246 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 5 



upon an electrode, the dynode D v where each incident electron 

 causes the emission of several secondary electrons. The same process 

 is repeated at the dynodes D 2 , D 3 , .... The electrons from the last 

 dynode stage are collected by a positive anode A , and the current I a 

 is measured. If the gain of each stage (the number of electrons 



.■&--- 





Fig. (5-1)13. Photomultiplier, schematic diagram. 



formed by secondary emission for each primary electron) is g and if 

 n dynode stages are used, the total amplification is 



A = g n 



The value of g varies with the voltage between successive dynodes 

 and with the surface composition and the geometry of the dynodes 

 from 0.5 to about 10. 



The focusing of the electrons from one stage to the next can be 

 accomplished with magnetic or electrostatic electron-optical sys- 

 tems. Recent constructions use electrostatic focusing systems 

 almost exclusively. 1 



Commercial photomultipliers are built with 9 to 14 stages and furnish a gain 

 between 10 5 and more than 10 7 (10 9 , EMI tube 6262): Nucleonicr, 10 (3), 34 

 (1952). Photomultipliers with a greater number of dynodes have been built: 

 18 stages, with a gain up to 10 12 , described by N. Schaetti, Helv. Physica Acta, 

 23, 108 (1950); 19 stages, by A. Lallemand, Le Vide, 4, 618 (1949). Refer to 

 Chem. Abstracts, 44, 381b (1950). 



The gain and the sensitivity of photomultipliers vary with the 

 voltages applied to the dynode stages. A typical characteristic for a 

 single stage showing the gain per stage over a wide range of applied 

 voltages is shown in Fig. (5-1)14; for continuous operation only the 

 part indicated by a solid line is used. In this range, the characteristic 

 can be expressed analytically, according to Larson and Salinger, 2 by 



g = kVW st 



1 For references on the construction of photomultipliers, see G. A. Morton, 

 "The Scintillation Counter," in L. Marton (ed.), "Advances in Electronics," 

 vol. IV, pp. 7 Iff., Academic Press, Inc., New York, 1952. 



2 C. C. Larson and H. Salinger, Rev. Sci. Instr., 11, 227 (1940). 



