Design Considerations for an Electron Energy Loss Spectroscopy Parallel Recording System

Scott, Colin P (1988) Design Considerations for an Electron Energy Loss Spectroscopy Parallel Recording System. PhD thesis, University of Glasgow.

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Abstract

This thesis describes the results of an investigation into the design of a parallel recording system for electron energy loss spectroscopy (EELS). The motivation behind the construction of such a system is the greatly enhanced detection efficiency which can be achieved, as compared to conventional serial recording systems. This is of great benefit in experimental situations where specimen drift, radiation damage, or signal to noise ratio are limiting factors. Chapter 1 provides a brief introduction to the method of EELS analysis in the transmission electron microscope (TEM) and discusses the instrumentation required to generate and record EELS spectra. Chapter 2 contains a detailed review of the theory of homogeneous field magnetic sector spectrometers, following the work of Enge, Brown, and Heighway. The matrix method used to calculate the optical properties of such spectrometers is introduced, and the focussing coefficients for an arbitrary magnetic sector are derived to second order. A spectrometer analysis program based on the theory of chapter 2 is described in chapter 3. The program is used to calculate the aberration coefficients of two well known 2nd order corrected spectrometer designs [Shuman 1983, Scheinfein and Isaacson 1984] and hence determine the nature of the electron intensity distribution at their dispersion planes. Post-spectrometer magnification of the dispersion plane is required in parallel EELS in order to overcome the resolution limiting effects of electron scatter within the detector. The requirement that the magnifications in the dispersive and non-dispersive planes be independent indicates the use of quadrupole lenses as the magnifying elements. Chapter 4 reviews the theory of quadrupoles and extends the matrix transfer method of chapters 2 and 3 to quadrupole lenses. The design of a four lens quadrupole system suitable for post-spectrometer magnification in EELS is described in chapter 5. The system can vary the magnification in the dispersive direction from 5x to 97x (at 100 keV), while maintaining an almost constant magnification in the non-dispersive direction. Chapter 6 considers the types of multielement detectors which could be applied to parallel EELS, and discusses the advantages of using wide aperture linear photodiode arrays operating in the indirect mode as detection elements. The design and construction of the instrumentation required to operate two such arrays, manufactured by Reticon and Hamamatsu, is also reported in this chapter. Experiments on the electrical and optical performance of both these arrays are described in chapter 7. The results of these experiments indicate that the Hamamatsu device is the more suitable for detection of EELS spectra. Chapter 8 contains experimental results on the evaluation of various scintillator screens laid on fibre-optic plates directly coupled to the fibre-optic input window of the Hamamatsu array. The most suitable of the scintillators tested was a screen made from a single crystal of yttrium aluminium garnet (YAG) polished down to a thickness of 30um. The detective quantum efficiency of a prototype detector consisting of the Hamamatsu photodiode array fibre-optically coupled to such a screen is shown to be greater than 0.25 for a range of input electron doses varying from 40 electrons / channel-second to greater than 10.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Applied physics, Optics
Date of Award: 1988
Depositing User: Enlighten Team
Unique ID: glathesis:1988-77677
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 14 Jan 2020 11:53
Last Modified: 14 Jan 2020 11:53
URI: http://theses.gla.ac.uk/id/eprint/77677

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