Ultraviolet Photon Counting Imaging System Key Technology

An Extreme Ultraviolet Camera is designed in the second and third project of Chinese Moon Program to study the distribution of plasma in Earth's plasmasphere by imaging the distribution of the He~+ ion through its emission at 30.4 nm.The Extreme ultraviolet imaging detector is the key part of the camera,and it is very importanc to study the extreme ultraviolet detector.The wedge and strip anode detector based on microchannel plate is adopted in this paper.Anode detectors have many merits,such as low background noise,high signal and nose ratio,high detection quantum efficiency,photon-counting sensitivity,long range,large format,radiation tolerance,high space resolution and time resolution.As a result,the anode detector has been widely used in space detection and astronomy observation.According to the electron readout mode,anode detectors can be divided into two types,one is direct collection readout mode,and the other is induction readout mode.The wedge and strip anode detector with direct collection readout and induction readout are studied.As the key device of anode detector,the(?)ndependent design and manufacture is absolutely critical.Utilizing the coating equipment and the lithography equipment,we design and process the wedge strip anode independently,and the cost and period are reduced significantly.There are two types of readout mode for anode detector.For direct collection readout anode detector,the performance of three wedge and strip anode detectors with different parameter is studied.The dark count,resolution,linearity and count rate of the detector are measured and the results show that the wedge and strip anode detector with the smallest coupling constants between interelctrodes has the best resolution. The resolution along X direction is close to 75μm,and the resolution along Y direction is better than 62.5μm.In addition,the linearity of detector is determined by the distance between the microchannel plate and the wedge and strip anode,which can be improved by modulating the distance of microchannel plate and anode.The three anode detectors have higher resolution when the count rates are less than 10000 counts/s.For induction readout anode detectors,the performance of the dectors with different pitch and different insulting gap are studied.For the anode detector with big pitch and narrow insulting gap(NO.2-WSA),when the distance of MCP and Ge layer is 10 mm and 1 mm,the performance of the detector under different Ge substrate,different thickness of Ge layer substrate,different sheet resistance of Ge layer and different thickness of Ge layer are tested.The results suggest that the thicker the Ge layer substrate is,the greater the image distortion is and the lower the count rate is;the higher the sheet resistance of Ge layer is,the lower the count rate is,and under high photon flux the less the image area is;under same light intensity,the thicker the Ge layer is,the lower the count rate is.For the anode detector with big pintch and wide insulting gap(NO.3-WSA),when the distance between microcharmel plate and Ge layer is 1 mm,the performance of anode detector with 1.5 mm ceramic substrate and 2.0 mm ceramic substrate is tested respectively and compared with the anode detector with big pintch and narrow insulting gap.The results of dark count image show that the image distortions of the two detectors are comparative when the thickness of substrate is same and the image distortions are increased with the substrate thickness increasing.The resolution results which is tested inΦ42 mm show that the resolution of the anode detector with big pintch and wide insulting gap with 2 mm ceramic substrate along the X direction is nearly 100μm and that along Y direction is better than 100μm.The linearity tests suggest that the linearity of the anode detector with big pintch and wide insulting gap is better when the thickness of ceramic substrate is 1.5 mm.In addition,as the light intensity is same and the thickness of ceramic substrate is 1.5 mm,the count rate of the anode detector with wide insulting gap is higher than that of the anode detector with narrow insulting gap.For the direct collection readout mode,there is distortion of photon counting image. In order to correct the image distortion,three different image processing methods are adopted.The first correction method is the physical correction.The image is corrected by modulating the capacitive coupling constants between electrodes W-S,W-Z,and S-Z(W,S,and Z represent the electrodes of wedge,strip,and Z respectively).The second correction method is the global correction,and the distorted image is corrected through a uniform transformation matrix.The third correction method is the local correction method,and the distorted image is divided into many small areas,each area is corrected by a local matrix.The performance comparision results show that the local correction method can provide the best visual inspections and performance evaluation values among the three correction methods.At last,the induction charge footprint size is estimated by theoretical calculation and experimental image.Both the theoretical calculation and experimental image estimation show that the charge footprint size is significant affected by the thickness of Ge layer substrate,and it is not sensitive to the distance between the microchanne plate and Ge layer.The designed extreme ultraviolet photon counting imaging detector based on direct collection readout can meet the requirement of the second project of Chinese Moon Program.It hasΦ48 mm effective area,less than 1 counts/s·cm~2 dark count rate, 16000 counts/s count rate and better than 75μm resolution under 1000 counts/s count rate.