Sec. 5-1] RADIATION TRANSDUCERS 249 



For the same application, windows and photocathodes of large di- 

 mensions (>4 in. in diameter, more than 15 in. 2 of area) have been 

 built. The sensitivity of the cathode is not always constant over its 

 entire surface but there may be zones of higher or lower sensitivity. 



output characteristics. For continuous operation and if long- 

 time stability is required, the output current should be limited to a 

 value of 10~ 5 to 10~ 4 amp. For a short duration of not more than 30 

 sec an average current of 10~ 3 or even 10~ 2 amp is allowable. Much 

 higher currents can be used for pulsed operation. The operation of a 

 tube furnishing output-current pulses as high as 0.2 to 0.3 amp is 

 described by Greenblatt; 1 a tube for pulsed operation with an output 

 current of the order of 1 amp is described by Post; 2 and Singer, 

 Neher, and Ruehle 3 have operated photomultiplier tubes with out- 

 put-current pulses as high as 15 amp. 



The output impedance is essentially that of the load resistor in the 

 anode circuit; it is, in general, in the megohm range, but for pulse 

 operation values as low as 100 ohms have been used. 4 



The output from a photomultiplier is a linear function of the inci- 

 dent light. The deviations from linearity are about 3 per cent for a 

 variation of the light flux between 10~ 9 and 10~ 3 lumen. Nonlinearity 

 resulting from space charges in the last stages may set in at higher 

 light levels. Such space charges frequently occur not at the anode 

 stage but at the third or second-to-the-last stage. Nonlinearity at 

 low levels (scintillation counters) may be due to statistical vari- 

 ations. 5 Nonlinearity of photomultipliers caused by the action of the 

 voltage divider which supplies the partial voltages for the dynode 

 stages is discussed below under Circuits for Photomultipliers. 



The response time of a photomultiplier is, in general, shorter than 

 10 -8 sec. In the time interval between 10~ 9 to .10~ 8 sec, dynamic 

 errors begin to become noticeable. Primarily two effects can be 

 observed: a time delay between input and output signal and a spread 

 of the transit time. A short light pulse will cause a longer pulse 

 of anode current (time dispersion). The time delay between input 

 and output signal is due to the finite transit time of the electrons 

 through the tube. The time dispersion is due (1) to differences in 

 the initial velocities of the emitted photoelectrons, (2) to differ- 

 ences in path lengths of the electrons, and (3) to a lesser extent, to 



1 M. H. Greenblatt et al., Nucleonics, 10, 44 (August, 1952). 



2 R. F. Post, Nucleonics, 10, 46 (May, 1952). 



3 S. Singer, L. K. Neher, and R. A. Ruehle, Rev. Sci. Instr., 27, 40 (1956). 



4 Post, loc. cit. 



5 G. A. Morton, op. cit., p. 78. 



