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A photoemission diode was used to measure total backwards electron emission with unfiltered and filtered photon spectra from the SPI-600 X-ray generator. The samples consisted of thick Cu, Ni and Sn foils, Sn and Ni plated Cu wires used in cable fabrication, and Cu foils plated with Au and Rh. For the filtered spectrum the Cu emission is found to be 1.8 times larger than that calculated with Dellin-MacCallum values of photo-Compton currents. The Sn foil emission is 2.6 times that of the Ni, but the Sn plated wire emission is found to be only 1.7 times that of the Ni plated wire. When difference in electron average energy is taken into account, cables constructed from the Sn plated wire should show a SGEMP vacuum response that is only 25% larger than cables constructed from the Ni plated wire if all other parameters, e.g. gaps, are held constant.