ULTRAVIOLET SPECTKOSCOPIC TECHNIQUE 



139 



At low levels of radiation intensity, the photociirrent generated will be 

 small. This current may be amplified external to the cell by conven- 

 tional vacuum tube circuits (Zworykin and Ramberg, 1949, Chaps. 12-14) 

 or it may be amplified within the cell, either by gas multiplication or by 

 the use of secondary emission, as in the photomuitiplier tubes. The 



30 



50 70 



10,000 



100 200 300 500 700 1000 2000 4000 



ACCELERATING VOLTAGE OF PRIMARY ELECTRONS 



Fig. 4-6. Secondary-emission characteristics of typical photosurface materials. 

 {Reproduction from Photoelectricity and Its Application, by V. K. Zioorykin and E. G. 

 Romberg, John Wiley d- Sons, Inc., 1949.) 



advent of photomuitiplier tubes has largely supplanted the use of gas- 

 filled tubes. The direct or amplified photocurrent may be measured with 

 a galvanometer or ammeter, may be recorded, may be integrated in dis- 

 crete quantities and counted (Douglas, 1947; Launer, 1949), or may be 

 used to operate such devices as relays and motors. 



Photomuitiplier Tubes. In the 

 photomuitiplier tubes, the primary 

 current from the photosurface is 

 multiplied by a factor which may 

 be as large as 10^ by repeated use 

 of multiplication at secondary emis- 

 sion surfaces. Many surfaces, in- 

 cluding those commonly used as 

 photosurfaces, will, when struck by 

 an electron of appropriate energy, 

 emit several electrons. The num- 

 ber given off per primary electron 

 depends on the surface and the 

 voltage applied to the primary elec- 

 tron (Fig. 4-6) (Zworykin et at., 

 1936; Morton, 1949). 



In the focused photomuitiplier tubes (Rajchman and Snyder, 1940), 

 the primary photocurrent is focused by an electrostatic field onto such a 

 surface, called a dynode. This process is repeated nine or ten times until 

 the vastly amplified current from the last secondary emitting surface is 

 collected on an anode (Fig. 4-7). 



FOCUSING 

 GRILL 



MICA SHIELD' 



Fig. 4-7. Construction of nine-dynode 

 focused photomuitiplier tube. 0, photo- 

 cathode; 10, anode; 1-9, dynodes. 

 {Engstrom, 1947a; Journal Optical Society 

 of America.) 



