Detectors photoemissive

Photoemissive detectors, such as the photocell or the photomuliplier, are based on the external photoeffect. The photocathode of such a detector is covered with one or several layers of materials with a low work function 0 (e.g., alkali metal compounds or semiconductor compounds). Under illumination with monochromatic light of wavelength X = cfv, the emitted photoelectrons leave the photocathode with a kinetic energy given by the Einstein relation 

They are further accelerated by the voltage Vq between the anode and cathode and are collected at the anode. The resultant photocurrent is measured either directly or by the voltage drop across a resistor (Fig. 4.95a). 

Two types of photoelectron emitters are manufactured opaque layers, where light is incident on the same side of the photocathode from which the photoelectrons are emitted (Fig. 4.95b) and semitransparent layers (Fig. 4.95c), where light enters at the opposite side to the photoelectron emission and is absorbed throughout the thickness d of the layer. Because of the two factors fia and He, the quantum efficiency of semitransparent cathodes and its spectral change are critically dependent on the thickness d, and reach that of the reflection-mode cathode only if the value of d is optimized. 

For most emitters the threshold wavelength for photoemission is below 0.85 xm, corresponding to a work function 0 1.4 eV. An example for such a material with 0 1.4eV is a surface layer of NaKSb [4.123]. Only some 

Different devices of photoemissive detectors are of major importance in modern spectroscopy. These are the the photomultiplier, the image intensifier, and the streak camera. 

The most commonly used photocathodes are metallic or alkaline (alkali halides, alkali antimonide or alkali telluride) cathodes. The quantum efficiency rj = is defined as the ratio of the rate of photoelectrons to the rate of incident photons 

For most emitters the threshold wavelength for photoemission is below 0.85 xm, corresponding to a work function (j) 1.4 eV. An example for such a material with (j) 1.4 eV is a surface layer of NaKSb [252]. Only some complex cathodes consisting of two or more separate layers have an extended sensitivity up to about X 1.2 jim. For instance, an InGaAs photocathode has an extended sensitivity in the infrared, reaching up to 1700 nm. The spectral response of the most commonly fabricated photocathodes is designated by a standard nomenclature, using the symbols SI to S20. Some newly developed types are labeled by special numbers, which differ for the different manufacturers [253]. Examples are SI = Ag — O — Cs (300-1200 nm) or S4 = Sb - Cs (300-650 nm). 

For experiments demanding high time resolution, the rise time of this anode pulse should be as small as possible. Let us consider which effects may contribute to the anode pulse rise time, eaused by the spread of transit times for the different electrons [4.129,4.130]. Assmne that a single photoelectron is emitted from the photoeathode, and is accelerated to the first dynode. The initial velocities of the seeondary electrons vary because these electrons are released at different depths of the dynode material and their initial energies, when leaving the dynode surface, are between 0 and 5 eV. The transit time between two parallel electrodes with distance d and potential difference F is obtained from d= at with a = eVI md), which gives 

The quantum efficiency n = g ph defined as the ratio of the rate of production of photoelectrons, n, to the rate of incidence of photons, np. It depends on the cathode material, on the form and thickness of the photoemissive layer, and on the wavelength x of the incident radiation. The quantum efficiency n = can be represented by the product of three 

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Some radiation detectors, i.e., photoemissive detectors (vacuum phototubes or photomultipliers) or semiconductor detectors (photodiodes or phototransistors) directly produce an electrical signal by quantum effects. Their output is strongly dependent on the wavelength of the detected radiation. Thermal detectors, i.e., thermocouples and thermopiles, bolometers, pyroelectric detectors, or pneumatic and photoacoustic detectors record a temperature increase through radiation and convert this into an electrical signal. This is proportional to the flux of the absorbed radiant power, independent of the wavelength. 

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. 

On the other hand, photoconductive devices in principle respond only to the number of photoexcited carriers, regardless of where they are generated within the material. Thus they will receive equal contributions of background noise from both hemispheres if both are at the same temperature. This will cause another reduction of Df and D (7 ) by the square root of two. Photoemissive detectors having translucent photocathodes will be sensitive also to radiation from both hemispheres. Those with opaque photocathodes will not. 

By cooling the detector element substantially below the background temperature, the radiation from the back hemisphere can be ignored. Thus the value of D Ts) and Df need not be reduced for the photoconductive and translucent photoemissive detectors when the operating temperature is well below ambient. Cooling to 77 K is sufficient for a 295 K background limit. 

Combined Effects of Mode of Operation and Operating Temperature—The expressions for Df and D Ts) given in (2.78) and (2.81) apply directly to photovoltaic detectors, and to photoemissive detectors having opaque photocathodes. 

The expressions apply to a cooled photoemissive detector employing a translucent photocathode, but must be reduced by the square root of two if the operating temperature equals the background temperature. 

A photoemissive detector contains not only the photoemissive sensor layer but also electron detection or gain stages plus a connection to external electronics. In the preceding sections we have limited discussion to the photocathode portion of the photodetector because its quantum efficiency, spectral uniformity, and wavelength limitations are the primary factors limiting the performance of a complete device advances in the capabilities of the photocathodes have also been most dramatic in recent years. 

Although photoemissive detectors with no internal electron gain stages are available in the form of photodiodes, most state of the art devices incorporate an electron multiplier to boost photocathode output before it is coupled to the external electronics [5.2]. There are many advantages of electron-multiplication... 

Fig. 4.95a-c. Photoemissive detector (a) principle arrangement of a photocell (b) opaque photocathode and (c) semitransparent photocathode... 

A significant improvement of the signal-to-noise ratio in detection of low levels of radiation can be achieved with single-photon counting techniques, which enable spectroscopic investigations to be performed at incident radiation fluxes down to 10 " W. These techniques are discussed in Sect. 4.5.5. More details about photomultipliers and optimum conditions of performance can be found in excellent introductions issued by EMI or RCA [4.131,4.132]. An extensive review of photoemissive detectors has been given by Zwicker [4.123] see also the monographs [4.133,4.134]. 

H.R. Zwicker Photoemissive detectors. In Optical and Infrared Detectors, 2nd edn., ed. by J. Keyes, Topics Appl. Phys., Vol. 19 (Springer, Berlin, Heidelberg 1980)... 

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