- Email: [email protected]
Determination of 238U in ground-water samples using gamma-ray spectrometry
Applied Radiation and Isotopes 69 (2011) 636–640
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Determination of 238U in ground-water samples using gamma-ray spectrometry M. Korun n, K. Kovacˇicˇ ‘‘Jozˇef Stefan’’ Institute, Jamova cesta 39, Ljubljana, Slovenia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 May 2010 Received in revised form 26 October 2010 Accepted 20 December 2010 Available online 25 December 2010
A method for measuring the low activity concentration of 238U in water is described. Samples of 50 L were evaporated and the dry residue after evaporation was measured. Typically, 30 g of material was obtained for the samples processed in this way. Based on measuring the samples using a gamma-ray spectrometer, equipped with a germanium crystal having a diameter of 8 cm and a beryllium window, the decision threshold resulting from the measurements was assessed to be 1.5 Bq/m3. A total of 26 samples of ground water were processed and activity concentrations of up to 20 Bq/m3 were measured. However, most of the results were in a range around 5 Bq/m3. & 2010 Elsevier Ltd. All rights reserved.
Keywords: Gamma-ray spectrometry Water samples Decision threshold 238 U 234 Th
1. Introduction Uranium is dissolved in ground waters because it leaches from the rock of the aquifer. Natural uranium is composed of three longlived uranium isotopes—238U, 235U and 234U. The isotopic ratio 235 U/238U in ground water is equal to the activity ratio in the host rock, whereas the activity ratio 234U/238U in the water depends on the type of mineral bearing the uranium and the geochemical conditions, which determines the solubility of the uranium. Usually, the concentration of 234U exceeds the concentration of its parent 238U in ground water since 234U may be displaced from its original position in the crystal lattice due to recoil when emitting an alpha particle (Osmond and Cowart, 1992). Consequently, the 234 U atoms are more available for leaching than the 238U atoms. The activity concentration of 238U in water cannot be measured by gamma-ray spectrometry directly, since the probability of the emission of gamma-rays in its decay is too low. Instead, it can be determined from the activity concentration of 235U, by supposing the natural activity ratio, or from the activity concentration of its daughter 234Th, by supposing a radioactive equilibrium. However, neither of these methods is particularly sensitive, because of low 235 U/238U activity ratio and low 234Th probabilities for the emission of gamma-rays. Since both the activity ratio and the gamma-ray emission probabilities amount to a few percent, large-volume samples have to be processed in order to arrive at a measurement sensitivity that makes it possible to measure reliably uranium n
Corresponding author. Fax: + 386 1 477 3151. E-mail address: [email protected] (M. Korun).
0969-8043/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2010.12.010
concentrations of a few Bq/m3 in aqueous samples. The reference value, representing the assessed average over the world population, for the 238U activity concentration in drinking water is 1 Bq/m3 (UNSCEAR, 2000); therefore, it is to be expected that in many samples the activity concentration of the uranium will remain below the detection limit. In measurements near the detection limit, the counting statistics is the main source of uncertainty of the measurement results. In order to decrease the statistical uncertainty in a spectral analysis, as much information as possible is extracted from the spectrum. In addition, near the detection limit, systematic influences originating in the background subtraction and the interference corrections may have a considerable influence on the results. However, efforts have been made to make these systematic influences smaller than the statistical uncertainties. It is the aim of this article to show how gamma-ray spectrometric methods can be used to measure uranium in water at low concentrations. Gamma-ray spectrometry is suitable for measurements of the radioactive contamination of water because the 238U activity concentrations of several other members of the uranium and thorium decay chains can also be measured simultaneously.
