Colour quenching corrections on the measurement of 90Sr through Cerenkov counting
Introduction
90Sr is an artificial radionuclide, β emitter, generated by fission or neutron activation and liberated in explosions or nuclear accidents. From a radioecological point of view, it is interesting for several reasons. On the one hand, its relatively long half-life (T1/2 = 28.6 years) assures its presence in the biosphere for a long time. On the other hand, due to the Sr2+–Ca2+ chemical similarity, it is possible to introduce 90Sr in the food chain associated with calcium. Once in the organism, 90Sr and calcium attach to the human skeleton. 90Sr is able to cause damage mostly due to the high energy of the β emission of its descendant 90Y.
90Sr can be separated following a wide range of methods, among them, solvent extraction, ion-exchange and various forms of chromatography [1]. It can be measured either through gas proportional counting [2], further detection by liquid scintillation counting (LSC) [3] or plastic scintillation counting [4], or, through the measurement of the Cerenkov radiation of 90Y, the high-energy beta emitter in secular equilibrium with 90Sr [5]. Due to their respective half-lives (28.6 years for 90Sr and 64.1 h for 90Y), we can consider both radionuclides to be in secular equilibrium in the environment. 90Y is preferred for Cerenkov measurement to 90Sr, due to its higher maximum β energy (2283 keV), vastly superior to the Cerenkov threshold in pure water (263 keV).
The radiochemical method developed in [6] for the determination of 90Sr through the measurement of 90Y is suitable for the measurement of organic samples overall [7]. Other advantages of this technique are the simplicity of the chemical method and the rapidity in the measurements. The radiochemical procedure consists of the isolation of 90Y by solvent extraction, using HDEHP as an extractor.
The liquid scintillation analyser Tri-Carb 3170 TR/SL is hardly used for measurements by Cerenkov counting. However, the work presented here shows that this counter is suitable not only for liquid scintillation counting but also for Cerenkov counting. For this reason, a study of two different parameters necessary for the calibration of a Cerenkov counter is included: background and counting efficiency.
Nevertheless, to obtain an accurate result based on a Cerenkov measurement, the experimental work must be extremely rigorous because Cerenkov counting efficiency is very sensitive to colour quenching [8]. There are two different kinds of quenching, chemical and colour. Chemical quenching is due to chemical substances that interfere in the energy transference from the solvent to the solute [9]. Nevertheless, chemical quenching does not exist in Cerenkov counting due to the nature of the effect [10]. However, any absorbing material that colours the sample and reduces the number of photons transmitted through the scintillation medium causes colour quenching [11]. In both cases, the number of photons detected by the photomultiplier is less than that detected for unquenched samples and, therefore, causes a reduction in the counting efficiency [9].
Environmental samples, after applying the radiochemical procedure, might be affected by colour quenching, and for that reason it is necessary to evaluate the quenching degree and correct its effects. Several methods can be used to evaluate the quenching degree in a liquid scintillation measurement; one of which is the channel ratio method (CR) [11].
In this work, we have focused on the study of the curves of calibration of the counting efficiency depending on the degree of colour quenching. The colour quenching degree was calculated by following the channel ratio method, obtained from different windows. Thus, an A window, which includes the whole spectrum, and a B window corresponding to a high-energy zone of the A window, are defined. The ratio between the net counts of these windows defines the CR. Counting efficiency is usually calculated on samples traced with known activity and affected by colour quenching. The shift of the spectrum, defined through the CR, is fitted versus counting efficiency and thus the correction curve is obtained.
Most authors working with this method define the B window from the channel with the highest number of counts. However, no study justifies this selection. This paper presents measurements 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. Through a study of the fitting parameters related to the curves, we have evaluated how to choose the windows to establish the optimum curve that corrects the efficiency decrease due to colour quenching.
Section snippets
Experimental
The Tri-Carb 3170 TR/SL is a liquid scintillation counter specially designed for detection of low level alpha and beta radioactivity. The measurements have been made using the TR-LSC™ device, which is the optimum counting mode for very low activity samples.
The complete radiochemical procedure is shown in Fig. 1. The method used in this work for separation and isolation of 90Y is based on the property of Yttrium, which always behaves with valence 3+, forming stable complexes with diverse organic
Background, counting efficiency, LLD and FOM
Proper control of certain parameters in Cerenkov counting, such as the type of vial or the final volume of the samples in the vial, might reduce the background of the measurement considerably [5]. In order to select the final sample volume in the vial, we have studied the relationship between sample volume and background, and its influence on counting efficiency, lower limit of detection, and FOM.
It is well known that polyethylene vials produce less Cerenkov background than glass vials due to
Conclusions
The measurement of 90Sr through the determination of 90Y by Cerenkov counting using Packard's Tri-Carb 3170 liquid scintillation counter has been optimized in relation to background and counting efficiency. The aim of this work is to perform a study to optimize the evaluation of colour quenching corrections through the channel ratio method according to statistical criteria. Through the study of fitting parameters related to the curves, the dependence of efficiency corrections versus a chosen
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<sup>90</sup>Sr determination in water samples using Čerenkov radiation
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