LIGHT SOURCES AND DETECTORS 



The purpose of the electron-multiplier type of cell is to amplify the primary 

 photocurrent within the cell itself. This is achieved by utilizing the emission 

 of secondary electrons from another electrode (or several other electrodes in 

 cascade) which is bombarded by primary electrons emitted from the photo- 

 cathode. Tubes are available with as many as 14 secondary electrodes, 

 usually called dynodes, and amplification factors as high as 10^ can be 

 readily achieved. 



Probably the best-known photomultiplier is the R.C.A. 931 A, which was 

 extensively used during World War II and is still available from war surplus 



10 Collector 



Figure 28.29 Electrode arrangement in R.C.A. type 931 photomultiplier 



{By courtesy of Electronics^^) 



equipment. Figure 28.29 shows the general arrangement of the electrodes 

 and electron pathways. Incident illumination passes through a wire grid to 

 fall on the photocathode. The emitted photoelectrons are then guided 

 electrostatically on to a secondary-emitting cathode, or dynode. An 

 increased stream of electrons then passes to the next dynode and so on 

 through the cascade to be collected at the ninth electrode, usually referred to 

 as the anode. With a good 931 A an amplification of 10^ may be achieved 

 with a voltage difference of 100 between adjacent dynodes. Thus a photo- 

 sensitivity of 20 A/lm is obtained from a cell whose photocathode sensitivity 

 is only 20 /^A/lm. 



Two useful types of unit for supplying a photomultiplier are shown in 

 Figure 28.30. If about 1,000 V is applied to the ends of the resistor chain, 

 each dynode will receive about 100 V. It is wise to limit the anode voltage to 

 about 50 V as this diminishes the danger of overheating the anode if an 

 excessive light input is accidentally apphed. The average anode current 

 should not exceed 1 milliamp. 



The logarithm of the amplification factor varies inversely with the log of 

 the applied volts between adjacent dynodes (or 'volts per stage'), as shown in 



362 



