Co₂ Gas Proportional Counting in Radiocarbon Dating — Review and Perspective

Abstract

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.