OPTIMIZING THE COUNTING CONDITIONS FOR CARBON-14 FOR THE SAMPLE OXIDIZER-LIQUID SCINTILLATION COUNTER METHOD Vesa-Pekka Vartti STUK-Radiation and Nuclear Safety Authority, Laippatie 4, 00880 Helsinki, Finland. Email:
[email protected]. ABSTRACT. Radiocarbon is a pure beta emitter with a half-life of 5730 yr and a maximum energy of 156 keV. 14C is produced in nuclear reactors through neutron-induced reactions with carbon, nitrogen, and oxygen isotopes and is mostly released into the environment in gaseous releases such as carbon dioxide, 14CO2. Nowadays, 14C is one of the major contributors to the airborne radioactive releases in Finland. In recent years, 14C measurements in the environment have become a part of the monitoring program in the vicinities of Finnish nuclear power plants. The sample preparation for the liquid scintillation counting (LSC) was done with a 307 Sample Oxidizer by PerkinElmer. The dried environmental samples were combusted completely in an oxygen atmosphere to carbon dioxide and water. The 14CO2 was absorbed by the special reagent CarboSorb E (3-methoxypropylamine) and mixed with the LS cocktail PermaFluor E+. The apparatus is almost fully automatic and the combustion of a sample takes only a few minutes. Its disadvantage is the small amount of sample that can be combusted, with a 1-g upper limit for samples. The small volume per sample is challenging when the 14C concentrations are quite low and one would like to obtain a representative sample from the environment. The samples were counted with a low-level liquid scintillation counter Quantulus 1220. One of the most interfering phenomena in LSC is the quenching. In our case, it was extremely important since the CarboSorb E is a very strongly quenching agent. When combusting the actual samples, most of the CarboSorb E was used to absorb the CO2 from the sample and the quenching of the sample was low. When preparing a background sample by combusting only a cellulose cup, almost no CarboSorb E was consumed and the background spectrum was quite strongly quenched. This leads to overestimation of the background of the samples if the background spectrum is not corrected for quenching. This paper presents results of optimizing measuring conditions for 14C measurements with the Sample Oxidizer and the Quantulus liquid scintillation counter.
INTRODUCTION
Radiocarbon is a pure beta emitter with a half-life of 5730 yr and a maximum energy of 156 keV. It is produced naturally in the upper layers of the troposphere and the stratosphere by thermal neutrons absorbed by nitrogen atoms. 14C is also produced artificially in nuclear reactors through neutron-induced reactions with carbon, nitrogen, and oxygen isotopes. The nuclear bomb tests in late 1950s and early 1960s rapidly increased the 14C concentration in the atmosphere, reaching the maximum in 1963 when the level was almost double that of the natural 14C concentration. Since then, the 14C concentrations have decreased but are still somewhat higher than before the tests (IAEA 2004). Today, 14C is one of the major contributors to the airborne radioactive releases from Finnish nuclear power plants. Studies of 14C concentrations in the environment of Finnish NPPs became a part of the monitoring program during 2003–2007. In order to evaluate 14C releases from nuclear facilities, a background activity of approximately 250 Bq of 14C per kilogram of stable carbon can be used (Otlet et al. 1997). MATERIALS AND METHODS
The samples for 14C measurements were collected from the vicinities of the Finnish nuclear power plants. Finland has 4 nuclear power units at 2 sites. Two 510-MWe pressurized-water reactors (PWR) are situated in Loviisa and two 870-MWe boiling-water reactors (BWR) in Olkiluoto. The third unit at Olkiluoto Nuclear Power Plant is under construction, and it is based on the French-German European pressurized-water reactor (EPR) concept. The thermal output of the reactor is 4300 MW with a net electric power output of ~1600 MW (STUK, www.stuk.fi).
© 2009 by the Arizona Board of Regents on behalf of the University of Arizona LSC 2008, Advances in Liquid Scintillation Spectrometry edited by J Eikenberg, M Jäggi, H Beer, H Baehrle, p 293–298
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The collected sample materials included leaves of birch (Betula pendula) and black alder (Alnus glutinosa), twigs and leaves of blueberry (Vaccinium myrtillus), pasture grass, grains of barley, rosehips (Rosa canina), and lyme grass (Leymus arenarius). The sample material, usually a few hundred grams in fresh weight to ensure a representative sample, was dried overnight in ovens at 105 °C and ground to fine powder with mills. 14 C
in the samples was determined by combusting the samples in an oxygen atmosphere to carbon dioxide and water, absorbing the carbon dioxide with a special reagent and counting the sample with a liquid scintillation (LS) counter. The combustion of the samples to carbon dioxide and water was performed with the 307 Sample Oxidizer and the special reagents were made by PerkinElmer. A few hundred milligrams of dried sample material were combusted with the Sample Oxidizer until no visible residue was seen, which usually took 1 to 2 min. After the combustion, 2 separate samples (a sample of 3H [water] and 14C [carbon dioxide]) were trapped at ambient temperature, thus reducing the cross-contamination. The oxidizer condensed the water in a cooled coil and the tritium sample was then washed into a vial where it was mixed with an appropriate LS cocktail (Monophase S). The 14CO2 was then trapped by a vapor-phase reaction with an amine (CarboSorb E), forming carbamate, which was mixed with an appropriate LS cocktail (PermaFluor E+). The flow diagram of the 307 Sample Oxidizer is presented in Figure 1.
