Sec. 5-2] 



RA DIA TIOX TEA XSD UCE RS 



281 



Fid. (5-1)42. Thermionic radia- 

 tion detector: E f , filament volt- 

 age source; E b , plate voltage. 



c. Thermionic Detector. The system is similar to a thermionic 

 diode with a directly heated cathode, as shown in Fig. (5-1)42. The 

 applied plate voltage and the filament temperature are chosen so 

 that operation in the temperature-limited region occurs. If radia- 

 tion strikes the cathode, its temperature 

 will be raised, and the plate current will 

 increase. For theory and references, 

 see Jones. 1 



5-2. Transducers for Ionizing Ra- 

 diation 



The transducers described in this 

 section are based upon the ion-pro- 

 ducing effect of radiation; they include 

 the ionization chamber (5-21) and the 

 gas-filled counters (5-22), as well as the crystal counter(5-23). Scin- 

 tillation counters and Cerenkov counters are omitted, since they 

 are not considered electrical instrumentation elements, but rather 

 combinations of systems that convert ionizing radiation into light 

 with transducers that convert light into electric signals. 



Ionizing radiation is either of an electromagnetic nature, such as 

 X raj' s or gamma rays, or of a corpuscular nature, such as alpha or 

 beta rays. 



Ionizing radiation can sometimes be converted into electric signals 

 by transducers for optical radiation. Photomultipliers, for instance, 

 can be sensitive for gamma radiation; 2 also the response of some 

 photoconductive and photogalvanic systems extends into the X-ray 

 and gamma-ray region, and some of them respond to alpha and beta 

 radiation (see 5-12 and 5-13). Radiation of charged particles can 

 be converted into electric signals also by transducers for free charges 

 (4-1). 



Ionization chambers and proportional counters (as well as scin- 

 tillation counters) measure the energy per cubic centimeter and per 

 second of the absorbed radiation, while Geiger counters measure the 

 number of particles without regard to their energy. 



5-21. Ionization Chambers 



The ionization chamber, schematically illustrated in Fig. (5-2)1, 



consists essentially of two electrodes, A and B, well insulated from 



1 R. C. Jones, in L. Marton (ed.), "Advances in Electronics," sec. IV, 4, 

 p. 50, Academic Press, Inc., New York, 1953. 



2 . J. D. Graves and G. E. Koch, Rev. Sci. Instr., 21, 304 (1950). 



