In matter, the positron of the Ps can annihilate with a foreign electron with opposite spin in a process called "pick-off' annihilation, which occurs with the emission of two photons (again, higher order events are ignored).
Both channels, self- and pick-off annihilation, are available to Ps trapped in voids. A comparison of the rates of each yields (convoluted) information on pore size and density.
The energies of the 2 photons created in a pick-off process are restricted to be around the restmass of the annihilation particles (511 keV ± Doppler shift), as in the annihilation of positrons, whereas the photon energies in 3y-events span the whole range below 511 keV due to the one extra degree of freedom. This allows for the parameterization of the annihilation ratio , which can be calibrated to measure Ps decay .
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