Instrumentation illuminating beam

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... 

FS instrumentation tends to be more complex than UV systems, with monochromators in both illuminating and detector beam paths and, sometimes, dual detectors. These features are necessary to separate rather intense illuminating beams scattered radiation from the sometimes weaker molecular fluorescence. 

The diffraction pattern obtained in the detector plane when the beam scan in a STEM instrument is stopped at a chosen point of the image comes from the illuminated area of the specimen which may be as small as 3X in diameter. In order to form a probe of this diameter it is necessary to illuminate the specimen with a convergent beam. The pattern obtained is then a convergent beam electron diffraction (CBED) pattern in which the central spot and all diffraction spots from a thin crystal are large discs rather than sharp maxima. Such patterns can normally be interpreted only by comparison with patterns calculated for particular postulated distributions of atoms. This has been attempted, as yet, for only a few cases such as on the diffraction study of the planar, nitrogen-rich defects in diamonds (21). 

Cosine smearing. Because instrument volume and experiment time must both be minimized for a planetary Mossbauer spectrometer, it is desirable in backscatter geometry to illuminate as much of the sample as possible with source radiation. However, this requirement at some point compromises the quality of the Mossbauer spectrum because of an effect known as cosine smearing [327, 348, 349] (see also Sects. 3.1.8 and 3.3). The effect on the Mossbauer spectrum is to increase the linewidth of Mossbauer peaks (which lowers the resolution) and shift their centers outward (affects the values of Mossbauer parameters). Therefore, the diameter of the source y-ray beam incident on the sample, which is determined by a... 

An instrument used for obtaining magnified images of small objects. For illumination of the object microscopes may use ordinary light, ultraviolet light or a beam of electrons. See Electron Microscope. 

As the beam travels down the column, a number of electromagnetic lenses are used to guide the beam to the sample [44], The condenser lenses are part of the illumination system and are used to deliver electrons from the electron gun crossover to the sample. The condenser lenses determine the beam current reaching the sample. The objective, or final, lens determines the final spot size of the beam. A set of scanning coils are also present in the instrument column to scan the beam in a raster pattern over an area of the sample. At each point, data is collected and the points are combined to form the image. More detail on the data collection is given in the image formation section. 

The Hellige turbidimeter is also a variable depth type of instrument using visual detection. A combination of vertical and horizontal illumination of die sample and a split ocular permit the eye to function merely to compare the intensities of two images simultaneously appearing in the ocular. This is a special form of double-beam operation, and the intensity emitted by the light source need not be extremely constant from one sample to another. Adjustment is made of a slit in the path of the direct, or vertical, illumination of the sample, and the calibration curve consists of a plot of this slit opening v,r. concentration. 

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