236 



RADIATION BIOLOGY 



lation counters by Marshall et at. (1948). Selected and paired tubes are 

 stable enough to be used for the detection of small changes in intensity 

 of 1 part in 10,000 (Oldenberg and Broida, 1950). At low intensities 

 fatigue is negligible (Engstrom, 1947). Since the photocurrent of the 

 photomultiplier is internally amplified 10"* to 10' times, thermal-resistor 

 noise and most of the leakage current present with vacuum photocells 

 are eliminated. The equivalent noise level of the photomultiplier referred 



I0-' 



Q. 



E 



z 



UJ 



<r 



O 



I0"3 



10 



-5 



o 

 o 



z 

 < 



10' 



10" 



10-11 



10 



(3 



10 



-14 



10" 



I0"'2 10"'° 10'^ ID'S lO""* 



RADIANT FLUX, lumens 

 Fig. 3-34. Linoarity characteristics of photomultiplier tubes. {From Engstrom, 1947.) 



to the photocathode is potentially much less than that of other photo- 

 cells. The dark current decreases rapidly with temperature, but there is 

 very little decrease in sensitivity {ibid.). Reducing the temperature from 

 20° to — 79°C with solid carbon dioxide results in a 20-fold decrease in 

 noise; at the temperature of liquid nitrogen, — 190°C, the noise is reduced 

 about 100-fold. 



Photocondudive Cells. The electrical conductivity of certain materials, 

 for example, selenium, increases when they are irradiated. A photo- 

 conductive cell is obtained when these materials are deposited on an 

 insulator in thin films and electrodes are attached to two opposite edges. 

 The phenomenon of photoconductivity is exhibited by the class of poor 

 conductors known as "semiconductors," which electrically occupy an 

 intermediate position between good insulators, such as quartz and poly- 

 styrene, and good conductors, such as the metals. The semiconductors 

 are the active materials in the dry-plate electrical rectifiers and transis- 

 tors. Those which have been used in photoconductive cells are selenium, 

 germanium, thallium sulfide (thalafide cells), lead sulfide, lead selenide, 

 and lead telluride. Lead sulfide and lead telluride (Moss, 1950; Simpson 

 and Sutherland, 1952; Zworykin and Ramberg, 1949) are of special inter- 

 est because they extend the range of photoelectric-cell response from the 



