Co2 Gas Proportional Counting in Radiocarbon Dating — Review and Perspective

Gas counting is a mature and powerful technique central to radiocarbon dating. The method was taken from the detection techniques used in nuclear physics and adapted to the special requirements of low-level counting of the carbon gases. A compilation made by W G Mook in 1983 lists 174 gas counters used in 14C dating, the counting gases being CO2 (115 counters), CH4 (38), C2H2 (20) and C2H6 (1). In the present contribution, the current status of CO2 gas counting is reviewed. The emphasis on CO2 is justified by several observations: 1) CO2 is the primary gas to be produced in all methods; 2) routine techniques are able to achieve high purity CO2 gas, so further conversion to hydrocarbons appears unwarranted; 3) the cryogenic properties of CO2 facilitates handling and thus minimizes contamination; 4) all gas counting laboratories involved in high-precision work, eg, for calibration, use CO2 as counting gas. Most of the techniques mentioned in this review were already available around 1975 (see the Proceedings of the Ninth International Radiocarbon Conference (Berger & Suess 1979)); since that time, gas proportional counting has become a routine operating technique in several laboratories. Wherever possible, reference is made to individual laboratories; however, as the more technical aspects of the technique that are central to this chapter are rarely fully documented in the literature, we take most examples from the Heidelberg laboratory, and are fully aware that this description gives heavy weight to a single installation.