The Effect of X-ray Absorption in Thin Film Microanalysis

[摘要] Accurate knowledge of the production of characteristic and Bremsstrahlung x-rays when an monoenergetic beam of electrons is incident on a thin sample is of considerable importance in the technique of x-ray microanalysis. Chapter 2 describes the production of both the characteristic as well as Bremsstrahlung x-rays. The main motivation of this work has been to investigate the possibility of developing a quantitative technique, based on the intensity of the characteristic x-rays, which can correct the x-ray lines for specimen self absorption effects. Chapter 3 first reviews the methods and problems of performing for the absorption of low energy lines and then describes an alternative approach of absorption corrections which is simple to use and is suitable for rapid minicomputer evaluation. Energy dispersive solid state detectors are commonly used to record the x-rays produced when the beam of an electron microscope is incident on a specimen. Understanding the characteristics of such detectors is important in quantitative thin film microanalysis. Chapter 4 describes the principles of their construction and some factors affecting the performance of such detectors. Absorption of x-rays strongly depends on mass absorption coefficients. In chapter 4 an investigation has been made to see how different formulations for calculating mass absorption coefficients affect the predicted detector parameters for a number of detectors. Extraction of the characteristic x-ray counts from a measured spectrum of a thin specimen involves separation of these x-rays from the Bremsstrahlung signal. Extraction of the characteristic x-ray counts from a measured spectrum of a biological specimen is more complicated as peaks of interest are not well-separated and most of the peaks lie in the low energy region where the rate of background variation is more rapid. Chapter 5 first reviews some conventional methods of separating the characteristic x-rays from the measured spectrum and then describes an alternative approach which was developed to separate the characteristic signal from the Bremsstrahlung signal in organic samples. This background modelling method was used with a particular biological sample and results are compared with a conventional method of separating x-rays from the background signal. Chapters 6 and 7 contain details of the experimental work carried out to investigate the technique of self absorption corrections discussed in chapter 3. Compound semiconductor multilayer systems were selected for this investigation as these are wedge shaped so that the thickness and hence the amount of self absorption varies significantly within the specimen and thus are ideal for such an investigation. Finally chapter 8 reviews the preceding chapters, and suggests improvement and areas for further study.

[发布机构] University:University of Glasgow

[关键词] Condensed matter physics, Optics