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ISOTOPIC TRACERS AND NUCLEAR RADIATIONS [Chap. 10 



secondary gas should be considerably less than 1 per cent of the gas mixture. 

 Combinations of permanent gases suggested by Simpson are given in Table 34. 



Table 34. Low-voltage Ccwnter Filling Mixtures 



Under most conditions, the minimum threshold voltage is found for primary 

 gas pressures of 5 to 10 cm Hg. The optimum amount of secondary gas 

 depends to some extent on the total pressure and kinds of gases used. For 

 the neon-argon mixture, the most carefully investigated, the minimum 

 threshold of approximately 120 volts is found for a neon pressure of ~5 cm 

 Hg and ~0.01 per cent argon. With argon concentrations up to 0.9 per cent 

 the threshold under the same conditions increases to approximately 160 volts. 



Construction, evacuation, and purity of the gases require particular con- 

 sideration. The cathode surface should be one that minimizes secondary 

 electron emission from both photoemission and ion bombardment. For 

 this reason a thin coating of cuprous oxide gave the most satisfactory results. 

 Before filling, a counter is thoroughly outgassed by pumping for 4 to 6 hr 

 while its temperature is maintained by a furnace at 300°C. Complete 

 outgassing is further ensured by heating the anode wire to incandescence and 

 by applying high- voltage discharges between anode and cathode. 



The addition of an organic vapor to make the counter self-quenching 

 raises the threshold to 230 volts or more, but this is still low compared with 

 that of conventional gas mixtures which have thresholds of 800 to 1,500 volts. 

 Specifications for a self-quenching low-voltage counter developed by Simpson 

 [30] are given below. 



Cathode: Copper, 1.9 cm diameter 

 Anode: 0.01 cm diameter tungsten wire 

 Filling gas: Neon, 5.7 cm Hg; argon 5 to 50 X 10 -4 cm Hg; distilled ethyl 



alcohol, ethyl ether, or amyl acetate, 0.02 to 0.04 cm Hg 

 Threshold: 230 to 270 volts 

 Plateau: 90 to 120 volts 

 Slope: 2 to 5 per cent 



