262 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 5 



the spectral response of the photocoimter. Locher 1 has investigated 

 a great number of photoelectric surfaces, liquids, solids, metals, and 

 insulators and has apparently succeeded in covering a spectral range 

 from 90 to 750 m/j,. 



Further work in this direction is described by K. H. Kreuchen, Z. Physik, 

 94, 549 (1935), and 97, 625 (1936); W. Christoph, Ann. Physik, 23, 747 (1935), 

 and Physikal.Z., 37, 265 (1936); O. S. Duffendack and W.E.Morris, J. Opt.Soc. 

 Am., 32, 8 (1942); M. V. Scherb, Phys. Rev., 73, 86 (1948). 



The greatest difficulty is the interaction of the gas in the counter 

 with the photoelectric surface. 2 Bauer 3 and Kiepenheuer 4 have tried 

 to overcome this difficulty by separating the counter from the photo- 

 electric system by means of thin metal foils or glass membranes. The 

 point counter has also been used for the measurement of small light 

 intensities. 5 



For review papers, see H. Neuert, Arch. tech. Messen, V 422, March, 1942, 

 and C. E. Mandeville and M. V. Scherb, Nucleonics, 7, 34 (November, 1950). 



5-12. Photoconductive Transducers 



Almost all semiconductors are photoconductive, i.e., their elec- 

 trical resistance decreases when they are exposed to light. The effect 



is particularly strong in selenium, in 

 metal sulfides, oxides, and halides, as 

 well as in germanium and silicon. The 

 basic transducer circuit is shown in Fig. 

 (5-1)28. While the photoconductive 

 layer P is kept in darkness, its resist- 

 ance is high, and only a small dark 

 current will be observed. Exposure to 

 light causes a decrease of resistance and, 

 consequently, an increase of the current (photocurrent). 



A review of the plrysical processes in photoconduction is given by 

 Rose. 6 The photoconductive process is initiated by the absorption 

 of an incident photon which causes the excitation of an electron or 

 other carrier to the level of the conduction band. The carrier is then 



A 



^- 



Light 



*s 



Fig. (5-1)28 

 basic circuit 



Photoconductor, 



1 G. 

 2 G. 

 3 H. 

 4 K. 



5 H. 



L. Locher, Phys. Rev., 53, 333 (1938), and Phys. Rev., 42, 525 (1932). 



W. Barnes and L. H. du Bridge, Phys. Rev., 49, 409 (1936). 



Bauer, Z. Physik, 71, 532 (1931). 



O. Kiepenheuer, Z. Physik, 107, 145 (1937). 



Bauer and B. Sturm, Z. Physik, 94, 85 (1935). 

 6 A. Rose, in R. G. Breckenridge, B. R. Russell, and E. E. Hahn (eds.), 

 "Photoconductivity Conference," John Wiley & Sons, Inc., New York, and 

 Chapman & Hall, Ltd., London, 1956. 



