PHOTOELECTRIC DETECTORS 



below that at which 'gas glow' occurs. It is therefore not possible to get a 

 linear response to illumination in a cell with an anode load. The degree of 

 gas amplification will also vary due to the absorption of the gas on to the 

 electrodes and the glass envelope. This results in shorter life and unpredict- 

 able variations in sensitivity although these may be of small amplitude. A 

 gas filled cell if continuously operated at its maximum rated voltage may 

 decrease in sensitivity by 50 per cent in 500 hours of use. Much longer life 



< 



6-0 

 5-0 

 4-0 

 3-0 



20 



1-0 



Figure 28.28 Performance data of the Mallard 20CG photoemissive cell. 

 Anode current plotted against anode voltage with total illumination as 



parameter 



can be obtained by reducing anode voltage or current, and by running the 

 cell intermittently. 



To summarize, it may be stated that the increased sensitivity of the gas 

 filled photoemissive cell is obtained only by some sacrifice of linearity, 

 maximum output current, stability, life and frequency response. 



Electron-multiplier photoemissive cells 



The simple types of photoemissive cell dealt with above usually require 

 some degree of voltage or power amplification before their output signals 

 can be examined or recorded. If the experiment only requires the observation 

 of an alternating component in the output this can be readily achieved by 

 means of a high gain a.c. amplifier whose input is capacitor-coupled to the 

 photocell anode. A load resistor is, of course, connected in series with the 

 cell. However, if d.c. components require amplification practical difficulties 

 arise in constructing a sensitive d.c. amplifier which is free from 'drifts'. 

 When dealing with low light intensity signals, whether alternating or not, 

 the photomultiplier tube comes into its own, as the necessity for high gain 

 amplifiers is avoided. 



361 



