  The intense emission of 511 keV photons from the Galactic center and within
terrestrial thunderstorms is attributed to the formation of parapositronia'
clouds. Unbound electron-positron pairs and positronia can be created by strong
electromagnetic fields produced in interactions of electrically charged
objects, in particular, in collisions of heavy nuclei. Kinematics of this
process favours abundant creation of the unbound electron-positron pairs with
very small masses and the confined parapositronia states which decay directly
to two 511 keV quanta. Therefore we propose to consider interactions of
electromagnetic fields of colliding heavy ions as a source of low-mass pairs
which can transform to 511 keV quanta. Intensity of their creation is enlarged
by the factor Z$^4$ (Z is the electric charge of a heavy ion) compared to
protons with Z=1. These processes are especially important at very high
energies of nuclear collisions because their cross sections increase
proportionally to cube of the logarithm of energy and can even exceed the cross
sections of strong interactions which may not increase faster than the squared
logarithm of energy. Moreover, production of extremely low-mass $e^+e^-$-pairs
in ultraperipheral nuclear collisions is strongly enhanced due to the
Sommerfeld-Gamow-Sakharov (SGS) factor which accounts for mutual Coulomb
attraction of non-relativistic electrons to positrons in case of low
pair-masses. This attraction may lead to their annihilation and, therefore, to
the increased intensity of 511 keV photons. It is proposed to confront the
obtained results to forthcoming experimental data at NICA collider.
