Detector Photoelectric detectors

In all this early work, the x-ray beam impinged upon a phosphor powder on the tube envelope. Detectors of this general kind will be called phosphor-photoelectric detectors to distinguish them from modern scintillation counters (2.11), also photoelectric, in which the light is often generated in a single crystal. The name phosphor-photoelectric detector/ though necessary, is clumsy and not entirely satisfactory. 

The phosphor-photoelectric detector is generally used with polychromatic beams the intensity of which is high enough to make the detector instantaneous. External amplification easily increases its otTtput currents to values that can be read on a micro- or milliammeter. Output currents thus amplified could be used through servo links to control operations such as blending. 

In the phosphor-photoelectric detector used as just described, the x-ray quanta strike the phosphor at a rate so great that the quanta of visible light are never resolved they are integrated into a beam of visible light the intensity of which is measured by the multiplier phototube. In the scintillation counters usual in analytical chemistry, on the other hand, individual x-ray quanta can be absorbed by a single crystal highly transparent to light (for example, an alkali halide crystal with thallium as activator), and the resultant visible scintillations can produce an output pulse of electrons from the multiplier phototube. The pulses can be counted as were the pulses-from the proportional counter. 

The photometer is adequately described in Figure 3-2. In the phosphor-photoelectric detector (2.10), the x-ray beam strikes a silver-activated zinc sulfide phosphor to produce blue-violet light that is changed by the multiplier phototube (Type 931-A) into an electric current that is amplified and read on a suitable micro- or milliammeter. A stable power supply for both x-ray tube and detector circuit are essential, as is clear from the circuit diagrams.10... 

Fig. 3-2. A, Phosphor-photoelectric detector B, sample cell C, sample D, CA-5 x-ray tube and housing E, milliammeter F, amplifier and rectifier vacuum tubes G, regulated power supply for amplifier tubes and multiplier phototube H, control panel.

The fire starts in the upholstered chair. The door to the fire room is closed until the photoelectric detector in the room alarms and the occupant after ten seconds leaves the room and leaves the door open. The occupants in the distant suite, after various delays, move down the corridor to escape. 

Photoelectric detectors are of the spot type or light-scattering type. In each, visible products of combustion partially obscure or reflect a beam of light between its source and a photoelectric receiving element. The disruption of the light source is detected by the receiving unit and as a result... 

The simplest types of photometric instrument are designed for measurements in the visible region of the spectrum only and rely on coloured filters and simple photoelectric detectors. The name colorimeter is often used to describe such instruments although this is not necessarily correct and the word should probably be reserved for visual comparators rather than photoelectric instalments. 

Photoelectric detectors produce a current which is proportional to the intensity of the light falling on them. 

The materials and design of the various photoelectric detectors available are such that the absorption of radiation results in the displacement of electrons and hence in the development of a potential difference between two electrodes. The main types of photoelectric detectors may be classified as either photovoltaic or photoconductive (Figure 2.24). 

Figure 2.24 Photoelectric detectors. Photovoltaic detectors measure the flow of electrons displaced by the absorption of radiation. Photoconductive detectors measure the changes in conductivity caused by the absorption of radiation.

Figure 3.11 Photo-excitation mechanisms in (a) intrinsic and (b) extrinsic photoelectric detectors.

Depending on the nature of the semiconductor material, photoelectric detectors can be classified as intrinsic or extrinsic detectors. Intrinsic photoelectric detectors are pure semiconductors, whereas in extrinsic photoelectric detectors some impurities are added to the semiconductor during the fabrication process. Energy diagrams showing the processes activated by photo-excitation in these two kinds of photoelectric detectors are shown in Figures 3.11 (a) and 3.11(b) for intrinsic and extrinsic detectors, respectively. 

In intrinsic photoelectric detectors, electrons are excited from the valence band to the conduction band by photon absorption. The conductivity increases due to the increment in the carrier densities in both the conduction and the valence bands. The excitation process is possible provided that the photon energy of the incident radiation is greater than the energy gap of the semiconductor. 

The main limitation of photoelectric detectors is the noise cansed by thermal excitation of the carriers from the valence band or from the impnrity levels. If there is a large dark current (a cnrrent generated by the detector in the absence of incident hght), the sensitivity of the photoelectric detector becomes poor (only very intense beams will indnce an appreciable change in the detector condnctivity). In order to rednce the dark cnrrent, photoelectric detectors are nsnally cooled dnring operation. 

There are two classes of photoelectric detectors photoconduction detectors and photodiodes. 

Another well-known type of photoelectric detector is the photodiode. Photodiodes... 

Although the photomultiplier can be considered to be a photoelectric detector, in this chapter we will describe it in a separate section, because of the special relevance of photomultipliers in the field of optical spectroscopy. This detector is more complicated and expensive than those described in previous sections. Nevertheless, photomultipliers are probably the most common detectors used in optical spectroscopy experiments. The reason for this is their high sensitivity and stability. 

There are several types of smoke detectors. The photoelectric type detects smoke particles which interrupt a light beam resulting in the generation of an electrical signal. Photoelectric detectors are used where very slow evolving, smoky fires are expected. 

Junkermann, W., U. Platt, and A. Volz-Thomas, A Photoelectric Detector for the Measurement of Photolysis Frequencies of Ozone and Other Atmospheric Molecules, . /. Atmos. Chem., 8, 203-227 (1989). 

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