Comparison of Different Methods of Environmental Radioactivity Measurements at the Zagreb Radiocarbon and Tritium Laboratory

Transcription

1 Comparison of Different Methods of Environmental Radioactivity Measurements at the Zagreb Radiocarbon and Tritium Laboratory B. Obelić, I. Krajcar Bronić, N. Horvatinčić, J. Barešić Rudjer Bošković Institute, P.O.Box 180, Zagreb, CROATIA Abstract. Three methods for low-level radiocarbon measurement and two methods for measurement of lowlevel tritium activity in environmental samples have been developed in the Zagreb Radiocarbon and Tritium Laboratory. Various sample preparation methods are used for 14 C (CH 4 synthesis, benzene synthesis, CO 2 absorption) and 3 H (CH 4 synthesis, water distillation). A gas proportional counter and a liquid scintillation counter are used for measurement. Basic parameters (sample quantity, background count rate, efficiency, figure of merit, limit of detection) of the methods are compared, as well as 14 C and 3 H activities determined by different techniques. 1. Introduction Radiocarbon and Tritium Laboratory of the Rudjer Bošković Institute in Zagreb developed gas proportional counter (GPC) techniques for radiocarbon and tritium low-activity measurements in 1968 and 1975, respectively. Since then, more than 3400 various radiocarbon samples (archaeological, geological, hydrogeological and environmental) have been measured. Most results have been regularly published as data lists in Radiocarbon [1]. Tritium activity has been measured in various natural waters: precipitation, groundwater, springs and rivers, and atmospheric water vapor [2]. Laboratory participated in different radiocarbon and tritium international intercomparison studies and proficiency tests as a part of the quality assurance and quality control system [3-5]. Liquid scintillation counter (LSC) Quantulus 1220 has been used since 2001 for low activity measurement of both isotopes. The LSC running and data acquisition was performed by using Wallac WinQ Windows Software for contolling Wallac 1220 Quantulus TM (Version 1.2) and for data processing we use Wallac EASY View Spectrum Analysis Program (Version 1.0). For radiocarbon measurement by using LSC two different sample preparation techniques have been developed: absorption of CO 2 (LSC-A) and benzene synthesis (LSC-B) methods. Here we present a comparison of basic parameters controlling both techniques of 3 H measurement (GPC and LSC) and all three techniques for 14 C measurement (GPC, LSC-A, and LSC-B), including the comparison of measured activities. 2. Tritium measurement For GPC tritium measurement, CH 4 is obtained by reaction of water with aluminium carbide at 150 C [6], purified and used as a counting gas in a multi-wire GPC. The counting energy window is set to energies between 1 kev and 10 kev to obtain the best figure of merit (FM). Gas quality control has been performed by simultaneous monitoring of the count rate above the tritium channel, i.e., above 20 kev [7]. The lowest tritium activity that can be distinguished from the background, i.e., the limit of detection (LOD) is 0.3 Bq/L. For tritium measurement by LSC, a certain amount of distilled water is mixed with HiSafe III scintillation cocktail in a 20-mL plastic vial. The scintillation mixture containing 8 ml of sample water and 12 ml of the HiSafe III cocktail resulted in better efficiency than other mixture 1

2 compositions. The tritium window, as measured by the LSC Quantulus, comprises channels from 18 to 220. However, the optimal counting parameters were determined for the counting window comprising channels from 25 to 187. In the counting window, 92% of the total tritium spectrum is recorded, while only 82% of the background is counted, and the corresponding FM is better than in the whole tritium window. The LOD for the LCS technique is 0.6 Bq/L. The basic parameters of both GPC and LSC techniques for tritium measurement are compared in Table I. The GPC technique requires larger amount of sample and shows slightly higher background count rate, but the count rate of active sample is higher which gives also better FM value and lower detection limit. Results of measurement of tritium activities by GPC and LSC in some environmental samples (precipitation, groundwater) are compared in Fig. 1. Good agreement is obtained for low tritium activities usually measured in the environment: the slope of the fitted line is (1.01 ± 0.01), the intercept is (-0.08 ± 0.15) Bq/L, and the regression coefficient 99.9%. Table I. Comparison of basic parameters of the GPC and LSC techniques for tritium activity measurement. Data for counting time of app. 1 day; cpm = counts per minute. GPC Sample quantity (ml) 50 8 LSC Background count rate, B (cpm) Active (93 Bq/L) sample net count rate, A m (cpm) Efficiency 23.6% FM = A m 2 /B LOD (Bq/L) LSC, A (Bq/L) GPC, A (Bq/L) FIG. 1. Comparison of tritium activity concentrations in environmental samples measured by two methods, GPC and LSC. Symbols: measured activities, line: linear fit. 2

