Sec. 12.2] IONIZATION CHAMBERS 347 



monoxide, carbon dioxide, and the halogens, which form negative ions. Air 

 at atmospheric pressure is the most frequently used gas. Chambers operated 

 at high pressures for the purpose of contracting the range of the radiation 

 are often filled with argon or krypton since these gases have greater stopping 

 power and large cross sections for ionization. Pulse chambers, on the other 

 hand, frequently exhibit erratic performance and provide poor pulse shapes 

 when filled with negative ion-forming gases. Usually they are operated with 

 nitrogen, argon, or methane at atmospheric pressure, and in many instances 

 it is convenient to allow the gas to stream through the chamber. This 

 latter method is especially useful when the source of radiation is placed in 

 the chamber and is frequently changed as in measuring radioactive samples. 



12.2. Applications. The ionization chamber has found more extensive 

 use and more diverse adaptations than any other device for measuring 

 ionizing radiations. Its simplicity and comparative ruggedness are factors 

 to be considered to its advantage but, most important, the ionization chamber 

 lends itself to measurements of radiation under conditions that cannot be 

 duplicated with other means for detecting ionization. In general, applica- 

 tions of the chamber may be divided into two categories. Although these 

 categories are somewhat superficial, there exists an important physical 

 distinction between them. The first includes measurements of radiations 

 that have ranges comparable to or smaller than the dimensions of the gas 

 volume, and the second applies to penetrating radiations that have ranges 

 very much greater than the dimensions of the chamber. 



The first category involves measurements on short-range charged particles 

 and slow neutrons. The ionization produced in the chamber can usually be 

 interpreted unequivocally in terms of the number of primary particles or as 

 relative activity of the source of radiation. The design of chambers for this 

 purpose is largely a matter of expediency, consistent with special features of 

 the application and acceptable instrument designing. The source of primary 

 radiation, because of the comparatively short range of charged particles, must 

 be placed within the chamber to permit the greatest possible portion of the 

 range to lie in the sensitive gas volume. In the case of slow neutrons the 

 source may be boron or a fissionable material (see Sec. 10.10). In nearly 

 all cases where the source material is solid, it can be deposited in a thin film 

 on one of the electrodes or on a suitable support that is mounted adjacent 

 to the sensitive volume of the chamber. For the highest efficiency the film 

 thickness should equal approximately the range of the particle in the sub- 

 stance. A somewhat greater ionization efficiency and sensitivity can be 

 achieved when the substance can be introduced as part of the chamber gas 

 since self-absorption is then eliminated and the geometrical efficiency is 

 increased to very nearly 100 per cent. Thus boron tritfuoride is most com- 

 monly used for detecting slow neutrons, and carbon dioxide and hydrogen- 



