Photo-Multiplier Tubes

Author: J.B. Hoag

The phenomenon of secondary emission was discussed earlier and the nature of "voltage" multiplier tubes was presented in Sec. 15.5.

Fig. 20 K. The shape of the electrodes in a photo-multiplier tube

	There's an interactive simulation available on the "Learning by Simulations" Web site which shows the operation of an electron multiplier.

Figures 20 K, L, and M show the arrangement of the electrodes in tubes which take advantage of the multiplication of electrons by secondary emission. Referring to Fig. 20 L, all of the metal plates except the collector have been sensitized so as to be good emitters of electrons. The number of photoelectrons from A is proportional to the intensity of the incident light. These are attracted to the next more positive electrode B. The metal plates are shaped so as to bring as many as possible to B. They strike with sufficient energy to cause the emission of more secondary than incident electrons (say in the ratio of N to 1). The secondaries from B are electrostatically focused onto C where, again, N secondaries are created by each incident electron. The total number which then moves toward D will be N · N. The process continues, building up more and more electrons until the collector is reached. In an n-stage multiplier tube, there will appear Nⁿ electrons at the collector for each electron released by the light at the cathode. Thus, if N = 4 and n = 6, the current will be increased 4⁶ fold or 4096 times. In the tube of Fig. 20 L, the gain is 70 db. more than for a normal gas-filled photocell.

Fig. 20 L. A 6-stage electrostatic photo-multiplier tube

For the tube of Fig. 20 L, voltages on any one electrode may be incorrectly adjusted by about 5 per cent without seriously changing the sensitivity of the tube. A peak voltage swing of 75 volts may be obtained without distortion.

As an example of the sensitivity of a multiplier, the output may be 40 milliamperes per lumen with 100 volts per stage. At a light level varying from zero to 0.05 lumen, the collector current would then change from zero to 2 milliamperes.

The chief advantage of photo-multiplier tubes lies in their ability to detect and measure very weak light intensities. With an ordinary phototube, the erratic "dark current" or no-light current is often so great as to mask any current set up by a small amount of light sent onto the cathode.

Fig. 20 M. The R.C.A. 931 multiplier phototube

Figure 20 M shows the structure of a 9-stage electrostatically-focused multiplier phototube. When adjusted so that 5 secondaries appear for each incident electron, the current amplification would be, theoretically, 5⁹ or approximately 2 million. The dark current is equivalent to that produced by about 10^-6 lumen; in other words, is exceedingly small. Approximately 1250 volts are used, 400 of which are applied between dynode No. 9 and the collector. With 100 volts per stage, the sensitivity is 0.6 amp./lumen and the amplification proves to be 60,000. With 125 volts per stage, the gain is approximately one-quarter million. The tube is unusually responsive in the blue part of the spectrum. A typical voltage supply system for the tube is shown in Fig. 20 N.

Fig. 20 N. Voltage supply for the tube of Fig. 20 M