CO2-CS2 Geiger Counter

Abstract

Experiments on the mechanism and characteristics of a CO₂‐CS₂ Geiger counter are reported, and circuits and methods of operation which satisfy the special requirements are described. Counter: The active part is 2⅞ in. in diameter and 16 in. long, filled to 1 atm, 95% CO₂ and 5% CS₂. The cathode is copper and the anode is 0.005‐in.‐diam Chromel A. Characteristics: The electrons released by an ionizing particle become attached, probably to CS₂. The self‐quenching action of the counter is excellent, no spurious counts are observed in the absence of electronic quenching. However, a dead time of several milliseconds must be imposed electronically because of the long interval during which the negative ions arrive at the anode. Method: An electronic quench is used which is triggered both by the CO₂‐CS₂ counter and the anticoincidence ring. This serves to impose the required dead time and also to prevent the firing of the CO₂‐CS₂ counter by mesons. Measurements: The plateau was measured to 1900 v above threshold and was found to be level to within 1% from 400 to 1600 v above threshold. The maximum drift time of the negative ions was found to be about 9 msec with a sharp cutoff. Tests with various combinations of gas indicated, but did not prove, that the charge carrier was CS₂. The effects of common contaminants were determined. At voltages over 400 above threshold, 1% O₂ gave no detectable effect; 0.3% SO₂ gave a 2 to 3% reduction in counting rate. Extensive tests of the efficiency were made. Comparisons of the CO₂‐CS₂ filling with an argon‐ethane filling gave identical results within the experimental error. No basis was found for supposing that there was any failure of the CO₂‐CS₂ counter to register counts. Reliability: The counter described has been used for C¹⁴ dating purposes over a long period of time. There has been no indication that there are variations in counting rate outside those expected on the basis of statistics. The counting rate for ``dead'' CO₂ in an iron shield 9 in. thick and after anticoincidence is 8 per min.