Appendixes

Summary

It is possible to prepare specimens for, and to use electron microscopes, by following established routines on commercial equipment without understanding what is going on inside the equipment. However, as with a great many practices, a knowledge of how and why things are happening can be of great assistance in making the best use of them and in appreciating the reasons for modifying them. For readers who would like some background information the following four appendixes provide introductions to the important topics of high vacuum, vacuum-deposition of thin films, the analysis of x-rays, and electron sources in electron microscopes. Each section can be no more than a précis of its subject; readers wishing to be more deeply involved are referred to more comprehensive publications.

Appendix 1: Production and measurement of high vacua

The operation of an electron microscope is dependent on electrons in the illuminating beam travelling along predictable paths from their source to their destination. They will only do this if they are not deflected by collision with any atoms or molecules other than those in the specimen. The inside of the microscope must therefore be kept sufficiently free from gas molecules, of which there are normally more than 10¹⁹ in each cm³ of air at atmospheric pressure. It is only by reducing the gas pressure to less than one-millionth of atmospheric that a sufficiently long collision-free path is obtained for the microscope to operate. This is, however, only a statement of the upper limit of pressure which can be tolerated before electron microscopy becomes impossible.