  The energy and momentum conservation laws prohibit positron-electron
annihilation in vacuum producing the single photon only. It can produce two
photons in vacuum, or the process is possible with a participation of the third
body. The situation is different in a single crystal with one of the leptons
(e.g. positron) moving in the channeling (or in the quasi-channeling) mode when
the transverse motion of a particle is bound or sufficiently uneven. It may
increase sharply the probability of the single photon annihilation process.
