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210bi detection in waters by cherenkov counting – perspectives and new possibilities
Radiation Physics and Chemistry 166 (2020) 108474
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Pb/210bi detection in waters by cherenkov counting – perspectives and new possibilities 210
T
Ivana Stojkovića, Nataša Todorovićb,∗, Jovana Nikolovb, Branislava Tenjovićb, Slobodan Gadžurićc, Aleksandar Totc, Milan Vranešc a
Faculty of Technical Sciences, University of Novi Sad, Novi Sad, Serbia Department of Physics, Faculty of Sciences, University of Novi Sad, Novi Sad, Serbia c Department of Chemistry, Biochemistry and Environmental Protection, Faculty of Sciences, University of Novi Sad, Novi Sad, Serbia b
A R T I C LE I N FO
A B S T R A C T
Keywords: 210 Pb/210bi detection Cherenkov counting Ionic liquids Quantulus 1220
The measurement of 210Pb activity concentration via its product 210Bi, high-energy beta emitter capable of producing Cherenkov radiation in waters, by liquid scintillation counter Quantulus 1220 has been explored. Without the chemical pre-treatment, the detection limits have been evaluated and possibilities to further decrease MDA value have been considered. Prolonging the counting time coupled to the sample's evaporation prior to counting can enable the achievement of the detection limits below maximal permitted levels recommended by the international legislation. The impact of sodium salicylate addition to the counting vials on the detection efficiency and consequently MDA value has been tested. Sodium salicylate application is associated with the certain complications during sample preparation, here we propose salicylate-based ionic liquids instead. We present an innovative experiment that explores the influence of the addition of a synthesized 2-hydroxypropan1-amminium salicylate to the counting vials in order to decrease MDA parameter. Furthermore, the interference with 226Ra Cherenkov spectra was assessed and the possibility to evaluate its contribution to 210Pb/210Bi spectra has been proposed. The potential of 210Pb screening in the samples without the chemical isolation of 210Pb has not been thoroughly considered in the literature so far. 210Pb screening of a few spiked samples and real samples collected in the vicinity of mine was carried out, thus the obtained results demonstrate the method's evaluation and set its limitations.
1. Introduction
Beta emitter 210Pb has a half-life of 22.23(12) years and decays via two branches, Eβ max = 17.0(5) keV and Eβ max = 63.5(5) keV with transition probabilities of 80.2% and 19.8%, respectively. The lower-energy beta decay is followed by 46.539 keV gamma transition (with probability Pγ = 4.05%) which mainly leads to the emission of conversion electrons (internal conversion coefficient: αT = 17.86) [Antohe et al., 2016]. The progeny of 210Pb is the high-energy beta emitter 210Bi (Bi (T1/2 = 5.012(5) d, ·Eβ max = 1162.2(8) keV ) ), that is able to produce Cherenkov radiation in water samples [Arinc et al., 2011]. Cherenkov radiation represents an optical light emitted when a charged particle travels at a speed greater than the phase velocity of light in the medium [Shu et al., 2018]. It is a well-known fact that if a radionuclide emits beta particle whose energy is higher than 263 keV, this particle can generate Cherenkov radiation when it passes through water [Ross, 1969]. Thus, 210Pb can be detected via its daughter 210Bi by Cherenkov counting after approximately 40 days, which is the necessary period for the establishment of a radioactive equilibrium between 210Pb and 210Bi
The earth's crust contains 222Rn and its progenies, naturally occurring radioisotopes that are part of the 238U decay series. Since it is in the gaseous form, 222Rn emanates through faults and fissures to aquifers, where its long-lived daughters 210Pb, 210Bi and 210Po can be dissolved. Determination of 210Pb in aqueous systems is being carried out for radiological safety estimations as well as in studies of different environmental and marine processes [Grahek et al., 2006]. It requires sophisticated equipment and methods which are rapid, sensitive and precise since its 210Pb natural levels can be very low. The measurement assumes previous chemical separations in the samples that can be carried out by the solid phase extraction via chromatographic resin (Sr resin based on a crown ether, modified for the lead extraction) [Horwitz et al., 1994], or by other techniques: precipitation (e.g. PbSO4), liquidphase extraction (e.g. Polex™ extractive scintillator) [Katzlberger et al., 2001], ion exchange etc.
∗
Corresponding author. Trg Dositeja Obradovića 4, 21000, Novi Sad, Serbia. E-mail address: [email protected] (N. Todorović).
https://doi.org/10.1016/j.radphyschem.2019.108474 Received 8 July 2019; Received in revised form 26 August 2019; Accepted 1 September 2019 Available online 06 September 2019 0969-806X/ © 2019 Elsevier Ltd. All rights reserved.
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[Mirenda et al., 2018]. The third part of the paper is dedicated to the validation of the method and defining its potential and limitations, by considering the results of measurements of three spiked samples with 210Pb standard and two real samples collected in the vicinity of mine.
in a sample. Quantification of 210Pb content in waters is challenging since its low levels in the environment, weak gamma transitions and low beta energies. The detection limits reached by gamma spectrometry in 0.5 L of an aqueous sample are in the order of 100 mBq L−1 [Johansson, 2008]. On the other hand, there are numerous methods for the indirect 210Pb measurements through its daughters, 210Po and 210Bi (assuming radioactive equilibrium between 210Pb and its progeny) that can provide several times to two orders of magnitude lower detection limits. After pre-concentration and separation, those methods come down to either deposition of 210Po on silver foil and determination of its activity via alpha spectrometry, or 210Bi separation from 210Pb and its activity determination via Liquid Scintillation Counting (LSC) or gas-proportional counting [Grahek et al., 2006]. Recent reports show that inductively coupled plasma mass spectrometry (ICP-MS) that follows three distinctive sample treatment methods (co-precipitation, extraction chromatography, derivatization) leads to 210Pb detection limits of approximately 90 mBq L−1. This paper explores the possibility to measure 210Pb activity concentration by Cherenkov counting in LS counter Quantulus 1220 in water samples without their chemical pre-treatment. The obvious advantages of such analysis – if feasible, would be its simplicity and lowcost, beside environmental-friendly storage of the analysed samples. Although Cherenkov counting discriminates alpha and low-energy beta emitters, Cherenkov spectra of other high-energy beta emitters that could be present in the sample would interfere with the spectra generated from 210Pb/210Bi. For this reason, there have been no reported attempts in the literature to evaluate the potential of 210Pb screening in the samples without the chemical isolation of 210Pb, which makes the following study interesting and innovative. The presented research is comprised of several parts. At first, investigation of a method for 210Pb determination through detection of 210Bi Cherenkov radiation in general has been performed, where the main parameters such as the spectral ROI (Region Of Interest), detection efficiency, utilized LSC equipment and minimal detectable activity (MDA ) have been evaluated. Detection limits achieved suggest that this method can be useful only to examine if the analysed sample contains elevated levels of 210Pb or not, which means that its eventual application would be limited only to health studies – to determine whether the sample poses a radiological concern in terms of 210 Pb content or not. Since there are no reports that explore the possibility to screen 210Pb content in waters without their pre-treatment, the results presented in this paper offer a unique approach. This method is applicable to the analysis of naturally occurring radioisotopes in waters, therefore the greatest problem would come from 226Ra interference. Consequently, the focus was on the assessment of overlapping between 226Ra and 210Pb Cherenkov spectra and finding the prospective solution to this problem. Secondly, the potential improvement of the method was explored – decrement of MDA value via increment of the detection efficiency. It has been reported that sodium salicylate addition could significantly increase efficiency for Cherenkov counting [Wang et al., 2018]. On the other hand, the novelty in this work was to consider the impact that salicylates in the form of the ionic liquids (IL) would have when added to the counting vials. The implementation of IL assumes many benefits due to their unique physicochemical properties, chemical and thermal stability, non-flammability, immeasurably low vapor pressure, the wide diversity of chosen constitutive ions and their full dissolution in polar substances such as water, while the properties such as melting temperature, thermal stability, refractive index, acid-base character, hydrophilicity, polarity, density, and viscosity can be tailored to a certain degree [Pinkert et al., 2009]. Recent research confirms that ionic liquids can be used for the detection and quantification of ionizing radiation, since they can act as solvents for Cherenkov measurements since their relatively high refractive index [Mirenda et al., 2014], but also can be utilized as the wavelength shifters of the Cherenkov photons
2. Materials and methods 2.1. Instrumentation and materials for LSC experiments All experiments have been performed on PerkinElmer's liquid scintillation spectrometer Quantulus 1220™, an ultra-low background system with both passive and active shielding, that was used for Cherenkov radiation detection. The background noise during LSC counting consists of high energy cosmic radiation that is dependent on the atmospheric pressure and humidity and other environmental radiation. Beside lead and copper that form passive shielding, Quantulus is equipped with an extra detector that identifies external radiation and eliminates it, further reducing the background level. This detector acts as the active shield, being the additional counter that operates in anticoincidence with signals detected in the counting chamber [Instrument Manual, 2002]. The spectra were acquired and evaluated by WinQ and EASYView software. Instrument's calibration was carried out with a standard radioactive source (aqueous 210Pb solution) produced by the Czech Metrology Institute, Inspectorate for Ionizing Radiation that has a certified activity A (210Pb) = 29.55 Bq mL−1 with combined standard uncertainty 1.0%, reference date October 1, 2013. Radium interference was observed via the addition of a standard radioactive source (aqueous solution of 226 Ra) produced from Czech Metrology Institute, Inspectorate for Ionizing Radiation as well, with a certified activity A (226Ra) = 39.67 Bq mL−1 with combined standard uncertainty 0.5%, reference date October 1, 2013. Cherenkov background rate depends on the type of counting vial and sample volume. Low diffusion polyethylene vials (Super PE vial Cat.No. 6008117) were selected since glass vials contain 4 K that induces higher background. During the initial experiments, the sample's volume was 20 mL, the vial's maximum capacity. However, increase in the analysed volume should proportionally reduce detection limits, therefore, the background sample's volume of 200 mL was evaporated slowly to 20 mL (the heat was adjusted to avoid spattering or boiling) in order to test the technique of pre-concentration of the lead content on MDA behaviour. Sodium salicylate was of 99% grade and purchased from HiMedia Laboratories Pvt. Ltd. For the precise mass measurements, AUW220 Analytical Balance from Shimadzu with readability 0.1 mg was used. 2.2. A method for
210
Pb determination
Establishment of the main parameters assumes preparation of the calibration samples, their counting and determination of the optimal spectral window (ROI) and detection efficiency as well as detection limit evaluation. If the reference activity of the calibration sample was A [Bq], the detection efficiency ε was calculated as:
ε=
Rc − R0 , A −1
(1) −1
where R c [s ] and R 0 [s ] are the count-rates of the calibration sample (reference standard) and the background, respectively. The sample activity concentration As [Bq L−1] should be calculated using the following expression:
As =
Rs − R 0 , Vε
(2) −1
where V [L] is the analysed volume of the water sample and RS [s 2
] is
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measurements are displayed in Fig. 3a, with the obtained efficiency 14.44(21)%. These findings are in good agreement with previous reports for 210Pb/210Bi detection through Cherenkov counting: it is in the range from 10% [Johansson, 2008] to 20% [Al-Masri et al., 1997]. Standards were stored for 50 days in the laboratory (where the temperature is maintained constant at 20 °C) in order to reach 210Pb/214Bi equilibrium and then counted in several cycles of 100 min under the same conditions as the analysed samples. Background samples were prepared with 20 mL of distilled water transferred into the vial, as well as 200 mL of distilled water evaporated to 20 mL of the final background sample. According to the expression (3), detection limit that can be reached is MDA = 0.85 Bq L−1 for 1000 min of counting. Fig. 3b shows MDA behaviour with the measurement duration.
