Factors Contributing to Incomplete Charge Collection in Si(Li) X-ray Detectors

Pears, Kevin A (1990) Factors Contributing to Incomplete Charge Collection in Si(Li) X-ray Detectors. PhD thesis, University of Glasgow.

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Abstract

This project is concerned with a detailed study of the properties and behaviour of lithium drifted silicon Si(Li) energy dispersive x-ray detectors. In particular, the factors which effect their performance in the detection of low energy x-rays. The Si(Li) detectors used in this project were a new generation of detector. These new detectors were known to exhibit an order of magnitude improvement in the low energy tails associated with the detection process. These low energy tails are caused by the effects of incomplete charge collection (ICC) of the charge carriers created during the detection process. The work discussed in this thesis describes a series of experiments which were performed to primarily investigate why electron generated x-ray spectra obtained from our transmission electron microscope were of a poorer quality, in terms of ICC, than had been predicted by a series of preliminary tests performed with this detector. Chapter 2 discusses the various theoretical considerations required throughout the project. This includes a brief outline of the processes involved during the detection of x-rays and discusses the various cross-sections involved for these processes. Chapter 3 then discusses the design and operation of the Si(Li) energy dispersive x-ray detector. Chapter 4 details the preliminary investigation performed to establish the quality of electron generated spectra obtainable with the Si(Li) detector. The quality of the spectra was established by the accuracy of the theoretical background fit obtainable for a particular spectrum. This investigation clearly showed that the spectra were of an inferior quality to that expected. Chapter's 5 and 6 then detail an investigation into possible reasons to explain why the spectra should have been of such a poor quality. This investigation considers what effects the microscope environment can have on the detectors performance. Whilst in use on the microscope, the detector is immersed in the strong magnetic field from the objective lens (~0.3 T). In addition, the detector is continually subjected to a high energy irradiation from the x-rays created during the collisions between the electron beam and the specimens. In certain applications it is also possible for electrons to be scattered onto the detector. Two similar detectors were studied in detail; this allowed direct comparisons of their performances to be made. This investigation revealed that several interesting phenomena occur with these detectors. Both detectors were found to have regions around their circumference in which a high trap density existed (this leading to the creation of excess ICC). However, the nature of these side regions was found to be different for both detectors. In the first detector, the traps were seen to form by themselves. In the second detector, traps were created by high energy irradiation from terbium characteristic photons. These traps were seen to decay with time. However, both detectors were found to behave in the desired manner when collimated down to a small central region. The performance of the first detector was seen to show a marked deterioration, in terms of the level of ICC, during the time this investigation took. Chapter 7 discusses some of the processes known to be occurring in the detectors which give rise to the low energy tails seen in all spectra. Some of these processes are fundamental and should not be confused with those which give rise to the ICC. The magnitudes of these processes were established whenever possible. This information is valuable to the detector manufacturers who are striving to produce the best detectors that physical constraints will allow. A knowledge of these fundamental limits makes it possible to predict levels in design improvement theoretically possible.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Optics, Nuclear physics and radiation
Date of Award: 1990
Depositing User: Enlighten Team
Unique ID: glathesis:1990-78151
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 30 Jan 2020 15:38
Last Modified: 30 Jan 2020 15:38
URI: http://theses.gla.ac.uk/id/eprint/78151

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