Unscattered beam

Absorption Coefficient—Fractional absorption of the energy of an unscattered beam of x- or gamma-radiation per unit thickness (linear absorption coefficient), per unit mass (mass absorption coefficient), or per atom (atomic absorption coefficient) of absorber, due to transfer of energy to the absorber. The total absorption coefficient is the sum of individual energy absorption processes (see Compton Effect, Photoelectric Effect, and Pair Production). 

Actual measurements deviate from theory at the large particle end because the angle of the diffracted flux is not sufficient to be distinguishable from the unscattered beam for very large particles. Deviation at the small particle end occurs as the particle size approaches the wavelength of the light source. These limitations... 

Figure 1. Test micrograph showing the displacement of the unscattered beam (small dots) in the selected area diffraction (SAD) pattern when it occurs in polar coordinates (Philips EM 300). The tilt has been fixed at the 002 Bragg angle for carbon ( 0.3°) and the azimuth changed by small increments. The 000 spot displaces along a practically perfect circle which corresponds to the 002 Debye Scherrer ring. Such a device allows exploration of any position in the SAD pattern, even when neither sharp nor intense hkl reflections are visible. The SAD pattern of an asphaltene heat-treated at 500°C has been superimposed to the test micrograph. Various positions of a 0.13 A aperture are shown.

Scattering. The entire beam of all of the gamma rays that reach the detector consists of two components an unscattered beam and a scattered beam. Commonly, it is convenient to express the entire beam in terms of a buildup function B, i.e.,... 

The unscattered beam (< ) consists of those photons that go through the shield without any interaction. If the source strength is 5( y/s), the intensity of the unscattered beam or the unscattered photon flux is given by the simple... 

In phase contrast scattered beams are allowed to pass through a large objective aperture and recombine with the unscattered beam to form the image. This would give no contrast if the objective lens was perfect, and perfectly in focus. The lens is not perfect, and often defocused, causing the scattered beams to be phase shifted. 

Some other parameters and techniques important for many types of microscopy should be defined. The depth of field is the depth or thickness of the specimen that is simultaneously in focus. As is explained in Chapter 3, the depth of field is large for electron microscopes and small for optical microscopes. (The depth of focus is the depth of the image that is in focus at one time and is not important in microscopy.) The field of view is the area of the specimen which is included in the image. Bright field (BF) is an imaging mode where the direct unscattered beam is allowed to... 

Mass thickness contrast is generally weak in polymers. Staining, shadowing or decoration methods (Chapter 4) have to be applied to enhance this contrast. Diffraction contrast, produced by the scattering of diffracted beams outside the objective aperture, is limited by radiation damage of crystallinity. This leaves phase contrast, where scattered beams (inside the objective aperture) are phaseshifted and recombined with the unscattered beam. 

As mentioned earlier, in the center of detector array there is either a mirror or a hole which guides the unscattered beam to a beam monitor (Figure 3.17). The beam monitor serves several purposes ... 

HRTEM exploits three different interactions of electron beam-specimen unscattered electrons (transmitted beam), elastically scattered electrons (diffracted beam) and inelastically scattered electrons. Different types of images are obtained in HRTEM. As a result, diffraction patterns are shown because of the scattered electrons. If the unscattered beam is selected, we obtain the Bright Field Image. Dark Field Images are obtained if beams are selected by the objective aperture. 

The light scattered by the optical elements can be reflected by the vacuum vessel, which is not isolated from seismic vibrations, and can recombine with the unscattered beam. The vacuum tubes are thus equipped with baffles to eliminate the reflected beam rays. 

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