particle physics Particle physics or high energy physics is the study of fundamental particles and forces that constitute matter and radiation. The fundamental particles in the universe are classified in the Standard Model as fermions (matter particles) an ...

, secondary emission is a phenomenon where primary incident particles of sufficient

energy In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of ...

, when hitting a surface or passing through some material, induce the emission of secondary particles. The term often refers to the emission of

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 n ...

s when charged particles like electrons or ions 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 difference has been applied. The type known as ...

strike a metal surface; these are called secondary electrons. 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 electron emission is used in photomultiplier tubes and

image intensifier An image intensifier or image intensifier tube is a vacuum tube device for increasing the intensity of available light in an optical system to allow use under low-light conditions, such as at night, to facilitate visual imaging of low-light proce ...

tubes to amplify the small number of photoelectrons produced by photoemission, making the tube more sensitive. It also occurs as an undesirable side effect in electronic

s when electrons from the

cathode A cathode is the electrode from which a conventional current leaves a polarized electrical device. This definition can be recalled by using the mnemonic ''CCD'' for ''Cathode Current Departs''. A conventional current describes the direction in whi ...

strike the

anode An anode is an electrode of a polarized electrical device through which conventional current enters the device. This contrasts with a cathode, an electrode of the device through which conventional current leaves the device. A common mnemonic is ...

, and can cause parasitic oscillation.

Applications

Secondary emissive materials

Commonly used secondary emissive materials include * alkali antimonide * Beryllium oxide (BeO) * Magnesium oxide (MgO) * Gallium phosphide (GaP) * Gallium arsenide phosphide (GaAsP) * Lead oxide (PbO)

Photo multipliers and similar devices

In a photomultiplier tube, one or more electrons are emitted from a

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 ...

and accelerated towards a polished metal electrode (called a

dynode A dynode is an electrode in a vacuum tube that serves as an electron multiplier through secondary emission. The first tube to incorporate a dynode was the dynatron, an ancestor of the magnetron, which used a single dynode.Albert W. Hull, E. F. ...

). They hit the electrode surface with sufficient energy to release a number of electrons through secondary emission. These new electrons are then accelerated towards another dynode, and the process is repeated several times, resulting in an overall gain ('electron multiplication') in the order of typically one million and thus generating an electronically detectable current pulse at the last dynodes. Similar

electron multiplier An electron multiplier is a vacuum-tube structure that multiplies incident charges. In a process called secondary emission, a single electron can, when bombarded on secondary-emissive material, induce emission of roughly 1 to 3 electrons. If an ...

s can be used for detection of fast particles like electrons or ''ions''.

Historic applications

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 access computer memory used a type of

cathode ray tube A cathode-ray tube (CRT) is a vacuum tube containing one or more electron guns, which emit electron beams that are manipulated to display images on a phosphorescent screen. The images may represent electrical waveforms ( oscilloscope), ...

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.

Undesirable effects - the tetrode

Secondary emission can be undesirable such as in the tetrode

thermionic valve 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 difference has been applied. The type known as a ...

(tube). In this instance the positively charged screen grid can accelerate the electron stream sufficiently to cause secondary emission at the

( 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, which could cause the tube to become unstable. This side effect could be put to use by using some older valves (e.g., type 77 pentode) as dynatron oscillators. This effect was prevented by adding a third grid to the tetrode, called the suppressor grid, to repel the electrons back toward the plate. This tube was called the pentode.

References

{{DEFAULTSORT:Secondary Emission Condensed matter physics Electronics concepts Vacuum tubes Electron