Sec. 5-1] 



RA DIA TIOX TliA XSDl '( 'h'h'S 



251 



750 



Supply voltoge, volts 

 1,000 1,250 1,500 



E10" 1 



,10"' 



rio 



1 



i 

 / 

 -— »« — 

 • 



than 10~ 10 sec. 1 The time dispersion decreases approximately with 

 the square root of the applied voltage per stage and can be reduced, 

 therefore, by operating the photomultiplier at higher than normal 

 voltage (see Pulsed Operation, below). 



Undesirable signals can be caused by dark current and noise. Dark 

 current, by itself, does not necessarily introduce an error if correc- 

 tions for it can be applied. However, the noise component of the dark 

 current is frequently objection- 

 able. The dark current in com- 

 mercial photomultipliers at room 

 temperature is generally of the 

 order of 10~ 7 amp. 



The dark current is caused by 

 three phenomena: (1) leakage cur- 

 rent, predominantly at the inside 

 of the tube and frequently caused 

 by traces of cesium, but also occur 

 ring at the outside of the tube and 

 in the tube socket ; ( 2 ) thermionic 

 emission from the photocathode 

 and, to a smaller extent, from 

 the first dynode stages, and (3) re- 

 generative ionization, i.e., the impact of positive ions upon the 

 cathode, but also the incidence of light from excited gas molecules 

 and from glass fluorescence upon the cathode. The relative magni- 

 tude of these three sources of dark current and their variation with 

 the voltage applied per stage are shown in Fig. (5-1)16. 



The dark current is frequently expressed by the equivalent light 

 input, i.e., by that amount of light at the cathode that causes an 

 anode current of the same magnitude as the dark current. The equiv- 

 alent light level is proportional to the dark current but inversely 

 proportional to the sensitivity. If plotted against the sensitivity, 

 which increases with increasing voltage per stage, a curve like that 

 shown in Fig. (5-1)17 will ensue. 



Leakage current on the outside can be reduced in some cases by 

 coating the outside of the tube with a conductive layer and applying 

 appropriate potentials. 2 



The thermionic component of the dark current can be reduced by 



1 Anon., Report on the Sixth Scintillation Counter Symposium, Washington, 

 Nucleonics, 16, 58-59 (June, 1958). 



2 Z. Naray, Acta Physica Acad. Sci. Hung., 5, 159 (1955), and H. J. Marrinan, 

 J. Opt. Soc. Am., 43, 1211 (1953). 



,1C) 



m 0.1 1 10 100 1,000 



Sensitivity, omp / lumens 



Fig. (5-1)17. Light input equivalent to 

 the dark current of a photomultiplier 

 (from RCA Tube Manual, Tube 6342; 

 by permission). 