its count-rate. The detection limit was evaluated via Currie relation as the Minimal Detectable Activity (MDA) parameter [Bq L−1] that is dependent on t0 [s], the background counting time:
MDA =
2.71 + 4.65 R 0 t0 . V ε t0
(3)
The measurement uncertainties for efficiency, activity concentration and MDA were calculated according to standard deviations of all relevant measured parameters in equations (1)–(3). 2.3. Ionic liquid synthesis An equimolar amount of salicylic acid, dissolved in methanol, was added dropwise to a 1-amino-2-propanol water solution and cooled in an ice-bath. After addition of the salicylic acid, the reaction mixture was stirred at room temperature for 2 h. The obtained ionic liquid was dried under vacuum for the next 3 h to remove any traces of methanol and water. The obtained liquid product was stored in vacuum desiccator under P2O5 for the next 48 h. When the final product was dried, determination of the water content in the IL was determined by the Karl Fisher titration via Metrohm 831 Karl Fischer coulometer. The water content was found to be less than 150 ppm. Obtained ionic liquid (structure presented in Fig. 1) was analysed by NMR spectroscopy to confirm its structure and purity, and the assignations are presented in supplementary materials. NMR spectra were recorded in D2O at 25 °C on a Bruker Advance III 400 MHz spectrometer. Tetramethylsilane was used as an accepted internal standard for calibrating chemical shift 1H and 13C. The purity of synthesized ionic liquid is more than 95%.
3.2. Detection efficiency improvement It is known that a considerable fraction of Cherenkov photons (generated in water by electrons in a wide range of energies) lies in the ultraviolet region so that photomultiplier tubes that operate inside LS counter cannot detect them [Wang et al., 2018]. Sodium salicylate can act as the wavelength shifter, absorbing ultraviolet photons and reemitting them at longer wavelengths, so that photomultiplier tubes would detect more light, which overall would increase the counting efficiency. It has been shown that the addition of sodium salicylate > 1 mg g−1 causes a significant increase for the detection efficiency of 210 Bi due to the combination of wavelength shifting and the production of more scintillation light [Wang et al., 2018]. In Fig. 4 the effect of sodium salicylate addition in calibration samples on 210Pb/210Bi detection efficiency is presented. All samples were prepared with the same activity concentration ( A (210Pb) = 4.97(7) Bq), but contained an increasing mass of sodium salicylate. On the inserted graph in Fig. 4, count-rates of background samples to which an increasing mass of sodium salicylate has been added are shown. It demonstrates that sodium salicylate has no impact on the background level, it remains constant, R 0 = 0.0161(9) s−1. From the values obtained on Fig. 4, it can be concluded that the addition of approximately 1 g of sodium salicylate to 20 mL counting vial sets detection efficiency high enough, which has been selected as the optimal mass for the routine analysis. The addition of sodium salicylate alters the total mass of the sample ~6% or less, therefore it does not affect sample geometry significantly, nor Cherenkov radiation detection. Its contribution to the total sample volume should be accounted for when the activity concentration or MDA are calculated, expressions (2)–(3). Certain complications exist in the procedure of sodium salicylate addition, however: samples should always be neutralized to pH 6–8 by NaOH solution to enhance its solubility in water which is poor on lower pH conditions. During the real sample analysis, when waters of different origin should be prepared for counting, its pH value could vary and should be adjusted individually for each sample. This fact has triggered another research – finding the other agents that would be fully miscible with water, independently on its pH value. Accordingly, ionic liquid (IL) was synthesized in the form of salicylate: 2-hydroxypropan-1-amminium salicylate. The addition of IL to counting vials has a similar effect on 210 Pb/210Bi detection efficiency like sodium salicylate. Few droplets have been added to the calibration vials with the same activity concentration ( A (210Pb) = 5.02(8) Bq). A series of 10 calibration vials has been prepared, containing the increasing mass of IL, and the obtained results are given in Fig. 5. Again, the addition of IL does not influence Cherenkov radiation detection since it increases the total mass of the sample ~7% or less, but its contribution to the total sample volume should be implemented in the expressions (2)–(3). The shape and position of Cherenkov spectra are consistent with the addition of the increasing mass of IL, which is demonstrated in Fig. 6a. Furthermore, the samples that contain only 210Pb standard and 210Pb
3. Results and discussion Here follows the report about the determination of optimal parameters of the method, its establishment and final evaluation. 3.1. System calibration The counting protocol for Cherenkov spectra analysis on Quantulus 1220 was set up manually, with the setup configuration given in the previous research [Todorović et al., 2017]. However, a substantial difference was discovered if the counting was carried out on high or low coincidence bias. The obtained spectra of 210Pb calibration sample and background sample, both counted on low and high coincidence bias are presented in Fig. 2. When the coincidence bias is set to high, 210Pb spectra are generated in the narrow energy region, while better spectral resolution and higher counting sensitivity are gained on coincidence bias set to low. The optimal energy window was selected based on the maximal ε2 FOM [cps−1]= , Figure Of Merit parameter. ROI was established R 0 [s−1] from 40 to 250 channels if the measurement is performed on low coincidence bias (on high coincidence bias ROI was set from 150 to 260 channels). The detection efficiency was determined with an asset of five calibration standards with different 210Pb activity prepared in three probes by adding different amounts of the certified 210Pb activity to distilled water in order to obtain the total volume of 20 mL. Results of
Fig. 1. Chemical structure of 2-hydroxypropan-1-amminium salicylate. 3
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Fig. 2. Cherenkov spectra of distilled water samples spiked with 210Pb standard ( A =25.1(4)Bq), generated on low coincidence bias and high coincidence bias counting protocol.
Fig. 3. Establishment of the method's main parameters: (a) Determination of the detection efficiency in the calibration samples (b) MDA dependence on the counting time.
Fig. 4. The influence of sodium salicylate addition on 210Pb/210Bi detection efficiency. Insert: background counts dependence on the mass of the added sodium salicylate.
Fig. 5. 210Pb/210Bi detection efficiency improvement with the addition of IL in the mass range (0–1.4) g.
4
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Fig. 6. Distilled water samples spiked with 210Pb standard ( A =5.02(8)Bq), with the addition of an increasing mass of IL (a) Cherenkov spectra (b) Alpha spectra generated on alpha/beta counting protocol (c) Beta spectra generated on alpha/beta counting protocol.
was added because IL acts as the wavelength shifter. The conclusion is that IL evince no scintillation effect (otherwise the count-rate of alpha spectrum would be also enhanced with IL addition), but cause spectral wavelength shifting (since IL presence significantly increases the countrate of beta spectra which contains also Cherenkov signals that were shifted). All these spectra considerations from Fig. 6 suggest that no other radiation than Cherenkov emissions from 210Bi have generated the excessive signals in the spectra with IL addition. It is clear that with the addition of IL in negligible amounts (up to 1.4 g) to 20 mL of water, the
standard+1.4 g of IL were counted on default alpha/beta counting protocol, and their gross alpha and beta spectra are displayed for comparison on Fig. 6b and c, respectively. Alpha spectrum does not increase with the addition of IL, as noticeable from the Fig. 6b, that means that IL does not function as the scintillator. Beta spectrum of the vial that contains IL was generated with significantly greater intensity compared to the beta spectrum of the pure 210Pb standard, Fig. 6c. This can be interpreted in the sense that the Cherenkov radiation signals from 210Bi contribute to gross beta spectrum, so the excessive Cherenkov radiation signals were produced to gross beta spectrum when IL
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3.4. Interference with radium spectra
refractive index of a sample cannot be significantly altered, so the possibility that the threshold for Cherenkov radiation detection has been lowered causing the efficiency increment at presented proportion should be discarded at once. This leads to a conclusion that IL behaves as the wavelength shifter as well as sodium salicylate. By comparing the effect of these two agents (Figs. 4 and 5), it can be noticed that a certain mass of sodium salicylate would cause more explicit increase in the counting efficiency than the same mass of IL added to the vial. This was expected since mole fraction of salicylate in sodium salt is significantly higher than the ionic liquid, 86 and 64%, respectively. The usage of IL proposed in this paper offers an interesting alternative to sodium salicylate since it is fully miscible with water independently on its pH value.
