Thermionic noise

Birch et al. have used the Philips XP2254B, an S20 version of the XP2020Q, to study the fluorescence lifetimes of a series of aminotetraphenylporphyrins in a multiplexed fluorometer. 83 The extended red response (S20R) version of this device, the XP2257B, has been used with IR spark source excitation to study the fluorescence lifetimes of carbocyanine dyes up to 930 nm emission in isotropic and anisotropic media. 55,561 % 84) An improved voltage divider network has been developed for linear focused photomultipliers which reduces thermionic noise from the photocathode by an order of magnitude by restricting the collection of photoelectrons to the center of the photocathode. 84 ... 

Thermionic dark current emission occurs whenever thermally produced electrons are accelerated in the dynode train. The detection of low light energy levels is limited by the thermionic dark current. Thermionic noise cannot be balanced out of the measurement. Thermionic noise can be decreased by operating the multiplier phototube at reduced temperatures and in some applications this is done. For most analytical spectroscopy applications, however, the lower thermionic noise obtained at lower temperatures is not worth the considerable inconvenience of operating at low temperatures. 

It is important that a photomultiplier gives low noise and a low dark current, i.e. low background signal in the absence of photons, usually caused by thermionic emission of photons from the cathode material. 

For photomultiplier tubes, the noise is caused primarily by thermionic emission from the cathode. The dark current produced by this process is proportional to the area of the photomultiplier cathode, and is typically in the range of 0.05 to 10 nA. 

For the three types of noise discussed above, the Fellgett advantage must be carefully evaluated. This multiplex advantage is an unquestioned benefit, for example, when detector noise dominates, as is the case in infrared Fourier transform spectroscopy. In visible/UV spectroscopy, the detector noise which is present can also be minimized with the multiplex advantage. Therefore it is not always necessary to cool photomultiplier tubes to reduce the thermionic emission for Fourier transform spectroscopy. 

Light reflected from the field of view is detected by a photomultiplier encased in a 6 mm thick water-cooled copper housing, to reduce both thermionic and radiation-induced noise. The output is amplified and sent via the 50 m umbilical cable to the video store housed in the electronics rack. This video frame store, which consists of 512 x 512 bytes of 8 bit grey scale, acts as a buffer between the picture frame rate of from 1 to 30 seconds, and the video monitor which displays the contents of the store at the normal video frame rate. Since the eye can resolve only 6 bits grey scale, this store is more than adequate for video reproduction. 

The use of the liquid scintillation counter for chemiluminescence in the out-of-coincidence mode, at phototube-amplifier gains such that single photoelectrons are detected also means that the thermionic emission of the phototube photocathode will be counted with equal efficiency. If N and N represent the thermionic electron noise of the phototubes, the signal-to-noise ratio for chemiluminescence measurements will be approximately... 

The so-called dark noise introduced by a discrete dynode SEM when operated under appropriate conditions is largely Johnson noise, the result of random thermionic emission of electrons from the dynode surfaces, and is minimized by appropriate choice of materials. When SEMs are used appropriately the dark noise is generally low (< lO A, negligible in applications of interest here) even when operated to provide Gsem = 10 . (The maximum output current from such devices can be as high as 10 A before saturation occurs.) In addition, some dark current can arise from leakage through the ceramic supports for the dynodes, particularly when they become... 

The main advantages of the CPM are its compact design, its greater dynamic range and its lower dark current (caused mainly by thermionic emission from the photocathode) which is smaller due to its reduced area. The noise caused by fluctuations in the multiplication factor is also smaller, due to the larger value of the secondary emission factor q. 

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