Smithsonian Report, 1964. — Cowan 



Plate 7 



One of the 112 photomultiplier tubes used in each large tank, shown with its mounting 

 socket. The 5-inch diameter face, equivalent to perhaps 100 human eyes, contains a thin, 

 photosensitive surface. When a photon of light falls on it, an electron is ejected from the 

 surface toward the interior of the tube. The electron strikes the first metal element 

 known as a "dynode" where it splashes several more electrons out of the metal. These, 

 in turn, repeat this over some nine more dynodes, multiplying the number each time until, 

 finally, several million electrons appear at the base for each one started from the tube 

 face. These produce a pulse of voltage in the circuit at the base which is then amplified 

 and analyzed by the equipment farther along the line. 