The obvious problem of the presented method that has not been resolved so far is the fact that if 210Pb is not separated prior to counting, the Cherenkov spectrum will be generated as the cumulative spectrum of all high-energy beta emitters that are present in the sample and able to produce Cherenkov radiation. Since the analysed samples are intended to be collected in the areas where one would not expect the occurrence of the artificial radionuclides, their interference will not be considered here. This means that the main problem would come from radium and its progenies which are naturally occurring radionuclides. This obstacle cannot be fully overcome without the chemical isolation of 210Pb in the samples. Still, for the investigation, one should examine separately Cherenkov spectra of distilled water samples spiked with 226Ra and 210Pb standard that are shown on the same chart in Fig. 7. The possibility of discriminating these two radionuclides lies in the fact that 226Ra Cherenkov spectrum counted on high coincidence bias is generated with more intensity and in a broader range of channels than 210Pb spectrum. When compared on low coincidence bias, these two spectra do not differ in the way that we could discern one from another. Radium presence can therefore be detected if the sample is counted on high coincidence bias in channels from 300 to 430 if the obtained counts are significantly higher than the counts of blank samples in the same region. Parameters obtained during the calibration of the detector for the whole spectrum generated on high coincidence bias and for the high-energy part where 210Pb/210Bi counts are not expected to be generated is displayed in Table 2. Since the efficiency for the part from 300 to 430 channels is too low, 3.52(6)%, it would be better to add sodium salicylate in order to achieve the adequate efficiency. For this purpose, several calibration samples have been prepared, with the same activity concentration ( A (226Ra) = 7.89(4) Bq), but contained an increasing mass of sodium salicylate in the range (0–1.1)g. The obtained dependence of the efficiency with sodium salicylate mass was: ε [%] = 0.0352(6) + 0.056(9) ∗ m [g ]. In Table 2 the calculation has been given for the addition of 1 g of sodium salicylate and it can be noticed that relatively adequate detection efficiency has been reached. Therefore, we propose the following technique: the sample should be counted both on low and high coincidence bias (after the addition of an agent that improves detection efficiency, if possible). From the count-rate in the spectrum from 300 to 430 channels generated on high coincidence bias, using the parameters given in Table 2, 226Ra activity concentration could be estimated. Calculations for the estimation of 226 Ra activity concentration are analogue to the expressions (1)–(3). Furthermore, from the obtained estimation of 226Ra activity, the one could assess the count-rate contribution of 226Ra to the cumulative Cherenkov spectrum in channels 130–400 counted on low coincidence bias that consists of 226Ra+210Pb signals (using the parameters given in Table 2 for the full energy window). This method allows quick estimation of 210Pb activity even in samples that contain other naturally occurring radioisotopes such as 226Ra and its progenies. However, the method is not a tool for the precise determination of 210Pb content but should be able to point to the high 210Pb activity (if present) in the water even in the presence of radium.
3.3. MDA considerations The international guidance levels established for the naturally occurring 210Pb content in drinking water is set to 200 mBq L−1 according to the European legislation [Council Directive 2013/51/Euratom], while the World Health Organization recommends this parameter to be 100 mBq L−1 [WHO, 2011]. The national legislation in Serbia recommends the maximal acceptable level for 210Pb in drinking water to be 200 mBq L−1 [Official Gazette of the Republic of Serbia 36/2018]. MDA value that has been reached in the presented method so far, 0.85 Bq L−1 is not acceptable but could be further reduced. Three possibilities to decrease MDA parameter have been considered and are presented in Table 1. At first, by prolonging the measurement time, the detectable limit can be slightly reduced, but not under the level required for the radiological assessment. Secondly, if a sample is evaporated, lead-210 content will be pre-concentrated. The experiments have shown that if the sample volume increases for 10 times, i.e. from 200 mL to 20 mL, the background count rate will not differ, which means that MDA could be decreased 10 times as well - to 0.085 Bq L−1 for 1000 min of counting. Thus, larger volume of samples, 1 L or more would cause a significant decrease in the detection limit. Thirdly, if the efficiency detection increases (by the addition of sodium salicylate or ionic liquid, as previously demonstrated), MDA value would decrease. In Table 1, some examples with calculations are given in order to evaluate the effect of these simple techniques on the detection limits of the method. If the combination of all these factors is applied, for example, if the counting time is 2000min, with the original sample volume 200 mL, and with 1 g of sodium salicylate added to the vial prior to counting, the archived limit comes down to MDA = 0.011 Bq L−1. These findings demonstrate the potential of this method for the reliable detection of 210Pb content in the areas where its natural levels are increased. Additionally, methods of analysis used must, as a minimum, be capable of measuring activity concentrations with a limit of detection 0.02 Bq L−1 [Council Directive 2013/51/Euratom], which evidently, can be accomplished. The detection limit of 11 mBq L−1 would not be satisfactory for the studies of the environmental processes however, so the application of this method would be restricted only to a screening technique, such as the monitoring of aquifers in the areas around mines, or other water sources with possibly elevated levels of the natural radioactivity. Table 1 Reduction of MDA parameter by three possible techniques. Method's parameters in general
ε [%] V [L] t0 [min] MDA [Bq L−1]
14.44(21) 0.02 1000 0.85
Longer measure-ment
14.44(21) 0.02 2000 0.60
Volume increment (evaporation, 200 mL → 20 mL)
14.44(21) 0.2 1000 0.085
6
Efficiency increment Addition of 1 g of sodium salicylate
Addition of 1 g of IL
82.5(17) 0.02 1000 0.15
56.0(10) 0.02 1000 0.22
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Fig. 7. Cherenkov spectra of distilled water samples spiked with 210Pb standard ( A =25.1(4)Bq) and 226 Ra standard ( A =15.78(8)Bq), generated both on low and high coincidence bias counting protocols.
Besides the interference with the other radioisotopes, the method's precision would also be dependent on the extent to which radioactive equilibrium has been established in samples. Determination of 210Pb level should be performed only after ~7 half-lives of 210Bi, in principle, in order to reach radioactive equilibrium, i.e. after 35 days. It is interesting to note, however, that in the case of 90Sr (T1/2 = 28.8 y ) and its progeny 90Y (T1/2 ≈ 2.67 d ), more than 80% of the equilibrium is achieved in 7 days [Stamoulis et al., 2007], although theoretically, 90 Sr/90Y equilibrium should be achieved after 18 days. This suggests that 210Pb measurement could be performed within 2–3 weeks after the sampling with reliable results.
Table 3 Results of the Aref [Bq L
0.199 (3) 0.298 (4) 0.993(15)
The relevance of the method was tested on three spiked samples with 210Pb standard, results of these measurements are given in Table 3. For the two of them the activities were set below the detection limit so the measurements could not be carried out without the evaporation of the sample or additions of the agents that would increase detection efficiency. The obtained activity of the third sample was much above the reference value, but with relatively high measurement uncertainty as well. After the addition of sodium salicylate, it became possible to detect lower activities of the first two samples, however, with poor accuracy and high measurement uncertainty, but with the acceptable zscores for all tested samples. It is interesting to note that the obtained activity concentration of the third sample, that was possible to be measured before sodium salicylate addition as well, was acquired with better precision and accuracy after its addition. On the contrary, better precision has led to higher z-score after the addition of sodium salicylate. It is evident that even higher activities would be measured with better accuracy. All these results demonstrate that the method is relevant only for the screening of the environmental samples to determine if the 210Pb levels are increased or not. Surface waters in the vicinity of mines can contain elevated
ε [%] MDA [Bq L−1] for 1000 min
226
]
210
Pb measurements of spiked samples.
Ameas [Bq L−1]
< MDA < MDA 1.7 (5)
z-score
– – 1.4
Measurement after the addition of sodium salicylate The added mass [g]
Ameas [Bq L−1]
z-score
1.0196 0.965 0.9981
0.30 (7) 0.42 (7) 1.21 (8)
1.4 1.7 2.7
concentrations of the naturally occurring radioisotopes, since mining of natural ores and minerals could bring forth natural radionuclides to the surface of the earth. Two real samples of the surface waters were collected in the vicinity of the closed uranium mine Gabrovnica located in the Eastern Serbia in the area surrounding the village Kalna on Stara Planina Mountain, which is defined as a location characterized by the increased content of natural radioactive elements [Nikolov et al., 2014]. Those samples were analysed by gamma-spectrometry (detailed description of the equipment and working parameters is given in [Todorović et al., 2012]) with the results given in Table 4. Results of 210 Pb determination by LSC method for the two mine samples are also given in Table 4. The samples have been prepared in three probes (with and without sodium salicylate addition) and counted in three cycles of 600 min long measurements. The generated Cherenkov spectra of the mine samples on Quantulus 1220, both on high and low coincidence bias, are presented in supplementary materials, where one can see the excess signals above the background level observable on both counting protocols. Lead content was possible to be obtained from the measurement (via counting protocol with low coincidence bias) even without the addition of agents that would improve the detection efficiency. 210Pb activity concentration was obtained based on the count-
3.5. Method trueness and comparison with gamma-spectrometry
Table 2 Parameters obtained during the calibration of the detector for
−1
Ra detection via Cherenkov radiation (high c.b.).
