© 2006 by the Arizona Board of Regents on behalf of the University of Arizona LSC 2005, Advances in Liquid Scintillation Spectrometry Edited by Stanis≥aw Cha≥upnik, Franz Schönhofer, John Noakes. Pages 423–428. OPTIMIZATION OF 14 C CONCENTRATION MEASUREMENT IN AQUEOUS SAMPLES USING THE DIRECT ABSORPTION METHOD AND LSC Carmen Varlam 1,2 • Ioan Stefanescu 1 • Mihai Varlam 1 • Cristina Bucur 3 • Irina Popescu 1 • Ionut Faurescu 1 ABSTRACT. We developed a radiocarbon measurement procedure for natural-level water samples. The characteristics and specifics of this new measurement technique are discussed and compared with concurrent methods. Several experiments were carried out for sample preparation; the direct absorption method was selected, with a few custom improvements, to increase the reproducibility and accuracy of this simple and quick method. Results confirmed the validity of the sample preparation and measurement procedures, and confirm that this method provides better reproducibility than traditional techniques. A val- idation test using CaCO 3 results from oyster shell and marble provided a mean value of 0.2137 ± 0.0039 Bq/g C, for an effi- ciency of ~64% and a background value of 2.22 cpm. This procedure was applied on different types of water. INTRODUCTION Radiocarbon is a radioactive isotope of carbon and is a pure beta emitter with a half-life of 5730 yr; it decays to 14 N by emitting low-energy beta radiation with an average energy of 49.5 keV and max- imum energy of 156 keV. 14 C is easily transferred during biological processes and soil-plant inter- action involving carbon compounds. Radioactive waste containing 14 C is continuously produced in nuclear reactors, spent-fuel reprocessing, and radioisotope production, as well as medical research. Cernavoda Nuclear Power Plant (NPP) is the only NPP in Romania and the only Canada Deuterium Uranium ® (CANDU) reactor operating in Europe. The Cernavoda Unit 1 is a pressurized heavy- water reactor (PHWR) fueled with natural uranium and moderated and cooled by heavy water. The radionuclide 14 C is a byproduct of this type of reactor and is predominantly produced by neutron activation of 17 O water molecules (heavy water is enriched not only in deuterium but also in 17 O). In heavy-water reactors such as the CANDU, the major portion of 14 C produced originates from the moderator. Routine operation of this type of reactor and its auxiliary process systems results in the production of a large variety of solid, liquid, and gaseous radioactive wastes. Therefore, monitoring 14 C around this important nuclear facility has become a necessity, with water as the component of primary interest. In order to evaluate the release of 14 C from nuclear facilities, a background activity of 250 Bq/kg of stable carbon has been allowed by regulatory bodies such as the Institute de protection et de sûreté nucléaire (France), the National Radiological Protection Board (United Kingdom), and the Inspectorate for Nuclear Reactor Safety (Switzerland). According to these regulations, any 14 C level above this background level, other than normal production of 14 C by cosmic radiation, will be considered pollution (International Atomic Energy Agency 2004). Every monitoring program must include a background level of 14 C measurement on a routine basis to detect environmental increases from a particular source. Since the 14 C/ 12 C ratio in the present bio- sphere and atmosphere is only ~10 –12 , substantial sample sizes are required to measure its activity with conventional β – decay counting equipment. The CO 2 absorption method for preparing samples for 14 C analysis by liquid scintillation counting (LSC) has been successfully employed by several laboratories for many years. The method in our laboratory was developed by Commonwealth Sci- entific and Industrial Research Organisation (CSIRO) Land and Water and its main advantage is its 1 National Institute R&D for Cryogenic and Isotopic Technologies, Rm. Valcea, Romania. 2 Corresponding author. Email: cvarlam@icsi.ro. 3 Environmental Laboratory of NPP Cernavoda, Romania.