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 produce two gamma-ray photons after an average lifetime of 125 picoseconds or three gamma-ray photons after 142 nanoseconds in vacuum, depending on the relative spin states of the positron and electron. 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 reduced mass, the frequencies associated with the spectral lines are less than half of those of the corresponding hydrogen lines.
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) and denoted 1S
0. 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 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 as the number increases: the branching ratio for decay into 4 photons is 1.439(2)×10−6.
Para-positronium (S = 0) lifetime in vacuum is
The triplet state with parallel spins (S = 1, Ms = −1, 0, 1) is known as ortho-positronium (o-Ps) and denoted 3S1. The triplet state in vacuum has a mean lifetime of 142.05±0.02 ns, and the leading mode of decay is three gamma quanta. Other modes of decay are negligible; for instance, the five-photons mode has branching ratio of ~1.0×10−6.
Ortho-positronium (S = 1) lifetime in vacuum is
Positronium in the 2S state is metastable having a lifetime of 1.1 µs against annihilation. The positronium created in such an excited state will quickly cascade down to the ground state, where annihilation will occur more quickly. Measurements of these lifetimes, as well as of the positronium energy levels, have been used in precision tests of quantum electrodynamics.
Annihilation can proceed via a number of channels, each producing one or more gamma rays. The gamma rays are produced with a total energy of 1022 keV (since each of the annihilating particles have mass of 511 keV/c2), the most probable annihilation channels produce two or three photons, depending on the relative spin configuration of the electron and positron. A single-photon decay is only possible if another body (e.g. an electron), to which some of the energy from the annihilation event may be transferred, is in the vicinity of the annihilating positronium. Up to five annihilation gamma rays have been observed in laboratory experiments, confirming the predictions of quantum electrodynamics to very high order.
The annihilation into a neutrino–antineutrino pair is also possible, but the probability is predicted to be negligible. The branching ratio for o-Ps decay for this channel is 6.2×10−18 (electron neutrino–antineutrino pair) and 9.5×10−21 (for each non-electron flavour) in predictions based on the Standard Model, but it can be increased by non-standard neutrino properties, like mass or relatively high magnetic moment. The experimental upper limits on branching ratio for this decay (as well as for a decay into any "invisible" particles) are <4.3×10−7 for p-Ps and <4.2×10−7 for o-Ps.
While precise calculation of positronium energy levels uses the Bethe–Salpeter equation or the Breit equation, the similarity between positronium and hydrogen allows a rough estimate. In this approximation, the energy levels are different between the two because of a different value for the mass, m*, used in the energy equation (see electron energy levels for a derivation)
- is the charge magnitude of the electron (same as the positron),
- is Planck's constant,
- is the electric constant (otherwise known as the permittivity of free space),
- is the reduced mass.
The reduced mass in this case is
- where and are, respectively, the mass of the electron and the positron (which are the same by definition of particles and antiparticles).
Thus, for positronium, its reduced mass only differs from the rest mass of the electron by a factor of 2. This causes the energy levels to also roughly be half of what they are for the hydrogen atom.
So finally, the energy levels of positronium are given by
The lowest energy level of positronium (n = 1) is −6.8 electronvolts (eV). The next lowest energy level (n = 2) is −1.7 eV. The negative sign implies a bound state. We also note that a two-body Dirac equation composed of a Dirac operator for each of the two point particles interacting via the Coulomb interaction can be exactly separated in the (relativistic) center-of-momentum frame and the resulting ground-state [clarify] been obtained very accurately using finite element methods of J. Shertzer. The Dirac equation whose Hamiltonian comprises two Dirac particles and a static Coulomb potential is not relativistically invariant. But if one adds the (or , where is the fine-structure constant, which is about 1/137) contributions, where n = 1,2,3,... to the Hamiltonian, then the result is relativistically invariant in the limit. So only the lead term in the Hamiltonian is included. The next (or ) contribution are the Breit terms; workers rarely go to (or ) because at one has the Lamb shift (which is a detailed calculation needing quantum electrodynamics).
Prediction and discovery
Croatian scientist Stjepan Mohorovičić predicted the existence of positronium in a 1934 article published in Astronomische Nachrichten, in which he called the substance "electrum". Other sources credit Carl Anderson as having predicted its existence in 1932 while at Caltech. It was experimentally discovered by Martin Deutsch at MIT in 1951 and became known as positronium.
The first observation of di-positronium molecules—molecules consisting of two positronium atoms—was reported on 12 September 2007 by David Cassidy and Allen Mills from University of California at Riverside.
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