130–400 ch Full energy window
300–430 ch High-energy part of spectra
300–430 ch High-energy part of spectra, Addition of 1 g of sodium salicylate
16.63(24) 0.49
3.52(6) 1.27
9.1 (9) 0.49
7
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precision, but can serve as the efficient screening tool for the estimation if the water sample contains the increased level of 210Pb content or not.
Table 4 Results of measurements of mine samples. Gamma spectrometry
LSC measurements (without sodium salicylate)
LSC measurements (with sodium salicylate)
Radionuclide
K-40 Cs-137 Tl-208 Pb-210 Bi-211 Bi-212 Pb-212 Bi-214 Pb-214 Ra-224 Ra-226 Ac-228 Th-232 Th-234 (U-238) U-235 210 Pb content 226 Ra content 210 Pb content (226Ra contribution subtracted) 210 Pb content 226 Ra content 210 Pb content (226Ra contribution subtracted)
Sample 1 A [Bq L−1]
Sample 2 A [Bq L−1]
< 1.9 < 0.21 0.46 (5) <5 2.0 (6) 2.3 (6) 1.06 (7) 2.23 (12) 1.46 (16) 1.6 (6) 2.0 (4) 0.54 (11) 0.93 (17) 3.6 (5) 0.55 (8) 6.9 (6) 1.9 (5) 4.7 (4)
< 1.3 < 0.16 0.50 (5) <4 < 0.9 1.4 (6) 1.56 (8) 2.54 (13) 3.25 (11) < 1.5 2.94 (21) 0.44 (10) 1.1 (3) < 1.4 3.4 (7) 7.6 (6) 1.9 (5) 5.4(4)
4.28(15) 0.70(20) 4.21(13)
6.02(18) 0.70(21) 5.94(16)
Acknowledgements This work has been financially supported by the Provincial Secretariat for Higher Education and Scientific Research under the project “Radioactivity in drinking water and cancer incidence in Vojvodina” No. 142-451-2447/2018 (as a part of the project No. 114451-2538/2014) and by the Ministry of education, science and technological development of the Republic of Serbia under the projects No. OI171002 and III43002. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.radphyschem.2019.108474. References Al-Masri, M.S., Hamwi, A., Mikhlallaty, H., 1997. Radiochemical determination of lead210 in environmental water samples using Cerenkov counting. J. Radioanal. Nucl. Chem. 2l9 (1), 73–75. https://doi.org/10.1007/BF02040268. Antohe, A., Sahagia, M., Luca, A., Ioan, M.-R., Ivan, C., 2016. Measurement of liquid scintillation sources of 210Pb obtained from 222Rn decay. Appl. Radiat. Isot. 109, 286–289. https://doi.org/10.1016/j.apradiso.2015.12.020. Arinc, A., Johansson, L.C., Gilligan Chris, R.D., Pearce, A., 2011. Standardization of 210Pb by Čerenkov counting. Appl. Radiat. Isot. 69 (5), 768–772. https://doi.org/10.1016/ j.apradiso.2011.01.007. Council Directive 2013/51/EURATOM of 22 October 2013 laying down requirements for the protection of the health of the general public with regard to radioactive substances in water intended for human consumption. J. Off. Eur. Union L 296/12. http://data.europa.eu/eli/dir/2013/51/oj. Grahek, Ž., Rožmarić Mačefat, M., Lulić, S., 2006. Isolation of lead from water samples and determination of 210Pb. Anal. Chim. Acta 560 (1–2), 84–93. https://doi.org/10. 1016/j.aca.2005.12.057. Horwitz, E.P., Dietz, M.L., Rhoades, S., Felinto, C., Gale, N.H., Houghton, J., 1994. A leadselective extraction chromatographic resin and its application to the isolation of lead from geological samples. Anal. Chim. Acta 292 (3), 263–273. https://doi.org/10. 1016/0003-2670(94)00068-9. Johansson, L.Y., 2008. Determination of Pb-210 and Po-210 in aqueous environmental samples. Dr. Diss. 1–196. Katzlberger, C., Wallner, G., Irlweck, K., 2001. Determination of Pb-210, Bi-210 and Po210 in drinking water. J. Radioanal. Nucl. Chem. 249 (1), 191–196. https://doi.org/ 10.1023/A:1013230124145. Instrument Manual, 2002. Wallac 1220 Quantulus – Ultra Low Level Liquid Scintillation Spectrometer. PerkinElmer, Finland 1220–931–06. Mirenda, M., Rodrigues, D., Arenillas, P., Gutkowski, K., 2014. Ionic liquids as solvents for liquid scintillation technology. Čerenkov counting with 1-Butyl-3-methylimidazolium chloride. Radiat. Phys. Chem. 98, 98–102. https://doi.org/10.1016/j. radphyschem.2014.01.010. Mirenda, M., Rodrigues, D., Ferreyra, C., Arenillas, P., Sarmiento, G.P., Krimer, N., Japas, M.L., 2018. Ionic liquids as solvents for Čerenkov counting and the effect of a wavelength shifter. Appl. Radiat. Isot. 134, 275–279. https://doi.org/10.1016/j. apradiso.2017.07.061. Nikolov, J., Forkapić, S., Hansman, J., Bikit, I., Vesković, M., Todorović, N., Mrđa, D., Bikit, K., 2014. Natural radioactivity around former uranium mine, Gabrovnica in Eastern Serbia. J. Radioanal. Nucl. Chem. 302 (1), 477–482. https://doi.org/10. 1007/s10967-014-3203-1. Pinkert, A., Marsh, K.N., Pang, S., Staiger, M.P., 2009. Ionic liquids and their interaction with cellulose. Chem. Rev. 109 (12), 6712–6728. https://doi.org/10.1021/ cr9001947. Ross, H.H., 1969. Measurement of beta-emitting nuclides using Čerenkov radiation. Anal. Chem. 41 (10), 1260–1265. https://doi.org/10.1021/ac60279a011. Shu, D., Tang, X., Geng, C., Zhang, X., Gong, C., Shao, W., Liu, Y., 2018. Novel method exploration of monitoring neutron beam using Cherenkov photons in BNCT. Radiat. Phys. Chem. 156, 222–230. https://doi.org/10.1016/j.radphyschem.2018.11.024. Stamoulis, K.C., Ioannides, K.G., Karamanis, D.T., Patiris, D.C., 2007. Rapid screening of 90 Sr activity in water and milk samples using Čerenkov radiation. J. Environ. Radioact. 93 (3), 144–156. https://doi.org/10.1016/j.jenvrad.2006.12.007. Todorović, N., Nikolov, J., Forkapić, S., Bikit, I., Mrdja, D., Krmar, M., Vesković, M., 2012. Public exposure to radon in drinking water in Serbia. Appl. Radiat. Isot. 70 (3), 543–549. https://doi.org/10.1016/j.apradiso.2011.11.045. Todorović, N., Stojković, I., Nikolov, J., Tenjović, B., 2017. 90Sr determination in water samples using Čerenkov radiation. J. Environ. Radioact. 169–170, 197–202. https:// doi.org/10.1016/j.jenvrad.2017.01.021. Wang, Y., Yang, Y., Song, L., Ma, Y., Luo, M., Dai, X., 2018. Effects of sodium salicylate on the determination of Lead-210/Bismuth-210 by Cerenkov counting. Appl. Radiat. Isot. 139, 175–180. https://doi.org/10.1016/j.apradiso.2018.05.013. WHO, 2011. Guidelines for Drinking-Water Quality, fourth ed. World Health Organization, WHO Press, Genève, Switzerland978 92 4 154815 1.
