© 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 19–29. OPTIMIZATION OF THE CHANNEL RATIO METHOD FOR COLOR QUENCHING CORRECTION FOR MEASUREMENT OF 90 Sr VIA CERENKOV COUNTING F Mosqueda 1,2 • F Vaca 1 • M Villa 3 • G Manjón 4 ABSTRACT. This paper presents a method for the determining 90 Sr via its β-emitting daughter, 90 Y, in environmental sam- ples by Cerenkov counting. The method is based on the isolation of 90 Y by solvent extraction, using HDEHP as an extractor. The liquid scintillation (LS) analyzer Tri-Carb 3170 TR/SL has been employed. After radiochemical treatment, environmen- tal samples might be colored, and this will lead to color quenching, which is one of the most important problems that affect Cerenkov counting. The channel ratio (CR) method has been employed to correct this effect. Thus, the A window, which includes the whole spectrum, and the B window, corresponding to the most energetic zone of the A window, are defined. The ratio between the net counts of these windows defines the CR. Most authors working with this method define the B window as starting from the channel with higher number of counts. However, no study justifies this selection. This paper presents mea- surements and criteria to optimize and justify the choice of the B window. Several “B windows” have been defined, and their respective efficiency versus CR curves has been determined. During a study of the fitting parameters for the curves, we eval- uated how to choose the windows to establish the optimum curve that corrects the efficiency decrease due to color quenching. Results have been checked by taking part in an exercise of analytical intercomparison among a great number of Spanish lab- oratories for the measurement of 90 Sr in aqueous samples. INTRODUCTION 90 Sr is an artificial radionuclide, a β emitter, generated by fission or neutronic activation and liberated in explosions or nuclear accidents. 90 Sr is interesting from a radioecological point of view for several reasons; on one hand, its relatively long half-life (T 1/2 = 28.6 yr) assures its presence in the biosphere for a long time. On the other hand, due to the chemical similarity of Sr +2 -Ca +2 , it is possible to introduce 90 Sr associated to calcium into the food chain. Once in the organism, 90 Sr and calcium are fixed in the materials of the human skeleton. Bjørnstad et al. (1992) estimate that the biological period of semi-elimination of 90 Sr is approximately 50 yr; 90 Sr is able to provoke damage mostly due to the high energy of the β emission of its descendant 90 Y. The objective of this work is the optimization of a liquid scintillation counter (LSC), model Tri-Carb 3170 TR/SL, for the determination of the concentrations of 90 Sr by Cerenkov counting in environ- mental samples. A radiochemical procedure is developed for the determination of 90 Sr through the Cerenkov radiation emitted by its descendant 90 Y. Due to their respective half-lives (28.6 yr for 90 Sr and 64.1 hr for 90 Y), we can assume that both radionuclides would be in secular equilibrium in the environment. 90 Y is preferred for Cerenkov measurement of 90 Sr due to its higher maximum β energy (2283 keV), which is superior to the Cerenkov threshold in pure water (263 keV). The method used in this work is based on the one developed by Soumela et al. (1993). The main advantages of this technique are the simplicity of the chemical method and the rapidity of the mea- surements. On the other hand, to obtain an accurate result based on a Cerenkov measurement, the experimental work must be very rigorous because Cerenkov counting efficiency is very sensitive to color quenching (Fujii and Takiue 1988). There are 2 different kinds of quenching, chemical and color. Chemical quenching is caused by chemical substances that interfere with the energy transfer- ence from the solvent to the solute (Brooks 1979). Any absorbing material that colors the sample and reduces the number of photons transmitted through the scintillation medium causes color 1 Dpto. de Física Aplicada, Facultad de Ciencias Experimentales, Universidad de Huelva, Campus de El Carmen, 21007 Huelva, Spain. 2 Corresponding author. Email: fernando.mosqueda@dfa.uhu.es. 3 Centro de Investigación, Tecnología e Innovación, Universidad de Sevilla, Av. Reina Mercedes 4B, E41012 Seville, Spain. 4 E.T.S. Arquitectura, Universidad de Sevilla, Dpto. Física Aplicada II, Av. Reina Mercedes 2, E41012 Seville, Spain.