Sec. 5-1] 



l;A DIATIOX Tit A XSDl ( 7-7,'.s 



255 



stray electric and magnetic fields. Even the magnetic field of the earth 

 can cause variations of the photomultiplier output. Adequate 

 shielding should be provided where such stray fields cause errors. 

 Stray light entering the tube can cause erratic results. 



operation or PHOTOMULTiPLiERS. Three different methods of 

 operating photomultipliers are generally distinguished : ( 1 ) The light 

 flux falling upon the cathode can be continuous; the output from the 

 photomultiplier is read directly or after d-c amplification. This 



10 



— I— 



10 3 



— I — 



10= 



10' 



-I— 



10 7 



lO 6 



h- 



10 



— y 



10 



10" 



—i — 



Quonto /sec 



10"' 5 10" 14 



10 

 — t 



-13 



10" 



10" 



V 10 



-9 



10" 



— (— 



10" 



— I— 



10" 6 10" 



10" 



— I 



Lumen 



2-10 



5-10 



1.6-10" 19 



I 



/ electron /sec 

 Tube 1P-21, Hqu. air tern p.^> 



17 



10 



■13 



Max. curr ent 

 cont. operation 



Max. current 



10 amp /„ 



10 amp I a 



mtermitt. opera t 



- Tube 931 A , room temp. - 



Counting ot 

 single electrons 



d-c omplif. 



- Chopped light beom t o - c omplif. - 



I p = Photo- [cathode) current 



I a - Anode [output) current 



Fig. (5-1)20. Operating ranges for photomultipliers [from M. Ploke, Arch. tech. 

 Messen, J 391-6, January 1954; by permission]. 



method is generally applicable when the incident light level is higher 

 than 10 -8 lumen. At lower levels, the unavoidable drift of d-c ampli- 

 fiers can cause serious errors. (2) The light beam can be periodically 

 interrupted or modulated by alternating current, and the output can 

 be fed into a-c amplifiers, which are inherently more stable. The 

 method has the further advantage that the band width of the 

 output-signal detector can be restricted so that the noise level is 

 reduced. The method is in general applicable for light levels as low 

 as 10 -13 lumen. (3) For lower light levels the bursts caused by single 

 photoelectrons are observed or counted. This method permits the 

 measurement of smallest light levels, down to the order of 2 x 10~ 16 

 lumen. A synopsis of the different methods of operation for photo- 

 multiplier tubes is shown in Fig. (5-1)20. 



circuits for photomultipliers. The simplest circuit to obtain 

 the appropriate voltages for the single stages is a resistive voltage 

 divider, as shown in Fig. (5-1)21 . The operating voltage applied to 

 the voltage divider is of the order of 1 kV ; for d-c output, the positive 

 pole is preferably grounded; the partial voltages for the single stages 