rates from the cumulative spectrum generated on low coincidence bias at first. Determination of 226Ra activity concentration was evaluated from the count-rates obtained in high-energy part of spectra in channels from 300 to 430, while the counting protocol was set to high coincidence bias. From 226Ra activity the count rates that contribute in the cumulative spectrum generated on low coincidence bias have been estimated and subtracted, so the 210Pb content has been obtained more precisely, when 226Ra contribution is subtracted. Still, those activities do not prepresent pure 210Pb content and therefore will often be an overestimation of the true 210Pb activity, which is obvious when they are compared to the results obtained via gamma spectrometry. The results presented in Tables 3 and 4 demonstrate that the method presented in this paper is able to screen 210Pb activity and detect its presence if 210Pb activity is beyond its natural levels in the water sample. 4. Conclusions This paper has presented a thorough investigation of the 210Pb determination possibility in the water samples without their chemical pretreatment via Cherenkov radiation produced by its progeny 210Bi by LS counter Quantulus. Instrument's calibration and the obtained parameters of the method have been presented. The method is simple and it does not demand the expensive equipment for the sample pre-treatment procedure. Minimal detectable limit (0.85 Bq L−1) could be lowered (for one order of the magnitude or even more) to the acceptable values below maximal permitted levels by the increment of the initial sample volume that should be evaporated to 20 mL, prolonging the sample measurement time and adding sodium salicylate in order to increase efficiency detection. New ionic liquid, 2-hydroxypropan-1-amminium salicylate, has been synthesized and tested and it showed a similar impact on the efficiency behaviour as sodium salicylate. Its application provides an interesting alternative to sodium salicylate with the advantage of full miscibility with the water sample independently on its pH value. The problem with radium interference could be resolved if its contribution is assessed from the high-energy part of the Cherenkov spectrum produced on high coincidence bias. The analysis of spiked and real samples demonstrated that this method has poor accuracy and low 8
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Pb/210bi detection in waters by cherenkov counting – perspectives and new possibilities 210
T
Ivana Stojkovića, Nataša Todorovićb,∗, Jovana Nikolovb, Branislava Tenjovićb, Slobodan Gadžurićc, Aleksandar Totc, Milan Vranešc a
Faculty of Technical Sciences, University of Novi Sad, Novi Sad, Serbia Department of Physics, Faculty of Sciences, University of Novi Sad, Novi Sad, Serbia c Department of Chemistry, Biochemistry and Environmental Protection, Faculty of Sciences, University of Novi Sad, Novi Sad, Serbia b
A R T I C LE I N FO
A B S T R A C T
Keywords: 210 Pb/210bi detection Cherenkov counting Ionic liquids Quantulus 1220
The measurement of 210Pb activity concentration via its product 210Bi, high-energy beta emitter capable of producing Cherenkov radiation in waters, by liquid scintillation counter Quantulus 1220 has been explored. Without the chemical pre-treatment, the detection limits have been evaluated and possibilities to further decrease MDA value have been considered. Prolonging the counting time coupled to the sample's evaporation prior to counting can enable the achievement of the detection limits below maximal permitted levels recommended by the international legislation. The impact of sodium salicylate addition to the counting vials on the detection efficiency and consequently MDA value has been tested. Sodium salicylate application is associated with the certain complications during sample preparation, here we propose salicylate-based ionic liquids instead. We present an innovative experiment that explores the influence of the addition of a synthesized 2-hydroxypropan1-amminium salicylate to the counting vials in order to decrease MDA parameter. Furthermore, the interference with 226Ra Cherenkov spectra was assessed and the possibility to evaluate its contribution to 210Pb/210Bi spectra has been proposed. The potential of 210Pb screening in the samples without the chemical isolation of 210Pb has not been thoroughly considered in the literature so far. 210Pb screening of a few spiked samples and real samples collected in the vicinity of mine was carried out, thus the obtained results demonstrate the method's evaluation and set its limitations.
1. Introduction
Beta emitter 210Pb has a half-life of 22.23(12) years and decays via two branches, Eβ max = 17.0(5) keV and Eβ max = 63.5(5) keV with transition probabilities of 80.2% and 19.8%, respectively. The lower-energy beta decay is followed by 46.539 keV gamma transition (with probability Pγ = 4.05%) which mainly leads to the emission of conversion electrons (internal conversion coefficient: αT = 17.86) [Antohe et al., 2016]. The progeny of 210Pb is the high-energy beta emitter 210Bi (Bi (T1/2 = 5.012(5) d, ·Eβ max = 1162.2(8) keV ) ), that is able to produce Cherenkov radiation in water samples [Arinc et al., 2011]. Cherenkov radiation represents an optical light emitted when a charged particle travels at a speed greater than the phase velocity of light in the medium [Shu et al., 2018]. It is a well-known fact that if a radionuclide emits beta particle whose energy is higher than 263 keV, this particle can generate Cherenkov radiation when it passes through water [Ross, 1969]. Thus, 210Pb can be detected via its daughter 210Bi by Cherenkov counting after approximately 40 days, which is the necessary period for the establishment of a radioactive equilibrium between 210Pb and 210Bi
The earth's crust contains 222Rn and its progenies, naturally occurring radioisotopes that are part of the 238U decay series. Since it is in the gaseous form, 222Rn emanates through faults and fissures to aquifers, where its long-lived daughters 210Pb, 210Bi and 210Po can be dissolved. Determination of 210Pb in aqueous systems is being carried out for radiological safety estimations as well as in studies of different environmental and marine processes [Grahek et al., 2006]. It requires sophisticated equipment and methods which are rapid, sensitive and precise since its 210Pb natural levels can be very low. The measurement assumes previous chemical separations in the samples that can be carried out by the solid phase extraction via chromatographic resin (Sr resin based on a crown ether, modified for the lead extraction) [Horwitz et al., 1994], or by other techniques: precipitation (e.g. PbSO4), liquidphase extraction (e.g. Polex™ extractive scintillator) [Katzlberger et al., 2001], ion exchange etc.
∗
Corresponding author. Trg Dositeja Obradovića 4, 21000, Novi Sad, Serbia. E-mail address: [email protected] (N. Todorović).
https://doi.org/10.1016/j.radphyschem.2019.108474 Received 8 July 2019; Received in revised form 26 August 2019; Accepted 1 September 2019 Available online 06 September 2019 0969-806X/ © 2019 Elsevier Ltd. All rights reserved.
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[Mirenda et al., 2018]. The third part of the paper is dedicated to the validation of the method and defining its potential and limitations, by considering the results of measurements of three spiked samples with 210Pb standard and two real samples collected in the vicinity of mine.
in a sample. Quantification of 210Pb content in waters is challenging since its low levels in the environment, weak gamma transitions and low beta energies. The detection limits reached by gamma spectrometry in 0.5 L of an aqueous sample are in the order of 100 mBq L−1 [Johansson, 2008]. On the other hand, there are numerous methods for the indirect 210Pb measurements through its daughters, 210Po and 210Bi (assuming radioactive equilibrium between 210Pb and its progeny) that can provide several times to two orders of magnitude lower detection limits. After pre-concentration and separation, those methods come down to either deposition of 210Po on silver foil and determination of its activity via alpha spectrometry, or 210Bi separation from 210Pb and its activity determination via Liquid Scintillation Counting (LSC) or gas-proportional counting [Grahek et al., 2006]. Recent reports show that inductively coupled plasma mass spectrometry (ICP-MS) that follows three distinctive sample treatment methods (co-precipitation, extraction chromatography, derivatization) leads to 210Pb detection limits of approximately 90 mBq L−1. This paper explores the possibility to measure 210Pb activity concentration by Cherenkov counting in LS counter Quantulus 1220 in water samples without their chemical pre-treatment. The obvious advantages of such analysis – if feasible, would be its simplicity and lowcost, beside environmental-friendly storage of the analysed samples. Although Cherenkov counting discriminates alpha and low-energy beta emitters, Cherenkov spectra of other high-energy beta emitters that could be present in the sample would interfere with the spectra generated from 210Pb/210Bi. For this reason, there have been no reported attempts in the literature to evaluate the potential of 210Pb screening in the samples without the chemical isolation of 210Pb, which makes the following study interesting and innovative. The presented research is comprised of several parts. At first, investigation of a method for 210Pb determination through detection of 210Bi Cherenkov radiation in general has been performed, where the main parameters such as the spectral ROI (Region Of Interest), detection efficiency, utilized LSC equipment and minimal detectable activity (MDA ) have been evaluated. Detection limits achieved suggest that this method can be useful only to examine if the analysed sample contains elevated levels of 210Pb or not, which means that its eventual application would be limited only to health studies – to determine whether the sample poses a radiological concern in terms of 210 Pb content or not. Since there are no reports that explore the possibility to screen 210Pb content in waters without their pre-treatment, the results presented in this paper offer a unique approach. This method is applicable to the analysis of naturally occurring radioisotopes in waters, therefore the greatest problem would come from 226Ra interference. Consequently, the focus was on the assessment of overlapping between 226Ra and 210Pb Cherenkov spectra and finding the prospective solution to this problem. Secondly, the potential improvement of the method was explored – decrement of MDA value via increment of the detection efficiency. It has been reported that sodium salicylate addition could significantly increase efficiency for Cherenkov counting [Wang et al., 2018]. On the other hand, the novelty in this work was to consider the impact that salicylates in the form of the ionic liquids (IL) would have when added to the counting vials. The implementation of IL assumes many benefits due to their unique physicochemical properties, chemical and thermal stability, non-flammability, immeasurably low vapor pressure, the wide diversity of chosen constitutive ions and their full dissolution in polar substances such as water, while the properties such as melting temperature, thermal stability, refractive index, acid-base character, hydrophilicity, polarity, density, and viscosity can be tailored to a certain degree [Pinkert et al., 2009]. Recent research confirms that ionic liquids can be used for the detection and quantification of ionizing radiation, since they can act as solvents for Cherenkov measurements since their relatively high refractive index [Mirenda et al., 2014], but also can be utilized as the wavelength shifters of the Cherenkov photons
2. Materials and methods 2.1. Instrumentation and materials for LSC experiments All experiments have been performed on PerkinElmer's liquid scintillation spectrometer Quantulus 1220™, an ultra-low background system with both passive and active shielding, that was used for Cherenkov radiation detection. The background noise during LSC counting consists of high energy cosmic radiation that is dependent on the atmospheric pressure and humidity and other environmental radiation. Beside lead and copper that form passive shielding, Quantulus is equipped with an extra detector that identifies external radiation and eliminates it, further reducing the background level. This detector acts as the active shield, being the additional counter that operates in anticoincidence with signals detected in the counting chamber [Instrument Manual, 2002]. The spectra were acquired and evaluated by WinQ and EASYView software. Instrument's calibration was carried out with a standard radioactive source (aqueous 210Pb solution) produced by the Czech Metrology Institute, Inspectorate for Ionizing Radiation that has a certified activity A (210Pb) = 29.55 Bq mL−1 with combined standard uncertainty 1.0%, reference date October 1, 2013. Radium interference was observed via the addition of a standard radioactive source (aqueous solution of 226 Ra) produced from Czech Metrology Institute, Inspectorate for Ionizing Radiation as well, with a certified activity A (226Ra) = 39.67 Bq mL−1 with combined standard uncertainty 0.5%, reference date October 1, 2013. Cherenkov background rate depends on the type of counting vial and sample volume. Low diffusion polyethylene vials (Super PE vial Cat.No. 6008117) were selected since glass vials contain 4 K that induces higher background. During the initial experiments, the sample's volume was 20 mL, the vial's maximum capacity. However, increase in the analysed volume should proportionally reduce detection limits, therefore, the background sample's volume of 200 mL was evaporated slowly to 20 mL (the heat was adjusted to avoid spattering or boiling) in order to test the technique of pre-concentration of the lead content on MDA behaviour. Sodium salicylate was of 99% grade and purchased from HiMedia Laboratories Pvt. Ltd. For the precise mass measurements, AUW220 Analytical Balance from Shimadzu with readability 0.1 mg was used. 2.2. A method for
210
Pb determination
Establishment of the main parameters assumes preparation of the calibration samples, their counting and determination of the optimal spectral window (ROI) and detection efficiency as well as detection limit evaluation. If the reference activity of the calibration sample was A [Bq], the detection efficiency ε was calculated as:
ε=
Rc − R0 , A −1
(1) −1
where R c [s ] and R 0 [s ] are the count-rates of the calibration sample (reference standard) and the background, respectively. The sample activity concentration As [Bq L−1] should be calculated using the following expression:
As =
Rs − R 0 , Vε
(2) −1
where V [L] is the analysed volume of the water sample and RS [s 2
] is
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measurements are displayed in Fig. 3a, with the obtained efficiency 14.44(21)%. These findings are in good agreement with previous reports for 210Pb/210Bi detection through Cherenkov counting: it is in the range from 10% [Johansson, 2008] to 20% [Al-Masri et al., 1997]. Standards were stored for 50 days in the laboratory (where the temperature is maintained constant at 20 °C) in order to reach 210Pb/214Bi equilibrium and then counted in several cycles of 100 min under the same conditions as the analysed samples. Background samples were prepared with 20 mL of distilled water transferred into the vial, as well as 200 mL of distilled water evaporated to 20 mL of the final background sample. According to the expression (3), detection limit that can be reached is MDA = 0.85 Bq L−1 for 1000 min of counting. Fig. 3b shows MDA behaviour with the measurement duration.
