Detector types photoemissive

Although not specifically delineated, the volume is also divided into three general sections. The first addresses the full spectrum of infrared detectors and contains a limited coverage of all the material presented in subsequent chapters. It serves as an introduction to the volume and presents to the reader an overall view of the present state of the infrared technology art. It also serves as the mortar between the more in-depth discussions which follow. The midsection. Chapters 3,4, and 5, is a detailed analysis of those detector types which are most widely used today thermal, photoconductive/photovoltaic and photoemissive. 

In order to be consistent with the earlier chapters of this volume the detector advances are segregated into general detector types (thermal, photoconductor and photovoltaic, photoemissive, charge transfer devices, and heterodyne). The final section of this chapter presents future detector and related technology advances (from the author s point of view) that would greatly enhance our ability to use optical and infrared detectors to solve basic problems of our society. 

Flame photometry (see also p. 168) is almost exclusively used for the determination of alkali metals because of their low excitation potential (e.g. sodium 5.14eV and potassium 4.34 eV). This simplifies the instrumentation required and allows a cooler flame (air-propane, air-butane or air-natural gas) to be used in conjunction with a simpler spectrometer (interference filter). The use of an interference filter allows a large excess of light to be viewed by the detector. Thus, the expensive photomultiplier tube is not required and a cheaper detector can be used, e.g. a photodiode or photoemissive detector. The sample is introduced using a pneumatic nebulizer as described for FAAS (p. 172). Flame photometry is therefore a simple, robust and inexpensive technique for the determination of potassium (766.5 nm) or sodium (589.0nm) in clinical or environmental samples. The technique suffers from the same type of interferences as in FAAS. The operation of a flame photometer is described in Box 26.2. 

As shown in Table 25-2, there are two general types of transducers one type responds to photons, the other to heat. All photon detectors are based on the interaction of radiation with a reactive surface either to produce electrons (photoemission) or to promote electrons to energy states in which they can conduct electricity (photoconduction). Only UV, visible, and near-IR radiation possess enough energy to cause photoemission to occur thus, photoemissive detectors are limited to wavelengths shorter than about 2 p.m (2000 nm). Photoconductors can be used in the near-, mid-, and far-IR regions of the spectrum. 

With phototubes and photomultiplier-type detectors (photoemissive detectors, ultraviolet to visible range), thermal noise becomes insignificant as compared to shot noise. Shot noise is the random fluctuation of the electron current from an electron-emitting surface (i.e., across a junction from cathode to anode), and in PM mbes that is amplified and becomes the noise-limiting fluctuation. In instruments with these detectors, the absolute error is not constant at all values of T, and the expressions for the spectrophotometric error become more complicated. It has been calculated that, for these cases, the minimal error should occur at 0.136 or A = 0.87. These instruments have a working range of about 0.1 to 1.5 A. 

There are two basic types of photon detectors photoemissive and solid state. The photoemissive type is generally represented by the photomultiplier tube detectors, whereas the solid-state type detectors are represented by photodiode detectors, pyroelectric detectors, and infrared detectors. 

Instead of the quadmpole mass analyzer, other types of detectors may be used, e g., other types of mass analyzers like TOF-MS, tandem mass spectrometers (TQ instmments), but also devices for optical and photoelectron spectroscopy or neutral product analysis, ion photodissociation, or photoemission [104]. 

Table 1.4 Basic types of vacuum photoemissive detectors according to photocathode type ...

Regarding its design, a photoemissive detector may be a vacuum tube, photomultiplier (a tube plus dynode converter), some types of camera tubes, image converters, image amplifiers, etc. The main shortcoming of photoemissive tubes is the limited choice of cathode materials, which does not permit their maximum wavelengths to reach the spectral ranges of interest. We mention them for the sake of completeness, as well as because future solutions could potentially overcome this problem. 

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