288 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 5 



electrons in the walls, which, in turn, cause the formation of ion 

 pairs in the gas. Ionization chambers and counters for the former 

 radiation should, therefore, be provided with thin windows or thin 

 walls, while the wall thickness for the more energetic radiation 

 should be made "greater than the maximum range of the secondary 

 electrons, yet not so thick as to produce any appreciable attenua- 

 tion of the primary gamma radiation." 1 



A multiple-plate ionization chamber for X rays is shown in Fig. 

 (5-2)6. The chamber has the advantage that its volume and, there- 

 fore, its sensitivity are large, yet the spacing between the electrodes 

 is small and the time required for the collection of electrons and 

 ions is short. Therefore, the resolving time for very fast X-ray pulses 

 is short (on the order of a fraction of a millisecond). 



A chamber type frequently used for X-ray dose determination 

 is illustrated in Fig. (5-2)7. Chambers of this kind can be built 

 quite small (less than 1 cm 3 ) for application in body cavities or for 

 "point-by-point" measurement of dose distribution. 



A chamber with a thin window W, primarily useful for the measure- 

 ment of a rays, is shown in Fig. (5-2)8. 



Extensive descriptions of ionization chambers for use in nuclear research 

 will be found in Rossi and Staub, loc. cit. Ionization chambers for use in X-ray 

 dosimetry are reviewed by R. Jaeger, Arch. tech. Messen, V 61—1, January, 

 1941. For further descriptions and extensive literature, see S. A. Korff, "Elec- 

 tron and Nuclear Counters," D. Van Nostrand Company, Inc., Princeton, N.J., 

 1946, and D. H. Wilkinson, "Ionization Chambers and Counters," Cambridge 

 University Press, London, 1950. 



5-22. Gas-filled Counters 



a. Geiger Counters with Cylindrical Geometry. The most common 

 form of counter is shown schematically in Fig. (5-2)9. The anode 

 consists of a thin tungsten or platinum wire held by insulators within 

 a cylindrical cathode. The cathode is thin enough, or has a window, 

 to permit radiation to enter the tube. The counter is filled with a 

 gas, such as argon, to which is added a small amount of polyatomic 

 gas (e.g., alcohol). The pressure in the counter is in general between 

 a few centimeters Hg and atmospheric pressure. A voltage is applied 

 through a resistance R and causes an electric field in the counter 

 which is strongest around the anode. If an ionizing particle enters 

 the counter and causes ionization, a discontinuous electric gaseous 



1 B. B. Rossi and H. H. Staub, "Ionization Chambers and Counters," 

 McGraw-Hill Book Company, Inc., New York, 1949. 



