Vacuum phototubes

Vacuum phototubes are preferred as detectors over barrier-layer photocells because of their higher signal-to-noise ratio, greater stability, longer life, and freedom from fatigue. Simple tubes are preferred over multiplier types because they are less costly, are more stable, and can be used in simpler circuits. 

Vacuum phototubes have two electrodes that have a maintained potential difference. The cathode (negative electrode) consists of a plate coated with a photosensitive metal. Radiation incident upon the photosensitive cathode causes an emission of electrons (the photoelectric effect), which are collected at the anode (the positive electrode). The resulting photocurrent can be readily amplified and measured (Fig. 1-5). 

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. 

A reverse-biased silicon diode can serve as a radiation detector because ultraviolet and visible photons are sufficiently energetic to create additional electrons and holes when they strike the depletion layer of a pn junction. The resulting increase in conductivity is easily measured and is directly proportional to radiant power. A silicon-diode detector is more sensitive than a simple vacuum phototube but less sensitive than a photomultiplier tube. 

They are more sensitive than a vacuum phototube but considerably less sensitive than a photomultiplier tube. Diode arrays can also be obtained commercially with front-end devices called image intensifiers to provide gain and allow the detection... 

Three types are used. (I) Barrier-layer cells. These are satisfactory only for simple filter instruments. (2) Vacuum phototubes. These tubes require an external power supply, unlike barrier-layer cells, and their output is usually amplified before measurement. (3) Photomultiplier tubes are easily the most satisfactory detectors for use in flame photometry. The photocurrent is amplified inside the tube in such a way that much lower light levels can be detected and measured accurately than is possible with vacuum phototubes with amplifiers. A stable source of high voltage up to perhaps 2000 volts is required to operate the photomultiplier tubes, but these tubes are almost universally used in high-performance instruments and are essential if the advantages of using narrow band width are to be obtained. 

This approach, when compared with (ii) with respect to signal-to-noise ratio, has the multi-channel advantage each wavelength is measured continuously and no loss in measurement time occurs. Nevertheless, these systems are inferior to normal photometers, as all photo-receptors capable of image-sensing operation (photodiode array, vidicon, etc.) show a poorer signal-to-noise ratio in comparison with vacuum phototubes and photomultipliers. 

Figure 7.13. Schematic diagram of three common detectors used in the ultraviolet-visible region. A The barrier-layer or photovoltaic cell. B A vacuum phototube. C The vacuum photomultiplier.

Detector Noise and Drift. Transducers for modem spectrophotometric systems are usually vacuum phototubes or photomultiplier tubes. Noise and drift from the transducer can be quite troublesome in kinetic methods, and special care is normally taken to ensure low-noise operation. 

The most common detector is the photomultiplier tube (PMT). A PMT is a sealed, evacuated transparent envelope (quartz or glass) containing a photoemissive cathode, an anode, and several additional electrodes called dynodes. The photoemissive cathode is a metal coated with an alkali metal or a mixture of elements (e.g., Na/K/Cs/Sb or Ga/As) that emits electrons when struck by photons. The PMT is a more sophisticated version of a vacuum phototube (Fig. 5.17), which contained only a photoemissive cathode and an anode the photocurrent was hmited to the electrons ejected from the cathode. In the PMT (Fig. 5.18), the additional dynodes multiply the available electrons. The ejected electrons are attracted to a dynode that is maintained at a positive... 

The first observation of the photoelectric effect was made in 1887 by Heinrich Hertz, who reported that a spark jumped more readily between two charged spheres when their surfaces were illuminated with light. Between the time of this observation and the theoretical explanation of the photoelectric effect by Einstein in 1905, several important studies of the photoelectric effect were performed with what is now known as a vacuum phototube. Einstein s explanation of the photoelectric effect was both simple and elegant but was far enough ahead of its time that it was not generally accepted until 1916, when Millikan s systematic studies confirmed the details of Einstein s theoretical conclusions. 

Figure 6-13 is a schematic of a vacuum phototube circuit similar to the one used by Millikan to study the photoelectric effect. The surface of the large photo-cathode on the left usually is coated with an alkali... 

Silicon diodes are more sensitive than vacuum phototubes but less sensitive than photomultiplier tubes (see curve fin Figure 7-27). Photodiodes have spectral ranges from about 190 to llOOnm. 

A second type of photoelectric device is the vacuum phototube, which consists of a semicylindrical cathode and a wire anode sealed inside an evacuated transparent envelope (see Figure 7-29). The concave surface of the electrode supports a layer of photoemissive material (Section 6C-1) that tends to emit electrons when it is irradiated. When a voltage is applied across the electrodes, the emitted electrons flow to the wire anode generating a photocurrent that is generally about one tenth as great as that a.ssociated with a photovoltaic cell for a given radiant intensity. In contrast. 

For a very praciical. although somcwhal ilaied, discussion of vacuum phototubes and phoiomultipiier tubes and circuits, see F. F.. Lytle, Amil Chem.. 1974. 46. >4x-. ... 

Further examples for limiting-current transducers are the vacuum phototube and the flame ionization detector. 

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

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