2. Methods For the measurements, the most sensitive gamma-ray spectrometer in the Laboratory for Radiological Measuring Systems and Radioactivity Measurements at the ‘‘Jozˇef Stefan’’ Institute was used. The detector has a planar crystal with a diameter of 8 cm and
M. Korun, K. Kovacˇicˇ / Applied Radiation and Isotopes 69 (2011) 636–640
a thickness of 3 cm. Its resolution is 0.7 keV at low energies and 2.05 keV at 1.33 MeV. The detector is mounted in a low-activity, U-style cryostat with a beryllium window. The spectrometer has a shielding of 50-year-old lead with a thickness of 15 cm. The shield’s cavity is lined with 2 mm of copper, 2 mm of cadmium and 10 mm of mercury. The spectrometer is equipped with a cosmic veto shield consisting of two plastic scintillator plates, reducing the count rate in the 511 keV annihilation peak to about one half. The overall background count rate in the energy region from 3 to 2700 keV is 0.8 s 1, and in the energy region from 40 to 2700 keV it is 0.6 s 1. The counting time for a single measurement is approximately two days. The main sources of the peaked background are the uranium in the beryllium window and the radon daughters in the shield’s cavity. The count rate in the strong uranium peaks is constant over time. It is determined as an average over many background measurements with an accuracy of about 1% in the peak at 63 keV, about 2% in the peak at 93 keV and about 3% in the peak at 186 keV. To reduce the count rate in the peaks belonging to radon and its daughters, the shield’s cavity is flushed with aged nitrogen. In spite of this the radon-induced background still exhibits seasonal variations. In Fig. 1 the count rate in the peak at an energy of 609 keV, belonging to the radon daughter 214Bi, is presented as a function of the average outside temperature. The temperature dependence occurs because the counting room is located in a basement below the surface and is at a constant temperature of 20 71 1C. During high outside temperatures there is a smaller exchange of air and the radon concentration in the air of the counting room rises. On the other hand, at low outside temperatures the exchange of air is large and radon concentration is smaller. To assess the count rate in the radon-induced peaks the empirical dependencies of the count rates in the peaks belonging to the radon daughters on the average outside temperature were established, the outside temperature was measured and the count rates were calculated as a function of the average outside temperature during the spectrum’s acquisition. The calculated background count rates are used in the background subtraction. To achieve a sensitivity that enables measurements of the activity concentrations of the members of the uranium and thorium decay chains in water from the environment, water samples of 50 L were collected. The samples were then acidified to a pH of less than 2 with nitric acid in order to retain the radionuclides in the solution (ISO, 2007). The samples were evaporated at a temperature of 6575 1C by forced ventilation. Care was taken to ensure that the fine particles, which eventually settled at the bottom of the sampling vessel, were not transferred to 0.0012
dn/dt (609 keV) / s-1
0.0010 0.0008 0.0006 0.0004 0.0002 0.0000
0
5
10
15
20
25
T / °C Fig. 1. Background count rate in 609 keV peak as a function of average air temperature outside building.
637
the evaporation cell. The time needed to evaporate an acidified sample of 50 L was 4–8 days. The samples of residue after evaporation were collected and pressed into plastic containers with a diameter of 6 cm. Typically, between 20 and 40 g of the residue was collected. In the calculation of the attenuation of gamma-rays in the sample it was supposed that the residue contains 50% of calcium carbonate and 50% of calcium nitrate. The samples were measured as close to the detector as possible in order to maximize the counting efficiency. For acidified samples with low content of dissolved substances the retention of dissolved uranium in the sampling vessel, decanting system and evaporation vessel, together with the possible loss of the sample because of spilling, is smaller than 5% (Glavicˇ-Cindro et al., 2010). It is assessed that the sources of uncertainty in the sample preparation process do not exceed 3%. Cross-contamination of samples was prevented by washing the decanting system and evaporation vessel with hydrochloric acid and ethanol, and rinsed with distilled water. To reduce as much as possible the influence of the statistical uncertainties on the measurement result 238U was determined from the activity concentrations of 235U and the first daughter of 238U which is 234Th. To improve the counting statistics and to assess the activity concentration of 234Th at the sampling time the samples were measured twice—as soon as possible after the evaporation and after a few half lives of 234Th. From the 234Th activity at both counting times its activity at the sampling time and its asymptotic activity, when it is in equilibrium with 238U, is calculated from aTh234 ðtÞ ¼ aTh234 ð0Þet ln 2=t1=2 þ aU238 ð1et ln 2=t1=2 Þ where aTh234(0) denotes the 234Th activity concentration at the sampling time, aU238 the 234Th activity concentration in equilibrium with its parent 238U and t1/2 the half life of 234Th. When determining the 235U activity from its strongest peak, occurring at 186 keV, the presence of 226Ra in the sample has to be taken into account. Both isotopes radiate at similar energies and their contributions to the spectrum cannot be separated. Since 226Ra is present in ground water its contribution to the peak at the energy of 186 keV has to be subtracted before calculating the activity of 235 U. 226Ra radiates only at an energy of 186 keV, and therefore its activity has to be assessed from the activities of its daughters. Its first daughter, 222Rn, emanates from the sample; therefore, the equilibrium with the radon daughters 214Pb and 214Bi, which can be measured using gamma-ray spectrometers directly, cannot be assumed. Also, during evaporation the radon is lost from the sample, which means its concentration in the residue changes with time. To calculate the concentration of the radon daughters in the sample as a function of time it was assumed that the radon daughters are in equilibrium with 222Rn. It was assumed further that radon begins to accumulate in the sample material in the evaporation vessel when the materials, dissolved in the water, start to aggregate. This time depends on the concentration of dissolved substances in the collected sample. At low concentrations this occurs about one day after the beginning of evaporation. Further it was assumed that the radon activity in the evaporation vessel increases linearly until the end of evaporation. It follows then that at an average evaporation time of 6 days the dry residue is approximately 2 days old by the end of evaporation and that the effective time, when 222Rn starts accumulating in the sample, is two days before the end of evaporation. It was assumed also that 29%76% of 222Rn emanates from the sample. Using these assumptions the correlation between the 226Ra activity from the first measurement and from the second measurement is presented in Fig. 2. Since the first measurement is performed as soon as possible after the evaporation, when its activity in the sample is still rising, and the second when it reaches a value close to equilibrium with 226Ra and is constant in time, the factors describing the disequilibrium between 226Ra and 222Rn differ by about a factor
M. Korun, K. Kovacˇicˇ / Applied Radiation and Isotopes 69 (2011) 636–640
638
20
a2 (Ra-226) [Bq/m3]
a2 (U-238, 186 keV U-235) / Bq/m3
10
1
1
15
10
5
0
-5
10
-5
a1 (Ra-226) / Bq/m3 Fig. 2. Comparison of 226Ra activity concentration determined from first and second measurements.
