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Scanning Electron Microscope (SEM) > Everhart Thornley detector

Description of Flash Animation

This animation simulates an Everhart Thornley detector.

This resource requires Flash Player 9 or above available to download from

The simulation only refers to the detection of secondary electrons. These will also be high energy backscattered electrons from the specimen. How will the detector respond to these?

What will such a detector, therefore, measure when the grid bias is:

  1. positive, and
  2. 100V negative?

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An Everhart Thornley detector detects secondary electrons. These strike a scintillator, e.g., phosphor, which emits light. The light is transmitted down a perspex pipe to a photo-multiplier. This produces a voltage which controls the CRT. The scintillator is at a high voltage to attract electrons. It is surrounded by a Faraday cage (a mesh at a ground potential), which prevents it from interfering with the electron beam.

Collection efficiency for secondary electrons can be improved by applying a small +ve voltage to the cage.

Use the controls in the box to start the simulation opposite.


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