Radon Elimination During Benzene Preparation for Radiocarbon Dating by Liquid Scintillation Spectrometry

Abstract

Radon gas is a serious contaminant in radiocarbon dating by radiometry. The low specific ionizations associated with the α-particle emitting radon and its β-particle emitting daughters overlap within the 14 C counting window. Elimination of radon is therefore imperative for precise 14 C age determinations. This paper deals with the sources and mechanism of incorporation of radon affecting 14 C dating by liquid scintillation (LS) counting, and reviews conventional radon elimination practices in 14 C laboratories. It demonstrates, based on rigorous multichannel and multiparameter α- and β-particle spectral analyses of some 1000 benzene samples, that parent radium is not present and that its daughter radon is quantitatively eliminated during dynamic vacuum recovery of benzene at −78°C. However, the radon-free benzene can be recontaminated by exposure to air containing traces of radon, such as is common in concrete or low-lying laboratories. The use of radon-free air, when exposing the benzene to the atmosphere, and the monitoring of radon counts from the environment and sample benzene in a fixed ‘radon window', are essential prerequisites to the quality control of 14 C age determinations in very low background systems.