10
15
20
Fig. 3. Comparison of 238U activity concentration determined from count rate of 235 U in 186 keV peak in first and second measurements for samples where 226Ra was identified.
20
15
a (U-238, Th-234) / Bq/m3
Fig. 3 presents the comparison between the 238U activity concentrates calculated from the 235U concentrations of the first and second measurements for the samples where 226Ra was determined. It should be noted that the activity concentration of 226 Ra in the samples is smaller than the activity concentration of 238 U. It can be observed that in these circumstances the influence of background variations, which are not described completely by the outside temperature, do not affect the measurement of the uranium activity concentration to a significant degree. Fig. 4 presents the comparison between the weighted average of both 238U activity concentrations obtained from 235U and the equilibrium activity concentration of 234Th. It is clear that both methods must yield the same activity concentration. Fig. 5 presents the frequency distribution of the number of samples analyzed over the 238U activity concentration and the uncertainty of its activity concentration as a function of the activity concentration itself. The 238U activity concentration was calculated as the weighted average of the asymptotic 234Th activity concentration and the concentration obtained from the peak at 186 keV. It is clear from Fig. 5b that the trend line denoting the dependence of the uncertainty of the 238U activity concentration on the activity concentration itself is nearly horizontal, indicating the uncertainty does not depend on the activity concentration. This agrees with the well-known property (Weise et al., 2006) that near the detection limit the uncertainty depends weakly on the measurement result. The average uncertainty, calculated for the confidence interval of 68%, is 0.92 Bq/m3. Since the measurement results distribute normally with the standard deviation described by the uncertainty it follows that for samples with negligible concentration of 238U the
5
a1 (U-238, 186 keV U-235) / Bq/m3
of 2. It can be observed that the largest relative differences between the first and second measurement results occur at small concentrations, which implies that their main source is the time dependency of the background, which was not fully under control. However, its influence on the 238U activity concentration is minor at 226Ra concentrations that are well below the 238U concentrations.
3. Results and discussion
0
10
5
0
-5
-5
0
5
10
15
20
a (U-238,186keV U-235) / Bq/m3 Fig. 4. Comparison of 238U activity concentration obtained from 186 keV peak of 235 U and equilibrium 234Th activity concentration.
measurement results distribute around zero activity concentration with the standard deviation of 0.92 Bq/m3. For a probability of 5% for making type-1 errors, i.e. for reporting false results with a probability of 5%, the decision threshold is assessed to be 1.5 Bq/m3. It is clear from the frequency distribution of Fig. 5a that its integral below the decision threshold is 2, indicating that 238U was determined in 24 samples of the 26 samples analyzed. Using the method described, the activity concentration of 234Th at the sampling time can be determined. Fig. 6a presents the distribution of the number of measurement results over the 234Th
M. Korun, K. Kovacˇicˇ / Applied Radiation and Isotopes 69 (2011) 636–640
6
639
10
5
8
4
N
N
6 3
4
2
2
0
0 Δa (Th-234) / Bq/m3
1
Δa (U-238) / Bq/m3
1.5
1.0
0.5
8 6 4 2 0 -10
-8
-6
-4
-2
0
2
4
6
8
10
3
a (Th-234) / Bq/m 0.0
-5
0
5
10
15
20
a (U-238) / Bq/m3 Fig. 5. Frequency distribution of determined 238U activity concentration (a) and dependence of uncertainty of 238U activity concentration on 238U activity concentration itself (b). Dashed line denotes average uncertainty.
