Secondary emission
Encyclopedia
Secondary emission in physics
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 primary particles are often charged particle
s like electron
s or ion
s. If the secondary particles are electrons, the effect is termed secondary electron
emission. In this case, the number of secondary electrons emitted per incident particle is called secondary emission yield. If the secondary particles are ions, the effect is termed secondary ion emission.
The effect can also be exploited to advantage such as in the photomultiplier
tube. In this instance the electrons (or an electron) emitted from a photocathode
are accelerated towards a polished metal electrode (called a dynode
). This electron or electrons strike with sufficient energy to 'knock' many more electrons from its surface through secondary emission. These new electrons are then accelerated towards another dynode where even more electrons are emitted. This process occurs (typically) 10 or so times. The result is that the tiny and normally undetectable current from the photocathode becomes a much larger and easily measurable current flowing in the final anode circuit. The current gain is typically many hundreds of millions.
For the first multiplication the electron is accelerated by 100 to 200 eV and hits the surface in grazing incidence so that a mean 10 secondary electrons are emitted and the chance that at least 2 electrons are emitted is very high. In this way every electron can be detected and the efficiency of about 0.3 is mostly governed by the generation of photoelectrons (one kind of secondary electron) and their ejection into the vacuum. Ions are detected by accelerating them onto a separate dynode, which suffers from sputtering
, and detecting their secondary electrons. Ions at keV kinetic energy generate about 30 secondary electrons.
computer memory used a type of cathode ray tube
called the Williams tube
that used secondary emission to store bits on the tube face. Another random access computer memory tube based on secondary emission was the Selectron tube
. Both were made obsolete by the invention of magnetic core memory
.
thermionic valve (tube). In this instance the positively charged screen grid can accelerate the electron stream sufficiently to cause secondary emission at the anode
(plate
). This can give rise to excessive screen grid current. It is also partly responsible for this type of valve (tube), particularly early types with anodes not treated to reduce secondary emission, exhibiting a 'negative resistance
' characteristic. This side effect could be put to use by using some older valves (e.g., type 77 pentode) as dynatron
oscillators.
Physics
Physics is a natural science that involves the study of matter and its motion through spacetime, along with related concepts such as energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.Physics is one of the oldest academic...
is a phenomenon where primary incident particles of sufficient energy
Energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...
, when hitting a surface or passing through some material, induce the emission of secondary particles. The primary particles are often charged particle
Charged particle
In physics, a charged particle is a particle with an electric charge. It may be either a subatomic particle or an ion. A collection of charged particles, or even a gas containing a proportion of charged particles, is called a plasma, which is called the fourth state of matter because its...
s like electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s or ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
s. If the secondary particles are electrons, the effect is termed secondary electron
Secondary electrons
Secondary electrons are electrons generated as ionization products. They are called 'secondary' because they are generated by other radiation . This radiation can be in the form of ions, electrons, or photons with sufficiently high energy, i.e. exceeding the ionization potential...
emission. In this case, the number of secondary electrons emitted per incident particle is called secondary emission yield. If the secondary particles are ions, the effect is termed secondary ion emission.
Secondary emissive materials
Most common used secondary emissive materials include :- alkali antimonide,
- beryllium oxide (BeO),
- magnesium oxide (MgO),
- gallium phosphide (GaP),
- gallium arsenide phosphide (GaAsP),
- lead oxide (PbO),
- etc.
Photo multipliers and similar devices
Secondary emission is a phenomenon where additional electrons, called secondary electrons, are emitted from the surface of a material when an incident particle (often, charged particle such as electron or ion) impacts the material with sufficient energy. The number of secondary electrons emitted per incident particle is called secondary emission yield.The effect can also be exploited to advantage such as in the photomultiplier
Photomultiplier
Photomultiplier tubes , members of the class of vacuum tubes, and more specifically phototubes, are extremely sensitive detectors of light in the ultraviolet, visible, and near-infrared ranges of the electromagnetic spectrum...
tube. In this instance the electrons (or an electron) emitted from a photocathode
Photocathode
A photocathode is a negatively charged electrode in a light detection device such as a photomultiplier or phototube that is coated with a photosensitive compound...
are accelerated towards a polished metal electrode (called a dynode
Dynode
A dynode is one of a series of electrodes within a photomultiplier tube. Each dynode is at a more positive electrical potential than its predecessor. Secondary emission occurs at the surface of each dynode...
