Everhart-Thornley detector

The Détecteur Everhart-Thornley is a detector of electron S used mainly in the electron microscopes with sweeping (MEB). It was developed in the Sixties by Thomas Eugene Everhart and RFM Thornley with the Université of Cambridge.

History

In 1960, two students of Charles Oatley, Thomas Everhart and RFM Thornley, had the idea, to improve the system of collection used originally by Vladimir Zworykin and which consisted of a phosphorescent screen Photomultiplicateur, to add a guide of light between this phosphorescent screen and this Photomultiplicateur. This guide allowed a coupling between the scintillator and the photomultiplier, which improved the performances largely. Invented more than one half-century ago, this detector is today that most frequently used.

Principle

The Everhart-Thornley detector is a detector of secondary electrons. It collects the secondary electrons emitted by the sample in a Electron microscope with sweeping under the impact of the primary electrons and converts each secondary electron into several million electronic loads available at exit of the detector.

A Everhart-Thornley detector is composed of a space of acceleration which amplifies the energy of the ions, of a Scintillateur which converts the electrons into photons, of a guide of light which transports the photons until a Photomultiplicateur which converts the photons into electrons and multiplies the electrons. The electric charge present at exit is then detected by an electronic amplifier.

The scintillator is carried to a tension of several kilovolts compared to a kind of Faraday screen room to which the potential is close to that of the sample so that the primary electrons are not too disturbed. In the normal functioning, the Faraday screen room is polarized with some +200 volts compared to the sample in order to create on the surface of the sample a sufficient electric field to drain the secondary electrons. A grid makes it possible the secondary electrons to penetrate in the space of acceleration which amplifies the energy of the electrons of a factor 50. This operating process is not possible in a MEB with weak vacuum since the potential of the scintillator would ionize the atmosphere of the room of observation.

With the entry of the guide of light, the scintillator, which converts the electrons into photons brings into play the scintillation which is a phenomenon of Fluorescence. On the outlet side of the guide of light, the conversion of the photons into electrons is ensured by a Photocathode bringing into play the photoelectric Effet.

Operation in positive tension

Under a positive tension being able to reach 250 Volt S (see diagram on the left), the Faraday screen room attracts with much effectiveness the secondary electrons coming from the sample. It is not only true for the electrons coming from the sample but also for the electrons coming from the room itself. It is because the Electric field generated by the Faraday screen room is strongly dissymmetrical that one can obtain an effect of relief.

Operation in negative tension

When the detector is used with a negative tension being able to go until - 50 Volt S (see diagram on the right), the detector is able to reject up to 90% of the secondary electrons because their energy is often lower than 10 EV. The Everhart-Thornley detector thus becomes in this case a detector of retrodiffused electrons.

The majority of the detectors detect electrons with an energy from 10 to 15 keV. It is in this order of magnitude of energies that one finds the retrodiffused electrons but not the secondary electrons.

See too

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