activity concentration. It is clear that the distribution has its maximum at an activity concentration of zero, which implies that most of the 234Th activity concentrations are under the decision threshold of the measurements. In Fig. 6b the dependence of the uncertainty of the 234Th activity concentration on the 234Th activity concentration itself is presented. A minimum in the uncertainty can be observed near the zero activity concentration. This is due to an additional source of uncertainty, which originates from the calculation of the 234Th activity concentration that is extrapolated to the sampling date. The uncertainty of the extrapolated concentration depends, in addition to the 238U activity concentration, on the time interval of the extrapolation. Since the time interval between the sampling date and the date of the first measurement depends on the evaporation time and availability of the measuring equipment, the decision threshold and measurement uncertainty for 234Th are not constant. Therefore large measurement uncertainties coincide with large measurement results. The dashed line denotes the relation between the measurement result and its uncertainty near zero activity. In the conditions of the measurements described, the smallest measurement uncertainty, attained at 234Th activity concentrations near zero, is about 2 Bq/m3. This corresponds to the interval between the sampling and the first measurement of 10 days. From this uncertainty the decision threshold in the most favorable measurement conditions is assessed to be about 3 Bq/m3. The average decision threshold for the 234Th activity-concentration measurements can be assessed from the standard deviation of the distribution of a number of 234Th measurement results over the activity concentration to be 4.3 Bq/m3.
Fig. 6. Frequency distribution of determined 234Th activity concentration at sampling time (a) and dependence of uncertainty of 234Th activity concentration on 234Th activity concentration itself (b). Dashed line denotes relation between measurement result and its uncertainty.
4. Conclusion The easiest way to measure 238U activity concentration in water samples by gamma-ray spectrometry is by measuring the equilibrium concentration of its daughter 234Th, i.e. to perform the measurement of 234Th activity concentration few half lives of 234Th after sampling. To decrease the delay needed to attain equilibrium two measurements, one as soon as possible after sampling and the other at least one half life after the first measurements, may be performed and the equilibrium concentration is calculated by extrapolation. In this case besides the 238U activity concentration also the 234Th activity concentration at sampling time can be calculated. When the activity concentration of 226Ra is smaller or comparable to that of 238U, to lower the sensitivity and to improve the reliability of the measurements, information on 238U can be extracted also from the 235U peak at 186 keV supposing the isotopic ratio. Since this peak interferes with the peak from 226Ra occurring at the same energy, the activity concentration of 226Ra must be determined from the concentration of radon daughters in order to correct the area of the peak at 186 keV for the 226Ra contribution. It is advantageous to use this method when two measurements are performed since then the values of the parameters determining the activity ratio between 226Ra and radon daughters, the effective age of the dry residue and the radon emanation rate, can be validated. The 238U activity concentration is calculated as the weighted average of the equilibrium activity concentration and the activity concentration obtained from 235U. The reliability of this method is assured since the 238U activity concentration is obtained from two independent sources. With this combined method, measurement uncertainties of about 1 Bq/m3 were attained. The time dependence of the spectrometer background was identified as the main source of systematic influence affecting the results near the detection limit,
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which is due to the presence of radon daughters in air in the cavity of the detector shield. A decision threshold of 1.5 Bq/m3 was obtained in the measurements. Simultaneously with the equilibrium activity concentration of 234 Th, its activity concentration at the sampling time was also determined. Here, the sensitivity of the measurements depends on the 238U activity concentration in the sample and on the time interval between the sampling time and the time of the first measurement. The smallest decision threshold achieved was 2 Bq/m3, but the average decision threshold in the measurement was about 4.3 Bq/m3. Due to the decision threshold exceeding the 238U decision threshold and its lower solubility in water, 234Th was not determined in the analyzed samples. In the samples of ground water that were analyzed, 238U activity concentrations up to 20 Bq/m3 were measured. In most of the samples, activity concentrations of approximately 5 Bq/m3 were determined. The sensitivity of the method described and its simplicity in comparison to methods involving the radiochemical separation of elements and subsequent charged-particle measurements make this gamma-ray spectrometric method a useful tool in the prospecting of ground-water sources and drinking-water supplies.
Acknowledgments This work was performed under contract J7-0363 of the Slovenian Research Agency. The authors want to express their thanks and recognition to the staff in charge of sample preparation, especially Dr. Marijan Necˇemer, Petra Maver Modec dipl. phys., Drago Brodnik and Helena Fajfar.
References Glavicˇ-Cindro, D., Necˇemer, M., Vodenik, B., 2010. Optimization of the preparation of samples of water residue by evaporation IJS-DP-Draft (in Slovenian). ISO, 2007. ISO10703:2007 (E) Water quality – Determination of the activity concentration of radionucledes – Method by high resolution gamma-ray spectrometry. Osmond, J.K., Cowart, J.B., 1992. Ground water. In: Ivanovich, M., Harmon, R.S. (Eds.), Uranium-series Disequilibrium: Application to Earth, Marine and Environmental Sciences. Clarendon Press, Oxford. UNSCEAR, 2000. Sources and effects of ionizing radiations. Report to the General Assembly with Scientific Annexes, YN, New York. Weise, K., et al., 2006. Bayesian decision threshold, detection limit and confidence limits in ionizing-radiation measurement. Radiat. Prot. Dosimetry 121, 52–63.
