Research On The Electron Transport And Noise Characteristics Of Image Intensifiers

Image intensifiers are used as the core commponets of low-light-level night vision system and ultraviolet(UV)imaging detector system.And their capabilities of detecting and imaging are mostly governed by the resolution and signal to noise ratio(SNR).The resolution of an image intensifier is directly related to the electron transport in photocathode and microchannel plate(MCP),moreover,the noise characterization of an image intensifier is characterized by SNR.Thus,in this thesis,the researches on the electron transport and noise characteristics of image intensifiers have been carried out.Based on the atomic structure model for uniform-doping GaAs photocathode and the ionization sacattering model,the effects of doping concentration and electron diffusion length on the electron transport in transmission-mode GaAs photocathode have been qualitative analysed.At the same time,the modulation transfer function(MTF)expressions for exponential-doping GaAs,GaAlAs,and GaN photocathodes are obtained from the electron transport equation.Furthermore,the effects of the electron diffusion length,the emission layer thickness,the absorption coefficient and the velocity of back-interface combination for these photocathodes on their resolution characteristics and quantum efficiencies have been theoretically calculated on the basis of MTF expressions.The results show that,compared with the uniform-doping structure,the exponential-doping one can not only obtain higher quantum efficiency but also upgrade the resolution.By using numerical method,the characteristics of electron transport and resolution of MCP have been researched analogously.First,the electric field distribution of MCP is calculated.Second,the trajectories of the electron transport in MCP are simulated by solving the electron transport equation.At last,the corresponding MTF curves are plotted.Furthermore,the effects of tilt angle ?t,channel diameter D,voltage VM across the MCP,and output electrode depth h on the characteristics of electron transport and of MTF have been analysed.The simulated results show that,for the case of VM =900V,?t =14°,D=5.0?m and h=10?m,the MCP has a good performance of electron transport and resolution.The noise characteristics of MCP have been researched through elvaluating its noise factor(NF).First,the main factors affecting the noise factor of MCP are analyzed theoretically.Then,by employing a method using the sine random signal with Poisson distribution as the noise excitation for electron source,the evaluation of NF has been implemented.The results show that the NF value of filmed MCP is lower than 1.8 which this value increases by approximately 14.7%,compared with unfilmed one.The NF value of unfilmed MCP with open area ratio(OAR)of 72%is lower than 1.3 which this value decreases by approximately 8.9%,compared with one with OAR of 64%.In addition,by treating MCP with electron brush moderately,the NF value of MCP can be reduced to a certain extent.Notably,the ion barrier film can prolong the life time of image intensifiers at the cost of degrading the noise characteristics of MCP,and thus the selection of optimum film thickness should be considered.By testing the signal to noise ratio at the output end(SNRo)and the halo effect of low-light-level image intensifier,the noise characteristics of image intensifiers have been studied.The influence of pro-proximity pulse voltage on the SNRo and on the halo effect of generation ? low-light-level image intensifier has been mainly researched.Meanwhile,the SNRo of super generation ? and generation ? image intensifiers drived by the direct current power supply have been measured and analyzed.The test results show that,when the high and low level for pro-proximity pulse voltage are-300V and 0.2V,respectively,the proximity pulse voltage with the duty cycle value of larger than 60%has a little influence on the SNRo and halo of generation ? image intensifiers.Besides,the SNRo of both super generation ?and generation ? image intensifiers drived can be improved with increasing voltage of photocathodes.But as the photocathode voltage reaches a certain value,both of SNRo incline to saturation.When the voltage across the MCP increases gradually in the range of 700V to 800V,the SNRo of super generation ? image intensifier improves obviously,however,the increase in SNRo is slowed down and even reduced as continuously increases the voltage across the MCP.The works in this thiese would provide a theoretical guide,an experimental support,and an evaluation tool in developing the image intensifiers with low noise and high resolution.