Photoproduction of relativistic positronium atoms on extended targets

It is shown that the yield of relativistic positronium atoms in photoproduction on extended targets is much larger than usually accepted. The cause of such a difference is transparent. At high energies, when the positronium atom formation time τf becomes greater than the target length $L,$ it is the free e⁺e^- pair that propagates through the target and the formation of positronium atoms takes place out of the target. In the framework of the light-cone technique we consider the interaction of an e⁺e^- pair with the screened Coulomb field of atoms in matter and obtain the analytical expressions for relativistic positronium atoms photoproduction cross sections on extended targets. The relation to free pair photoproduction on extended targets is discussed.