Research Of Photon Counting Detector Based On Vernier Anode

The World Space Observatory/Ultraviolet （WSO/UV） is a worldwidemulti-national cooperation ultraviolet astronomy program. The long-slit spectrograph（LSS） is a spectral imaging payload with medium spectral resolution capability （R=1500to2500） at wavelength of102³20nm. The core component of the LSS is aphoton counting detector based on microchannel plate （MCP）.In this paper, the principles and methods of the UV ultra-weak light imaging anddetection technology, and the present research status and development trends at homeand abroad in the photon counting detection field are introduced. In order to meet therequirements of sensitivity and spatial resolution for LSS, the photon countingdetector solution based on Vernier anode is determined, and the prototype ismanufactured. The detector is mainly composed by the input window, photocathode,MCPs and the Vernier anode, and the entire detection system is composed by the UVlight source （254nm）, weakener and filters, detector, readout electronics and dataacquisition system.The design software of Vernier anode was developed, and the one andtwo-dimensional Vernier anode were fabricated. The detector imaging software wasdeveloped, and the imaging results of one and two-dimensional Vernier anodedetectors were gained. In the conditions of the anode structure being optimized, thedetector resolution experimental results revealed that: the resolution of No.1anodewas up to70μm （36×36mm）, and the resolution of No.2anode was up to88μm （@30×30mm）. On this basis, the influence of the count rate, gain of MCPs, andacceleration electric field of MCP to anode on the spatial resolution is provided, theresults revealed that: the voltage of two cascaded MCPs should be more than1800V,if the count rate is at the range of0.5¹5kHz, its influence on the resolution can beignored. The image drift due to long time acquisition and the image distortionphenomenon are analyzed in theory. Using photocathode can improve the quantum efficiency of the detection system. The Au cathode experimental results revealed that:improving the quantum efficiency can improve the resolution of the imaging system.Using the grid between the MCP and the anode can inhibit the secondary electrons onthe anode. Grid experiment: if the voltage of MCP to the grid is too high, modulationdistortion occurs, and grid has little contribution to the spatial resolution. Theexperimental calibrations of the count sensitivity and dynamic range for the detectionsystem were finished, the count sensitivity calibration results show that: the lowerthreshold of MCP voltage and data acquisition both affect the measurement of thecounting sensitivity; the dynamic range tests focused on the system imaging, theresults: the normalized dynamic range is about10⁵.While the detection system was built and the ultra-weak radiation image wasreconstructed, the analysis of the quantum properties and statistical properties ofphoton counting detector, the time characteristics of the detector, the charge clouddistribution characteristics of anode, the secondary electron emission characteristics,noise characteristics, quantum efficiency and dynamic range were provided. We foundthat the count rate was proportional to incident photon rate within a certain range. Ifthe optical power was too high, the counting loss caused by the pulse pile and deadtime of the system was serious, and the linear relationship may be damaged. ThePreliminary quantum efficiency of the Bare MCP detector using two cascaded MCPswas measured. It was found that the quantum efficiency of the detector was very low,which was approximately10^-7（at254nm）. Through the calculation of the timecharacteristics of the detector, it was found that the time resolution （FWHM） ofdetector was about a few nanoseconds, and the dead time of detection system wasabout10μs if considering the readout electronics. Without considering theinter-electronic coulomb interaction and diffusion effects, the charge cloud radius wascalculated as about1.4mm （at15mm,300V MCP to anode） using "ballistic" model.According to the experimental results, the SNR of the Bare MCP detector using twocascaded MCPs was about2000, the noise equivalent power （NEP） was about dozensof picowatts. A new estimation method of minimum detectable optical power isproposed, and the results show that: the minimum input optical power is about 10^-1410^-13W for limiting imaging. According to dead time of system and thedistribution characteristics of photon counting, the calculation results of the maximumdetection rate, the minimum detectable power and the maximum record rate of photonevents for the system were about1MHz, several picowatts and37kHz respectively.The decoding principles of the one and two-dimensional Vernier anodes areprovided. According to the experimental phenomenon of ghosting, the noise model ofghosting was established, and the theoretical calculations showed that: the distributionof the phase deviations was Gaussian distribution, the phase deviation threshold ofghosting was0.1rad or the total charge noise of ghosting was about10⁴e rms （n=4）.The Monte Carlo simulation for System imaging process was provided, and theimaging and simulation software was developed based on VC6.0. The resolutiontheoretical calculation results for different simulation electronic gains showed that:the simulation electronic gain had a greater impact on the spatial resolution of theX-direction. The resolution for Monte Carlo simulation was about300μm （at G=10⁵）.According to the noise transmission model and Gaussian noise model, the spatialresolution of detector was estimated in theory. It was revealed that spatial resolutionwas related with the anode structure, size, the SNR of signal and so on.