PHOTOELECTRIC DETECTORS 



Set 

 zero 



Rl 



< 



-i 



HT+ 



— II 



r 



Amp 



/'^ 



0-5 M' 



T 



<:y 



_ Ganged 



IM 

 1M 

 IM 

 IM 

 IM 

 IM 



IM 

 IM 



IM 



Stabilized 



EHT Power 



pack 



Battery operated 

 photomultiplier 



-950 V 



Mains operated Photomultiplier 



In this case the current in the resistor chain 

 is 100 /<A and the circuit is suitable for 

 anode currents up to, say, 10 /<A. Greater 

 light intensities necessitate lower resistor 

 values : a tenfold reduction would give a 

 chain current of 1 mA, making an upper 

 anode current of, say, 100 //A reasonable. 



Figure 28.30 Supply circuits for securing linear amplification from a photo- 

 multiplier cell. The sensitivity is proportional to the anode load i?^, whose upper 

 limit is set by the upper limit of frequency response required. IfC is the amplifier 

 input and stray capacitance, including the capacitance of any cable joining P.E. 

 cell to amplifier, and if the upper turnover frequency is to be «Jniax> '/'^" 

 ■'^imax = l/CCoimax)- Practical Rj^s range from 10 kD. to 1 MO 



Figure 28.31. If a constant sensitivity is required, care must therefore be 

 taken to supply the multipHer tube with a constant voltage. Conversely this 

 relationship between voltage supply and amplification can be used to advan- 

 tage for the design of a wide range photometer as shown in the circuit of 

 Figure 28.32. Here the supply voltage is altered to give a constant anode 

 current for varying light intensities. The supply voltage required to give 

 this constant current can then be measured; it will bear a logarithmic 

 relationship to the light intensity. 



If a multiplier cell is placed in total darkness a residual current will be 

 found to flow. This is usually called the 'dark current', and is of the order 

 of 0-1 jiiA in a 931 A at 100 V/stage, although big variations between indivi- 

 dual tubes occur. Some of this dark current is due to leakage at the base of 



363 



