232 



RADIATION BIOLOGY 



the other in the near infrared at 800 m/i. It has a high sensitivity to 

 beyond 1000 m/x. Because of the high near-infrared response the S-1 

 surface has the highest thermionic emission and therefore the highest 



Table 3-17. Typical Compositions of the Photosensitive Surfaces 

 Giving the Various Standard Spectral-response Curves 



Response Designation Typical Composition 



S-1 Silver-cesium oxide-cesium 



S-3 Silver-rubidium oxide-rubidium 



S-4 Antimony-cesium 



S-5 Antimony-cesium in high-silica glass, as Vycor or quartz 



S-6 Sodium in high-silica glass 



S-8 Bismuth-cesium 



dark current. The S-3 response is high throughout the visible and most 

 nearly approximates the spectral response of the eye. The S-4 and S-5 

 response are due to the same photosensitive surfaces and differ only in 

 the transmittance of the envelope. 



s-i 



c 



>-l20 



u 

 o 



.■t 100 



l_ 

 O 



v 80 



t 60 

 m 



UJ 



S 20 



lu 

 q: 



■t 140 



^120 



o 



t_ 



2 100 

 \- 

 o 



> 80 



> 



t 60 

 en 



^ "0 



UJ 



> 



400 



800 

 S-5 



1200 400 



800 

 S-6 



1200 



20 





200 



600 



1000 



200 600 



WAVE LENGTH 



1000 

 m Li 



400 



800 



1200 



Fig. 3-29. Relative spectral sensitivity of various types of photocathodes. {Courtesy 

 of Radio Corporation of America.) 



The vacuum photocell is thoroughly evacuated in order to eliminate 

 collisions between electrons and gas molecules. These collisions produce 

 ions that usually increase the noise level and affect the hnearity of the 

 response. The vacuum photocell has an extremely linear response, since 