Figure 1 Flow diagram of sample preparation with the 307 Sample Oxidizer (http://las.perkinelmer.com)
After oxidation, the samples were measured with the low-background liquid scintillation counter Quantulus 1220 by PerkinElmer. The quench curves, recovery and memory tests were performed with the liquid SpecChec standards by PerkinElmer. The lower limit of detection (LLD) was calculated using the formula (L’Annunziata 2003): 4.65 B LLD = --------------------60EVTX where B is background counts, E the fractional detection efficiency (cpm/dpm), V the sample mass (kg), T the counting time in minutes, and X is any factor that is relevant (e.g. chemical
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yield or decay correction). With a 600-min counting time, a background of 3.0 cpm, a detection efficiency of 0.75, and a sample volume of 1 g, the LLD is 7.3 Bq/kg. RESULTS AND DISCUSSION
One of the major causes for interference in LSC is the quenching, which means the loss of counts due to sample or cocktail characteristics. The CarboSorb E, used for trapping carbon dioxide, is a heavily quenching agent, which can cause problems in the LSC process. If the carbon content in the sample is not enough to use up all of the CarboSorb, the samples may be strongly quenched, reducing the counting efficiency. The quenching was determined by making the samples with 10 mL of PermaFluor LS cocktail and different amounts (0, 2, 4, 6, 8, and 10 mL) of CarboSorb E and then adding a known activity of 14C into the sample vials. These samples were measured with the Quantulus and the counting efficiency was recorded and plotted as a function of the external standard quenching parameter, SQP(E) (Table 1). The quench curves were recorded at 4 different windows (130–350, 130–400, 130–450, and 130–500); an example is seen in Figure 2. The best correlation was found with the second-order polynomial function, E% = a * SQP(E)2 + b * SQP(E) + c. Table 1 Results from quench curve measurements (added 14C 91600 dpm). 130–500 130–450 130–400
130–350
Samplea
SQP(E) net cpm Eff %
net cpm Eff %
net cpm Eff %
net cpm Eff %
0 CS + 10 PF 0 CS + 10 PF 0 CS + 10 PF 2 CS + 10 PF 2 CS + 10 PF 2 CS + 10 PF 4 CS + 10 PF 4 CS + 10 PF 4 CS + 10 PF 6 CS + 10 PF 6 CS + 10 PF 6 CS + 10 PF 8 CS + 10 PF 8 CS + 10 PF 8 CS + 10 PF 10 CS + 10 PF 10 CS + 10 PF 10 CS + 10 PF
807.9 802.5 806.1 761.1 762.7 761.8 724.3 729.7 726.4 692.6 684.7 679.1 663.3 664.2 666.2 649.0 651.4 648.2
73101 72826 72922 70744 70342 70371 64938 64981 65016 56667 56693 56568 50081 50386 50105 48365 48354 48392
60110 59934 59970 64622 64272 64385 63449 63491 63531 56554 56577 56454 50063 50364 50086 48358 48351 48385
42988 42892 42836 50486 50234 50280 54889 55024 55013 53753 53752 53640 48862 49198 48930 47665 47668 47718
a CS
77814 77412 77518 71403 70997 70998 64959 65005 65034 56667 56693 56567 50081 50386 50106 48365 48355 48393
85.0% 84.5% 84.6% 78.0% 77.5% 77.5% 70.9% 71.0% 71.0% 61.9% 61.9% 61.8% 54.7% 55.0% 54.7% 52.8% 52.8% 52.8%
79.8% 79.5% 79.6% 77.2% 76.8% 76.8% 70.9% 70.9% 71.0% 61.9% 61.9% 61.8% 54.7% 55.0% 54.7% 52.8% 52.8% 52.8%
65.6% 65.4% 65.5% 70.5% 70.2% 70.3% 69.3% 69.3% 69.4% 61.7% 61.8% 61.6% 54.7% 55.0% 54.7% 52.8% 52.8% 52.8%
46.9% 46.8% 46.8% 55.1% 54.8% 54.9% 59.9% 60.1% 60.1% 58.7% 58.7% 58.6% 53.3% 53.7% 53.4% 52.0% 52.0% 52.1%
= CarboSorb; PF = PermaFluor.
The recovery of the method was tested by combusting a known activity of 14C and comparing those samples with non-combusted samples with the same activity. The recovery was found to be 98.4 ± 1.9% (Table 2); thus, it was decided that the method does not need correction for recovery but it is taken into account in the uncertainty estimations. If samples with high activity are combusted, there is a possibility that some of the activity is retained in the Oxidizer and released when combusting the next samples. This is called the memory effect, and it was checked by combusting samples of a known activity of 14C and by combusting an inactive standard sample after each active sample. The activity was checked with the Quantulus and the memory was found to be 0.03 ± 0.01% (Table 2). Memory does not need to be corrected when measuring samples with normal environmental 14C levels.