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Determination of 238U in ground-water samples using gamma-ray spectrometry M. Korun n, K. Kovacˇicˇ ‘‘Jozˇef Stefan’’ Institute, Jamova cesta 39, Ljubljana, Slovenia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 May 2010 Received in revised form 26 October 2010 Accepted 20 December 2010 Available online 25 December 2010
A method for measuring the low activity concentration of 238U in water is described. Samples of 50 L were evaporated and the dry residue after evaporation was measured. Typically, 30 g of material was obtained for the samples processed in this way. Based on measuring the samples using a gamma-ray spectrometer, equipped with a germanium crystal having a diameter of 8 cm and a beryllium window, the decision threshold resulting from the measurements was assessed to be 1.5 Bq/m3. A total of 26 samples of ground water were processed and activity concentrations of up to 20 Bq/m3 were measured. However, most of the results were in a range around 5 Bq/m3. & 2010 Elsevier Ltd. All rights reserved.
Keywords: Gamma-ray spectrometry Water samples Decision threshold 238 U 234 Th
1. Introduction Uranium is dissolved in ground waters because it leaches from the rock of the aquifer. Natural uranium is composed of three longlived uranium isotopes—238U, 235U and 234U. The isotopic ratio 235 U/238U in ground water is equal to the activity ratio in the host rock, whereas the activity ratio 234U/238U in the water depends on the type of mineral bearing the uranium and the geochemical conditions, which determines the solubility of the uranium. Usually, the concentration of 234U exceeds the concentration of its parent 238U in ground water since 234U may be displaced from its original position in the crystal lattice due to recoil when emitting an alpha particle (Osmond and Cowart, 1992). Consequently, the 234 U atoms are more available for leaching than the 238U atoms. The activity concentration of 238U in water cannot be measured by gamma-ray spectrometry directly, since the probability of the emission of gamma-rays in its decay is too low. Instead, it can be determined from the activity concentration of 235U, by supposing the natural activity ratio, or from the activity concentration of its daughter 234Th, by supposing a radioactive equilibrium. However, neither of these methods is particularly sensitive, because of low 235 U/238U activity ratio and low 234Th probabilities for the emission of gamma-rays. Since both the activity ratio and the gamma-ray emission probabilities amount to a few percent, large-volume samples have to be processed in order to arrive at a measurement sensitivity that makes it possible to measure reliably uranium n
Corresponding author. Fax: + 386 1 477 3151. E-mail address: [email protected] (M. Korun).
0969-8043/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2010.12.010
concentrations of a few Bq/m3 in aqueous samples. The reference value, representing the assessed average over the world population, for the 238U activity concentration in drinking water is 1 Bq/m3 (UNSCEAR, 2000); therefore, it is to be expected that in many samples the activity concentration of the uranium will remain below the detection limit. In measurements near the detection limit, the counting statistics is the main source of uncertainty of the measurement results. In order to decrease the statistical uncertainty in a spectral analysis, as much information as possible is extracted from the spectrum. In addition, near the detection limit, systematic influences originating in the background subtraction and the interference corrections may have a considerable influence on the results. However, efforts have been made to make these systematic influences smaller than the statistical uncertainties. It is the aim of this article to show how gamma-ray spectrometric methods can be used to measure uranium in water at low concentrations. Gamma-ray spectrometry is suitable for measurements of the radioactive contamination of water because the 238U activity concentrations of several other members of the uranium and thorium decay chains can also be measured simultaneously.
