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The photocathode contains combinations of materials such as caesium, rubidium and antimony specially selected to provide a low work function, so when illuminated even by very low levels of light, the photocathode readily releases electrons.
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photocathode and contains
The device contains a low absorbency / scatter input window, typically aluminum, input fluorescent screen, photocathode, electron optics, output fluorescent screen and output window.
photocathode and combinations
Various combinations of photocathode and window materials were assigned " S-numbers " ( spectral numbers ) ranging from S-1 through S-40, which are still in use today.
photocathode and materials
Besides the different photocathode materials, performance is also affected by the transmission of the window material that the light passes through, and by the arrangement of the dynodes.
Typically the materials have low work function and are therefore prone to thermionic emission, causing noise and dark current, especially the materials sensitive in infrared ; cooling the photocathode lowers this thermal noise.
The most common photocathode materials are:
Since Ag-O-Cs has a higher dark current than more modern materials photomultiplier tubes with this photocathode material are nowadays used only in the infrared region with cooling.
With the discovery of more effective photocathode materials, which increased in both sensitivity and quantum efficiency, it became possible to achieve significant levels of gain over Generation 0 devices.
Second generation image intensifiers use the same multialkali photocathode that the first generation tubes used, however by using thicker layers of the same materials, the S25 photocathode was developed, which provides extended red response and reduced blue response, making it more suitable for military applications.
* Discovery of other photocathode materials
photocathode and such
Both the photocathode and the image plane of such an electrode configuration are curved concave as seen from the anode aperture.
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.
* Phototubes containing a photocathode which emits electrons when illuminated, such that the tube conducts a current proportional to the light intensity.
photocathode and caesium
Instead, a caesium iodide phosphor is deposited directly on the photocathode of the intensifier tube.
photocathode and specially
It has an efficient photocathode that transforms the scene light into an electron image ; the latter is then accelerated towards a target specially prepared for the emission of secondary electrons.
The scene image is projected onto an efficient continuous-film semitransparent photocathode that transforms the scene light into a light-emitted electron image, the latter is then accelerated ( and focused ) via electromagnetic fields towards a target specially prepared for the emission of secondary electrons.
photocathode and low
A photocathode usually consists of alkali metals with very low work functions.
At room temperatures, this photocathode operates with very low dark current, making it ideal for use in photon counting applications.
The image dissector has no " charge storage " characteristic ; the vast majority of electrons emitted by the photocathode are excluded by the scanning aperture, and thus wasted rather than being stored on a photo-sensitive target, as in the iconoscope or image orthicon ( see below ), which largely accounts for its low light sensitivity.
The high sensitivity of this photocathode, greater than 900 µA / lm, allows more effective low light response, though this was offset by the thin film, which typically blocked up to 50 % of electrons.
photocathode and work
The electrons leave the photocathode, having the energy of the incoming photon ( minus the work function of the photocathode ).
Optoelectronic streak cameras work by directing the light onto a photocathode, which when hit by photons produces electrons via the photoelectric effect.
photocathode and when
* Photomultiplier tubes containing a photocathode which emits electrons when illuminated, the electrons are then amplified by a chain of dynodes.
photocathode and by
The photocathode sensitivities S, phosphor efficiencies P, and anode potentials V of the individual stages shall be distinguished by means of subscripts 1, and 2, in the text, where required.
The luminous gain of a single stage with Af ( flux gain ) is, to a first approximation, given by the product of the photocathode sensitivity S ( amp / lumen ), the anode potential V ( volts ), and the phosphor conversion efficiency P ( lumen/watt ).
The luminous efficiency Af of a photocathode depends on the maximum radiant sensitivity Af and on the spectral distribution of the incident light Af by the relation: Af where Af is normalized radiant photocathode sensitivity.
This is known as the electron affinity of the photocathode and is another barrier to photoemission other than the forbidden band, explained by the band gap model.
" The Soviet device used a magnetic field to confine the secondary electrons and relied on the Ag-O-Cs photocathode which had been demonstrated by General Electric in the 1920s.
Also in 1936, a much improved photocathode, Cs < sub > 3 </ sub > Sb ( caesium-antimony ), was reported by P. Görlich.
The caesium-antimony photocathode had a dramatically improved quantum efficiency of 12 % at 400 nm, and was used in the first commercially successful photomultipliers manufactured by RCA ( i. e., the 931-type ) both as a photocathode and as a secondary-emitting material for the dynodes.
No suitable photoemissive surfaces have yet been reported to detect wavelengths longer than approximately 1700 nanometers, which can be approached by a special ( InP / InGaAs ( Cs )) photocathode.
Electrons are generated by a cold cathode, a hot cathode, a photocathode, or radio frequency ( RF ) ion sources.
The long wavelength response can be extended to 930 nm by a special photocathode activation processing.
Such an arrangement is able to amplify the tiny current emitted by the photocathode, typically by a factor of one million.
* by emission mechanism ( thermionic, photocathode, cold emission, plasmas source ),
The entire electron image is deflected and a scanning aperture permits only those electrons emanating from a very small area of the photocathode to be captured by the detector at any given time.
On average, each image electron ejects several " splash " electrons ( thus adding amplification by secondary emission ), and these excess electrons are soaked up by the positive mesh effectively removing electrons from the target and causing a positive charge on it in relation to the incident light in the photocathode.
Using a simple lens, an image was focused on the photocathode and a potential difference of several thousand volts was maintained across the tube, causing electrons dislodged from the photocathode by photons to strike the fluorescent screen.
photocathode and very
It is obvious that the careful choice of photocathode which maximizes Af for a given input E ( in the case of the second stage, for the first phosphor screen emission ) is very important.
In the image store, light falls upon the photocathode which is a photosensitive plate at a very negative potential ( approx.
This makes the photocathode very efficient at creating photoelectrons from photons.
photocathode and levels
Due to the high electrostatic field stresses in the tube and the operation of the MicroChannel Plate, this led to failure of the photocathode within a short period-as little as 100 hours before photocathode sensitivity dropped below Gen2 levels.
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