Abstract
Outgassing of internal materials is one of the main factors that cause vacuum deterioration of low-light-level image intensifiers and shorten their lifetime (especially storage lifetime). To perceive the attenuation mechanism of the vacuum degree of image intensifiers from the microscopic view, the outgassing characteristics of lead silicate glass used for the fabrication of microchannel plates (MCPs), which are the core components of image intensifiers, were numerically simulated by giant canonical Monte Carlo and molecular dynamics methods. A glass structure model of the third-generation MCPs was constructed, and the diffusion coefficients and molecular motion trajectories of hydrogen, carbon dioxide, carbon monoxide, water vapor, oxygen, and methane gases, together with their outgassing rates and cumulative outgassing amounts, were calculated based on the condensed-phase optimized molecular potentials for atomistic simulation studies force field. The calculation results show that the rising temperature of MCP glass augments the diffusion coefficients of these gases, which makes the outgassing rates increase in the initial stage but then decrease relatively quickly. Among these six kinds of gas molecules, hydrogen molecules have the largest skip distance and the highest diffusion coefficient because MCP glass can supply more effective diffusion paths for the gas molecules with a relatively small size. When an MCP glass lies in vacuum, first, the cumulative outgassing amounts of various gases from it increase rapidly and then gradually reach stable values, and the cumulative outgassing amount of hydrogen tends to reach a stable value faster than that of other gases due to its highest diffusion coefficient.
Highlights
Low-light-level image intensifiers are ultrahigh vacuum electronic devices, which have a vacuum microcavity with a pressure of less than 1.0 × 10−6 Pa.1 In order to maintain the ultrahigh vacuum condition, it is necessary to place a certain amount of getter in the microcavity
Glass structures with dissolved gas molecules were constructed under the conditions of an absolute external pressure of 50 kPa and a glass temperature of 298 K with the Sorption module, which is usually used to analyze the adsorption isotherms, binding sites, binding energies, diffusion routes, and molecular selectivity for studying the adsorption properties of molecules in solid materials, and the initial concentrations of dissolved gases in an microchannel plates (MCPs) glass were calculated
It can be concluded that the final stable value of the cumulative outgassing amount of a gas depends on its initial concentration in the MCP glass, and the diffusion coefficient determines the speed taken for its cumulative outgassing amount to reach a stable value
Summary
Low-light-level image intensifiers are ultrahigh vacuum electronic devices, which have a vacuum microcavity with a pressure of less than 1.0 × 10−6 Pa. In order to maintain the ultrahigh vacuum condition, it is necessary to place a certain amount of getter in the microcavity. The gas entering the microcavity will adsorb on the surface of the photocathode and the inner walls of microchannels in the microchannel plate (MCP), which reduces the performances of the photocathode and MCP and thereby shortens the lifetime of image intensifiers. During the storage and usage of image intensifiers, these gases will be released gradually through diffusion and desorption and enter the ultra-high vacuum microcavities, leading to the device performance degradation. The previous research mainly focused on the macroscopic outgassing characteristic measurement, lacking deep explorations on the microscopic outgassing process and mechanism of MCPs. In this work, the outgassing behavior of the third-generation MCP glass in ultrahigh vacuum, which had undergone degassing treatment, was studied by numerical simulation based on grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) methods. The diffusion coefficients and molecular motion trajectories of various gases in the MCP glass, along with their outgassing rates and cumulative outgassing amounts, were calculated, and the outgassing mechanism of the MCP glass was further elucidated
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