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Let us start with the wiki article: The singlet state with antiparallel spins (S = 0, Ms = 0) is known as para-positronium (p-Ps) and denoted 1S0. It has a mean lifetime of 125 picoseconds and decays preferentially into two gamma quanta with energy of 511 keV each (in the center of mass frame). Detection of these photons allows for the reconstruction of ...

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Positronium is a quantum mechanical entity. It is a bound state in the potential create between two elementary "particles". Even though the wiki article draws a nice picture of he electron and positron orbiting about their center of mass this is a representation as if they are classical particles . They are not. The electron and positron are not in orbits, ...

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I'm afraid the responses so far are either misleading or do not answer the question. In fact in a dense medium a positron when it forms the longer lived ortho-positronium can "pick-up" an electron from an adjacent atom and then decay into 2 x 511 keV photons long before the 3-photon ortho-positronium state would have decayed. Look up "pick-up effect".

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