Secondary electron (SE) emission from thin carbon foils induced by 1-20 keV positrons has been investigated over a range of nominal foil thicknesses from 1.0 to 5.0 μg/cm2. The measurement of SEs was carried out in forward geometry using a microchannel plate as a detector. The SE yield γ has been measured as a function of beam energy and compared with some Monte Carlo simulation results. We also present in this thesis the material parameter Λ=? / (dE / dx) and the emitted SE energy spectra. Forincident positron energy of 5 keV or higher, the distribution is found to be characterizedby the Sickafus form, AEm and m is close to 1. For low energy incident positrons however, another form, Bexp(E / t) , is proposed for describing the SE distribution. The maximum scattering angle for SEs emitted from 5.0 μg/cm2 is found to be around 60°. Measurements of energy loss and energy loss straggling for 1-10 keV positrons passing through thin carbon foils of different thicknesses ranging from 1.0 to 5.0 μg/cm2 are present in this thesis. The stopping power dE / dx and positron transmission coefficient have also been investigated as a function of incident positron energy and foil thickness. Particularly, our experimental results are compared with those from Monte Carlo simulation and theory with a view to providing a way to determine the real thickness of carbon foil. The ratio of the energy loss straggling to the foil thickness seems to have a linear relation with the beam energy. The transmitted positrons after passing through 5.0 μg/cm2 C-foil have a small scattering angle which is less than 10°. Titanium dioxide nanotube arrays fabricated by anodization of titanium foil and annealed at different temperatures were studied using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and positron annihilation spectroscopy (PAS). The crystallization process and morphological change of the nanotubes have been discussed. It is found that anatase (101) only appeared on the walls of the nanotube. The atomic concentration of fluoride and the ratio of Ti/O decreased when the annealing temperature increased. Vacancy type defects were found to diffuse toward the surface when the samples were annealed at 200°C and 400°C and healing of vacancies occurred at 600°C. In addition, the fluoride may form some complexes with vacancies on the surface hence lowering the value of the S parameter. A new design of the lifetime positron beam based on the SE emission is proposed. The observed experimental results and simulation results make it possible to construct a new type of high resolution (<250ps) lifetime positron beam. In addition, how rastering of the positron beam can be used to accurately locate the position of the C-foil is also shown; a feature that will be of value in setting up a positron lifetime system. Such a lifetime system is expected to be much simpler in construction than existing beam based lifetime spectroscopy systems. / published_or_final_version / Physics / Doctoral / Doctor of Philosophy

published_or_final_version / Physics / Master / Master of Philosophy

published_or_final_version / Physics / Master / Master of Philosophy

Thesis (M. Phil.)--University of Hong Kong, 2002. / Includes bibliographical references (leaves 117-121).

published_or_final_version / Physics / Doctoral / Doctor of Philosophy

This thesis presents measurements of charged current deep inelastic scattering cross sections in e+p collisions with longitudinally polarised positron beams. The measurements are based on data taken by the ZEUS detector at the HERA collider during the 2006-2007 run- ning period. The data sample has an integrated luminosity of 132 pb-1 and was taken at a centre-of-mass energy of 318 GeY. The total cross section has been measured at positive and negative values of the longitu- dinal polarisation of the positron beam (Pe). In addition, the single differential cross sections dδ / dQ2, dδ / dx and dδ / dy have been measured for Q2 > 200 Ge y2, also using both positively and negatively polarised positron beams. The reduced cross section has been measured in nine bins of Q2 in the kinematic range 280 < Q2 < 30000 Gey2 and 0.0078 < x < 0.42. The results are compared against the descriptions provided by the CTEQ6.6, MSTW 2008, HEARPDF1.0 and ZEUS-JETS PDFs. In general, the measured cross sections are well described by these predictions. Based on the measurement of the total cross section as a function of the polarisation of the positron beam, a lower limit on the mass of a hypothetical right-handed W boson has been extracted from the upper limit of the cross-section at Pe = -1. This limit is complementary to the limits obtained from direct searches (for example at CDF and D0) because the limit presented herein is for a space-like vV, whereas for direct searches, the limit on the mass of a time-like W boson is obtained. The results of this analysis have been published and have been included ill the determination of the HERAPDF theoretical prediction and also in HI and ZEUS combined results.

Thesis (Ph. D.)--University of Texas at Austin, 2000. / Vita. Includes bibliographical references (leaves 163-171). Available also in a digital version from Dissertation Abstracts.

No description available.

Surface-analysis and treatment apparatus have been variously designed, manufactured, developed, and commissioned or re-commissioned, for characterising the surfaces and efficiency of positron moderators based around 3 µm thick polycrystalline-tungsten foil. These include XPS and AES, based around a CLAM2 hemispherical analyser, electron-beam heating, ion bombardment, mass spectroscopy, UHV sample mounting, UHV manipulation, gas-handling lines, and entry-lock apparatus. The CLAM2 electron spectrometer is additionally adapted for operation as a bipolar charged-particle spectrometer. All control software, and much data-analysis software, is implemented in Labview. Apparatus and techniques for safely storing, handling, transferring into vacuum, and manipulating in vacuo, a nominally 1 mCi 22Na, UHV-compatible positron source, are designed, constructed, and implemented. The efficacy of cleaning and surface-analysis apparatus are demonstrated, with some limitations and instrument malfunction identified, and solutions implemented. Methods for passivating positron-trapping states in polycrystalline tungsten are proposed, based on the current understanding of positron moderation and trapping. Improved moderator geometries have been designed and an alternative, simpler, and easier to implement solid-gas moderator proposed.

No description available.