Electron multiplier
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An electron multiplier is a vacuum-tube structure that multiplies incident charges. In a process called
secondary emission In particle physics, secondary emission is a phenomenon where primary incident particles of sufficient energy, when hitting a surface or passing through some material, induce the emission of secondary particles. The term often refers to the emi ...
, a single
electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no kn ...
can, when bombarded on secondary-emissive material, induce emission of roughly 1 to 3
electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no kn ...
s. If an
electric potential The electric potential (also called the ''electric field potential'', potential drop, the electrostatic potential) is defined as the amount of work energy needed to move a unit of electric charge from a reference point to the specific point in ...
is applied between this metal plate and yet another, the emitted electrons will accelerate to the next metal plate and induce
secondary emission In particle physics, secondary emission is a phenomenon where primary incident particles of sufficient energy, when hitting a surface or passing through some material, induce the emission of secondary particles. The term often refers to the emi ...
of still more electrons. This can be repeated a number of times, resulting in a large shower of electrons all collected by a metal anode, all having been triggered by just one.


History

In 1930, Russian physicist Leonid Aleksandrovitch Kubetsky proposed a device which used
photocathode A photocathode is a surface engineered to convert light (photons) into electrons using the photoelectric effect. Photocathodes are important in accelerator physics where they are utilised in a photoinjector to generate high brightness electron be ...
s combined with
dynode A dynode is an electrode in a vacuum tube A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current flow in a high vacuum between electrodes to which an electric potential diff ...
s, or secondary electron emitters, in a single tube to remove secondary electrons by increasing the electric potential through the device. The electron multiplier can use any number of dynodes in total, which use a coefficient, σ, and created a gain of σn where n is the number of emitters.


Discrete dynode

Secondary electron emission begins when one electron hits a dynode inside a vacuum chamber and ejects electrons that cascade onto more dynodes and repeats the process over again. The dynodes are set up so that each time an electron hits the next one it will have an increase of about 100 electron Volts greater than the last dynode. Some advantages of using this include a response time in the picoseconds, a high sensitivity, and an electron gain of about 108 electrons.


Continuous dynode

A continuous dynode system uses a horn-shaped funnel of glass coated with a thin film of semiconducting materials. The electrodes have increasing resistance to allow secondary emission. Continuous dynodes use a negative high voltage in the wider end and goes to a positive near ground at the narrow end. The first device of this kind was called a Channel Electron Multiplier (CEM). CEMs required 2-4 kilovolts in order to achieve a gain of 106 electrons.


Microchannel plate

Another geometry of continuous-dynode electron multiplier is called the microchannel plate (MCP).. It may be considered a 2-dimensional parallel array of very small continuous-dynode electron multipliers, built together and powered in parallel. Each microchannel is generally parallel-walled, not tapered or funnel-like. MCPs are constructed from lead glass and carry a resistance of 109 Ω between each electrode. Each channel has a diameter of 10-100 μm. The electron gain for one microchannel plate can be around 104-107 electrons.


Applications


Instruments

In
mass spectrometry Mass spectrometry (MS) is an analytical technique that is used to measure the mass-to-charge ratio of ions. The results are presented as a ''mass spectrum'', a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is use ...
electron multipliers are often used as a detector of ions that have been separated by a mass analyzer of some sort. They can be the continuous-dynode type and may have a curved horn-like funnel shape or can have discrete dynodes as in a
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 ...
. Continuous dynode electron multipliers are also used in NASA missions and are coupled to a gas chromatography mass spectrometer ( GC-MS) which allows scientists to determine the amount and types of gasses present on Titan, Saturn's largest moon.


Night-vision

Microchannel plates are also used in night-vision goggles. As electrons hit the millions of channels, they release thousands of secondary electrons. These electrons then hit a phosphor screen where they are amplified and converted back into light. The resulting image patterns the original and allows for better vision in the dark, while only using a small battery pack to provide a voltage for the MCP.


See also

*
Faraday cup A Faraday cup is a metal (conductive) cup designed to catch charged particles in vacuum. The resulting current can be measured and used to determine the number of ions or electrons hitting the cup. The Faraday cup was named after Michael Faraday w ...
*
Daly detector A Daly detector is a gas-phase ion detector that consists of a metal "doorknob", a scintillator (phosphor screen) and a photomultiplier.N. R. DalyScintillation Type Mass Spectrometer ion Detector. ''Rev. Sci. Instrum.'' 31(3), 264–267 (1960). ...
*
Phototube A phototube or photoelectric cell is a type of gas-filled or vacuum tube that is sensitive to light. Such a tube is more correctly called a 'photoemissive cell' to distinguish it from photovoltaic or photoconductive cells. Phototubes were previ ...
*
Photo-multiplier tube Photomultiplier tubes (photomultipliers or PMTs for short) are extremely sensitive detectors of light in the ultraviolet, visible, and near-infrared ranges of the electromagnetic spectrum. They are members of the class of vacuum tubes, more speci ...
*
Scintillation counter A scintillation counter is an instrument for detecting and measuring ionizing radiation by using the excitation effect of incident radiation on a scintillating material, and detecting the resultant light pulses. It consists of a scintillator w ...
*
Lucas cell A Lucas cell is a type of scintillation counter. It is used to acquire a gas sample, filter out the radioactive particulates through a special filter and then count the radioactive decay. The inside of the gas chamber is coated with ZnS( Ag) - a ...
*
Zoltán Lajos Bay Zoltán Lajos Bay (July 24, 1900 in Gyulavári – October 4, 1992 in Washington, D.C.)"Fizikai Szemle 1999/5 - Zsolt Bor: OPTICS BY HUNGARIANS" (with Zoltán Bay), József Attila University, Szeged, Hungary, 199KFKI-Hungary-Bor/ref> was a Hu ...
(developer)


References


External links


Olympus Tutorial

How Discrete Dynode Electron Multipliers work
{{Mass spectrometry Measuring instruments Radio electronics Mass spectrometry Electron Analytical chemistry de:Sekundärelektronenvervielfacher