296 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 5 



cent methane) flows in a slow stream through the counter. The 

 relative yield of these counters for the soft /5 radiation of carbon 14 

 (0.155 MeV) is approximately 



Rodiooctive 

 sample 



Anodes 



Cathode 



inner 



cylinder 



Outer 

 cylinder 



A well-type counter is illustrated in Fig. (5-2)15. The radioactive 

 sample is inserted in the center; the counter is, in effect, a multi- 

 plicity of counters, which surround the sample so that radiation 

 emitted from the specimen in almost every direction is counted. 



Cathodes for gamma-ray counters should preferably be made of a 

 material of high atomic number Z, since the number of photo- 

 electrons produced by incident 

 gamma radiation increases with Z. 

 For instance, counters with bis- 

 muth cathodes (Z = 83) have a 

 higher counting efficiency for 

 gamma rays than those with cop- 

 per cathodes (Z =29). The superi- 

 ority of the bismuth counter 

 depends upon the energy of the 

 incident quanta; at low gamma- 

 ray energies, the bismuth counter 

 can be superior by a factor of 5 or 

 more; 1 at gamma-ray energies be- 

 yond about 1 MeV, the superiority 

 of the high-Z cathode counter is only of the order of ten to thirty per 

 cent. 2 



Cathodes in counters filled with chlorine are made with titanium 

 and zirconium. 3 Counters with a semiconducting cathode of tin 

 oxide coating fused on glass have been described by Clark. 4 The 

 thin tin oxide layer, which is transparent, is notattackedby chlorine. 

 Counters of this type can be made from thin wall glass of about 

 3 mg/cm 2 . 5 



1 Rossi and Staub, op. cit., chap. 5.2. 



2 R. D. Evans and R. A. Mugele, Rev. Sci. Instr., 7, 441 (1936); also H. 

 Maier-Leibnitz, Z. Naturforsch., 1, 243 (1946). 



3 A. L. Ward and A. D. Krumbein, Rev. Sci. Instr., 26, 341 (1955). 



4 L. B. Clark, Rev. Sci. Instr., 24, 641 (1953). 



5 Bulletin, Radiation Counter Laboratories, Skokie, 111. 



Fig. (5-2)15. Well-type counter. 



