Imaging Photon Detectors and their use with Single and Multiple Fabry-Perot Etalon Systems for Atmospheric Wind Measurements by Ian MeWhirter Department of Physics and Astronomy University College London Thesis presented for the degree of Doctor of Philosophy of the University of London July 1993 ProQuest Number: 10106689 All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. uest. ProQuest 10106689 Published by ProQuest LLC(2016). Copyright of the Dissertation is held by the Author. All rights reserved. This work is protected against unauthorized copying under Title 17, United States Code. Microform Edition © ProQuest LLC. ProQuest LLC 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106-1346 This thesis is dedicated to the memory of Hilda MeWhirter ABSTRACT Imaging Photon Detectors are extremely sensitive imaging devices capable of detecting single photons of light. The Fabry-Perot étalon, a multiple beam interference device, is capable of detecting very small changes in the wavelength of light and is thus well suited to the determination of Doppler shifts. The combination of these two devices has enabled the realisation of a series of operational interferometers for the measurement of wind velocities in the upper atmosphere. This is achieved by measuring Doppler shifts in optical phenomena occurring at high altitudes. The instruments have been successfully deployed in Northern Scandinavia, North America and other locations world-wide. A full description of the component parts of the IPD and its associated electronics is presented. The theory and practical limitations of the device are discussed, together with a critical performance analysis of the complete imaging system. In addition to the IPDs for the Fabry-Perot interferometers, special types have been built for high time-resolution atmospheric lidar and a rocket-borne auroral imager. The Fabry-Perot étalon is described and the practical aspects of incorporating it into an interferometer are considered. The instruments are required to run unattended for extended periods, so particular care has been paid to long term aspects of stability, reliability and safe operation. Etalons can be tuned using piezo-electric transducers to vary the cavity length, in conjunction with capacitance sensors which determine the precise amount of movement. Such devices are termed capacitance stabilised étalons. These étalons can be combined in multiple étalon systems which provide greatly improved optical filtering. This allows measurements to be made against the higher background illumination encountered during daylight hours. Triple étalon interferometers have been built which have been flown on balloons in Texas and operated from the ground in Northern Sweden. CONTENTS Abstract............................................................... 3 List of Illustrations................................................ 11 List of Acronyms..................................................... 15 CHAPTER 1 - INTRODUCTION............................................. 17 1.1 Overview........................................................ 17 1.2 The aim of this thesis.......................................... 17 1.3 The primary scientific objectives.............................. 18 1.4 The requirements for the instrumentation....................... 19 1.4.1 Detector development.....................................20 1.4.2 Etalon development....................................... 21 1.5 The subjects discussed in this thesis.......................... 22 1.5.1 The Imaging Photon Detector............................. 22 1.5.2 The Fabry-Perot Interferometer.......................... 23 1.5.3 Capacitance Stabilised Etalons.......................... 23 1.5.4 The Triple Etalon Interferometer........................ 23 1.5.5 Future developments...................................... 24 1.6 Division of responsibilities....................................24 CHAPTER 2 - THE MICROCHANNEL PLATE...................................25 2.1 Introduction.................................................... 25 2.2 Theory of operation............................................. 26 2.3 Fabrication..................................................... 26 2.4 Gain and plate configuration....................................27 2.4.1 Single plates............................................ 27 2.4.2 Two and three plate assemblies.......................... 28 2.4.3 Five plate assemblies....................................30 2.4.4 Curved microchannel plates...............................30 2.5 The ion barrier................................................. 31 2.6 MicroChannel plate lifetime.....................................33 2.7 Magnetic field immunity......................................... 33 2 . 8 Dead time....................................................... 34 2.9 Dark count...................................................... 36 CHAPTER 3 - THE PHOTOCATHODE.........................................44 3.1 Introduction.................................................... 44 3.2 Photoemission in metals.........................................45 3.2.1 Long wavelength response limit...........................46 3.2.2 Quantum efficiency....................................... 47 3.2.3 Thermionic emission...................................... 48 3 . 3 Semiconductor photocathodes.....................................48 3.4 Fabrication of multi-alkali photocathodes...................... 50 3.5 Negative electron affinity photocathodes....................... 50 3.6 Fabrication of negative electron affinity photocathodes....... 51 3.7 Measurement of photocathode sensitivity........................ 51 3.8 The proximity focussed lens.....................................53 3.8.1 The Gaussian distribution................................ 54 3.8.2 Radial energy............................................ 55 3.8.3 Electron ballistics...................................... 55 3.8.4 Optimisation of the gap size and potential.............. 58 CHAPTER 4 - POSITION SENSING ANODES.................................. 63 4.1 Introduction.................................................... 63 4.2 Resistive line theory........................................... 63 4.3 The rise-time method............................................ 64 4.4 The charge amplitude ratio method.............................. 64 4.5 The two dimensional resistive anode............................ 65 4.6 The distortionless resistive anode............................. 67 4.7 Pulse amplification............................................. 68 4.8 Noise considerations............................................ 70 4.9 Noise performance of the distortionless anode.................. 71 4.10 The Wedge and Strip anode....................................... 71 CHAPTER 5 - SIGNAL PROCESSING ELECTRONICS FOR THE RESISTIVE ANODE... 77 5.1 Methods of signal processing....................................77 5.2 Development of the rise-time method............................ 77 5.3 Development of the charge amplitude ratio method...............78 5.3.1 First generation charge ratio electronics............... 78 5.3.2 Second generation charge ratio electronics.............. 79 5.3.3 Third generation charge ratio electronics............... 81 5.3.4 Fourth generation charge ratio electronics.............. 82 5.4 Operation of the Signal Processing Unit........................ 83 5.5 Consideration of the positional algorithms..................... 85 5.6 Dead time and its effect on photon counting efficiency......... 86 5.6.1 Efficiency of a non-paralysable system.................. 86 5.6.2 Efficiency of a paralysable system...................... 87 5.6.3 Systems combining paralysable and non-paralysable stages................................... 87 5.7 Count rate limitations of the IPD system....................... 88 5.7.1 Processing electronics dead time.........................88 5.7.2 Moderate pulse pile-up................................... 89 5.7.3 Severe pulse pile-up..................................... 89 5.7.4 MOP local dead time...................................... 90 5.7.5 Speed limitations in data transfer...................... 90 5.8 The dual port memory............................................ 91 5.9 Dynamic range and pulse height distribution.................... 93 5.9.1 Implications of the quasi-Gaussian distribution........ 93 5.9.2 Gain matching............................................ 94 5.10 Amplifier design considerations................................ 96 CHAPTER 6 - PRACTICAL REALISATION OF THE IMAGING PHOTON DETECTOR...106 6.1 Introduction................................................... 106 6.2 The manufacture of the detector............................... 106 6.2.1 The prototype device....................................106 6.2.2 Devices with opaque photocathodes.......................106 6.2.3 Devices with transparent photocathodes................. 107 6.2.4 The ITT double stack devices........................... 109 6.3 The commissioning process for the tubes....................... 109 6.4 Operating potentials for the IPD electrodes................... 110 6.4.1 Voltages for the single stack device.................... 110 6.4.2 Voltages for the double stack device...................