its count-rate. The detection limit was evaluated via Currie relation as the Minimal Detectable Activity (MDA) parameter [Bq L−1] that is dependent on t0 [s], the background counting time:
MDA =
2.71 + 4.65 R 0 t0 . V ε t0
(3)
The measurement uncertainties for efficiency, activity concentration and MDA were calculated according to standard deviations of all relevant measured parameters in equations (1)–(3). 2.3. Ionic liquid synthesis An equimolar amount of salicylic acid, dissolved in methanol, was added dropwise to a 1-amino-2-propanol water solution and cooled in an ice-bath. After addition of the salicylic acid, the reaction mixture was stirred at room temperature for 2 h. The obtained ionic liquid was dried under vacuum for the next 3 h to remove any traces of methanol and water. The obtained liquid product was stored in vacuum desiccator under P2O5 for the next 48 h. When the final product was dried, determination of the water content in the IL was determined by the Karl Fisher titration via Metrohm 831 Karl Fischer coulometer. The water content was found to be less than 150 ppm. Obtained ionic liquid (structure presented in Fig. 1) was analysed by NMR spectroscopy to confirm its structure and purity, and the assignations are presented in supplementary materials. NMR spectra were recorded in D2O at 25 °C on a Bruker Advance III 400 MHz spectrometer. Tetramethylsilane was used as an accepted internal standard for calibrating chemical shift 1H and 13C. The purity of synthesized ionic liquid is more than 95%.
3.2. Detection efficiency improvement It is known that a considerable fraction of Cherenkov photons (generated in water by electrons in a wide range of energies) lies in the ultraviolet region so that photomultiplier tubes that operate inside LS counter cannot detect them [Wang et al., 2018]. Sodium salicylate can act as the wavelength shifter, absorbing ultraviolet photons and reemitting them at longer wavelengths, so that photomultiplier tubes would detect more light, which overall would increase the counting efficiency. It has been shown that the addition of sodium salicylate > 1 mg g−1 causes a significant increase for the detection efficiency of 210 Bi due to the combination of wavelength shifting and the production of more scintillation light [Wang et al., 2018]. In Fig. 4 the effect of sodium salicylate addition in calibration samples on 210Pb/210Bi detection efficiency is presented. All samples were prepared with the same activity concentration ( A (210Pb) = 4.97(7) Bq), but contained an increasing mass of sodium salicylate. On the inserted graph in Fig. 4, count-rates of background samples to which an increasing mass of sodium salicylate has been added are shown. It demonstrates that sodium salicylate has no impact on the background level, it remains constant, R 0 = 0.0161(9) s−1. From the values obtained on Fig. 4, it can be concluded that the addition of approximately 1 g of sodium salicylate to 20 mL counting vial sets detection efficiency high enough, which has been selected as the optimal mass for the routine analysis. The addition of sodium salicylate alters the total mass of the sample ~6% or less, therefore it does not affect sample geometry significantly, nor Cherenkov radiation detection. Its contribution to the total sample volume should be accounted for when the activity concentration or MDA are calculated, expressions (2)–(3). Certain complications exist in the procedure of sodium salicylate addition, however: samples should always be neutralized to pH 6–8 by NaOH solution to enhance its solubility in water which is poor on lower pH conditions. During the real sample analysis, when waters of different origin should be prepared for counting, its pH value could vary and should be adjusted individually for each sample. This fact has triggered another research – finding the other agents that would be fully miscible with water, independently on its pH value. Accordingly, ionic liquid (IL) was synthesized in the form of salicylate: 2-hydroxypropan-1-amminium salicylate. The addition of IL to counting vials has a similar effect on 210 Pb/210Bi detection efficiency like sodium salicylate. Few droplets have been added to the calibration vials with the same activity concentration ( A (210Pb) = 5.02(8) Bq). A series of 10 calibration vials has been prepared, containing the increasing mass of IL, and the obtained results are given in Fig. 5. Again, the addition of IL does not influence Cherenkov radiation detection since it increases the total mass of the sample ~7% or less, but its contribution to the total sample volume should be implemented in the expressions (2)–(3). The shape and position of Cherenkov spectra are consistent with the addition of the increasing mass of IL, which is demonstrated in Fig. 6a. Furthermore, the samples that contain only 210Pb standard and 210Pb
3. Results and discussion Here follows the report about the determination of optimal parameters of the method, its establishment and final evaluation. 3.1. System calibration The counting protocol for Cherenkov spectra analysis on Quantulus 1220 was set up manually, with the setup configuration given in the previous research [Todorović et al., 2017]. However, a substantial difference was discovered if the counting was carried out on high or low coincidence bias. The obtained spectra of 210Pb calibration sample and background sample, both counted on low and high coincidence bias are presented in Fig. 2. When the coincidence bias is set to high, 210Pb spectra are generated in the narrow energy region, while better spectral resolution and higher counting sensitivity are gained on coincidence bias set to low. The optimal energy window was selected based on the maximal ε2 FOM [cps−1]= , Figure Of Merit parameter. ROI was established R 0 [s−1] from 40 to 250 channels if the measurement is performed on low coincidence bias (on high coincidence bias ROI was set from 150 to 260 channels). The detection efficiency was determined with an asset of five calibration standards with different 210Pb activity prepared in three probes by adding different amounts of the certified 210Pb activity to distilled water in order to obtain the total volume of 20 mL. Results of
Fig. 1. Chemical structure of 2-hydroxypropan-1-amminium salicylate. 3
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Fig. 2. Cherenkov spectra of distilled water samples spiked with 210Pb standard ( A =25.1(4)Bq), generated on low coincidence bias and high coincidence bias counting protocol.
Fig. 3. Establishment of the method's main parameters: (a) Determination of the detection efficiency in the calibration samples (b) MDA dependence on the counting time.
Fig. 4. The influence of sodium salicylate addition on 210Pb/210Bi detection efficiency. Insert: background counts dependence on the mass of the added sodium salicylate.
Fig. 5. 210Pb/210Bi detection efficiency improvement with the addition of IL in the mass range (0–1.4) g.
4
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Fig. 6. Distilled water samples spiked with 210Pb standard ( A =5.02(8)Bq), with the addition of an increasing mass of IL (a) Cherenkov spectra (b) Alpha spectra generated on alpha/beta counting protocol (c) Beta spectra generated on alpha/beta counting protocol.
was added because IL acts as the wavelength shifter. The conclusion is that IL evince no scintillation effect (otherwise the count-rate of alpha spectrum would be also enhanced with IL addition), but cause spectral wavelength shifting (since IL presence significantly increases the countrate of beta spectra which contains also Cherenkov signals that were shifted). All these spectra considerations from Fig. 6 suggest that no other radiation than Cherenkov emissions from 210Bi have generated the excessive signals in the spectra with IL addition. It is clear that with the addition of IL in negligible amounts (up to 1.4 g) to 20 mL of water, the
standard+1.4 g of IL were counted on default alpha/beta counting protocol, and their gross alpha and beta spectra are displayed for comparison on Fig. 6b and c, respectively. Alpha spectrum does not increase with the addition of IL, as noticeable from the Fig. 6b, that means that IL does not function as the scintillator. Beta spectrum of the vial that contains IL was generated with significantly greater intensity compared to the beta spectrum of the pure 210Pb standard, Fig. 6c. This can be interpreted in the sense that the Cherenkov radiation signals from 210Bi contribute to gross beta spectrum, so the excessive Cherenkov radiation signals were produced to gross beta spectrum when IL
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3.4. Interference with radium spectra
refractive index of a sample cannot be significantly altered, so the possibility that the threshold for Cherenkov radiation detection has been lowered causing the efficiency increment at presented proportion should be discarded at once. This leads to a conclusion that IL behaves as the wavelength shifter as well as sodium salicylate. By comparing the effect of these two agents (Figs. 4 and 5), it can be noticed that a certain mass of sodium salicylate would cause more explicit increase in the counting efficiency than the same mass of IL added to the vial. This was expected since mole fraction of salicylate in sodium salt is significantly higher than the ionic liquid, 86 and 64%, respectively. The usage of IL proposed in this paper offers an interesting alternative to sodium salicylate since it is fully miscible with water independently on its pH value.
