250 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 



fluctuating time lags in secondary emission. This latter time dis- 

 persion is probably of the order of 10~ n to 10~ 10 sec. Sard, 1 in a 

 theoretical study, has shown that for a single electron a current pulse 

 in the anode circuit of a duration of about 6 X 10 -10 sec is to be ex- 

 pected. Practically observed time spreads under normal operating 



1.0 



E 

 o 

 o 



0.01 



o 



"O 



o 

 <t 



0.001 



0.0001 







20 



40 



60 80 

 Volts per stoge 



100 



120 



140 



Fig. (5-1)16. Dark current in a photomultiplier tube [from 

 R. W. Engstrom, Rev. Set. Instr., 37, 420 (1947); by per- 

 mission]. 



conditions are of the same order of magnitude. The effect of the 

 different initial velocities of the photoelectrons can be reduced by 

 first accelerating the electrons when they leave the cathode and 

 decelerating them before they reach the first dynode; the effect of 

 varying path lengths can be overcome by the use of a spherical 

 cathode surface and spherical electron-optical geometry behind the 

 cathode. Tubes having these features show time dispersions of less 



1 R. D. Sard, J. Appl. Physics, 17, 768 (1947). 