3 3. Radiocarbon measurement Complete procedures for radiocarbon measurement by all three methods are shown in Fig. 2. The first step in radiocarbon measurement is preparation of CO 2 by combustion of an organic sample or by dissolving a carbonate sample in hydrochloric acid. For GPC measurement, CO 2 is then converted to CH 4 by catalytic reaction (Ru as a catalyst) with H 2 at 470 C. Purified CH 4 is stored for 2 weeks to allow radon to decay [8], and used as a counting gas [6, 9]. For LSC measurement, CO 2 can be either absorbed in an absorption mixture (LSC-A method) or used for benzene synthesis (LSC-B method). ORGANIC SAMPLE INORGANIC SAMPLE Pretreatment (A-B-A) Combustion Dissolution HCl CO 2 H 2 Li Catalytic reaction (Ru) Carbosorb + Permafluor Carbidization Absorption Li 2 C 2 CH 4 Carbamate Hydrolisis H 2 O C 2 H 2 GPC LSC-A Catalytic trimerization C 6 H 6 Butyl PBD LSC-B FIG. 2. Three procedures for radiocarbon sample preparation and measurement in the Zagreb Radiocarbon and Tritium Laboratory. 3

4 Absorption of CO 2 (LSC-A) is performed in a vacuum line directly to a 20-mL low-potassium glass vial filled with the mixture of Carbosorb E (the absorbing medium) and Permafluor E (the scintillator) [10]. The amount of CO 2 absorbed in the absorption mixture is determined by weighting the glass vial with the mixture before and after the absorption. The optimal conditions of sample preparation were determined after tests with different CO 2 flow rates and various compositions of the absorption mixture. A flow rate of about 70 ml CO 2 per minute was chosen for routine sample preparation. Under such a flow rate it takes about 25 min to obtain the absorption mixture saturated with CO 2. The best counting efficiency (E) was obtained for the mixture containing 10 ml of Carbosorb E and 10 ml of Permafluor E. The sample preparation for LSC-B method involves the reaction of CO 2 with lithium at 700ºC - 900ºC, subsequent hydrolysis of lithium carbide to acetylene, and its catalytic trimerization on the vanadium catalyst to benzene [11]. The results of tests showed that the optimal time of carbide and hydrolysis reaction is 20 min and min, respectively. Temperature of catalytic reaction is between 60 C and 90 C with duration of 1-2 hours, depending on the quantity of acetylene. The acetylene yields range from 89% to 98%, and benzene yields range from 77% to 90%, while the benzene purity is 98.9 to 99.5%. To prepare the scintillation cocktail for LSC measurements, we add 15 mg of scintillator (butyl-pbd) per 1 g of benzene and measure 14 C activity of benzene in 7-mL low-potassium glass vials. For samples prepared by both LSC-A and LSC-B methods the optimal counting parameters for measurement in LSC were determined, as well as the SQP value as a test of sample quality. (The SQP is an index of quenching, Standard Quench Parameter, which represents the end point of the external standard spectrum, i.e., the channel number beyond which 1% of the total counts are found. As external standard we use a built-in 37 kbq 152 Eu capsule.) The number of 30-minute cycles in a run was taken to be 30 (or more), resulting in 900 (or more) minutes per sample measurement. The counting efficiency (E) was determined from the ratio of measured net count rates (A m, expressed in cpm) and the known 14 C activity of the standard (A, expressed in dpm). The counting efficiencies in the so-called " 14 C windows", encompassing the channels and for LSC-A and LSC- B, respectively, are 70% and 90%, respectively. The counting window for the LSC-A technique after optimization comprises channels between 144 and 372 and it encompasses 93% of the total 14 C spectrum and 77% of the background spectrum. The total efficiency for the LSC-A is therefore 65%. The FM value has been improved from 16 to 18.4 after optimization. The dating limit of the method is 31,800 yr. Optimization of the counting window for the LSC-B technique resulted in background reduction to 64% of that in the 14 C window, while 92% of the 14 C spectrum has remained in the window. Therefore, the total efficiency of the LSC-B technique is 82.4% and the FM value is considerably improved (from 2200 to 2908). Under adopted counting condition, the 14 C dating limit is 52,160 yr (a 5-mL benzene sample counted 1 day). The limit can be improved to 54,800 yr by using larger amount of benzene, e.g., 7 ml, if the quantity of available sample allows it. Longer counting time (e.g., 2 days) will also increase the maximal age that can be determined to 57,600 yr. Comparison of some basic parameters of the measurement of 14 C in the counting windows is shown in Table II, where also data for our GPC system are shown for comparison. The comparison of conventional 14 C ages [12] of benzene samples prepared as described above (LSC- B method) and those of the same samples measured by the GPC is shown in Fig. 3. The LSC-B ages were corrected also for different quenching, by using the quenching curve. (Small amount of acetone was added to the spike benzene, and the SQP and the count rate of such samples have been measured. For our synthesized benzene samples of purity 98.9%-99.5% the efficiency correction due to quenching was less than 1%.) The agreement between the LSC-B and GPC ages is good, giving the slope of the fitted line equal to 1.01 ± No systematic difference between the two sets is observed, as shown also by the intercept of the fitted line equal to (39 ± 41) yr. 4

