Positron Annihilation on Atoms and Molecules, Ph.D

UNIVERSITY OF CALIFORNIA SAN DIEGO Positron Annihilation on Atoms and Molecules A dissertation submitted in partial satisfaction of the requirements for the degree of Do ctor of PhilosophyinPhysics by Ko ji Iwata Committee in charge Cliord M Surko Chair John M Go o dkind William E Mo erner Lu J Sham John D Simon Copyright Ko ji Iwata All rights reserved The dissertation of Ko ji Iwata is approved and it is acceptable in quality and form for publication on microlm Chairman University of California San Diego iii iv Contents Signature Page iii Table of Contents iv List of Figures ix List of Tables xi Acknowledgments xiii Vita Publications and Fields of Study xv Abstract xviii Intro duction Simple illustration of a p ositron interacting with a molecule Background in p ositron physics Prediction and discovery of the p ositron Positron sources Physics involved in the interaction of a p ositron with a molecule Long range p olarization and dip olecharge interactions Shortrange interactions r a Potential from atomic nuclei Pauli exclusion principle Positronium atom formation Annihilation Interaction of p ositrons with solids liquids and gases Solids Positron mo derators Liquids Gases Outline of dissertation Overview of previous atomic and molecular physics studies us ing p ositrons Densegas and lowpressure exp eriments Densegas exp eriments Lowpressure exp eriments Annihilation rates v Anomalously high annihilation rates Momentum distribution measurements Ion mass sp ectrum measurements Description of the exp eriments Source and mo derator Beamline Positron trap Gashandling system Gases Liquids and solids Annihilation rate measurements ray sp ectral measurements Exp erimental pro cedure Sp ectral analysis Calibration and detector resp onse Annihilation rates Theoretical considerations Longlived resonances Large cross sections from the resonance collision Results Noble gases Inorganic molecules Alkanes and substituted alkanes Alkenes Oxygencontaining hydro carb ons Aromatics and saturated rings Deuterated hydro carb ons Deuterated alkanes Deuterated b enzenes Partially uorinated hydro carb ons Discussion Empirical scaling of annihilation rates Dep endence of annihilation rates on p ositron temp erature Recent theoretical work Semiempirical approach Large scale calculations for molecules Other issues Noble gas atoms Repro ducibil ity of measurements Delo calized b onds in b enzene vi Determination of p ositron annihilation sites in a molecule Current state of annihilation rate studies Concluding remarks on annihilation rate measurements Annihilation ray sp ectra Introduction Momentum distribution measurements Annihilation ray angular correlation measurements Dopplerbroadened ray sp ectral measurements Results Noble gases Helium Neon argon krypton and xenon Previous measurements from other exp eriments Inorganic molecules Hydrogen Other gases Alkanes Alkane alkene and alkyne Aromatics Fully halogenated carb ons Partially uorinated hydro carb ons Other organic molecules Annihilation on innershell electrons Sp ectral analysis b eyond the Gaussian approximation Discussion Concluding Remarks Positron annihilation with innershell electrons in noble gas atoms Introduction Exp eriment Results Concluding remarks for innershell electron annihilation Positron annihilati on in the interstellar media Introduction Positrons in the interstellar media Interstellar molecules Astrophysical p ositron annihilation simulation Description of the exp eriment Exp erimental results vii Summary of p ositron annihilation in the interstellar media Conclusion Summary of this dissertation Future work Concluding remarks App endices A Ion gauge sensitivity calibration for gases and molecular va p ors BTable of physical parameters of atoms and molecules CTable of annihilation ray sp ectra from atoms and molecules References viii List of Figures Schematic diagram a lowenergy p ositron interacting with a molecule Schematic illustration of p ositronium atom formation Schematic diagram of p ositron trapping in a defect Apparatus for measuring annihilation time sp ectra in highpressure exp eriments Schematic drawing of annihilation time sp ectrum in a highpressure exp eriment Annihilation time sp ectrum for helium gas DACAR sp ectra of noble gases Schematic overview of the exp erimental setup Schematic diagram of the source and coldhead assembly Schematic diagram of a PenningMalmb erg trap Schematic diagram of the p ositron trap electro de structure Gas pressure in the third stage of the p ositron trap during a pump out cycle Schematic diagram of the p ositron trap Aschematic diagram of a gashandling system for test substances that exist in gas form at atmospheric pressure Aschematic diagram of a gashandling system for test substances which are in liquid or solid form at atmospheric pressure Annihilation rate data for b enzened ray sp ectrum of the keV line from a Sr source tal values of Z Z plotted against Z Exp erimen e Theoretical and exp erimental values of Z for noble gases e Values of Z and structures of alkenes with six carb ons e Values of Z and structures of cyclohexane and substituted b ezenes e Ratio of Z for deuterated alkanes to those for protonated alkanes e Values of Z and structures of deuterated b enzenes studied e Empirical scaling of Z for nonp olar molecules containing only e single b ond ix Empirical scaling of Z for oxygencontaining hydro carb ons and e small molecules with a dip ole moment andor double b onds Empirical scaling of Z for alkenes ring hydro carb ons and sub e stituted b enzenes Dep endence of annihilation rates of noble gases on p ositron tem p erature Illustration of the momentum of an annihilating electronp ositron pair and the resulting raymomenta Observed sp ectra from H and Ne Annihilation ray sp ectrum from helium atoms Exp erimentally measured ray sp ectra from noble gases ray sp ectrum from p ositron annihilation on molecular hydrogen ray sp ectra for p ositrons annihilating with CO and CO The Gaussian linewidth for alkane molecules ray sp ectrum from p ositron annihilation on uoro ethane Fraction of p ositrons annihilating on uorine atoms in partially uorinated hydro carb ons rayspectrumofH ray sp ectrum of hexane The ray sp ectrum resulting from p ositrons annihilating on xenon atoms The ray sp ectrum resulting from p ositrons annihilating on kryp tonatoms The ray sp ectrum resulting from p ositrons annihilating on ar gon atoms Positron annihilation ray sp ectrum from the Galactic center Positron annihilation ray sp ectrum from the Galactic center using a Ge detector Molecular structures and Z of p olycyclic aromatic hydro carb ons e ray line for p ositrons annihilating on mixture of hydrogen and naphthalene molecules x List of Tables Parameters of the neon and tungsten mo derators and the p ositron trap Values of Z for noble gases e Measured values of Z for inorganic molecules e Measured values of Z for alkanes and substituted alkanes e Measured values of Z for isomers of p entane e Measured values of Z for alkenes e Measured values of Z for alcohols carb oxylic acids and ketones e Measured values of Z for ring molecules and aromatics e Measured values of Z for other large organic molecules e Measured values of Z for protonated and deuterated alkanes e Measured values of Z for deuterated b enzenes e Measured values of Z for partially uorinated hydro carb ons e The ray linewidths for noble gases The ray linewidths for H The ray linewidths for inorganic molecules The ray linewidths for hydro carb ons The ray linewidths for fully halogenated carb ons The ray linewidths for partially uorinated hydro carb ons The ray linewidths for other organic molecules Values of from ts to various mo dels r The annihilation

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