Fluorescent screening

The basic device is very simple. A tip of refractory metal, such as tungsten, is electrically heat-polished to yield a nearly hemispherical end of about 10" cm radius. A potential of about 10 kV is applied between the tip and a hemispherical fluorescent screen. The field, F, falls off with distance as kr, and if the two radii of curvature are a and b, the total potential difference V is then... 

The final grid is positively charged to accelerate the accepted electrons onto the fluorescent screen. The diffraction pattern may then be photographed. 

X-ray fluoroscopy is used for direct on-line examination. A fluorescent screen is used to convert x-ray photons into visible light photons. A television camera receives the visible image and displays it on a television screen (see Fig. 19). This type of system is used for security screening of carry-on luggage at airports. 

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... 

TEM offers two methods of specimen observation, diffraction mode and image mode. In diffraction mode, an electron diffraction pattern is obtained on the fluorescent screen, originating from the sample area illuminated by the electron beam. The diffraction pattern is entirely equivalent to an X-ray diffraction pattern a single crystal will produce a spot pattern on the screen, a polycrystal will produce a powder or ring pattern (assuming the illuminated area includes a sufficient quantity of crystallites), and a glassy or amorphous material will produce a series of diffuse halos. 

Cathodoluminescence (CL), i.e., the emission of light as the result of electron-beam bombardment, was first reported in the middle of the nineteenth century in experiments in evacuated glass tubes. The tubes were found to emit light when an electron beam (cathode ray) struck the glass, and subsequendy this phenomenon led to the discovery of the electron. Currendy, cathodoluminescence is widely used in cathode-ray tube-based (CRT) instruments (e.g., oscilloscopes, television and computer terminals) and in electron microscope fluorescent screens. With the developments of electron microscopy techniques (see the articles on SEM, STEM and TEM) in the last several decades, CL microscopy and spectroscopy have emerged as powerfirl tools for the microcharacterization of the electronic propenies of luminescent materials, attaining spatial resolutions on the order of 1 pm and less. Major applications of CL analysis techniques include ... 

Surface-sensitive diffraction is, for the most part, restricted to analysis of surfaces of single crystals and overlayers and films on such surfaces. If a polycrystalline sample is illuminated using a beam of low-energy electrons, each crystallite surfiice exposed will create its own diffraction pattern, all of which will be superimposed on the fluorescent screen detector. If more than a few orientations are illuminated by the beam, the pattern becomes too complicated to analyze. Flowever, if the size of the... 

For detection of secondary ions a laterally resolving detector is necessary. In the first step a channel plate for amplification is used secondary electrons from the output of this device are accelerated either to a fluorescent screen or to a resistive anode. If a fluorescent screen is used the image is picked up by a CCD camera and summed frame by frame by use of a computer. The principal advantage of this system is unlimited secondary ion intensities, but compared with the digital detection of the resistive anode encoder the lateral and intensity linearity is not as well-defined. 

The atom probe field-ion microscope (APFIM) and its subsequent developments, the position-sensitive atom probe (POSAP) and the pulsed laser atom probe (PLAP), have the ultimate sensitivity in compositional analysis (i.e. single atoms). FIM is purely an imaging technique in which the specimen in the form of a needle with a very fine point (radius 10-100 nm) is at low temperature (liquid nitrogen or helium) and surrounded by a noble gas (He, Ne, or Ar) at 10 -10 Pa. A fluorescent screen or a... 

Fig. 14-4. An electric discharge tube, very low pressure. Electrons travel from the negative electrode to the positive electrode some of them pass through the triangular hole to produce a triangular spot on the fluorescent screen.

This experiment does have some features of a direct vision observation. First, the glowing spot is directly visible. Second, it is easy to imagine an invisible stream of particles hurtling through the triangular hole in the electrode to crash against the fluorescent screen in a burst... 

Consider the apparatus shown in Figure 14-6. The equipment is similar to that shown in Figure 14-4 except a fluorescent screen within the tube reveals the trajectory of the particles that pass through the slot in the positive electrode. When a magnetic field is added, the electron trajectory is curved. A mathematical analysis of the curvature permits an interpretation of this experiment that leads to a determination of e/m. 

In the Sunbury x-ray photometer, described by Cranston, Matthews, and Evans,25 commutation between standard and unknown (25 times a second) is achieved in a somewhat different way. This instrument uses two x-ray beams from a single source—one through a standard hydrocarbon, the other through an unknown hydrocarbon on which sulfur is to be determined. The beams strike fluorescent screens in a light-tight box. A chopping disk interposed between the screens and a single multiplier phototube is used to accomplish commutation betw fen standard and unknown. 

Oscillograph. A cathode-ray oscilloscope in which a photographic or other permanent record is produced by the electron beam of a cathode-ray tube. A cathode-ray oscilloscope is a test instrument that uses a cathode-ray tube to make visible on a fluorescent screen the instantaneous values and waveforms of electrical quantities that are rapidly varying as a function of time... 

Since the wavelength is of the order of lattice distances, electrons that are scattered elastically undergo constructive and destructive interference (as with X-rays in XRD). The back-scattered electrons form a pattern of spots on a fluorescent screen from which the symmetry and structure of the surface may be deduced. 

---