Sec. 5-1] 



/,'. IDIA TION TRA XSDUCEliS 



247 



where k is a constant and E 8i is the voltage applied to the stage. A 

 characteristic showing the over-all current amplification of a photo- 

 multiplier as a function of the applied voltage is shown in Fig. (5-1)15. 

 The amplification can be controlled, within wide limits, by chang- 

 ing the supply voltage applied to the photomultiplier. A further 



12 



o 



* 



e 6 



10' 



,io fc 



10 ; 



10' 



10 ; 



10' 



1.000 



100 



10 



0.1 



0.01 



6 

 o 



200 400 600 800 

 Volts per stage 



1,000 



Fig. (5-1)14. Gain per stage of a photo- 

 multiplier (typical for RCA 931 A- and 

 IP 21 -type dynode surfaces )[from R.F. 

 Post, Nucleonics, 10, 46 (May, 1952); by 

 permission]. 



250 500 750 1,000 1,250 

 Supply voltage, volts 



Fig. (5-1)15. Over-all amplification and 

 sensitivity of a photomultiplier, voltage 

 per stage =isof supply voltage {from 

 RCA Tube Manual, Tube IP 21; by 

 permission). 



method to control (reduce) the amplification is to defocus the electron 

 stream in any one of the sections of the tube. 



The luminous sensitivity of commercial photomultipliers is gener- 

 ally in the vicinity of 20 amp/lumen but varies widely for different 

 types and even for different specimens of the same type. At its 

 highest, it is 20,000 amps/lumen. 1 The sensitivity characteristic is 

 parallel to that showing the amplification, as illustrated in Fig. 

 (5-1)15. Figures for the radiant sensitivity for normal continuous 

 operation reported in the literature vary between 400 and 80,000 

 juA/juW for incident radiation of a wavelength at which the cathode 

 has a maximum sensitivity. 



input characteristics. The spectral sensitivities of photomulti- 

 plier tubes are determined by the characteristics of their cathodes 

 and are similar to those described in Figs. (5-1)3 to 5. Photomulti- 

 pliers that are sensitive in the gamma-ray region are described by 

 Graves and Koch. 2 The sensitivity of the photocathodes in photo- 

 multipliers is, in general, of the order of 60 ^A/lumen. The light- 

 intensity level for which a photomultiplier is suitable is determined 



1 J. D. McGee, Nucleonics, 10, 34 (March, 1952). 



2 J. D. Graves and G. E. Koch, Rev. Sci. Instr., 21, 304 (1950). 



