Shutters Used in EELS and EFTEM Measurements
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In EELS measurements, an internal exposure shutter can cause a streak at an intense zero-loss peak in the spectra taken with a short exposure time. However, a beam deflector placed above the specimen can be used as an exposure shutter in order to avoid the streaking.

Because the CCD exposure time in EELS measurements cannot be too short, Kimoto et al. [1] employed a so-called streak imaging technique to obtain high-speed spectrum acquisition. The streak imaging technique that was originally applied to visualize energy instability [2] is similar to a chart recorder, as shown in Figure 2598, where the high-speed spectrum acquisition is achieved by exposing a full-frame CCD during the readout period. Another EEL spectrum is continuously projected on the CCD camera during the parallel shift (i.e. exposure time) of the well electrons. This allows acquiring a series of time-resolved spectra as a two-dimensional image. The time resolution depends on the duration of a well-electron transfer to the adjacent line. Additional benefit is that this spectrum acquisition technique is effective in avoiding artifacts due to the slow beam blanking of an exposure shutter.

Schematic illustration showing the similarity between a chart recorder (a) and a streak imaging technique (b)

Figure 2598. Schematic illustration showing the similarity between a chart recorder (a) and a streak imaging technique (b). [3]

 

 

 

 

[1] Kimoto, K., Kothleitner, G., Grogger, W., Matsui, Y., Hofer, F., 2005. Advantages of a monochromator for bandgap measurements using electron energy-loss spectroscopy. Micron 36, 185–189.
[2] Brink, H.A., Barfels, M.M.G., Burgner, R.P., Edwards, B.N., 2003. A sub-50 meV spectrometer and energy filter for use in combination with 200 kV monochromated (S)TEMs. Ultramicroscopy 96, 367–384. 
[3] Koji Kimoto, Kazuo Ishizuka, Toru Asaka, Takuro Nagai, Yoshio Matsui, 0.23 eV energy resolution obtained using a cold field-emission gun and a streak imaging technique, Micron 36 (2005) 465–469.

 

 

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