Beam formation

The features from which one has to choose In building a SIMS Instrument are as numerous and varied as the problems which the technique can address. Shown conceptlonally In Figure 1 are the various choices for primary bombarding beam formation and emitted particle detection, all encompassed within a vacuum system. 

Table 10.1. Beam formation data for carbon dioxide flowing through a collimated-hole structure and observed at a distance of 40.6 cm. From [GWa60, Zan66].

Figure 1 shows the basic optimal system of the optical rotation detector, which is based on the nonmodulated polarized beam-splitting method. The light radiated from the light source is straightened by the plane polarizer, then to the lens for beam formation and concentration, and then to the flow cell. 

Molecular beam formation from flames at 1 atmosphere has been discussed. Greene et used several slits together with differential pumping between... 

Figure 16.20 FAB and MALDI techniques, (a) The principle of fast-atom beam formation with xenon (b) The formation of fast atoms of argon in a collision chamber and subsequent bombardment of the sample by this atom beam, usually of 5-10 kV kinetic energy (c) MALDI or ionization by effect of illumination with a beam of laser generated light onto a matrix containing a small proportion of analyte. The impact of the photon is comparable with that of a heavy atom. Through a mechanism, as yet not fuUy elucidated, desorption and photoionization of the molecules is produced. These modes of ionization by laser firing are particularly useful for the study of high molecular weight compounds, especially in biochemistry, though not for routine measurements.

Fig. 2.2 Schematic representation of the nested-pair of cones used for sampling and molecular-beam formation. Species entering the sampling cone are entrained in a free-jet expansion into the vacuum stage (5 X 10 Pa). Only those species having moments directed along the cone axis enter the skimmer cone. (Reproduced from [16], with permission.)...

Beam formation can be considered to be composed of two separate processes beam steering and focusing (Macovski, 1983). The implementation of these two functions may or may not be separated, depending on the system design. Focusing will be discussed first. 

Some of the drawbacks associated with mechanical steering involve the inertia associated with the transducer, the mechanism, and the fluid within the nosepiece of the transducer. The inertia introduces limitations to the frame rate and clearly does not permit random access to look angles as needed (the electronically steered approaches supply this capability). The ability to steer the beam at will is important in several situations but most importantly in Doppler applications. Further, electronic beam formation affords numerous advanced features to be implemented such as the acquisition of multiple lines simultaneously and elimination of the effects due to variations in speed of sound in tissue. 

With most of today s systems, there is a logarithmic compression of the amplified signal after beam formation amplification. The goal of this is to emphasize the subtle gray level differences between the scatterers from the various types of tissues and from diffuse disease conditions. 

This chapter has reviewed the fundamentals of the design of ultrasound scanners with a particular focus on the beam formation process. Different types of image data acquisition methods are described. 

As is well known, very cold molecules are produced in a supersonic expansion, which allows for the formation of weakly bound van der Waals molecules (Levy, 1980). Early molecular beam studies were limited to gas-phase species, where, by the use of co-expansion with an inert gas, the amount of cooling could be enhanced, allowing the formation of van der Waals clusters. This technique was used in the early 1970s to study the reactivity of (CHal) with alkali atoms (e.g. see Gonzalez Urena et al. (1975)). The basic formulae of cluster-beam formation are summarized in Box 24.1. 

As CO2 laser implies, carbon dioxide is the active component of this laser type. The laser gas also contains helium and nitrogen. Besides these main components, some special CO2 lasers require admixing of oxygen, hydrogen, carbon monoxide, and/or xenon, which additionally supports the physical and chemical laser-beam formation. 

Niu, H. (1994). Fundamental studies of the plasma extraction and ion beam formation processes in inductively coupled plasma mass spectrometry. Unpublished Ph.D. Thesis, Iowa State University, Ames. 

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