10 



RADIATION HIOLOGY 



An alternative design, such as is used in the " Venetian bhnd" photo- 

 multiplier tubes (Sommer and Turk, 1950), does not attempt to focus the 

 electrons from each dynode upon the next, but merely uses an accelerating 

 field to draw the majority of secondary electrons (70-8o per cent) to the 

 succeeding dynode (Fig. -4-8). 



Thus a conventional photomultiplier tube consists of a photocathode, 

 a series of 9-11 secondary emission dynodes, the first of which is main- 

 tained at a potential 75-150 volts above that 

 of the photocathode while each succeeding 

 dynode is elevated another 75-150 volts in 

 potential in sequence, and a final anode which 

 is maintained 50-100 volts above the potential 

 of the last dynode. With a multiplication of 

 3-5 per dynode, the over-all amplification of a 



3- 



2- 



0-1 



I 



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 — KSNWssr- 

 —ryyyyy'yr- 



— kWVCsV^— 



—vyyyyyyr- 

 — K\\\x\si— 

 —vyyyyy^~ 



— KX\\\V1— 



—Y///y/r- 



— KVX\XV\I — 



> i?| to 27„ 



or 







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umM 



9 dynode tube can range from 3* to . 

 20,000 to 2,000,000. 



Because the current capacity of the last 

 dynode or anode is limited, there is, at normal 

 gain, a maximum current which may be drawn 

 from the photosurface, and hence a maximum 

 illumination to which it should be exposed. 

 This limit, which will be less than 1 /xw for a 

 1P28 photomultiplier tube operated at a cur- 

 rent amplification of 10^, may be raised if the 

 voltage applied per stage is reduced. 



The response of photomultiplier tubes is a 

 linear function of light intensity over many 

 decades. Fatigue is inappreciable at low light 

 Jevels. Because of the variations in secondary 

 emission with dynode voltage, the voltage 

 supply for the photomultiplier tube potentials 

 must be held stable to an order of magnitude better than the stability 

 desired in the output current. Batteries may be used, or regulated elec- 

 tronic supplies have been described (Ply male and Hansen, 1950; 

 Higinbotham, 1951; Hill, 1945; Mautner, 1947). 



The over-all amplification of the photomultiplier is very closely a 

 logarithmic function of the voltage applied per dynode, over several 

 decades of gain (Fig. 4-9). As a consequence of this circumstance, it can 

 be shown that, for varying levels of illumination, the voltage per dynode 

 necessary to maintain a constant output current is proportional to the 

 logarithm of the reciprocal of the intensity of the illumination. This 

 property may be used in the design of circuits intended to measure absorp- 

 tion directly in terms of optical density (Sweet, 1946). 



The time resolution of a photomultiplier tube is limited only by the 



Fifi. 4-8. Design of "Vene- 

 tian blind" type photomul- 

 tiplier tube: T, photosensi- 

 tive surface; D, dynodes; E, 

 collecting anode. (Sommer 

 and Turk, 1950; Journal of 

 Scientific Instruments.) 



