The Design of the Emission Layer for Electron Multipliers

The electron multipliers gain is closely related to the secondary electron emission coefficient (SEE) of the emission layer materials. The SEE is closely related to the thickness of the emission layer. If the emission layer is thin, the low SEE causes the low gain of electron multipliers. If the emission layer is thick, the conductive layer can't timely supplement charge to the emission layer, the electronic amplifier gain is low too. The electron multipliers usually choose Al₂O₃ and MgO film as the emission layer because of the high SEE level. MgO easy deliquescence into Mg(OH)₂ Mg₂(OH)₂CO₃ and MgCO₃ resulting in the lower SEE level. The SEE level of Al₂O₃ is lower than MgO, but Al₂O₃ is stable. We designed a spherical system for testing the SEE level of materials, and proposed to use low-energy secondary electrons instead of low-energy electron beam for neutralization to measuring the SEE level of Al₂O₃, MgO, MgO/Al₂O₃, Al₂O₃/MgO, and precisely control the film thickness by using atomic layer deposition. We propose to compare the SEE under the adjacent incident electrons energy to partition the SEE value of the material, and obtain four empirical formulas for the relationship between SEE and thickness. Since the main materials that cause the decrease in SEE are Mg₂(OH)₂CO₃ and MgCO₃, we use the C element atomic concentration measured by XPS to study the deliquescent depth of the material. We propose to use the concept of transition layer for SEE interpretation of multilayer materials. Through experiments and calculations, we put forward a new emission layer for electron multipliers, including 2-3 nm Al₂O₃ buffer layer, 5-9 nm MgO main-body layer, 1 nm Al₂O₃ protective layer or 0.3 nm Al₂O₃ enhancement layer. We prepared this emission layer to microchannel plate (MCP), which significantly improved the gain of MCP. We can also apply this new emission layer to channel electron multiplier and separate electron multiplier.