5 Table II. Comparison of characteristic parameters of all three radiocarbon measurement techniques for the counting time of app. 1 day. LSC-A: absorption of CO 2 and LSC measurement; LSC-B: benzene synthesis and LSC measurement; GPC: methane preparation and gas proportional counting. The data for LSC-A and LSC-B techniques correspond to the optimized counting windows. Radiocarbon activity of the standard is 100 pmc = dpm/g of carbon. LSC-A LSC-B GPC Amount of carbon (g) Spectrum area (counting window, channel from to) Real activity of standard, A (dpm) Net count rate of standard, A m (cpm) Count rate of background, B (cpm) Efficiency, E = A m /A (%) Figure of merit, FM = A 2 m /B Maximum 14 C age T max * Tmax = 8033 ln A meas t meas B 31,800 52,160 37,500 The comparison of 14 C activities expressed in pmc (percent of modern carbon, defined as 100 pmc = dpm/g of carbon = Bq/g of carbon) obtained by the GPC and LSC-A methods is presented in Fig. 4. The agreement between the two sets of data is again good (the slope of the fitted line is 1), but the uncertainties of the LSC-A results are 2-3 times larger that those of the GPC method for the same counting time (1 day per sample). Due to high background count rate and low efficiency (Table II), the maximal determinable 14 C age for LSC-A is 31,800 yr. However, these characteristics are good enough for certain applications that do not require high precision (e.g., environmental monitoring), and also for samples having high 14 C activity LSC-B, years (BP) GPC, years (BP) FIG. 3. Comparison of the conventional 14 C ages of various samples measured by LSC-B and GPC methods. 5

