CHAPTER 10 



GEIGER-MULLER COUNTERS 



10.1. General Properties. Geiger-Miiller tubes are a form of high-gain, 

 gas-filled, amplifying diode operated at potentials below the continuous dis- 

 charge voltage. Their essential function is that of a triggering device in 

 which a voltage pulse is produced by a discharge initiated by an ionizing 

 particle. The accumulation of negative charge, reaching a magnitude of the 

 order of 10 -8 coulomb, is collected at an anode where it can be detected by 

 appropriate instruments. The usual form of Geiger-Miiller tube consists 

 of a cylindrical cathode and a coaxially mounted wire anode sealed in a tube 

 containing one of various possible gas mixtures, usually at reduced pressure. 

 Designs, materials, and gas mixtures of many varieties are used, the choice 

 depending largely on the kind of radiation to be detected and to some extent 

 on special purposes. The tubes commonly used vary in size from 0.3 to 

 10 cm in diameter and from 2 to 50 cm in length, and for most tubes the anode 

 wire, usually tungsten, is about 0.02 to 0.1 mm in diameter. 



The size, duration, and general character of the discharge in a counter 

 tube is independent of the specific ionizing power of the initial particle. 

 Thus an electron and an alpha particle produce the same pulse as observed 

 on an oscilloscope screen although the latter particle produces 10 3 to 10 5 

 times as many ion pairs per centimeter of path. One ion pair, if formed in 

 the sensitive region of the tube, is sufficient to trigger the discharge which sub- 

 sequently involves roughly 10 10 ion pairs. 



The first part of the discharge occurs rapidly. Electrons released by the 

 initial ionizing particle drift rapidly to the central anode wire. In the high 

 electric field close to the wire the electrons acquire sufficient energy between 

 collisions to ionize the neutral gas molecules releasing additional electrons 

 which further contribute to a Townsend avalanche. Thus the initial number 

 of free electrons is increased by a factor of 10 8 to 10 10 . Because of the high 

 mobility of electrons, this part of the process is completed in a microsecond or 

 less. The less mobile positive ions, however, remain as a positive space 

 charge surrounding the anode along its entire length [1,2], and the subsequent 

 behavior of the discharge depends upon the composition of the gas used to 

 fill the counter, more generally, depending upon whether it is a quenching or 

 nonquenching gas. 



10.2. Non-self-quenching Counters. When a counter tube contains 

 monatomic, diatomic, or certain triatomic gases, the discharge tends to 



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