Sec. 5-1] RADIATION TRANSDUCERS 239 



The thermionic emission from the cathode can be computed from 

 the Richardson law. If the applied plate voltage is high enough to 

 cause all emitted electrons to move to the anode (no space charge), 

 the thermionically emitted current is 



i T = Aat 2 e~^ IKt) 



where A is a constant (for pure metals about 1.2 X 10 6 amp/ra 2 deg 

 K 2 ), a the area of the cathode, / the temperature in degrees Kelvin 

 <p the work function of the emitting metal 

 in volts, e the electron charge in coulombs, 

 and K is Boltzmann's constant. The therm- 

 ionic emission increases rapidly as 99 dimin- 

 ishes, i.e., as the threshold wavelength is 

 shifted toward the infrared part of the 

 spectrum. The thermionic emission can be Fig. (5-1)6. Photoelectric 

 reduced considerably by cooling of the cel1 with g uard rin 8 to re - 



.... i i. j a r • i.- 1 i_ duce the effect of leakage 



emitting electrode. A refrigerating chamber rrent 

 for this purpose is described by Engstrom. 1 



Leakage current over the glass wall of the tube can be reduced by 

 reducing the voltage applied between the electrodes or by cleaning 

 and drying the glass surface or coating it with a water-repellent com- 

 pound. A very good means to avoid the influence of the leakage 

 current in the output consists in the application of a guard ring 

 around one of the electrodes. A circuit diagram for the guard ring 

 arrangement is shown in Fig. (5-1)6. 



The current caused by the impact of positive ions upon the cathode 

 and current caused by field emission are, in general, less important 

 than leakage current and thermionic emission. Both can be reduced 

 by reducing the applied voltage; apparently, the current caused by 

 ion impact can be reduced by cooling. Static charges on the glass 

 wall are usually avoided by coating the walls of the tube with a 

 conductive layer. 



Several methods have been used to separate the dark current from 

 the photoelectric current: The light source can be modulated or 

 periodically interrupted (light chopper) before entering the photo- 

 electric cell. The resulting photoelectric a-c component in the output 

 from the photoelectric device can be separated from the d-c com- 

 ponent, i.e., thermionic and leakage current. 



Kalmus and Striker 2 have placed the photoelectric cell in the field 



1 R. W. Engstrom, Rev. Sci. Instr., 18, 587 (1947). 



2 H. P. Kalmus and G. O. Striker, Rev. Sci. Instr., 19, 79 (1948). 



