Photon detectors photo-multiplier

The Raman effect is due to the same vibrations that give rise to the infrared spectrum. Raman scattering describes the inelastic scattering of incident light by certain vibrational transitions (described as Raman active). Note that this is not a fluorescence effect. The molecule is not electronically excited and the incident photon interacts with the vibration of the molecule on a time-scale of the order of 10 seconds. The Raman effect is also weak—except for resonant transitions, no more than one photon in a million is inelastically scattered in this way. Hence the need for powerful sources of monochromatic radiation (lasers) and sensitive detectors (photo-multiplier tubes or charge-coupled devices). 

The photo-multiplier tube (PMT) as a one-channel detector could not be an alternative. Even though the PMT is an almost perfect photon detector, offering shot-noise limited measurements for almost every photon flux, but its resfliction to exclusive sequential registration of spectral intervals makes it, and also the different types of dissector tubes, unsuitable for application in CS AAS. 

The secondary electron multiplier (SEM) detector is the key to the role of mass spectrometry as an extremely sensitive analytical technique with wide dynamic range and compatibility with on-line coupling to fast chromatographic separations. The SEM was a natural development from the invention of the photomultiplier (Zworkin 1936, 1939), in which photoelectrons produced by photons falling on a conversion dynode with a photo-sensitive surface are amplified in an avalanche fashion by accelerating the original (first strike) photoelectrons on to a... 

Standard equipment makes use of photodiodes working with the internal photo effect. The charge current is linear proportional to the number of photons falling onto the sensitive area. These photodiodes allow an arrangement in arrays. This type of detector is used in modem simultaneous detection systems (see Sec. 3.2.) and in chromatography [1,18]. Photomultipliers use the external photo effect. Incident radiation catapults out of the sensitive surface (photocathode) electrons which are multiplied (scintillation) and accelerated between dynodes on their way to the anode. Their detectivity is better and prices are higher [23]. 

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