A Novel Design of a Variable Energy Positron Lifetime Beam

This dissertation, "A Novel Design of a Variable Energy Positron Lifetime Beam" by Dan, Chen, 陳丹, was obtained from The University of Hong Kong (Pokfulam, Hong Kong) and is being sold pursuant to Creative Commons: Attribution 3.0 Hong Kong License. The content of this dissertation has not been altered in any way. We have altered the formatting in order to facilitate the ease of printing and reading of the dissertation. All rights not granted by the above license are retained by the author. Abstract: Abstract of thesis entitled A NOVEL DESIGN OF A VARIABLE ENERGY POSITRON LIFETIME BEAM Submitted by Chen Dan for the degree of Master of Philosophy at The University of Hong Kong in December 2007 Positron annihilation lifetime spectroscopy (PALS) is known to be a sensitive method for characterizing vacancy type defects in materials and thus tends to be the most popular of the positron annihilation techniques. Conventional PALS, which is 22 normally based on the emission of positrons from Na, works only on bulk samples since the penetration depth of the positrons from this isotope is large and not controllable. In order to study the defects in thin films or at material surfaces, however, it is highly desirable to employ PALS with a slow positron beam which has the capability of varying the incident positron energy and hence the mean positron implantation depth. Such a depth profiling PALS system is referred to in the present work as Variable Energy Positron Annihilation Lifetime Spectroscopy (VEPALS). The problem of constructing a VEPALS system, however, is the absence of a start signal for a positron entering a sample when using a positron beam. One of the ways of furnishing the necessary start signal is the usage of secondary electrons emitted when the positron strikes the sample. This method, however, suffers from two drawbacks. First the efficiency of the secondary emission varies from sample to sample. Second the timing resolution is usually poor (typically 400 to 800ps) due to the variation of secondary electron energies. The new VEPALS design presented in this thesis offers a novel approach to solving these problems. It is based on the emission of secondary electrons produced when the positron beam passes through a 25nm thick carbon foil located in front of the sample. A needle of 30μm diameter is positioned on-axis right behind the foil, creating a radial electric field that deflects the secondary electrons radially outwards sufficiently so as to miss the sample and to hit a micro channel plate (MCP) detector placed down beam. The MCP signal provides the start-signal for the positron lifetime spectrometer. The design has a cylindrical symmetry that reduces the experimental complexity. Moreover the small variations in secondary electron path length results in good timing resolution (= 500eV) of the positron beam. DOI: 10.5353/th_b3955879 Subjects: Positron beams Positron annihilation Electrons - Emission