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Research on the key technology of ultraviolet photon counting detectors based on MCP

Author: ZhaoFeiFei
Tutor: ZhaoBaoSheng
School: Xi'an Optics and Fine Mechanics
Course: Physical Electronics
Keywords: Photon counting imaging detector (PCID) Microchannel plate (MCP) Wedge and strip anode (WSA) Charge induction readout
CLC: TN23
Type: PhD thesis
Year: 2010
Downloads: 112
Quote: 1
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Abstract

The World Space Observatory–Ultraviolet （WSO-UV） is a space telescope projectled by Russia, with contributions from a number of other countries in the world. In themission, Long Slit Spectrograph （LSS） instrument will provide low resolution spectrain the range102-320nm, the study of which is ongoing in China. Therefore, a task oftwo photon counting detectors respectively in the range102-170nm and160-320nmis being carried out.The ultraviolet photon counting imaging detector based on microchannel plate withinduction readout is adopted in this paper. Because it has many merits, such as lowbackground noise, high signal and nose ratio, high detection quantum efficiency,photon-counting sensitivity, long range, large format, radiation tolerance, high spaceresolution and time resolution, it is fit for being used in space detection and astronomyobservation. The research of this paper is focused on the key technology of the nearultraviolet detector （NUVD）.The photocathodes with high resistance must be prepared on transparent conductivesubstrate in order that electrons are supplemented during photocathodes are emittingphotoelectrons. Five kinds of conductive thin films were prepared on MgF2substrateby vacuum deposition. The surface morphology, sheet resistance, microstructure andtransmittance curves in the wave band of190～800nm are investigated by opticalmicroscope （OM）, four-probe method, high resistance meter, X-ray diffractomer（XRD） and spectrophotometer. Variation range of transmittance is acquired in thewave band of160～320nm when the sheet resistance reaches107/□. It is shownthat the average transmittance of Ni thin film is highest and is stable. Therefore, Nifilm with10nm is adopted for conductive substrate of Cs₂Te cathode.Through analysis on the performances of CsI, KBr and KI photocathodes, it isdetermined that CsI is the best one sensitive to the wavelength range of102-170nm.The preparation method, emission mode, evaporation rate, substrate temperature and some measures are also determined. Through analysis on the performances of Cs₂Teand Rb₂Te photocathodes, it is determined that Cs₂Te is the best one sensitive to thewavelength range of160-320nm. We prepared two prototypes with fluorescencescreen as readout and tested the uniformity of photocathodes, which shows that thepreparation technology is feasible.When the detector adopts induction readout method, the preparation parameters ofGe film will influence the performances of the photon counting imaging system. TheGe thin films were fabricated by electron beam evaporation. The structures of Ge thinfilms deposited on ceramic and quartz glass substrates and influences of technicalparameters on resistance were studied. The XRD analysis of thin films deposited onthe two substrates show that the thin films both have cubic amorphous Ge structure.The resistance can be controlled by anneal, depositing rate, or thickness of film. Theperformance of the system, which adopted Ge layers with different resistance，wasstudied. These results suggest that resistance of the charge induced layer influencesspatial resolution less than the counting rate.The imaging principle of induction readout mode is studied. In this paper, weestablished the theoretical model of induction readout circuit, explained the influenceof Ge film sheet resistance and substrate capacitance on the performances of thesystem, and validated it by experiments. In addition, the influence of charge cloudfootprint on this system was also discussed here, and Modulation,"S" distortion andtheir cause are focused. Then，Monte Carlo Simulations for anode decode withdifferent charge cloud size are presented.Annealing temperature during NUVD making process will affect properties of Gefilm, and consequently deteriorate the performance of detector. Therefore, theinfluence of annealing temperature on Ge films and detector is studied in order todetermine the crucial parameters. The Ge films are prepared on ceramic and quartzglass by electron gun, and then analyzed by scanning electron microscope （SEM）,high resistance meter and XRD. The results suggest that the optimum choices ofsubstrate and annealing temperature are ceramic plate and250℃respectively.Presently, the detector with induction readout all adopted Ge film as charge induction layer. In this paper, we first adopted Si film and researched on theinfluences of film parameters on this system. Moreover, the performances of thissystem with two kinds of films were compared. The experiment testified that thecharge induction layer is not limited to Ge film. If only the sheet resistance is properand the structure is more uniform and compact, the kind of films can be used.The Au film with15nm thickness was adopted as transmission photocatode,because it is stable and matches the light source in this experiment （Mercury lamp andnarrow band optical filter of253.7nm）. The results indicate that this measure canefficiently improve the resolution and count rate of this system.During the preparation process of image intensifiers, the technique of photocathodetransfer and indium seal not only makes device design more flexible but also canimprove gain, time and space resolution. However, it needs to be improved further inthe aspect of gas tightness. In this paper, metal multilayer films were prepared onglass substrate by vacuum deposition in order to improve the wettability of In-Snalloy with substrate. The wettability and spreading properties of In-Sn alloy with glassand the surfaces of five film structures were compared. The experiments of indiumseal indicated that the chance of leak is little when the film structure of glass/Cr/Ni/Cu/Ag was adopted.The prototype of ultraviolet photon counting imaging detector based on inductionreadout was proposed, which adopted Wedge and Strip Anodes （WSA） due to itsstructure is relatively simple. It has1375counts/s count rate and better than350μmresolution under this count rate. Aiming at the problems in the test, a test systemproject is proposed in order to acquire the more precise data of resolution and linearityof the detectors.

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