). This electron or electrons strike with sufficient energy to 'knock' many more electrons from its surface through secondary emission. These new electrons are then accelerated towards another dynode where even more electrons are emitted. This process occurs (typically) 10 or so times. The result is that the tiny and normally undetectable current from the photocathode becomes a much larger and easily measurable current flowing in the final anode circuit. The current gain is typically many hundreds of millions.
For the first multiplication the electron is accelerated by 100 to 200 eV and hits the surface in grazing incidence so that a mean 10 secondary electrons are emitted and the chance that at least 2 electrons are emitted is very high. In this way every electron can be detected and the efficiency of about 0.3 is mostly governed by the generation of photoelectrons (one kind of secondary electron) and their ejection into the vacuum. Ions are detected by accelerating them onto a separate dynode, which suffers from sputtering
Sputtering
Sputtering is a process whereby atoms are ejected from a solid target material due to bombardment of the target by energetic particles. It is commonly used for thin-film deposition, etching and analytical techniques .-Physics of sputtering:...
, and detecting their secondary electrons. Ions at keV kinetic energy generate about 30 secondary electrons.
Special amplifying tubes
In the 1930s special amplifying tubes were developed which deliberately "folded" the electron beam, by having it strike a dynode to be reflected into the anode. This had the effect of increasing the plate-grid distance for a given tube size, increasing the transconductance of the tube and reducing its noise figure. A typical such "orbital beam hexode" was the RCA 1630, introduced in 1939. Because the heavy electron current in such tubes damaged the dynode surface rapidly, their lifetime tended to be very short compared to conventional tubes.Early computer memory tubes
The first random accessRandom access
In computer science, random access is the ability to access an element at an arbitrary position in a sequence in equal time, independent of sequence size. The position is arbitrary in the sense that it is unpredictable, thus the use of the term "random" in "random access"...
computer memory used a type of cathode ray tube
Cathode ray tube
The cathode ray tube is a vacuum tube containing an electron gun and a fluorescent screen used to view images. It has a means to accelerate and deflect the electron beam onto the fluorescent screen to create the images. The image may represent electrical waveforms , pictures , radar targets and...
called the Williams tube
Williams tube
The Williams tube or the Williams-Kilburn tube , developed in about 1946 or 1947, was a cathode ray tube used to electronically store binary data....
that used secondary emission to store bits on the tube face. Another random access computer memory tube based on secondary emission was the Selectron tube
Selectron tube
The Selectron was an early form of digital computer memory developed by Jan A. Rajchman and his group at the Radio Corporation of America under the direction of Vladimir Zworykin, of television technology fame...
. Both were made obsolete by the invention of magnetic core memory
Magnetic core memory
Magnetic-core memory was the predominant form of random-access computer memory for 20 years . It uses tiny magnetic toroids , the cores, through which wires are threaded to write and read information. Each core represents one bit of information...
.
Undesirable effects
Secondary emission can be undesirable such as in the tetrodeTetrode
A tetrode is an electronic device having four active electrodes. The term most commonly applies to a two-grid vacuum tube. It has the three electrodes of a triode and an additional screen grid which significantly changes its behaviour.-Control grid:...
thermionic valve (tube). In this instance the positively charged screen grid can accelerate the electron stream sufficiently to cause secondary emission at the anode
Anode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....
(plate
Plate electrode
A plate is a type of electrode that formed part of a vacuum tube. The plate is impressed with a positive charge so that it may capture and flow electrons within a circuit....
). This can give rise to excessive screen grid current. It is also partly responsible for this type of valve (tube), particularly early types with anodes not treated to reduce secondary emission, exhibiting a 'negative resistance
Negative resistance
Negative resistance is a property of some electric circuits where an increase in the current entering a port results in a decreased voltage across the same port. This is in contrast to a simple ohmic resistor, which exhibits an increase in voltage under the same conditions. Negative resistors are...
' characteristic. This side effect could be put to use by using some older valves (e.g., type 77 pentode) as dynatron
Dynatron
For the brand, see Dynatron Radio LtdThe dedicated dynatron vacuum tube was invented by Albert Hull in 1918. It has three electrodes: a thermionic cathode, a perforated anode, and a supplementary anode or plate, and its characteristic curves have a region exhibiting negative resistance, which is...
oscillators.