Sec. 10.7] GEIGER-MULLER COUNTERS 313 



been successfully employed to increase by several times the useful life of very 

 small counters [8]. By attaching a bulb of larger dimension to the counter 

 outside the sensitive region to serve as a reservoir, the percentage decom- 

 position of quenching gas per discharge can be made very much smaller. 



10.7. Low-voltage Counter Tubes. A considerable reduction in the 

 Geiger-Muller threshold of counter tubes containing conventional gas mix- 

 tures can be achieved by reducing the total gas pressure. The minimum 

 threshold voltage of most counters containing permanent gases is found at 

 pressures near a few centimeters of mercury, below this the threshold again 

 increases very rapidly with decreasing pressure. With a quenching gas 

 present, the minimum threshold for counters of moderate size is near 500 volts 

 and without a quenching gas, somewhat lower. In practice, however, a 

 substantial reduction in the threshold by this means is impracticable since 

 the reduced pressure is accompanied by a marked decrease in the counter-tube 

 efficiency and ultimately, below 5 to 10 cm Hg, by the alteration or even loss 

 of the Geiger-Muller plateau. It is well known that the threshold voltage 

 also decreases with the diameter of the cathode and to some extent with the 

 diameter of the anode wire, but the reduction effected by the use of very small 

 anode-wire diameters is not very great and is severely limited for mechanical 

 reasons. 



Geiger-Muller and proportional counters with very low thresholds can 

 be made, however, with highly specific mixtures of permanent gases and 

 suitable cathode surfaces. Such counters, with operating voltages of 130 to 

 250 volts and normal characteristics, have been developed and exhaustively 

 investigated by Simpson [30]. 



The discharge mechanism of these counters is similar to that described in 

 Sees. 10.1 and 10.2 for ordinary gas mixtures but with one important differ- 

 ence. In conventional gas mixtures or with a single permanent gas, a con- 

 siderable fraction of the energy derived from the electric field during a dis- 

 charge is taken up by atoms that are raised to metastable excited states 

 rather than being ionized. The energy absorbed by these atoms therefore is 

 largely lost since they do not contribute to the total ionization in the dis- 

 charge. With the admixture of a small quantity of a second permanent gas, 

 the metastable states can be rapidly reduced by collisions of the second kind, 

 i.e., by inelastic collisions in which a neutral secondary gas atom is ionized 

 and the metastable atom reduced to the ground state [30,31]; thus, more 

 efficient utilization of the electric field for the formation of ions is possible and 

 the threshold voltage is accordingly reduced. 



The mechanism described above is possible when the first ionization 

 potential of the secondary gas is less than but near the energy of the meta- 

 stable state of the primary component. In addition, the concentration of the 