The obvious problem of the presented method that has not been resolved so far is the fact that if 210Pb is not separated prior to counting, the Cherenkov spectrum will be generated as the cumulative spectrum of all high-energy beta emitters that are present in the sample and able to produce Cherenkov radiation. Since the analysed samples are intended to be collected in the areas where one would not expect the occurrence of the artificial radionuclides, their interference will not be considered here. This means that the main problem would come from radium and its progenies which are naturally occurring radionuclides. This obstacle cannot be fully overcome without the chemical isolation of 210Pb in the samples. Still, for the investigation, one should examine separately Cherenkov spectra of distilled water samples spiked with 226Ra and 210Pb standard that are shown on the same chart in Fig. 7. The possibility of discriminating these two radionuclides lies in the fact that 226Ra Cherenkov spectrum counted on high coincidence bias is generated with more intensity and in a broader range of channels than 210Pb spectrum. When compared on low coincidence bias, these two spectra do not differ in the way that we could discern one from another. Radium presence can therefore be detected if the sample is counted on high coincidence bias in channels from 300 to 430 if the obtained counts are significantly higher than the counts of blank samples in the same region. Parameters obtained during the calibration of the detector for the whole spectrum generated on high coincidence bias and for the high-energy part where 210Pb/210Bi counts are not expected to be generated is displayed in Table 2. Since the efficiency for the part from 300 to 430 channels is too low, 3.52(6)%, it would be better to add sodium salicylate in order to achieve the adequate efficiency. For this purpose, several calibration samples have been prepared, with the same activity concentration ( A (226Ra) = 7.89(4) Bq), but contained an increasing mass of sodium salicylate in the range (0–1.1)g. The obtained dependence of the efficiency with sodium salicylate mass was: ε [%] = 0.0352(6) + 0.056(9) ∗ m [g ]. In Table 2 the calculation has been given for the addition of 1 g of sodium salicylate and it can be noticed that relatively adequate detection efficiency has been reached. Therefore, we propose the following technique: the sample should be counted both on low and high coincidence bias (after the addition of an agent that improves detection efficiency, if possible). From the count-rate in the spectrum from 300 to 430 channels generated on high coincidence bias, using the parameters given in Table 2, 226Ra activity concentration could be estimated. Calculations for the estimation of 226 Ra activity concentration are analogue to the expressions (1)–(3). Furthermore, from the obtained estimation of 226Ra activity, the one could assess the count-rate contribution of 226Ra to the cumulative Cherenkov spectrum in channels 130–400 counted on low coincidence bias that consists of 226Ra+210Pb signals (using the parameters given in Table 2 for the full energy window). This method allows quick estimation of 210Pb activity even in samples that contain other naturally occurring radioisotopes such as 226Ra and its progenies. However, the method is not a tool for the precise determination of 210Pb content but should be able to point to the high 210Pb activity (if present) in the water even in the presence of radium.
3.3. MDA considerations The international guidance levels established for the naturally occurring 210Pb content in drinking water is set to 200 mBq L−1 according to the European legislation [Council Directive 2013/51/Euratom], while the World Health Organization recommends this parameter to be 100 mBq L−1 [WHO, 2011]. The national legislation in Serbia recommends the maximal acceptable level for 210Pb in drinking water to be 200 mBq L−1 [Official Gazette of the Republic of Serbia 36/2018]. MDA value that has been reached in the presented method so far, 0.85 Bq L−1 is not acceptable but could be further reduced. Three possibilities to decrease MDA parameter have been considered and are presented in Table 1. At first, by prolonging the measurement time, the detectable limit can be slightly reduced, but not under the level required for the radiological assessment. Secondly, if a sample is evaporated, lead-210 content will be pre-concentrated. The experiments have shown that if the sample volume increases for 10 times, i.e. from 200 mL to 20 mL, the background count rate will not differ, which means that MDA could be decreased 10 times as well - to 0.085 Bq L−1 for 1000 min of counting. Thus, larger volume of samples, 1 L or more would cause a significant decrease in the detection limit. Thirdly, if the efficiency detection increases (by the addition of sodium salicylate or ionic liquid, as previously demonstrated), MDA value would decrease. In Table 1, some examples with calculations are given in order to evaluate the effect of these simple techniques on the detection limits of the method. If the combination of all these factors is applied, for example, if the counting time is 2000min, with the original sample volume 200 mL, and with 1 g of sodium salicylate added to the vial prior to counting, the archived limit comes down to MDA = 0.011 Bq L−1. These findings demonstrate the potential of this method for the reliable detection of 210Pb content in the areas where its natural levels are increased. Additionally, methods of analysis used must, as a minimum, be capable of measuring activity concentrations with a limit of detection 0.02 Bq L−1 [Council Directive 2013/51/Euratom], which evidently, can be accomplished. The detection limit of 11 mBq L−1 would not be satisfactory for the studies of the environmental processes however, so the application of this method would be restricted only to a screening technique, such as the monitoring of aquifers in the areas around mines, or other water sources with possibly elevated levels of the natural radioactivity. Table 1 Reduction of MDA parameter by three possible techniques. Method's parameters in general
ε [%] V [L] t0 [min] MDA [Bq L−1]
14.44(21) 0.02 1000 0.85
Longer measure-ment
14.44(21) 0.02 2000 0.60
Volume increment (evaporation, 200 mL → 20 mL)
14.44(21) 0.2 1000 0.085
6
Efficiency increment Addition of 1 g of sodium salicylate
Addition of 1 g of IL
82.5(17) 0.02 1000 0.15
56.0(10) 0.02 1000 0.22
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Fig. 7. Cherenkov spectra of distilled water samples spiked with 210Pb standard ( A =25.1(4)Bq) and 226 Ra standard ( A =15.78(8)Bq), generated both on low and high coincidence bias counting protocols.
Besides the interference with the other radioisotopes, the method's precision would also be dependent on the extent to which radioactive equilibrium has been established in samples. Determination of 210Pb level should be performed only after ~7 half-lives of 210Bi, in principle, in order to reach radioactive equilibrium, i.e. after 35 days. It is interesting to note, however, that in the case of 90Sr (T1/2 = 28.8 y ) and its progeny 90Y (T1/2 ≈ 2.67 d ), more than 80% of the equilibrium is achieved in 7 days [Stamoulis et al., 2007], although theoretically, 90 Sr/90Y equilibrium should be achieved after 18 days. This suggests that 210Pb measurement could be performed within 2–3 weeks after the sampling with reliable results.
Table 3 Results of the Aref [Bq L
0.199 (3) 0.298 (4) 0.993(15)
The relevance of the method was tested on three spiked samples with 210Pb standard, results of these measurements are given in Table 3. For the two of them the activities were set below the detection limit so the measurements could not be carried out without the evaporation of the sample or additions of the agents that would increase detection efficiency. The obtained activity of the third sample was much above the reference value, but with relatively high measurement uncertainty as well. After the addition of sodium salicylate, it became possible to detect lower activities of the first two samples, however, with poor accuracy and high measurement uncertainty, but with the acceptable zscores for all tested samples. It is interesting to note that the obtained activity concentration of the third sample, that was possible to be measured before sodium salicylate addition as well, was acquired with better precision and accuracy after its addition. On the contrary, better precision has led to higher z-score after the addition of sodium salicylate. It is evident that even higher activities would be measured with better accuracy. All these results demonstrate that the method is relevant only for the screening of the environmental samples to determine if the 210Pb levels are increased or not. Surface waters in the vicinity of mines can contain elevated
ε [%] MDA [Bq L−1] for 1000 min
226
]
210
Pb measurements of spiked samples.
Ameas [Bq L−1]
< MDA < MDA 1.7 (5)
z-score
– – 1.4
Measurement after the addition of sodium salicylate The added mass [g]
Ameas [Bq L−1]
z-score
1.0196 0.965 0.9981
0.30 (7) 0.42 (7) 1.21 (8)
1.4 1.7 2.7
concentrations of the naturally occurring radioisotopes, since mining of natural ores and minerals could bring forth natural radionuclides to the surface of the earth. Two real samples of the surface waters were collected in the vicinity of the closed uranium mine Gabrovnica located in the Eastern Serbia in the area surrounding the village Kalna on Stara Planina Mountain, which is defined as a location characterized by the increased content of natural radioactive elements [Nikolov et al., 2014]. Those samples were analysed by gamma-spectrometry (detailed description of the equipment and working parameters is given in [Todorović et al., 2012]) with the results given in Table 4. Results of 210 Pb determination by LSC method for the two mine samples are also given in Table 4. The samples have been prepared in three probes (with and without sodium salicylate addition) and counted in three cycles of 600 min long measurements. The generated Cherenkov spectra of the mine samples on Quantulus 1220, both on high and low coincidence bias, are presented in supplementary materials, where one can see the excess signals above the background level observable on both counting protocols. Lead content was possible to be obtained from the measurement (via counting protocol with low coincidence bias) even without the addition of agents that would improve the detection efficiency. 210Pb activity concentration was obtained based on the count-
3.5. Method trueness and comparison with gamma-spectrometry
Table 2 Parameters obtained during the calibration of the detector for
−1
Ra detection via Cherenkov radiation (high c.b.).
