The Everhart-Thornley Detector is a detector used in scanning electron microscopes (SEMs). It is named after its designers, T Everhart and RFM Thornley. The Everhart-Thornley Detector has been available since the fifties, but remains the most frequently used detector in SEMs.
The basis component of the detector is a scintillator that emits photons when hit by high-energy electrons. The emitted photons are collected by a lightguide and transported to a photomultiplier for detection.
Most scintillators require the incident electrons to have an energy of 10-15keV to be detected. This will typically be fulfilled by the backscatttered electrons, but not by the secondary electrons created by interactions between the sample and the electron beam.
The detector is placed within a Faraday cage to prevent the interaction of electrons from incident beam with the scintilator. The potential of this Faraday cage can be adjusted to allow for the detection of secondary electrons. If the Faraday cage has a large positive potential, secondary electrons will be accelerated to sufficient energies to allow detection. This mode of operation is not possible in low-vacuum SEM, as the high potential on the Faraday cage would ionize the atmosphere of the SEM.