252 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 5 



cooling of the tube. Roughly, each 10°C decrease of temperature 

 causes a reduction of the thermionic current to one-half. The ther- 

 mionic current may also be reduced by reducing the sensitive area of 

 the photo-cathode, as described by Naray. 1 By electron-optical 



ioo no 



Volts per stage 



Fio. (5-1)18. Noise output from three photomultiplier 

 tubes as a function of the voltage applied per stage [from 

 R. W. Engstrom, Rev. Sci. Instr., 37, 420 (1947); by per- 

 mission]. 



means an image of a small area of the photocathode is formed on the 

 first dynode. Thermal electrons from outside this area do not reach 

 the dynode, and the anode current due to thermionic emission is 

 reduced by a factor of IOO. 2 



Noise in photomultipliers is of twofold origin: the random varia- 

 tion of the thermionic emission and that of the photoemission. The 

 rms noise current, measured at the anode, is 



(J*)* = G[2e Af(i t + i p )f 



where G is the amplification (gain) of the photomultiplier tube, e the 

 charge of the electron, A/ the bandwidth of the detecting system, i t 

 the thermal emission from the cathode (at room temperature of the 



1 Naray, loc. cit. 



2 See also G. E. Kron, Astrophys. J., 103, 324 (1946). 



