Sec. 10.6] GEIGER-MULLER COUNTERS 311 



6. Continuous-discharge region. This is characterized by corona, glow, 

 and finally, arc discharge. 



10.5. Filling Gases. The most frequently used filling gas is a mixture of 

 argon (80 to 95 per cent) and alcohol (5 to 20 per cent) at a total pressure of 

 5 to 40 cm Hg. Although nearly all permanent gases have been used, either 

 alone or in various mixtures, argon is probably the most useful since it has 

 a large cross section for ionization, a sufficiently high ionization potential to 

 allow electron transfer with most polyatomic molecules, does not form 

 negative ions, and is readily available. Many quenching gases other than 

 ethyl alcohol can be used such as ethane, amyl acetate, ethyl ether, and 

 tetraethyl lead. Heavy polyatomic molecules may also be used, and some 

 increase in useful counter life will usually be found because of the greater 

 number of decompositions required to reduce it to a nonquenching gas. 

 However, their use in most counters is undesirable since they lead to a longer 

 resolving time due to the low mobility of heavy ions. Gases that have strong 

 tendencies to form negative ions should normally be avoided [4,9]. These 

 include primarily oxygen, carbon dioxide, water vapor, and the halogens. 

 However, under special conditions they can be used successfully in small 

 amounts [11,14]. It has also been pointed out by Present [36] that chlorine 

 and bromine can under certain conditions be used as quenching gases. 



10.6. Counter-tube Life. The useful life of self-quenching counters 

 appears to be dependent to some extent upon the volume of gas in the 

 counter [8]. With each discharge approximately 10 10 ion pairs are formed, 

 and in the process a small fraction of the polyatomic molecules are decom- 

 posed into smaller fragments, some of which still retain quenching properties. 

 Ultimately, however, an appreciable fraction of the complex molecules is 

 reduced to free oxygen, hydrogen, carbon, and simple molecules which remain 

 partly as nonquenching gases and partly as an accumulation of crud on the 

 cathode. The counter performance tends to become erratic and finally 

 no longer self-quenching and must be refilled. Assuming that 10 8 to 10 10 

 quenching gas molecules are decomposed per discharge and an initial number 

 of molecules of approximately 10 20 , the approximate life of a counter is then 

 of the order of 10 10 counts. In practice it is found that the smallest counters 

 retain their quenching property for only a few months at best when in con- 

 stant use, whereas larger counters, with volumes of the order of 75 cc, have 

 useful lives of a year or more under similar conditions. Counters containing 

 only permanent gases should exhibit no change with time, barring leaks or 

 deterioration of the mechanical and electrical properties of the tube itself. 



The effective life of self-quenching counters can be prolonged somewhat 

 by operating the counter at a voltage in the lower end of the plateau and 

 by increasing the total gas volume without, at the same time, increasing the 

 sensitive volume in which the discharge takes place. The latter method has 



