Photomultiplier tube electron multiplication

In addition to multielectrode photomultiplier tubes, electron multiplication can be obtained using a reverse-biased silicon diode as the anode. At photocathode-to-anode voltages of several thousand volts, the impinging photoelectron can produce about 1000 hole-electron pairs. This gain of 1000 will again have fluctuations of the order of the square root of the gain, about 30 or 3%. This type of multiplication as well as modifications of the photomultiplier electrode structure are used extensively in imaging devices discussed in Section VI. 

FIGURE 5.5 Schematic illustration of a photomultiplier tube. A single photon ejects an electron from the photocathode. The electron is accelerated by voltage differences, and knocks multiple electrons off each successive surface. The burst of electrons is collected at the end. 

DC amplifiers are the simplest and least expensive of the electronic measurement systems. They are most commonly found in commercially available fluorimeters. Ideally, the amplifier stage of a circuit contributes little or no noise to the system the photomultiplier should produce only the noise associated with perfect performance of the photocathode and the electron multiplication process described above. However, the anode dark current of the photomultiplier adds to the noise in the signal, and the amplifier makes its own contribution to the total noise. It is therefore imperative to select the proper photomultiplier tube with low dark current so as to have a higher signal-to-noise ratio. 

Photomultiplier tube (PMT) based on electron multiplication in multichannel plates. 

FIGURE 26.3 Photomultiplier tube. Incident photons cause the photocathode to emit electrons which collide with the first dynode which emits additional electrons. Multiple dynodes provide sufficient gain to produce an eas-iiy measurabie eiectric pulse from a single photon. 

CEM Channel Electron Multiplier detects electrons, ions, soft X rays directly. Multiplication of the signal is a similar process to that of a photomultiplier tube. 

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