256 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 5 



are between 70 and 150 volts; the output signal E appears across 

 the load resistance R L and is usually amplified in successive stages. 

 Higher voltages are frequently applied to the initial stage to improve 

 the collection of photoelectrons emitted from the cathode; high 

 collection efficiency (95 to 100 per cent) at this stage is important to 

 improve the signal-to-noise ratio. In photomultipliers with a large 

 number of stages (n > 10), the last stage or stages also are frequently 



operated at higher voltages to 

 reduce space charges. An example 

 of a nonuniform voltage distri- 

 bution and its effect on pulse- 

 height distribution curves is given 

 by Greenblatt. 1 



The output signal follows a linear 

 function of the incident light flux 

 for small and moderate light levels 

 when the current i b through the 

 photomultiplier tube is small com- 

 pared to the current i t through the 

 resistive network. If the incident 

 light flux increases, the current 

 through the tube increases too, in 

 particular that between the last 

 dynode and the anode. If this cur- 

 rent approaches the voltage divider 

 current i t , the voltage distribution of the voltage-dividing network 

 will be upset ; the voltages between the last stages will be diminished, 

 those between the initial stages will be increased (under some con- 

 ditions to the extent that an internal breakdown occurs). The result 

 may be an increase or decrease of the gain as shown in Fig. (5-1)22. 

 The nonlinearity may be useful; for instance, the increase of gain in 

 the region A may be desirable, or the output-current limitation in the 

 region B may protect the photomultiplier tube from damage caused 

 by exposure to excessive light. A circuit with approximately linear 

 response where such protection is enhanced by the use of series resis- 

 tors is shown in Fig. (5-1)23. 2 The transfer characteristic of a photo- 

 multiplier used in connection with this circuit is not entirely linear 

 and has peaks and valleys caused by electrostatic defocusing. 



Since the amplification of photomultiplier tubes varies strongly 

 with the supply voltage, operation at constant voltage and the use of 



1 Greenblatt et al., op. cit., p. 47. 



2 R. W. Engstrom and E. Fisher, Rev. Sci. Instr., 28, 525 (1957). 



Fig. (5-1)21. Photomultiplier wit;.. 

 resistive voltage divider: C, cathode; 

 A, anode; 1 to 9, dynodes; E s , supply 

 voltage; E , output voltage across load 

 resistor R L . 



