MgO thin films of different thicknesses were prepared on polished Ag-3wt%Mg alloy by an activation process at 500-650 degrees C under oxygen pressures of 0.5-10.0 Pa. The influence of the thickness of MgO film on the secondary electron yield of the Ag-3wt%Mg alloy is investigated. The alloy with an MgO film of moderate thickness of 65 +/- 1 nm shows the highest secondary electron yield of 10. Oxygen pressure below 10.0 Pa is crucial for the formation of intact MgO film on the top surface. Computational simulation analysis reveals that the maximum yield occurs when the penetration depth of primary electrons is 13-18 nm less than the thickness of MgO film. Electron supply of this extra MgO film is allowed by an electron tunneling mechanism under certain bias voltage. The replenishment of new electrons to the electron deficient surface is crucial for obtaining optimized secondary electron yields for Ag-Mg alloy cathodes. (C) 2016 Elsevier Ltd. All rights reserved.