Electron beam current

For a simplified case, one can obtain the rate of CL emission, =ft GI /e, where /is a function containing correction parameters of the CL detection system and that takes into account the fact that not all photons generated in the material are emitted due to optical absorption and internal reflection losses q is the radiative recombination efficiency (or internal quantum efficiency) /(, is the electron-beam current and is the electronic charge. This equation indicates that the rate of CL emission is proportional to q, and from the definition of the latter we conclude that in the observed CL intensity one cannot distii pish between radiative and nonradiative processes in a quantitative manner. One should also note that q depends on various factors, such as temperature, the presence of defects, and the... 

Fig. 42 —AES surface survey of elements in the disk substrate surface after polishing. The slurry contains 6 wt % Si02 particles with a diameter of 30 nm, 1 wt % oxidizer and 2 wt % lubricant in Dl water, and pH value of the slurry is 1.8. (a) Elements in the disk surface, (b) deep distribution of the elements. (The contents of elements and their deep distribution in the polished surface were analyzed by using a PHI 680 auger nanoprobe under determining conditions as follows ion beam current of 1 u,A, ion beam voltage of 2 kV, electron beam current of 10 nA, electron beam voltage of 10 kV and scan area of 20 fj.m by 20...

Analysis of individual catalyst particles less than IMm in size requires an analytical tool that focuses electrons to a small probe on the specimen. Analytical electron microscopy is usually performed with either a dedicated scanning transmission electron microscope (STEM) or a conventional transmission electron microscope (TEM) with a STEM attachment. These instruments produce 1 to 50nm diameter electron probes that can be scanned across a thin specimen to form an image or stopped on an image feature to perform an analysis. In most cases, an electron beam current of about 1 nanoampere is required to produce an analytical signal in a reasonable time. 

Modern spectrometers only require electron beam currents in the range 0.1 lOnA and hence probe sizes of 20-200 nm may be readily achieved with thermionic sources and 5-15 nm with a FEG. Spatially resolved compositional information on heterogeneous samples may be obtained by means of the Scanning Auger Microprobe (SAM), which provides compositional maps of a surface by forming an image from the Auger electrons emitted by a particular element. 

Thoriated tungsten, whose surface has been carburized at high temperatures, have lower work functions than pure tungsten and emit equivalent electron beam currents at lower temperatures thereby extending the life and stability when compared to regular tungsten sources. These sources require a stable, high vacuum (10 to 10 torr) and are more difficult to fabricate. 

Figure 7. Efficiency of green-emitting (Zn,Cd)S Cu,Al relative to LasO S.-Tb " as a function of electron beam current density (O) 30 kV, 25 kV, (A) 20 kV. Electron beam dwell is 0.5 fis and pulse repetition rate is 60 pps.

The basic electrical parameters of an electron beam processor are its acceleration voltage, the electron beam current, and the electron beam power. The ratio of electron beam power and the input electrical power defines the efficiency of an electron accelerator. The acceleration voltage determines the energy of the electrons, as pointed out in Section 2.2. 

Electron beam current Number of electrons per second emitted from the cathode, measured in mA (1 mA = 6.25 x 10 electrons per second) at the high-voltage unit. 

Dose rate Dose per unit time, expressed in Gy s = J kgr s E At a constant accelerating voltage, it is proportional to the electron beam current. 

The electron gun consists of a spiral-shaped tungsten cathode and a Wehnelt cylinder. These two components not only constitute the electrodes of the acceleration gap, but also form the optical assembly to control and shape the electron beam. Current signals are linear and have a repetition frequency... 

Electron beam power Product of the acceleration voltage and of the electron beam current, expressed in kW (1 kW =10 mA x 100 kV). 

The electron gun consists of a spiral-shaped tungsten cathode and a Wehnelt cylinder. These two components not only constitute the electrodes of the acceleration gap, but also form the optical assembly to control and shape the electron beam. Current signals are linear and have repetition frequency about 800 Hz. They are used to deflect the electron beam horizontally and vertically over the exit window plane. The scanner can be equipped by two cathodes for maximum output. Then, the width of the exit window is more than double that of a standard unit with a single cathode. The exit window containing the 12-15 prn-thick titanium foil is relatively large to assure an effective cooling of the foil. 

Figure 5. Effect of electron beam current on incremental creep rate of polystyrene during irradiation...

The beam-scan length was continuously variable from 3 to 15 inches, and the beam was about % inch wide. Electron current was adjustable to a maximum of 250 /xa. The material was carried under the beam on a variable-speed, continuous belt, the dry powder being exposed to air at all times. Dosage was controlled by varying scan width, electron beam current, or belt speed. To ensure complete penetration, the polymer bed depth was kept at 0.5 inch or less. 

Since for a given measuring time nlrut It, and acc /e2i> where /el is the electron beam current, it can be seen that when acc /Jtrue the statistical error is optimal and independent of ItV Second, the coincidence rate is restricted by the maximum allowable singles count rates in either channel (—106— 107 cps, as discussed earlier). 

An interesting result was the appearance of F+ in the residual gas analysis when the electron beam in AES was placed on the sample, suggesting easy desorption and a very unstable surface. In fact, when electron beam currents were not minimized, the fluorine frequently was desorbed completely, and did not appear in the Auger spectrum. 

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