Spin-averaged annihilation rate

A given system in a given state has only one annihilation rate. The terms two-photon annihilation rate, three-photon annihilation rate , and spin-averaged annihilation rate, sometimes seen in the literature, have no operational meaning and are not measurable [5]. 

Despite this, the spin-averaged annihilation rate is a useful concept. If a positron is immersed in an electron-rich medium in which all electron... 

Binding energies and annihilation rates for polyleptons are given in Table 2.2. The current values for positronium are listed for completeness. Since Wheeler s seminal 1946 paper, the 3- and 4-particle polyleptons have been the subject of many studies, and their properties are well understood today. The annihilation rate of diatomic positronium, Ps2, is about twice the spin-averaged rate in Eq. (2.4) because there are two positrons and each of them sees spin-paired electrons. Recently a calculation of the 5-particle... 

The annihilation rates of all the two-electron systems except PsCu are close to the spin-averaged rate, 2 ns-1, and show only a weak monotonic dependence on the binding energy. The only exception, PsCu, is also the only bound system in Table 2.3 that was studied by CL We have already noted the slow convergence of annihilation rates for this method. 

The calculated annihilation rate of Ps2Li+ is close to twice the spin-averaged rate, suggesting a structure in which a relatively well-defined diatomic positronium molecule is bound to the Li+ core. 

Fig. 3. The cross-section for annihilation in flight in H + H collisions (in atomic units). Dashed curve leptonic annihilation in the coUisional singlet state, o- = (4.0 X 10 /y )uo- Dash-dotted curve leptonic annihilation in the coUisional triplet state, a- = (3.5 X 10 /y )ao- Solid line the total leptonic annihilation rate for spin-polarized H - - H collisions (e.g., in the magnetic trap), taken as the statistical average of the cross-sections mentioned above. The upper curve (dotted) represents the cross-section = (0.14/y/ )ao for the hadronic annihilation in flight.

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