Demagnified source size

Consider now the wide angle scattering resolution. According to Rosenbaum and Holmes the width of a diffraction spot is determined by the demagnified source size — a —, the intrinsic width of a reflection and a dispersion term to account for the wavelength spread ... 

Once the primary electron beam is created, it must be demagnified with condenser lenses and then focused onto the sample with objective lenses. These electron lenses are electromagnetic in nature and use electric and magnetic fields to steer the electrons. Such lenses are subject to severe spherical and chromatic aberrations. Therefore, a point primary beam source is blurred into a primary beam disk to an extent dependent on the energy and energy spread of the primary electrons. In addition, these lenses are also subject to astigmatism. AH three of these effects ultimately limit the primary beam spot size and hence, the lateral resolution achievable with sem. 

To produce micrometer sized focused beams, one employs highly demagnifying optics to image the source onto the sample. Such optics can include Kirkpatrick-Baez mirrors, Fresnel zone plates, tapered capillaries, and compound refractive lenses, all of which have been used to produce submicron focal spots at third generation storage rings. 

A standard TEM is shown in F 1. Its general construction resembles that of the light microscope. The electron sovuce at the top of the column emits electrons, which are accelerated, in this case, to 200 kV. The effective sovuce size is small, some 25 pm in diameter for a thermionic source, down to 2.5 run for a field emitter. This source is further demagnified by the multiple lens system forming the illuminating beam. An illuminated area ranging from 3 mm, the size of a specimen grid, down to 0.5 nm, can be produced at the specimen plane located at the center of the air gap of the objective lens (OL) as shown in F 1. The objective lens lies at the heart of the instrument, as it alone determines the resolution that can be achieved. The objective is provided... 

The crossover of a thermionic gun or the virtual point sources of Schottky or field-emission guns are demagnified by two to three electromagnetic lenses (Fig. 75), so that a geometric probe size [Pg.1117]

In the TEM the strongly excited first condenser forms a demagnified image of the filament (actually the cross-over of the electron gun is the effective source). Since beam divergence increases as the spot size falls, much of the flux... 

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