We report a measurement of photon impact ionization of $K$ and $L$ shell of Au and $K$ shell of Ag targets in the 1-GeV energy range. We show that the cross section is dominated by a contribution from a new channel called vacuum-assisted photoionization. In this process the energy-momentum balance associated with the removal of the innershell electron is obtained by conversion of a high-energy photon into an electron-positron pair. This measurement is consistent with the theoretical prediction that vacuum-assisted photoionization is the most probable ionization mechanism at very high energies.

• Received 30 September 2002

DOI:https://doi.org/10.1103/PhysRevLett.90.153002