Photoemissive vacuum detectors

In this type of detectors, measured optical radiation causes emission of electrons from the surface of a photocathode in a vacuum tube. Electrons may reach direcdy the anode or first be multiplied on dynodes (photomultiplicators). Historically, these have been the first photodetectors. Table 1.4 presents the basic types of vacuum detection tubes according to the 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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Figure 4.23. Photoemission process and UPS spectrum of a semiconducting material. F vacCs) and vac(d) represent the sample and detector vacuum levels, respectively. The Fermi edge of the metal is represented by the discontinuous line.

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. 

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 internal photoelectric effect, just as infrared radiation, was also first observed in the 19th century, when certain minerals such as selenium or lead sulfide were found to increase their electrical conductivity in the presence of light. These photoconductors depend upon the photoexcitation of bound electrons and/or holes into the conduction and/or valence bands of the material. Then, at the turn of the century, external photoemission was discovered in vacuum diodes. As first explained by Einstein, the photoelectric effect was found to have a threshold wavelength determined by the relation hv = he lk>E, where E is the energy required for the electron to exit the material. In the case of a semiconductor, the excitation energy, E, is that of the gap between the valence and conduction bands or the ionization energy of an impurity in the material. The electronic detector family has two main branches, the first being the vacuum photodiode and its more useful... 

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

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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