2. Methods For the measurements, the most sensitive gamma-ray spectrometer in the Laboratory for Radiological Measuring Systems and Radioactivity Measurements at the ‘‘Jozˇef Stefan’’ Institute was used. The detector has a planar crystal with a diameter of 8 cm and
M. Korun, K. Kovacˇicˇ / Applied Radiation and Isotopes 69 (2011) 636–640
a thickness of 3 cm. Its resolution is 0.7 keV at low energies and 2.05 keV at 1.33 MeV. The detector is mounted in a low-activity, U-style cryostat with a beryllium window. The spectrometer has a shielding of 50-year-old lead with a thickness of 15 cm. The shield’s cavity is lined with 2 mm of copper, 2 mm of cadmium and 10 mm of mercury. The spectrometer is equipped with a cosmic veto shield consisting of two plastic scintillator plates, reducing the count rate in the 511 keV annihilation peak to about one half. The overall background count rate in the energy region from 3 to 2700 keV is 0.8 s 1, and in the energy region from 40 to 2700 keV it is 0.6 s 1. The counting time for a single measurement is approximately two days. The main sources of the peaked background are the uranium in the beryllium window and the radon daughters in the shield’s cavity. The count rate in the strong uranium peaks is constant over time. It is determined as an average over many background measurements with an accuracy of about 1% in the peak at 63 keV, about 2% in the peak at 93 keV and about 3% in the peak at 186 keV. To reduce the count rate in the peaks belonging to radon and its daughters, the shield’s cavity is flushed with aged nitrogen. In spite of this the radon-induced background still exhibits seasonal variations. In Fig. 1 the count rate in the peak at an energy of 609 keV, belonging to the radon daughter 214Bi, is presented as a function of the average outside temperature. The temperature dependence occurs because the counting room is located in a basement below the surface and is at a constant temperature of 20 71 1C. During high outside temperatures there is a smaller exchange of air and the radon concentration in the air of the counting room rises. On the other hand, at low outside temperatures the exchange of air is large and radon concentration is smaller. To assess the count rate in the radon-induced peaks the empirical dependencies of the count rates in the peaks belonging to the radon daughters on the average outside temperature were established, the outside temperature was measured and the count rates were calculated as a function of the average outside temperature during the spectrum’s acquisition. The calculated background count rates are used in the background subtraction. To achieve a sensitivity that enables measurements of the activity concentrations of the members of the uranium and thorium decay chains in water from the environment, water samples of 50 L were collected. The samples were then acidified to a pH of less than 2 with nitric acid in order to retain the radionuclides in the solution (ISO, 2007). The samples were evaporated at a temperature of 6575 1C by forced ventilation. Care was taken to ensure that the fine particles, which eventually settled at the bottom of the sampling vessel, were not transferred to 0.0012
dn/dt (609 keV) / s-1
0.0010 0.0008 0.0006 0.0004 0.0002 0.0000
0
5
10
15
20
25
T / °C Fig. 1. Background count rate in 609 keV peak as a function of average air temperature outside building.
637
the evaporation cell. The time needed to evaporate an acidified sample of 50 L was 4–8 days. The samples of residue after evaporation were collected and pressed into plastic containers with a diameter of 6 cm. Typically, between 20 and 40 g of the residue was collected. In the calculation of the attenuation of gamma-rays in the sample it was supposed that the residue contains 50% of calcium carbonate and 50% of calcium nitrate. The samples were measured as close to the detector as possible in order to maximize the counting efficiency. For acidified samples with low content of dissolved substances the retention of dissolved uranium in the sampling vessel, decanting system and evaporation vessel, together with the possible loss of the sample because of spilling, is smaller than 5% (Glavicˇ-Cindro et al., 2010). It is assessed that the sources of uncertainty in the sample preparation process do not exceed 3%. Cross-contamination of samples was prevented by washing the decanting system and evaporation vessel with hydrochloric acid and ethanol, and rinsed with distilled water. To reduce as much as possible the influence of the statistical uncertainties on the measurement result 238U was determined from the activity concentrations of 235U and the first daughter of 238U which is 234Th. To improve the counting statistics and to assess the activity concentration of 234Th at the sampling time the samples were measured twice—as soon as possible after the evaporation and after a few half lives of 234Th. From the 234Th activity at both counting times its activity at the sampling time and its asymptotic activity, when it is in equilibrium with 238U, is calculated from aTh234 ðtÞ ¼ aTh234 ð0Þet ln 2=t1=2 þ aU238 ð1et ln 2=t1=2 Þ where aTh234(0) denotes the 234Th activity concentration at the sampling time, aU238 the 234Th activity concentration in equilibrium with its parent 238U and t1/2 the half life of 234Th. When determining the 235U activity from its strongest peak, occurring at 186 keV, the presence of 226Ra in the sample has to be taken into account. Both isotopes radiate at similar energies and their contributions to the spectrum cannot be separated. Since 226Ra is present in ground water its contribution to the peak at the energy of 186 keV has to be subtracted before calculating the activity of 235 U. 226Ra radiates only at an energy of 186 keV, and therefore its activity has to be assessed from the activities of its daughters. Its first daughter, 222Rn, emanates from the sample; therefore, the equilibrium with the radon daughters 214Pb and 214Bi, which can be measured using gamma-ray spectrometers directly, cannot be assumed. Also, during evaporation the radon is lost from the sample, which means its concentration in the residue changes with time. To calculate the concentration of the radon daughters in the sample as a function of time it was assumed that the radon daughters are in equilibrium with 222Rn. It was assumed further that radon begins to accumulate in the sample material in the evaporation vessel when the materials, dissolved in the water, start to aggregate. This time depends on the concentration of dissolved substances in the collected sample. At low concentrations this occurs about one day after the beginning of evaporation. Further it was assumed that the radon activity in the evaporation vessel increases linearly until the end of evaporation. It follows then that at an average evaporation time of 6 days the dry residue is approximately 2 days old by the end of evaporation and that the effective time, when 222Rn starts accumulating in the sample, is two days before the end of evaporation. It was assumed also that 29%76% of 222Rn emanates from the sample. Using these assumptions the correlation between the 226Ra activity from the first measurement and from the second measurement is presented in Fig. 2. Since the first measurement is performed as soon as possible after the evaporation, when its activity in the sample is still rising, and the second when it reaches a value close to equilibrium with 226Ra and is constant in time, the factors describing the disequilibrium between 226Ra and 222Rn differ by about a factor
M. Korun, K. Kovacˇicˇ / Applied Radiation and Isotopes 69 (2011) 636–640
638
20
a2 (Ra-226) [Bq/m3]
a2 (U-238, 186 keV U-235) / Bq/m3
10
1
1
15
10
5
0
-5
10
-5
a1 (Ra-226) / Bq/m3 Fig. 2. Comparison of 226Ra activity concentration determined from first and second measurements.
