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An Experimental Study of Thin Foil Positron Moderators and Positronium Interactions in Gases

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An Experimental Study of Thin Foil Positron Moderators and Positronium Interactions in Gases AN EXPERIMENTAL STUDY OF THIN FOIL POSITRON MODERATORS AND POSITRONIUM INTERACTIONS IN GASES A thesis submitted to the University of London for the degree of Doctor of Philosophy Nazrene Zafar Department of Physics and Astronomy University College London February 1990 ProQuest Number: 10797775 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 com plete 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 10797775 Published by ProQuest LLC(2018). Copyright of the Dissertation is held by the Author. All rights reserved. This work is protected against unauthorized copying under Title 17, United States C ode Microform Edition © ProQuest LLC. ProQuest LLC. 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106- 1346 ABSTRACT The use of thin single crystal tungsten, W(100), and nickel, Ni(lOO), foils as slow positron moderators in the transmission mode has been investigated. A simple annealing technique has been developed, employing resistive heating in low vacuum conditions of 6x10“2 Torr, to treat the foils of thicknesses in the range 1000-18000A. The maximum efficiencies obtained were (8.8±1.2)xl0^ from a 2000A W(100) foil and (6.511.OJIO"4 from a 5000A Ni(100) film in a system with a base pressure of 10~7 Torr. The considerations required to determine the efficiency, defined as the ratio of the slow positrons emitted to the number of fast positrons striking the foil, have been discussed in the context of the present arrangement. Comparisons show that the results obtained are a factor of 2.5-4.5 lower than calculated yields, however are in good agreement with values from experimental studies using more complex annealing procedures in higher vacuum conditions (>10"® Torr). The full width at half maximum of the energy distribution of the emitted positrons were found to be 1.7eV from W and 0.3eV from Ni compared to 2.8eV from W mesh. The timed positronium beam system developed by Laricchia et al (1988b) has been modified to allow the observation of n=2 state Ps formed in helium and argon gases. The excited state component of the positronium flux was quantified at incident positron energies of 16-52.5eV and found to compose, at maximum, 33% (He) and 42% (Ar) of the total positronium beam without correction for in-flight decay. Comparisons made with theoretical studies indicate that metastable S state positronium may constitute a large proportion of the excited state flux (*70%). Total positronium scattering cross-sections were extracted and the results obtained ranged from (4.5±0.8) to (7.610.8) xlO-20m2 in Ar for positronium energies of 17-41eV and fluctuated about *3xl0_20m2 in He in the regime 7-35eV from a minimum of (2.4±0.8) to a maximum of (3.511.3) xlO-20m2. The assumptions and corrections required to determine these values have been detailed and comparisons have been made with theory and with cross-sections for hydrogen scattering from helium and argon. Possible extensions of both the slow positron moderator and the positronium scattering investigations have been discussed. CONTENTS Page Abstract 2 Figure Captions 5 Table Captions 10 Acknowledgements 11 Chapter 1 Background 13 1.1 Introduction 13 1.2 Basic Properties of the Positron and Positronium 16 1.3 Positron Swarm Experiments 21 1.3.1 Lifetime technique 21 1.3.2 Two photon angular correlation technique 25 1.3.3 Doppler broadening technique 27 1.4 Slow Positron Beam Technique 29 1.4.1 Outline of technique 29 1.4.2 Moderator development 30 1.4.3 Source and moderator considerations 40 1.5 Slow Positron Beam Experiments in Gases 45 1.6 Motivation of the Present Study 56 Chapter 2 Positron Moderation 58 2.1 Introduction 58 2.2 Bulk Processes 60 2.2.1 Slowing down 60 2.2.2 Diffusion 66 2.3 Surface Processes 69 2.3.1 Free positron emission 69 2.3.2 Ps emission, trapping and reflection 76 2.4 Thin Film Moderators 79 2.4.1 Summary of processes 79 2.4.2 Use of thin film moderators 82 2.4.3 Aim of the study 85 Chapter 3 Experimental Study On The Use Of Thin Single Crystal Foils As Primary Transmission Moderators 87 3.1 Introduction 87 3.2 Foil Fabrication 88 3.2.1 W(100) foils 88 3.2.2 Ni(100) foils 89 3.3 Annealing 90 3.3.1 W(100) foils 90 3.3.2 Ni(lOO) foils 92 3.4 Test Beam 94 3.5 Deduction of B+ Flux 98 3.5.1 Source calibration 98 3.5.2 Backscattering coefficient 98 3.5.3 Solid angle effect 99 3.6 Transport Efficiency 100 3.7 Detection Efficiency 101 3.8 Results and Discussion 105 3.8.1 W(100) foils 105 3.8.2 Ni(100) foils 111 3.9 Summary 113 Chapter 4 Positronium Beam Development 120 4.1 Introduction 120 4.2 Positronium