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 inner-shell 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.
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