130–400 ch Full energy window
300–430 ch High-energy part of spectra
300–430 ch High-energy part of spectra, Addition of 1 g of sodium salicylate
16.63(24) 0.49
3.52(6) 1.27
9.1 (9) 0.49
7
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precision, but can serve as the efficient screening tool for the estimation if the water sample contains the increased level of 210Pb content or not.
Table 4 Results of measurements of mine samples. Gamma spectrometry
LSC measurements (without sodium salicylate)
LSC measurements (with sodium salicylate)
Radionuclide
K-40 Cs-137 Tl-208 Pb-210 Bi-211 Bi-212 Pb-212 Bi-214 Pb-214 Ra-224 Ra-226 Ac-228 Th-232 Th-234 (U-238) U-235 210 Pb content 226 Ra content 210 Pb content (226Ra contribution subtracted) 210 Pb content 226 Ra content 210 Pb content (226Ra contribution subtracted)
Sample 1 A [Bq L−1]
Sample 2 A [Bq L−1]
< 1.9 < 0.21 0.46 (5) <5 2.0 (6) 2.3 (6) 1.06 (7) 2.23 (12) 1.46 (16) 1.6 (6) 2.0 (4) 0.54 (11) 0.93 (17) 3.6 (5) 0.55 (8) 6.9 (6) 1.9 (5) 4.7 (4)
< 1.3 < 0.16 0.50 (5) <4 < 0.9 1.4 (6) 1.56 (8) 2.54 (13) 3.25 (11) < 1.5 2.94 (21) 0.44 (10) 1.1 (3) < 1.4 3.4 (7) 7.6 (6) 1.9 (5) 5.4(4)
4.28(15) 0.70(20) 4.21(13)
6.02(18) 0.70(21) 5.94(16)
Acknowledgements This work has been financially supported by the Provincial Secretariat for Higher Education and Scientific Research under the project “Radioactivity in drinking water and cancer incidence in Vojvodina” No. 142-451-2447/2018 (as a part of the project No. 114451-2538/2014) and by the Ministry of education, science and technological development of the Republic of Serbia under the projects No. OI171002 and III43002. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.radphyschem.2019.108474. References Al-Masri, M.S., Hamwi, A., Mikhlallaty, H., 1997. Radiochemical determination of lead210 in environmental water samples using Cerenkov counting. J. Radioanal. Nucl. Chem. 2l9 (1), 73–75. https://doi.org/10.1007/BF02040268. Antohe, A., Sahagia, M., Luca, A., Ioan, M.-R., Ivan, C., 2016. Measurement of liquid scintillation sources of 210Pb obtained from 222Rn decay. Appl. Radiat. Isot. 109, 286–289. https://doi.org/10.1016/j.apradiso.2015.12.020. Arinc, A., Johansson, L.C., Gilligan Chris, R.D., Pearce, A., 2011. Standardization of 210Pb by Čerenkov counting. Appl. Radiat. Isot. 69 (5), 768–772. https://doi.org/10.1016/ j.apradiso.2011.01.007. Council Directive 2013/51/EURATOM of 22 October 2013 laying down requirements for the protection of the health of the general public with regard to radioactive substances in water intended for human consumption. J. Off. Eur. Union L 296/12. http://data.europa.eu/eli/dir/2013/51/oj. Grahek, Ž., Rožmarić Mačefat, M., Lulić, S., 2006. Isolation of lead from water samples and determination of 210Pb. Anal. Chim. Acta 560 (1–2), 84–93. https://doi.org/10. 1016/j.aca.2005.12.057. Horwitz, E.P., Dietz, M.L., Rhoades, S., Felinto, C., Gale, N.H., Houghton, J., 1994. A leadselective extraction chromatographic resin and its application to the isolation of lead from geological samples. Anal. Chim. Acta 292 (3), 263–273. https://doi.org/10. 1016/0003-2670(94)00068-9. Johansson, L.Y., 2008. Determination of Pb-210 and Po-210 in aqueous environmental samples. Dr. Diss. 1–196. Katzlberger, C., Wallner, G., Irlweck, K., 2001. Determination of Pb-210, Bi-210 and Po210 in drinking water. J. Radioanal. Nucl. Chem. 249 (1), 191–196. https://doi.org/ 10.1023/A:1013230124145. Instrument Manual, 2002. Wallac 1220 Quantulus – Ultra Low Level Liquid Scintillation Spectrometer. PerkinElmer, Finland 1220–931–06. Mirenda, M., Rodrigues, D., Arenillas, P., Gutkowski, K., 2014. Ionic liquids as solvents for liquid scintillation technology. Čerenkov counting with 1-Butyl-3-methylimidazolium chloride. Radiat. Phys. Chem. 98, 98–102. https://doi.org/10.1016/j. radphyschem.2014.01.010. Mirenda, M., Rodrigues, D., Ferreyra, C., Arenillas, P., Sarmiento, G.P., Krimer, N., Japas, M.L., 2018. Ionic liquids as solvents for Čerenkov counting and the effect of a wavelength shifter. Appl. Radiat. Isot. 134, 275–279. https://doi.org/10.1016/j. apradiso.2017.07.061. Nikolov, J., Forkapić, S., Hansman, J., Bikit, I., Vesković, M., Todorović, N., Mrđa, D., Bikit, K., 2014. Natural radioactivity around former uranium mine, Gabrovnica in Eastern Serbia. J. Radioanal. Nucl. Chem. 302 (1), 477–482. https://doi.org/10. 1007/s10967-014-3203-1. Pinkert, A., Marsh, K.N., Pang, S., Staiger, M.P., 2009. Ionic liquids and their interaction with cellulose. Chem. Rev. 109 (12), 6712–6728. https://doi.org/10.1021/ cr9001947. Ross, H.H., 1969. Measurement of beta-emitting nuclides using Čerenkov radiation. Anal. Chem. 41 (10), 1260–1265. https://doi.org/10.1021/ac60279a011. Shu, D., Tang, X., Geng, C., Zhang, X., Gong, C., Shao, W., Liu, Y., 2018. Novel method exploration of monitoring neutron beam using Cherenkov photons in BNCT. Radiat. Phys. Chem. 156, 222–230. https://doi.org/10.1016/j.radphyschem.2018.11.024. Stamoulis, K.C., Ioannides, K.G., Karamanis, D.T., Patiris, D.C., 2007. Rapid screening of 90 Sr activity in water and milk samples using Čerenkov radiation. J. Environ. Radioact. 93 (3), 144–156. https://doi.org/10.1016/j.jenvrad.2006.12.007. Todorović, N., Nikolov, J., Forkapić, S., Bikit, I., Mrdja, D., Krmar, M., Vesković, M., 2012. Public exposure to radon in drinking water in Serbia. Appl. Radiat. Isot. 70 (3), 543–549. https://doi.org/10.1016/j.apradiso.2011.11.045. Todorović, N., Stojković, I., Nikolov, J., Tenjović, B., 2017. 90Sr determination in water samples using Čerenkov radiation. J. Environ. Radioact. 169–170, 197–202. https:// doi.org/10.1016/j.jenvrad.2017.01.021. Wang, Y., Yang, Y., Song, L., Ma, Y., Luo, M., Dai, X., 2018. Effects of sodium salicylate on the determination of Lead-210/Bismuth-210 by Cerenkov counting. Appl. Radiat. Isot. 139, 175–180. https://doi.org/10.1016/j.apradiso.2018.05.013. WHO, 2011. Guidelines for Drinking-Water Quality, fourth ed. World Health Organization, WHO Press, Genève, Switzerland978 92 4 154815 1.
rates from the cumulative spectrum generated on low coincidence bias at first. Determination of 226Ra activity concentration was evaluated from the count-rates obtained in high-energy part of spectra in channels from 300 to 430, while the counting protocol was set to high coincidence bias. From 226Ra activity the count rates that contribute in the cumulative spectrum generated on low coincidence bias have been estimated and subtracted, so the 210Pb content has been obtained more precisely, when 226Ra contribution is subtracted. Still, those activities do not prepresent pure 210Pb content and therefore will often be an overestimation of the true 210Pb activity, which is obvious when they are compared to the results obtained via gamma spectrometry. The results presented in Tables 3 and 4 demonstrate that the method presented in this paper is able to screen 210Pb activity and detect its presence if 210Pb activity is beyond its natural levels in the water sample. 4. Conclusions This paper has presented a thorough investigation of the 210Pb determination possibility in the water samples without their chemical pretreatment via Cherenkov radiation produced by its progeny 210Bi by LS counter Quantulus. Instrument's calibration and the obtained parameters of the method have been presented. The method is simple and it does not demand the expensive equipment for the sample pre-treatment procedure. Minimal detectable limit (0.85 Bq L−1) could be lowered (for one order of the magnitude or even more) to the acceptable values below maximal permitted levels by the increment of the initial sample volume that should be evaporated to 20 mL, prolonging the sample measurement time and adding sodium salicylate in order to increase efficiency detection. New ionic liquid, 2-hydroxypropan-1-amminium salicylate, has been synthesized and tested and it showed a similar impact on the efficiency behaviour as sodium salicylate. Its application provides an interesting alternative to sodium salicylate with the advantage of full miscibility with the water sample independently on its pH value. The problem with radium interference could be resolved if its contribution is assessed from the high-energy part of the Cherenkov spectrum produced on high coincidence bias. The analysis of spiked and real samples demonstrated that this method has poor accuracy and low 8