Links in this section:

Introduction
Fundamentals of the TEM technique
Beam-sample interaction
The Analytical TEM
Detector Protection
Qualitative Analysis
Quantitative Analysis
Microanalysis Examples (1)
Microanalysis Examples (2)
Microanalysis Examples (3)
Summary

Detector Protection

If a specimen support grid passes under the beam or the sample is viewed under low magnification conditions where the whole grid is exposed to high beam currents, a large flux of backscattered electrons is generated. The close proximity of the detector to the sample can cause an excess of these backscattered electrons to enter the detector. This not only overloads the electronics but may also deposit excess charge in the detector. As a result, the detection system is effectively paralyzed and may take many minutes to recover.

It is possible to initiate automatic detector withdrawal if counts exceed an acceptable value. However, this can cause vibration and image distortion, and due to the fact that the detector actually protrudes into the polepiece, it may also change the stigmation conditions of the microscope at high magnifications. Another option is to use a shutter between the sample and the crystal that closes when a high count rate is exceeded (Figure 4). The shutter does not cause the mechanical disturbance or stigmation problems associated with withdrawal and will prevent overload of the detection system. The detector is therefore ready to use immediately on return to normal analysis conditions and analysis throughput is improved.

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