Sec. 5-1] RADIATION TRANSDUCERS 259 



obtained with the standard resistive voltage divider (at the same 

 supply voltage) is described by Sherr and Gerhart. 1 



If continuous operation of the photomultiplier is not essential, the 

 multiplier may be operated at alternating current of line frequency, 

 or higher frequencies, or even at radiofrequencies. Inductive voltage 

 dividers and circuits for a-c operation are described by Zworykin 

 and Ramberg. 2 Since the sensitivity of the photomultiplier increases 

 strongly with applied voltages, the photomultipliers supplied with 

 alternating current will be operative only during the short times 

 when the a-c voltage is near the positive peak value. For this reason, 

 the signal-to-noise ratio of an a-c operated photomultiplier is less 

 than for d-c operation. 



pulsed operation. The operation of photomultipliers at high 

 supply voltages is desirable because it improves the amplification, 

 the current output, and the time-dispersion of the amplified pulses. 

 Frequently, the applicable voltage is limited by the effects of ion 

 impact upon the cathode or the earlier dynode stages, leading to 

 noise, instability, and breakdown of the tubes. Since the ion mo- 

 bility is small compared to that of the electrons, these undesirable 

 effects can be reduced by application of the supply voltage for very 

 short times. Post, 3 has operated photomultipliers with pulses of a 

 duration of 2.5 x 10 -6 sec or less (repetition rate, 60 per second) and 

 a voltage of four to five times the maximum specified value for d-c 

 operation and has increased the over-all amplification from 5 x 10 5 

 to 10 9 . About 10 to 20 per cent of the commercial tubes investigated 

 could withstand this method of operation. The operation of tubes 

 where the application of high voltage causes field emission cannot be 

 improved by pulsed operation. Singer, Neher, and Ruehle 4 have 

 operated photomultipliers with pulses of 10 -7 sec duration and an 

 over-all dynode voltage of 10 kV. 



PHOTOMULTIPLIER CIRCUITS FOR LOGARITHMIC CHARACTERISTIC. 



The absorption of light follows an exponential law; the optical den- 

 sity D of a light-absorbing layer is defined by 



D = log 10 ^ 



1 R. Sherr and J. B. Gerhart, Rev. Sci. Instr., 23, 770 (1952). 



2 V. K. Zworykin and E. G. Ramberg, "Photoelectricity," pp. 259 and 260, 

 John Wiley & Sons, Inc., New York, and Chapman & Hall, Ltd., London, 

 1956. 



3 Post, loc. cit. 



4 Singer, Neher, and Ruehle, loc. cit. 