6 100 LSC-A, a 14 C (pmc) GPC, a 14 C (pmc) FIG. 4. Comparison of 14 C activities measured by the LSC-A and GPC methods. 4. Conclusion The presented methods of measurements of environmental isotopes 3 H and 14 C including sample preparation differ in complexity, time consume, price, and in precision of measured results. GPC technique for tritium measurement is more complex than the LSC one, but the counting characteristics are better. The GPC method is therefore preferred in applications requiring higher precision and lower detection limit, e.g., in hydrogeological studies. LSC-A method for radiocarbon measurement is quick, cheep and simple and requires less carbon (~0.5 g) than the other two methods (~4 g). It is accurate enough for certain applications, although it is not as precise as either GPC or LSC-B method. Chemical preparations of samples for GPC and LSC-B methods of 14 C measurement are more complex, but the methods give better precision, and therefore they can be applied for dating in archaeology and geochronology ( 14 C dating). To summarize, LSC measurement techniques of tritium in water and of 14 C in various samples by CO 2 absorption are suitable for quick and accurate determination of environmental contamination (they could be used for quick determination of increased environmental 14 C and 3 H contamination in case of a nuclear accident), although both are not recommendable for applications requiring higher precision. Acknowledgement Work performed under Project of the Ministry of Science, Republic of Croatia, and with the help of IAEA TC Project CRO/2/002 "Nuclear Spectroscopy Techniques and Ion Beam Analysis in Environmental and Industrial Applications ". 6

7 References 1. Obelić, B., Krajcar Bronić, I., Horvatinčić, N., Rudjer Bošković Institute radiocarbon measurements XV. Radiocarbon 44/2: , (2002), and references therein. 2. Krajcar Bronić, I., Horvatinčić, N., Obelić, B., Two decades of environmental isotope record in Croatia: reconstruction of the past and prediction of future levels. Radiocarbon 40/1-2: , (1998). 3. Horvatinčić, N., Srdoč, D., Obelić, B., Krajcar Bronić, I., Radiocarbon dating of intercomparison samples at the Zagreb Radiocarbon Laboratory. Radiocarbon 32: , (1990). 4. Krajcar Bronić, I., Horvatinčić, N., Obelić, B., Bistrović, R., Radiocarbon intercomparison studies at the Rudjer Bošković Institute. Radiocarbon 37: , (1995). 5. Obelić, B., Horvatinčić, N., Krajcar Bronić, I., in Proceedings of the IRPA Regional Congress Dubrovnik, Croatia, 2001, (CD-ROM), edited by B. Obelić, M. Ranogajec-Komor, S. Miljanić, I. Krajcar Bronić (CRPA, Zagreb, 2002), paper no. 1o Horvatinčić, N., Radiocarbon and tritium measurements in water samples and application of isotopic analyses in hydrology. Fizika 12 (S2): , (1980). 7. Krajcar Bronić, I., Obelić, B., Srdoč, D., The simultaneous measurement of tritium activity and the background count rate in a proportional counter by the Povinec method: Three years experience at the Ruđer Bošković Institute. Nuclear Instrum. Meth. in Physics Research B17: , (1986). 8. Horvatinčić, N., Obelić, B., Krajcar Bronić, I., Srdoč, D., Bistrović, R., Sources of radon contamination in C-14 dating. Radiocarbon 37: , (1995). 9. Srdoč, D., Obelić, B., Horvatinčić, N., Sliepčević, A., Measurement of the 14 C Activity of the ANU Sucrose Secondary Standard by Means of the Proportional Counter Technique. Radiocarbon 21/3: , (1979). 10. Barešić, J., Krajcar Bronić, I., Horvatinčić, N., Obelić, B., in Proceedings of the Fifth symposium of the Croatian Radiation Protection Association, Stubičke Toplice, Croatia, 2003, edited by I. Krajcar Bronić, S. Miljanić, B. Obelić (CRPA, Zagreb, 2003), p Horvatinčić, N., Barešić, J., Krajcar Bronić, I., Obelić, B., Measurement of low 14 C activities in liquid scintillation counter in the Zagreb Radiocarbon Laboratory. 18 th International Radiocarbon Conference, Wellington, New Zealand, 2003, submitted to Radiocarbon. 12. Stuiver, M., Polach, H.A., Discussion: Reporting of 14 C data. Radiocarbon 19: , (1977). 7