PHOTOELECTRIC DETECTORS 



into the red than type A, which has Httle response beyond 630 mju. However, 

 it is not so sensitive to the shorter wavelength. 



Type S cathode— Type SI in the U.S.A.: the overall sensitivity is much 

 less than types A or B but it has the great advantage of being usable out to 



6,000 

 Wavelength 



Figure 28.23 The spectral sensitivity curves for type A, B and S photocathodes 



■30 





>« 



?0 



S 10 



4.000 6.000 



8,000 10,000 

 Wavelength 



12.000 



Figure 28.24 The spectral sensitivity curve of the type S cathode shown in 

 Figure 28.23 redrawn on a larger scale 



about 1,100 m^ where type A and B photocathodes are insensitive. In 

 Figure 28.24 the spectral sensitivity of the type S cathode is redrawn on a 

 larger scale. 



In Figures 28.23 and 28.24 the photoelectric current produced at each 

 wavelength has been calculated for an 'ideal' light source radiating equal 

 amounts of energy at each wavelength; but in Figure 28.25 the relative 

 sensitivities of the various photocathodes to a tungsten light source of C.T. 

 2,600°K are shown. As such a source radiates maximally at 1,000 m// and 

 little in the violet, the type S cathode is preferable to the types A and B, in 

 spite of the much greater quantum efficiency of the latter. The ratio of the 

 total areas covered by the curves in Figure 28.25 represents the ratio of the 

 sensitivities of the respective photocathodes in mA/lm to a tungsten source. 

 Although there are large individual variations of sensitivity among cells, 



357 



