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 suffici ...

and back-scattered electron

detector A sensor is a device that produces an output signal for the purpose of sensing a physical phenomenon. In the broadest definition, a sensor is a device, module, machine, or subsystem that detects events or changes in its environment and sends ...

used in scanning electron microscopes (SEMs). It is named after its designers,

Thomas E. Everhart Thomas Eugene Everhart FREng (born February 15, 1932, Kansas City, Missouri) is an American educator and physicist. His area of expertise is the physics of electron beams. Together with Richard F. M. Thornley he designed the Everhart–Thornley ...

and

Richard F. M. Thornley Richard is a male given name. It originates, via Old French, from Old Frankish and is a compound of the words descending from Proto-Germanic ''*rīk-'' 'ruler, leader, king' and ''*hardu-'' 'strong, brave, hardy', and it therefore means 'stron ...

, 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 sho ...

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

Vladimir Zworykin Vladimir Kosma Zworykin; or with the patronymic as ''Kosmich''; or russian: Кузьмич, translit=Kuz'mich, label=none. Zworykin anglicized his name to ''Vladimir Kosma Zworykin'', replacing the patronymic with the name ''Kosma'' as a middle na ...

and

Jan A. Rajchman Jan Aleksander Rajchman (London, 10 August 1911 – 1 April 1989) was a Polish 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 absorbed ...

inside a

Faraday cage A Faraday cage or Faraday shield is an enclosure used to block 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 cage ...

inside the specimen chamber of the microscope. A low positive

voltage Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to m ...

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 Microscopes