10
15
20
Fig. 3. Comparison of 238U activity concentration determined from count rate of 235 U in 186 keV peak in first and second measurements for samples where 226Ra was identified.
20
15
a (U-238, Th-234) / Bq/m3
Fig. 3 presents the comparison between the 238U activity concentrates calculated from the 235U concentrations of the first and second measurements for the samples where 226Ra was determined. It should be noted that the activity concentration of 226 Ra in the samples is smaller than the activity concentration of 238 U. It can be observed that in these circumstances the influence of background variations, which are not described completely by the outside temperature, do not affect the measurement of the uranium activity concentration to a significant degree. Fig. 4 presents the comparison between the weighted average of both 238U activity concentrations obtained from 235U and the equilibrium activity concentration of 234Th. It is clear that both methods must yield the same activity concentration. Fig. 5 presents the frequency distribution of the number of samples analyzed over the 238U activity concentration and the uncertainty of its activity concentration as a function of the activity concentration itself. The 238U activity concentration was calculated as the weighted average of the asymptotic 234Th activity concentration and the concentration obtained from the peak at 186 keV. It is clear from Fig. 5b that the trend line denoting the dependence of the uncertainty of the 238U activity concentration on the activity concentration itself is nearly horizontal, indicating the uncertainty does not depend on the activity concentration. This agrees with the well-known property (Weise et al., 2006) that near the detection limit the uncertainty depends weakly on the measurement result. The average uncertainty, calculated for the confidence interval of 68%, is 0.92 Bq/m3. Since the measurement results distribute normally with the standard deviation described by the uncertainty it follows that for samples with negligible concentration of 238U the
5
a1 (U-238, 186 keV U-235) / Bq/m3
of 2. It can be observed that the largest relative differences between the first and second measurement results occur at small concentrations, which implies that their main source is the time dependency of the background, which was not fully under control. However, its influence on the 238U activity concentration is minor at 226Ra concentrations that are well below the 238U concentrations.
3. Results and discussion
0
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a (U-238,186keV U-235) / Bq/m3 Fig. 4. Comparison of 238U activity concentration obtained from 186 keV peak of 235 U and equilibrium 234Th activity concentration.
measurement results distribute around zero activity concentration with the standard deviation of 0.92 Bq/m3. For a probability of 5% for making type-1 errors, i.e. for reporting false results with a probability of 5%, the decision threshold is assessed to be 1.5 Bq/m3. It is clear from the frequency distribution of Fig. 5a that its integral below the decision threshold is 2, indicating that 238U was determined in 24 samples of the 26 samples analyzed. Using the method described, the activity concentration of 234Th at the sampling time can be determined. Fig. 6a presents the distribution of the number of measurement results over the 234Th
M. Korun, K. Kovacˇicˇ / Applied Radiation and Isotopes 69 (2011) 636–640
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a (U-238) / Bq/m3 Fig. 5. Frequency distribution of determined 238U activity concentration (a) and dependence of uncertainty of 238U activity concentration on 238U activity concentration itself (b). Dashed line denotes average uncertainty.
