Positronium
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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 (greater than one) 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. The lower bound of two is a result of relativistic momentum conservation. Up to five annihilation gamma rays have been observed in laboratory experiments, confirming the predictions of quantum electrodynamics to very high order.
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 (mean lifetime = 125 ps). 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 lives about 10-7 seconds (mean lifetime = 140 ns).
While the orthopositronium state is the most common (75% of the positronium formed from positrons begins as "ortho"), this triplet state easily and quickly de-excites to become the singlet state parapositronium, which annihilates rapidly to two gamma rays, which are always oriented 180° from each other. This property is used in positron emission tomography.