GENERATION, CONTROL, AND MEASUREMENT 231 



and (2) the kinetic energy of the released electrons is inversely propor- 

 tional to the wave length (proportional to the frequency) and independent 

 of the incident flux. These relations arise from the fact that an electron 

 must have an energy in excess of a minimum value W in order to be 

 ejected. The quantum energy hv must then be equal to or greater than 



ANODE 



^ INCIDENT 



^ FLUX 



ANODE WIRE 



CYLINDRICAL 

 PHOTOCATHODE 



ENVELOPE 



fir 



Fig. 3-28. The photoemission cell and the circuit used to couple it to a vacuum-tube 

 amplifier. 



W. The excess energy resulting from the interaction between a photon 

 and an electron at a photosurface is converted into kinetic energy, and 



y^mv'^ = hv — W. 



At absolute zero W has a minimum value. At higher temperatures some 

 of the electrons will have acquired additional energy as a result of thermal 

 agitation, and a few will have sufficient energy to leave the surface in the 

 absence of any radiant flux. This very weak emission constitutes the 

 thermionic emission, which becomes a large part of the dark current at 

 room temperature for cells sensitive to the near infrared. The minimum 

 energy requirement is a constant for each type of surface and determines 

 the long-wave-length limit, or photoelectric threshold, since the photon 

 (quantum) energy must be equal to or greater than W. 



The photosensitive surfaces of photoemissive cells are prepared by 

 evaporating various metals onto a sheet-metal surface which serves also 

 as the cathode. The near-infrared, visible, and near-ultraviolet sensitive 

 cells have surfaces that consist of layers of hahde and other metals and 

 their oxides. American manufacturers have standardized on a limited 

 number of spectral-response curves which are typical of certain types of 

 photosensitive surfaces. The compositions of representative photosensi- 

 tive surfaces are given in Table 3-17, and the relative spectral sensitivity 

 in Fig. 3-29. It should be noted that the .spectral-response designation S 

 apphes only to the spectral-response curve and that compositions other 

 than those given in Table 3-17 may give the same curve. 



The S-1 response has two maxima, one in the middle ultraviolet and 



