Sec. 5-1] 



RADIATION TRANSDUCERS 



261 



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to 20 per cent. This deviation can be corrected, for instance, by 

 appropriate division of the scale of the indicating meter. 



The circuit shown in Fig. (5- 1 )26 automatically keeps the output 

 current nearly constant. An increase of the photoinultiplier anode 

 current causes the grid of the triode to assume a more negative 

 potential, resulting in a reduction of the voltage applied to the resis- 

 tive voltage divider and the different stages of the photoinultiplier. 



Logarithmic densitometers composed from photoelectric cells and 

 elements with logarithmic characteristics are also described by 

 Miller, 1 and by Tiedman. 2 



For review articles and references on photomultipliers, see S. Rodda, "Photo- 

 electric Multipliers," McDonald & Co., Ltd., London, 1953; Zworykin and 

 Ramberg, op. cit., chap. 8; Morton, op. cit.; Gorlich, loc. cit. 



d. Photocounters. The Geiger counter (5-216) is essentially a photo- 

 electric receiver, which is normally limited to very energetic quanta 

 which cause photoionization and 

 the liberation of electrons either in 

 the walls of the counter or in the 

 gas volume. A counter for ultra- 

 violet and visible light has first 

 been described by Rajewsky. 3 



A schematic diagram of the light 

 counter is shown in Fig. (5-1)27. 

 A cylindrical tube C has a window 

 W that permits ultraviolet and 

 visible light to enter the inside of the counter. The anode A consists 

 of a fine wire held in the center of the tube by means of two insulators 

 /. The counter is filled with a suitable gas plus a quenching agent of 

 a pressure of a few centimeters Hg. A photoemissive layer P is 

 arranged opposite the window. Electrons liberated from the photo- 

 emissive layer migrate toward the anode and cause ionization by 

 collision on their way to the anode. In its original form the counter 

 could be used only for the detection of ultraviolet incident light. 

 Rajewsky indicates that he was capable of measuring as little as 1 2 

 quanta/(cm 2 )(sec) in an observation time of 10 to 12 min. The 

 quantum yield is low, about 10~ 3 to 10 -4 electron/quantum. 



Different authors have tried to improve the quantum yield and 



1 C. W. Miller, Rev. Sci. Instr., 6, 125 (1935). 



2 J. A. Tiedman, Electronics, 14, 48 (March, 1941). 



3 B. Rajewsky, Z. Physik, 63, 576 (1930); Phijsikal. Z., 32, 121 (1931); Ann. 

 Physik, 20 (5), 13 (1934). 



I 



Fig. (5-1)27. Photocounter, schematic 

 diagram. 



