Magnetic lenses

In TEM, a focused electron beam is incident on a thin (less than 200 nm) sample. The signal in TEM is obtained from both undeflected and deflected electrons that penetrate the sample thickness. A series of magnetic lenses at and below the sample position are responsible for delivering the signal to a detector, usually a fluorescent screen, a film plate, or a video camera. Accompanying this signal transmission is a... 

The final set of magnetic lenses beneath the specimen are jointly referred to as post-specimen lenses. Their primary task is to magnify the signal transferred by the objective lens. Modern instruments typically contain four post-specimen lenses diffraction, intermediate, projector 1, and projector 2 (in, order of appearance below the specimen). They provide a TEM with its tremendous magnification flexibility. 

Equipment for producing and controlling the electron beam. Magnetic lenses are often used for the controlling. 

Magnesium, determination by x-ray emission spectrography, 32 in cements, 260, 261 Magnetic lenses, use, 263, 293 Major constituents, determination by comparison with a standard, 179-185... 

The usual source of electrons is a tungsten filament electron gun held at a negative potential (typically 10-30 kV), and magnetic lenses focus the beam into a fine probe incident on the surface of the specimen. A probe diameter of 0.2-1 pm is typical, with a current of 1-100 nA. 

Auger analysis of small features. The primary electron beam column is similar to that in electron microscopes, and it may contain both electrostatic and magnetic lenses for beam focussing as well as quadrupole deflectors for beam steering and octopole lenses for beam shaping. 

In a simple electron microscope, a primary beam of electrons is produced using a conventional electron gun, where a heated cathode, maintained at ground potential, emits electrons which are drawn out by a positive potential (typically 30 kV) to form a high energy electron beam. This beam is easily electrostatically and/or magnetically focused (since electrons are charged particles) to a few microns across, and can be directed to any point on the sample by a series of magnetic lenses. The system must be evacuated to reduce attenuation and scatter of the electron beam. When an electron beam strikes the sample, a number of processes take place (Fig. 5.6 Pollard and Heron 1996 51). 

Electron diffraction patterns are usually produced with transmission electron microscopes. These instruments are composed of several magnetic lenses. The main lens is the objective lens, which, in addition to forming the first magnified image of the specimen, also produces the first diffraction pattern. This original pattern is then magnified by the other lenses of the microscope so as to produce the final diffraction patterns on the screen or on a camera. 

In contrast to X-rays, electrons can be foeussed by magnetic lenses to give images of the investigated objeets. This is the basic principle behind every transmission electron microscope (TEM). As shown by the sketch in figure 9 the central part of every TEM is the objeetive lens. This lens eolleets all diffracted electron beams from the erystal and sorts them in the baek foeal... 

In an electron microscope, a stream of electrons is formed by the electron source and is accelerated toward the specimen using a positive electric potential. The stream is focused into a thin, monochromatic beam by using metal apertures and magnetic lenses. The electron beam interacts with the specimen and the effects of these interactions are detected and transformed into an image. 

Like their glass analogues, magnetic lenses suffer from such defects as coma, distortion, astigmatism, and chromatic and spherical aberration. 

The transmission electron microscope (TEM) images specimens a fraction of a micrometer or less in thickness. In a TEM, the beam passes through the q)eci-men so that some of the electrons are absorbed and seme scattered. The remaining electrons are focused ento a fluorescent screen or special photographic plates via the use of magnetic lenses. The resulting image is in black and white. 

Without the need for complicated magnetic lenses and electron beams, the STM is far less complex than the electron microscope. The tiiiy tunneling current can be simply amplified through electronic circuitry similar to that which is used in other electronic equipment, such as a stereo. In addition, the sample preparation is usually less tedious. Many samples can be im ed in air with essentially no preparation. For more sensitive samples which react with air, imaging is done in vacuum. A requirement for the STM is that the samples be electrically conductir, such as a metal. 

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