© 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 217–227. DIRECT 210 Pb DETERMINATION IN ENVIRONMENTAL SAMPLES BY LIQUID SCINTILLATION COUNTING AND ITS VALIDATION THROUGH γ-RAY SPECTROMETRY M Villa 1,2 • S Hurtado 1 • G Manjón 3 • R García-Tenorio 3 • M García-León 4 ABSTRACT. 210 Pb is a natural radionuclide belonging to the 238 U series and is one of the most important isotopes to be determined in environmental samples because it is considered an excellent tracer of several processes in nature. However, direct determination of this nuclide via traditional radiometric techniques is far from straightforward since 210 Pb emits only relatively soft beta particles and a low-energy gamma ray with a low probability of emission. We propose a rigorous method for direct 210 Pb determination in sediment samples by LSC. The method includes careful calibration and adaptation of the experimental system based on a low-background spectrometer, Quantulus 1220™. There are 2 objectives to the calibration: efficiency improvement and optimum alpha/beta separation using a pulse-shape discrimination system (PSA). On one hand, a detailed study of the PSA system is essential because, after the isolation of 210 Pb from the environmental matrix, some traces of its alpha-emitting daughter 210 Po or other natural alpha emitters might be present during the measurement. Moreover, the appearance of quenching in the sample affects not only counting efficiency but α/β discrimination as well. The quenching effects are also studied in both efficiency considerations and for α/β separation and are included in the calibration. On the other hand, a new approach is proposed based on calculation of an effective radiochemical yield, allowing the determination of 210 Pb levels in the analyzed samples without a previous direct determination of the counting efficiency. 210 Pb counting effi- ciency determination is avoided because in most of the available standards, 210 Pb is in secular equilibrium with its daughter 210 Bi. As the 210 Bi low-energy tail overlaps in the 210 Pb spectrum, it is difficult to precisely determine the 210 Pb efficiency. The optimized method was applied for determining 210 Pb concentrations in a set of riverbed sediments affected to a quite variable degree by anthropogenic inputs of this nuclide, which has been validated by comparing those results to the ones obtained by applying γ-ray spectrometry. Although it is a direct, non-destructive technique, precise determination of the activity requires knowing the full energy peak efficiency and self-absorption correction for the low-energy gamma ray emitted. This task was performed using Monte Carlo simulation. INTRODUCTION 210 Pb determination in environmental samples is of crucial importance in radiological impact studies and for dating sediments and other environmental compartments. For this reason, accurate measure- ment techniques are needed to correctly determine 210 Pb in environmental samples. 210 Pb can be measured through the determination of 210 Po by alpha spectrometry (Moser 1993) or directly by gamma spectrometry (Cannizzaro et al. 1999; San Miguel et al. 2002), measuring the 46.5-keV pho- ton emitted with an intensity of 4.0% during its decay. Cerenkov counting of 210 Bi, once secular equilibrium with 210 Pb is reached, is another technique used to measure 210 Pb. This technique has an easy sample treatment (Hurtado et al. 2003) but has the drawback of lower counting efficiencies than liquid scintillation counting (LSC). The use of LSC for directly determining 210 Pb in environmental samples is a promising alternative method (see Wallner 1997; Blanco et al. 2004). Nevertheless, due to the complexity of the 210 Pb decay scheme, which includes low-energy beta particles from 210 Pb and high-energy beta particles from 210 Bi as well as alpha particles from 210 Po, it is still necessary to calibrate the detector for proper 210 Pb counting (Villa et al. 2003). Currently, some liquid scintillation counters, such as the Quantulus 1220™, allow for simultaneous alpha and beta detection and their subsequent separation. 1 Centro de Investigación, Tecnología e Innovación, Universidad de Sevilla, Av. Reina Mercedes 4B, E41012 Seville, Spain. 2 Corresponding author. Email: mvilla@us.es. 3 E.T.S. Arquitectura, Universidad de Sevilla, Dpto. Física Aplicada II, Av. Reina Mercedes 2, E41012 Seville, Spain. 4 Facultad de Física, Universidad de Sevilla, Dpto. Física Atómica, Molecular y Nuclear, Av. Reina Mercedes s/n, E41012 Seville, Spain.