Sec. 10.8] GEIGER-MULLER COUNTERS 315 



10.8. Active Gas -filled Counters. A higher order of sensitivity as com- 

 pared with the thin window counter can be attained in measuring the activity 

 of very low-energy beta-emitting substances by introducing the isotope in 

 some gaseous form into the counter as part of the filling gas. This is par- 

 ticularly useful for measurements of weak activities of C u (154 kev) and for 

 all activities of H 3 (17 kev). In principle, any radioactive isotope that can 

 be put into gaseous form can be measured by this means. If, however, the 

 maximum energy is greater than 0.2 mev or if gamma rays are emitted, a 

 procedure using a thin-window beta counter or a gamma counter is in general 

 faster and more convenient. 



Beta emitters with maximum energies as low as tritium (17 kev) neces- 

 sarily must be measured under conditions that avoid any absorber including 

 self-absorption. This is possible with counter tubes only by introducing 

 tritium directly into the sensitive region of the tube. Satisfactory pro- 

 cedures of this kind have been developed by Black and Taylor [10], Pace [11], 

 Allen [12], and Cornog [13]. The tritium, is introduced as part of the gas 

 filling mixture in the form of HTO vapor. A conventional design of glass- 

 walled tube is used which has a total volume of 1 liter, a 5-cm-diameter 

 copper-screen cathode, and a 0.010-in. -diameter tungsten anode. 



The disturbing effects of water vapor in counter tubes are avoided by 

 limiting the HTO vapor pressure in the tube to 2 mm Hg. To this is added 

 a mixture of 2.5 cm Hg of anhydrous ethyl alcohol and 2 cm Hg of argon. A 

 plateau of 300 volts and a threshold of approximately 1,200 volts are reported 

 for this tube by Pace [11]. 



Memory effects due to absorption of water vapor are considerably more 

 serious than for most other substances, but a procedure suggested by Pace is 

 successful in reducing the counting rate to normal background after each 

 sample measurement. Between samples the tube is alternately evacuated 

 and flushed eight times with inactive water vapor and then followed by an air 

 rinse and finally evacuation to a pressure of 0.3 jj.. 



A successful type of gas counter for measuring C 14 in the form of carbon 

 dioxide has been developed by Miller [14]. A conventional gamma counter 

 tube is constructed of either metal or glass with a 0.006-in.-diameter tungsten 

 anode and, in the glass tubes, a chemically deposited silver cathode coated 

 with aqua dag. Carbon dioxide containing the C 14 is introduced at pressures 

 from 10 to 50 cm Hg. Satisfactory Geiger-Muller characteristics are obtained 

 by the addition of 2 cm Hg of carbon disulfide which provides a 200-volt 

 plateau with a 2 per cent slope. The threshold voltage varies from 1,800 

 to 4,500 depending upon the cathode diameter (1 to 4 cm) and gas pressure. 

 The optimum operating voltage was found to be approximately 160 volts 

 above threshold voltage. 



Memory effect appears to be negligible for carbon dioxide. 



