CHAPTER 11 

 PROPORTIONAL COUNTERS 



11.1. General Features and Use. The proportional counter is a variable 

 gas-amplifying diode, in general, similar to a Geiger-Muller discharge tube in 

 construction and materials. The particular usefulness of this form of counter 

 derives from the nearly linear relation between the output pulse size from 

 the counter and the total ionization produced initially in the counter by a 

 charged particle. With appropriate electronic circuits, it is possible to 

 discriminate between kinds of particles traversing the counter and frequently 

 between similar heavy particles of different energies. Thus, in decreasing 

 order of magnitude, definite pulse size for a particular counter can be asso- 

 ciated with fission fragments, alpha particles, protons, and electrons. The 

 property of discrimination can be used to advantage for reducing the observed 

 background from light particles and gamma radiation when low intensities 

 of heavily ionizing particles are measured, by adjusting the counter circuit to 

 amplify and register only pulses larger than a predetermined size. 



The design of proportional counters depends mainly upon the radiation 

 to be measured. For most applications three forms are commonly used: 



1. "Bell" shaped counters with tungsten wire anodes and with thin 

 windows for detecting alpha and beta radiations. 



2. Cylindrical glass or metal tubes with coaxial tungsten wire anodes for 

 neutrons and cosmic radiations. 



3. Point counters [1] similar to "bell" counters but with a spherical metal 

 anode 1 to 2 mm in diameter supported near the counter window. The 

 sensitive region of these counters lies in only a small volume surrounding the 

 ball anode. 



In addition a design developed by Zipprich [2], consisting of two parallel 

 plate electrodes and an accelerating grid placed between the plates, is less 

 often used but possesses the advantage of a well-defined sensitive region. 



Filling gases for proportional counters need not contain polyatomic, or 

 quenching, gases although the inclusion of such gases provides greater stabil- 

 ity, particularly for large pulses. The gases most frequently used are a 

 mixture of argon, 10 to 25 per cent, and methane, 75 to 90 per cent, or various 

 mixtures of nitrogen, methane, ethane, and ether. Pressures commonly used 

 vary from a few centimeters to half an atmosphere. 



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