a singlet state with antiparallel spins (S=0, Ms=0) known as parapositronium (term symbol 1S0). The singlet state lasts about 10-10 seconds. The other case, a triplet state, with spins parallel (S=1, Ms=-1,0,1) is known as orthopositronium (term symbol 3S1). The triplet state lasts about 10-8 seconds.

As the orthopositronium state is the most common (75%), the reaction typically emits two gamma rays, which are always oriented 180° from each other. This property is used in positron emission tomography.

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