activity concentration. It is clear that the distribution has its maximum at an activity concentration of zero, which implies that most of the 234Th activity concentrations are under the decision threshold of the measurements. In Fig. 6b the dependence of the uncertainty of the 234Th activity concentration on the 234Th activity concentration itself is presented. A minimum in the uncertainty can be observed near the zero activity concentration. This is due to an additional source of uncertainty, which originates from the calculation of the 234Th activity concentration that is extrapolated to the sampling date. The uncertainty of the extrapolated concentration depends, in addition to the 238U activity concentration, on the time interval of the extrapolation. Since the time interval between the sampling date and the date of the first measurement depends on the evaporation time and availability of the measuring equipment, the decision threshold and measurement uncertainty for 234Th are not constant. Therefore large measurement uncertainties coincide with large measurement results. The dashed line denotes the relation between the measurement result and its uncertainty near zero activity. In the conditions of the measurements described, the smallest measurement uncertainty, attained at 234Th activity concentrations near zero, is about 2 Bq/m3. This corresponds to the interval between the sampling and the first measurement of 10 days. From this uncertainty the decision threshold in the most favorable measurement conditions is assessed to be about 3 Bq/m3. The average decision threshold for the 234Th activity-concentration measurements can be assessed from the standard deviation of the distribution of a number of 234Th measurement results over the activity concentration to be 4.3 Bq/m3.
Fig. 6. Frequency distribution of determined 234Th activity concentration at sampling time (a) and dependence of uncertainty of 234Th activity concentration on 234Th activity concentration itself (b). Dashed line denotes relation between measurement result and its uncertainty.
4. Conclusion The easiest way to measure 238U activity concentration in water samples by gamma-ray spectrometry is by measuring the equilibrium concentration of its daughter 234Th, i.e. to perform the measurement of 234Th activity concentration few half lives of 234Th after sampling. To decrease the delay needed to attain equilibrium two measurements, one as soon as possible after sampling and the other at least one half life after the first measurements, may be performed and the equilibrium concentration is calculated by extrapolation. In this case besides the 238U activity concentration also the 234Th activity concentration at sampling time can be calculated. When the activity concentration of 226Ra is smaller or comparable to that of 238U, to lower the sensitivity and to improve the reliability of the measurements, information on 238U can be extracted also from the 235U peak at 186 keV supposing the isotopic ratio. Since this peak interferes with the peak from 226Ra occurring at the same energy, the activity concentration of 226Ra must be determined from the concentration of radon daughters in order to correct the area of the peak at 186 keV for the 226Ra contribution. It is advantageous to use this method when two measurements are performed since then the values of the parameters determining the activity ratio between 226Ra and radon daughters, the effective age of the dry residue and the radon emanation rate, can be validated. The 238U activity concentration is calculated as the weighted average of the equilibrium activity concentration and the activity concentration obtained from 235U. The reliability of this method is assured since the 238U activity concentration is obtained from two independent sources. With this combined method, measurement uncertainties of about 1 Bq/m3 were attained. The time dependence of the spectrometer background was identified as the main source of systematic influence affecting the results near the detection limit,
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which is due to the presence of radon daughters in air in the cavity of the detector shield. A decision threshold of 1.5 Bq/m3 was obtained in the measurements. Simultaneously with the equilibrium activity concentration of 234 Th, its activity concentration at the sampling time was also determined. Here, the sensitivity of the measurements depends on the 238U activity concentration in the sample and on the time interval between the sampling time and the time of the first measurement. The smallest decision threshold achieved was 2 Bq/m3, but the average decision threshold in the measurement was about 4.3 Bq/m3. Due to the decision threshold exceeding the 238U decision threshold and its lower solubility in water, 234Th was not determined in the analyzed samples. In the samples of ground water that were analyzed, 238U activity concentrations up to 20 Bq/m3 were measured. In most of the samples, activity concentrations of approximately 5 Bq/m3 were determined. The sensitivity of the method described and its simplicity in comparison to methods involving the radiochemical separation of elements and subsequent charged-particle measurements make this gamma-ray spectrometric method a useful tool in the prospecting of ground-water sources and drinking-water supplies.
Acknowledgments This work was performed under contract J7-0363 of the Slovenian Research Agency. The authors want to express their thanks and recognition to the staff in charge of sample preparation, especially Dr. Marijan Necˇemer, Petra Maver Modec dipl. phys., Drago Brodnik and Helena Fajfar.
References Glavicˇ-Cindro, D., Necˇemer, M., Vodenik, B., 2010. Optimization of the preparation of samples of water residue by evaporation IJS-DP-Draft (in Slovenian). ISO, 2007. ISO10703:2007 (E) Water quality – Determination of the activity concentration of radionucledes – Method by high resolution gamma-ray spectrometry. Osmond, J.K., Cowart, J.B., 1992. Ground water. In: Ivanovich, M., Harmon, R.S. (Eds.), Uranium-series Disequilibrium: Application to Earth, Marine and Environmental Sciences. Clarendon Press, Oxford. UNSCEAR, 2000. Sources and effects of ionizing radiations. Report to the General Assembly with Scientific Annexes, YN, New York. Weise, K., et al., 2006. Bayesian decision threshold, detection limit and confidence limits in ionizing-radiation measurement. Radiat. Prot. Dosimetry 121, 52–63.