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Page "Photomultiplier" ¶ 16
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caesium-antimony and photocathode
Also in 1936, a much improved photocathode, Cs < sub > 3 </ sub > Sb ( caesium-antimony ), was reported by P. Görlich.
For example, S-11 uses the caesium-antimony photocathode with a lime glass window, S-13 uses the same photocathode with a fused silica window, and S-25 uses a so-called " multialkali " photocathode ( Na-K-Sb-Cs, or sodium-potassium-antimony-caesium ) that provides extended response in the red portion of the visible light spectrum.

caesium-antimony and .
A caesium-antimony cathode gives a device that is very sensitive in the violet to ultra-violet region with sensitivity falling off to blindness to red light.

photocathode and had
" 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.
These devices used an S1 photocathode or " silver-oxygen-caesium " photocathode, discovered in 1930 which had a sensitivity of around 60 µA / lm ( Microampere per Lumen ) and a quantum efficiency of around 1 % in the ultraviolet region and around 0. 5 % in the infrared region.
Of note, the S1 photocathode had sensitivity peaks in both the infrared and ultraviolet spectrum and with sensitivity over 950 nm was the only photocathode material that could be used to view infrared light above 950 nm.
It was not until the development of the bialkali antimonide photocathodes ( potassium-cesium-antimony and sodium-potassium-antimony ) discovered by A. H. Sommer and his later multialkali photocathode ( sodium-potassium-antimony-cesium ) S20 photocathode discovered in 1956 by accident, that the tubes had both suitable infra-red sensitivity and visible spectrum amplification to be useful militarily.

photocathode and improved
To overcome the ion-poisoning problems, they improved scrubbing techniques during manufacture of the MCP ( the primary source of positive ions in a wafer tube ) and implemented autogating, discovering that a sufficient period of autogating would cause positive ions to be ejected from the photocathode before they could cause photocathode poisoning.

photocathode and quantum
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.
It has a typical sensitivity of around 230 µA / lm and a higher quantum efficiency than S20 photocathode material.

photocathode and efficiency
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.

photocathode and %
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.
The electron source for the ILC will use 2-nanosecond laser light pulses to eject electrons from a photocathode, a technique allowing for up to 80 % of the electrons to be polarized ; the electrons then will be accelerated to 5 GeV in a 250-meter linac stage.

photocathode and at
Finally, the electrons reach the anode, where the accumulation of charge results in a sharp current pulse indicating the arrival of a photon at the photocathode.
There are two common photomultiplier orientations, the head-on or end-on ( transmission mode ) design, as shown above, where light enters the flat, circular top of the tube and passes the photocathode, and the side-on design ( reflection mode ), where light enters at a particular spot on the side of the tube, and impacts on an opaque photocathode.
directed at the photomultiplier tube's photocathode which is connected to the negative of a high voltage source.
The wavelength of maximum emission is at 420 nm, well matched to the photocathode sensitivity of bialkali PMTs.
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.
In the image store, light falls upon the photocathode which is a photosensitive plate at a very negative potential ( approx.
Overall, the amplification at the image front and at the electron multiplier, plus the wise use of secondary emission wherever possible make the Image Orthicon an excellent camera tube, with a typical illumination on photocathode for maximum signal output of 0. 01ft-c ( 0. 1lx ).
Secondly, the photocathode exhibits negative electron affinity ( NEA ) which provides photoelectrons that are excited to the conduction band a free ride to the vacuum band as the Cesium Oxide layer at the edge of the photocathode causes sufficient band-bending.
This makes the photocathode very efficient at creating photoelectrons from photons.
The Modulation Transfer Function on an image intensifier is a measure of the output amplitude of dark and light lines on the display for a given level of input from lines presented to the photocathode at different resolutions.
Factors that affect the MTF include transition through any fiber plate or glass, at the screen and the photocathode and also through the tube and the microchannel plate itself.

photocathode and nm
The long wavelength response can be extended to 930 nm by a special photocathode activation processing.
This photocathode material covers a wider spectral response range than multialkali, from ultraviolet to 930 nm.
With special manufacturing techniques this photocathode can operate up to 1700 nm.
Firstly, they used a GaAs / CsO / AlGaAs photocathode which is more sensitive in the 800 nm-900 nm range than second generation photocathodes.

photocathode and was
This was the first compound photocathode material, developed in 1929.
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.
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.

photocathode and used
The RCA prototype photomultipliers also used a Ag-O-Cs ( silver oxide-caesium ) photocathode.
High dark current ; used mainly in near-infrared, with the photocathode cooled.
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.

photocathode and first
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.
Photomultipliers with large distances between the photocathode and the first dynode are especially sensitive to magnetic fields.
A photographic comparison between a first generation cascade tube and a second generation wafer tube, both using electrostatic inversion, a 25mm photocathode of the same material and the same F2. 2 55mm lens.

photocathode and photomultipliers
The windows of the photomultipliers act as wavelength filters ; this may be irrelevant if the cutoff wavelengths are outside of the application range or outside of the photocathode sensitivity range, but special care has to be taken for uncommon wavelengths.

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 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.
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.
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.
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.
Electrons are generated by a cold cathode, a hot cathode, a photocathode, or radio frequency ( RF ) ion sources.
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 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.
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.

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