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Figure 2 The quench curve of 14C samples in window 130–500 (channels)
Table 2 Recovery and memory tests. Non-combusted standards Recovery test
Memory test
Recovery test
Memory test
Avg. (cpm)
std. dev. (cpm)
Avg. (cpm)
std. dev. (cpm)
Avg. (cpm)
std. dev. (cpm)
%
±
%
±
78905
1114
77665
1054
22.2
6.4
98.4%
1.9%
0.03%
0.01%
In every set of samples, also a background sample was combusted with similar reagent amounts as in the samples. This was found problematic because practically no CarboSorb was used in the combustion process due to the low carbon content in the cellulose cups used as sample containers. Thus, the background sample consisted of 10 mL of pure CarboSorb and the background spectrum was strongly quenched; the background counts in the beginning of the spectrum (where the 14C was recorded) were increased. The effect can clearly be seen by comparing 2 background spectra with different CarboSorb amounts (Figure 3). Due to this effect, the background may be overestimated by 20%, which leads to underestimating the activities of 14C in the samples.
Figure 3 Background spectra with 2 mL (left) and 10 mL (right) of CarboSorb, and 10 mL of PermaFluor, respectively
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The background must be corrected. The best way would usually be to combust an organic background sample with inactive carbon, e.g. with ancient wood material (over 50,000 yr old) with no 14C in it. This would use up the CarboSorb and the quenching in the background spectrum would be at the same level as in the measured samples. Another way to correct the background would be to adjust the CarboSorb amount in the background sample to match the quench level in the samples. A third way would be to mathematically correct the background counts to correspond the quench level of the samples. By plotting the measured background counts against SQP(E), it was assumed that the background depends on the SQP(E) and the background counts in the chosen window can be expressed as cpm = a * SQP(E) + b (Figure 4). By assuming that the slope (a) remains constant, one can solve out the variable b when the cpm and the SQP(E) of the measured background sample is known. This way the background counts can be corrected to correspond to the SQP(E) of each individual sample. This method is based on certain assumptions but probably describes the background better than an uncorrected background.
Figure 4 Background cpm vs. SQP(E)
One of the problems in the Sample Oxidizer process is the relatively small sample size that can be used. The maximum CarboSorb E amount (10 mL) can take up 48 mmoles of carbon, which limits the sample size for environmental samples to about 1 g. With such a small sample size, it is very challenging to create a representative environmental sample, whereupon the homogeneity of the sample material must be carefully checked and parallel samples must be measured. Results of the measured samples from the vicinity of the Finnish NPPs are presented in Table 3. The total carbon content in the samples was not measured, but if the carbon content is assumed to be 50%, most results are in a good agreement with the background activity of 250 Bq of 14C per kilogram of stable carbon. The uncertainties are high due to large variation in the parallel sample measurements.
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V-P Vartti Table 3 Results of the 14C activity concentrations in environmental samples from the vicinity of the Finnish NPPs in 2006–2007. 14C
Sample
Place
Reference date
Bq/kg (dry weight)
±% (2σ)
Rosehip Blueberry twigs and leaves Leaves of black alder Blueberry twigs and leaves Leaves of black alder Pine needles Grass Leaves of birch Leaves of birch Grass Pine needles
Olkiluoto Olkiluoto Olkiluoto Loviisa Loviisa Loviisa Loviisa Loviisa Olkiluoto Olkiluoto Olkiluoto
7.9.2006 7.9.2006 7.9.2006 31.8.2006 31.8.2006 30.8.2007 30.8.2007 30.8.2007 17.7.2007 17.7.2007 17.7.2007
72 127 90 77 82 122 123 146 116 119 152
20 26 20 18 20 60 76 18 12 50 36
CONCLUSIONS
In our opinion, this method must be further developed, but the first results show it can be used for monitoring purposes at environmental levels. The quenching in the background sample should be approximately at the same level as in the actual samples. If the background sample has a higher quench level than the samples, the background correction is too large and the results are too small. In such a case, the background can be corrected mathematically, but care must be taken when evaluating the results. The sample size that can be used for one analysis is so small that the homogenization must be thorough, and a few replicate samples are needed to check the variation in the samples. REFERENCES International Atomic Energy Agency (IAEA). 2004. Management of waste containing tritium and carbon14. Technical Reports Series 421. Vienna: IAEA. L’Annunziata M. 2003. Handbook of Radioactivity Analysis. 2nd edition. London: Academic Press.
Otlet RL, Walker AJ, Fulker MJ, Collins C. 1997. Background carbon-14 levels in UK foodstuffs, 1981– 1995, based upon a 1992 survey. Journal of Environmental Radioactivity 34(1):91–101.
