Search for Two-Quantum Annihilation of Positrons in Flight with K -Shell Electrons

The two-quantum annihilation of positrons in flight with the $K$-shell electrons of silver has been investigated using 300-keV positrons. The experimental result (7.7 \ifmmode\pm\else\textpm\fi{} 6.4) \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}27}$ ${\mathrm{cm}}^{2}$/${\mathrm{sr}}^{2}$ has been obtained for the double-differential angular cross section of this process, at 30 and - 100\ifmmode^\circ\else\textdegree\fi{} for each annihilation photon with respect to the incident positron direction.