A search for low-mass short-lived states coupling to the electron and the positron and a study of the single-quantum annihilation of positrons.
Item
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Title
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A search for low-mass short-lived states coupling to the electron and the positron and a study of the single-quantum annihilation of positrons.
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Identifier
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AAI9325166
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identifier
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9325166
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Creator
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Wu, XiaoYe.
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Contributor
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Advisers: Kelvin G. Lynn | L. O. Roellig
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Date
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1993
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Language
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English
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Publisher
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City University of New York.
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Subject
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Physics, Atomic | Physics, Elementary Particles and High Energy
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Abstract
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Two experiments performed at Brookhaven National Laboratory, using a 3 MeV energy-tunable, mono-energetic positron beam, are described in this thesis.;i. A search for low-mass short-lived states coupling to {dollar}e\sp+e\sp-{dollar}. The experimental search for a previously unobserved light neutral particle in the mass (1560 keV {dollar}\sbsp{lcub}\sim{rcub}{lcub}<{rcub}\ M\sb{lcub}X\sp0{rcub}\ \sbsp{lcub}\sim{rcub}{lcub}<{rcub}\ M\sb{lcub}X{rcub} 1860{dollar} keV)--lifetime ({dollar}\tau\ \sbsp{lcub}\sim{rcub}{lcub}<{rcub}\ M\sb{lcub}X{rcub}3.5\times 10\sp{lcub}-3{rcub}{dollar} sec) region was motivated by the observations of anomalous mono-energetic {dollar}e\sp+{dollar} and {dollar}e\sp-{dollar} emissions in heavy-ion atom collisions near the Coulomb barrier at Gesellschaft fur Schwerionenforschung (GSI), Germany. Resonances in the Bhabha ({dollar}e\sp+e\sp-{dollar}) scattering cross section have been investigated in the beam energy range 1350 to 2350 keV. A thin ({dollar}\sim{dollar}3 keV energy loss) lithium target was used to achieve a minimum electron-momentum spread to enhance the resonance signal. The detector system employed in this experiment consisted of multi-wire proportional chambers (MWPC's) and plastic scintillators for the complete kinematic reconstruction of the {dollar}e\sp+e\sp-{dollar} scattering events. Within experimental uncertainties (0.27%), no evidence was observed for deviations from the smooth Bhabha scattering cross section, over the energy region suggested by the GSI data. On average, this experiment precludes at 90% confidence level the existence of an elementary or a composite particle ({dollar}J=0{dollar}) with a lifetime shorter than 3.3 {dollar}\times{dollar} 10{dollar}\sp{lcub}-13{rcub}{dollar} sec.;ii. Single-quantum annihilation of positrons with shell-bound atomic electrons. The single-quantum annihilation of positrons was studied experimentally in a thin lead target (3.52 mg/cm{dollar}\sp2{dollar}), at energies of 1 MeV and higher. Photo-peaks corresponding to the annihilation of the incident positrons with K, L, and M shell electrons were resolved and observed clearly for the first time. The cross section ratios L/K and M/K were determined. An analysis of the L peak profile yielded the ({dollar}L\sb{lcub}II{rcub}+L\sb{lcub}III{rcub})/L{dollar} ratio. The first measurements were also made of the directional distribution of the annihilation quanta of the three individual electron shells. The results were in good agreement with theory. These observations indicate the potential for applying the phenomenon to the development of a tunable, highly directional gamma-ray source.
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Type
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dissertation
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Source
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PQT Legacy CUNY.xlsx
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degree
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Ph.D.
