Measurement of the carbon 14 activity at natural level in air samples

¹ Marine Nationale, École des Applications Militaires de l'Énergie Atomique, Département Groupe d'Études Atomiques, BP. 19, 50115 Cherbourg Armées, France
² Institut de Radioprotection et de Sûreté Nucléaire, Laboratoire de Radioécologie de Cherbourg-Octeville, 50130 Cherbourg-Octeville, France
³ Laboratoire des Sciences du Climat et de l'Environnement, UMR 1572-CEA/CNRS, Domaine du CNRS, 91198 Gif-sur-Yvette, France

Abstract

The aim of the study was to measure the carbon 14 activity at natural level in air samples using classical methods of radiochemistry and beta counting. Three different methods have been tested in order to minimise the detection limit. In the three methods, the first step consists in trapping the atmospheric carbon 14 into NaOH (1N) using a bubbling chamber. The atmospheric carbon dioxide reacts with NaOH to form Na₂CO₃. In the first method the Na₂CO₃ solution is mixed with a liquid scintillate and is directly analysed by liquid scintillation counting (LSC). The detection limit is approximately 0.3 Bq.m^-3 of air samples. The second method consists in evaporating the carbonate solution and then counting the solid residue with a proportional gas circulation counter. The detection limit obtained is equivalent to the first method (0.36 Bq.m^-3 of air samples). In the third method, Na₂CO₃ is precipitated into CaCO₃ in presence of CaCl₂. CaCO₃ is then analysed by LSC. This method appears to be the most appropriate, the detection limit is 0.04 Bq.m^-3 of air samples.