The Everhart–Thornley detector (E–T detector or ET detector) is a

secondary electron Secondary electrons are electrons generated as ionization products. They are called 'secondary' because they are generated by other radiation (the ''primary'' radiation). This radiation can be in the form of ions, electrons, or photons with suffi ...

and back-scattered electron

detector A sensor is often defined as a device that receives and responds to a signal or stimulus. The stimulus is the quantity, property, or condition that is sensed and converted into electrical signal. In the broadest definition, a sensor is a devi ...

used in

scanning electron microscope A scanning electron microscope (SEM) is a type of electron microscope that produces images of a sample by scanning the surface with a focused beam of electrons. The electrons interact with atoms in the sample, producing various signals that ...

s (SEMs). It is named after its designers, Thomas E. Everhart and Richard F. M. Thornley, who in 1960 published their design to increase the efficiency of existing secondary electron detectors by adding a light pipe to carry the photon signal from the scintillator inside the evacuated specimen chamber of the SEM to the

photomultiplier A photomultiplier is a device that converts incident photons into an electrical signal. Kinds of photomultiplier include: * Photomultiplier tube, a vacuum tube converting incident photons into an electric signal. Photomultiplier tubes (PMTs for sh ...

outside the chamber. Prior to this Everhart had improved a design for a secondary electron detection by

Vladimir Zworykin Vladimir Kosma Zworykin (1888/1889July 29, 1982) was a Russian-American inventor, engineer, and pioneer of television technology. Zworykin invented a television transmitting and receiving system employing cathode-ray tubes. He played a role in t ...

and

Jan A. Rajchman Jan Aleksander Rajchman (10 August 1911 – 1 April 1989) was a Polish-American electrical engineer and computer pioneer. Biography Jan Aleksander was son of Ludwik Rajchman and Maria Bojańczyk. His father was a Polish bacteriologist and ...

by changing the electron multiplier to a photomultiplier. The Everhart–Thornley Detector with its lightguide and highly efficient photomultiplier is the most frequently used detector in SEMs. The detector consists primarily of a

scintillator A scintillator ( ) is a material that exhibits scintillation, the property of luminescence, when excited by ionizing radiation. Luminescent materials, when struck by an incoming particle, absorb its energy and scintillate (i.e. re-emit the ab ...

inside a

Faraday cage A Faraday cage or Faraday shield is an enclosure used to block some electromagnetic fields. A Faraday shield may be formed by a continuous covering of conductive material, or in the case of a Faraday cage, by a mesh of such materials. Faraday cag ...

inside the specimen chamber of the microscope. A low positive

voltage Voltage, also known as (electrical) potential difference, electric pressure, or electric tension, is the difference in electric potential between two points. In a Electrostatics, static electric field, it corresponds to the Work (electrical), ...

is applied to the Faraday cage to attract the relatively low energy (less than 50 eV by definition) secondary electrons. Other electrons within the specimen chamber are not attracted by this low voltage and will only reach the detector if their direction of travel takes them to it. The scintillator has a high positive voltage (in the nature of 10,000 V) to accelerate the incoming electrons to it where they can be converted to light photons. The direction of their travel is focused to the lightguide by a metal coating on the scintillator acting as a mirror. In the light pipe the photons travel outside of the microscope's vacuum chamber to a photomultiplier tube for amplification. The E-T secondary electron detector can be used in the SEM's back-scattered electron mode by either turning off the Faraday cage or by applying a negative voltage to the Faraday cage. However, better back-scattered electron images come from dedicated BSE detectors rather than from using the E–T detector as a BSE detector.

References

{{DEFAULTSORT:Everhart-Thornley detector Microscope components