Formation In: 121 4.2.1 Solids 121 4.2.2 Gases 125 4.2.3 Gases - excited states 129 4.3 Developments in the Ps Beam Technique 131 4.4 Positronium Scattering 138 4.5 Aim of the study 147 Chapter 5 Experimental Study On The Production Of Positronium And Its Interactions With Atomic Gases 149 5.1 Introduction 149 5.2 Slow positron Production 149 5.3 Time of Flight Method 155 5.3.1 Remoderation and detection 157 5.3.2 Timing electronics 159 5.4 Results and Discussion 161 5.4.1 Preliminaries 161 5.4.2 Lifetime measurements 169 5.4.3 Yield measurements 171 5.4.4 Cross-section measurements 182 5.4.5 Summary 194 Chapter 6 Conclusions 197 References 202 FIGURE CAPTIONS Figure Page 1.1 Feynman diagrams of annihilation into one, two and 17 three photons 1.2 Energy distribution of y-ray emission from o-Ps decay 19 (Ore and Powell, 1949, Chang et ah 1982, 1985) 1.3 Energy level diagrams of hydrogen and positronium 20 1.4 Decay process of “ Na 22 1.5 Schematic diagram of a typical lifetime system 22 and Ar gas lifetime spectrum 1.6 Schematic diagram of ACAR system and spectrum 26 1.7 Two dimensional e' momentum distribution in: 26 (a) single crystal quartz (Manuel, 1981) (b) single crystal Cu (Haghgooie et al, 1978) 1.8 Schematic diagram of system for measuring the Doppler 28 broadening of annihilation radiation 1.9 Comparison of positron yield from a W(110) moderator 29 with the B+ spectrum from a 58Co source (Schultz and Lynn, 1988) 1.10 Schematic diagram of positron beam apparatus using the 34 boron self-moderator (Kauppila et al, 1981) 1.11 Moderator arrangements: (a) backscattering, (b) vane, 42 (c) grid (d) cup - solid Ne, (e) cone and (f) transmission 1.12 Schematic diagram of high brightness e+ beam 44 at Brandeis (Canter et al, 1987) 1.13 Smoothed e^H e total scattering cross-sections 49 (from Charlton, 1985a) 1.14 Values of gt for e+-K and e"-K collisions 49 (Stein et ah 1987) 1.15 Schematic diagram of electrostatic crossed beam apparatus 51 used in differential cross-section measurements (Hyder et ah 1986) 1.16 Schematic diagram of the proposed arrangement for 53 the BNL high intensity e+ beam system incorporating the e+-H experiment (from Lubell, 1987) 1.17 Experimental arrangement of e+-H ionisation cross-section 55 measurement study (Raith et al, 1989) 1.18 Values of e+-H ionisation cross-sections 55 (Raith et al, 1989) 2.1 The interaction of a positron beam of energy <100keV with 59 the near-surface region of a solid (Schultz and Lynn, 1988) 2.2 Positron energy loss, de/dt, versus energy, E in Al 61 (Nieminen and Oliva, 1980) 2.3 Positron implantation profile in Ni using a “ Na source 63 (Hansen et al, 1982) 2.4 Implantation profiles, P’(z) and P(z) of a “Na B+ 65 spectrum in Al, Cu and W (Vehanen and Makinen, 1985) 2.5 Stopping (implantation) profile for 3 and 5keV positrons 65 in semi-infinite Cu (Valkealathi and Nieminen, 1984) 2.6 Potential energy seen by electrons and positrons 70 at a surface ( Mills, 1981a) 2.7 Variation of re-emitted positron energy distribution from 74 Ni(100) and CO with incident energy (Fischer et a/,1986) 2.8 Slow positron yield versus positron workfunction of Cu 74 (Murray and Mills, 1980) 2.9 Normal component of kinetic energy for Ps emitted from 78 A l(lll) (Mills et al, 1983) 2.10 Ps fraction versus temperature for clean and oxygen 78 exposed A l(lll) surfaces (Mills and Pfeiffer, 1985) 2.11 Slow positron remoderation efficiencies versus 81 incident energy in C u(lll) (a) and W(110) (b) (Vehanen and Makinen, 1985) 2.12 (a) Differentiated energy spectrum of forward re-emitted 83 positrons from 2500A W(100) foil at incident energy of 12keV after two cleaning cycles (b) Differentiated energy spectrum of forward re-emitted positrons from 1000A W(100) foil at 5.85keV incident energy (Chen et al, 1985) 3.1 Diagram of "oven" arrangement employed in the annealing 91 procedure 3.2 Schematic diagram of test beam apparatus 93 3.3 Cross section of source and sample arrangement 95 3.4 Exploded view of channeltron holder and associated grids 96 3.5 Block diagram of the timing electronics employed in the 102 deduction of the channeltron efficiency 3.6 Variation of measured and calculated yield with 106 thickness for W foils 3.7 (a) Retarding spectrum of 5000A W(110) foil 109 (b) Differentiated spectrum of 5000A W(110) foil 3.8 Variation of measured and calculated yield with 112 thickness for Ni foils 3.9 (a) Retarding spectrum of 3000A Ni(100) foil 114 (b) Differentiated spectrum of 3000A Ni(100) foil 4.1 Time-of-flight spectra of Ps emission from A l(lll), 123 Cu(100), Ni(100) and Au(100) at 50eV incident e+ energy (Howell et al, 1986) 4.2 Energy spectra of the Ps detected using the 123 beam-foil technique (Mills and Crane, 1985) 4.3 Ps formation cross-sections in (a) He and (b) Ar 126 (Charlton et
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