- Email: [email protected]
A new method for secondary standard measurements with the aid of liquid scintillation counting
ARTICLE IN PRESS
Applied Radiation and Isotopes 64 (2006) 1459–1464 www.elsevier.com/locate/apradiso
A new method for secondary standard measurements with the aid of liquid scintillation counting K. Kossert Physikalisch-Technische Bundesanstalt (PTB), Department 6.1, Bundesallee 100, D-38116 Braunschweig, Germany
Abstract A new secondary standard measuring procedure based on liquid scintillation counting has been developed at PTB. In this procedure, the efficiency curve, i.e. the counting efficiency of the nuclide to be investigated as a function of the tritium counting efficiency, is—in contrast to the CIEMAT/NIST method—not calculated but determined by experiment. For the measurements, gamma-emitting activity standards are used which were calibrated by other methods. The procedure was successfully used to calibrate solutions of the radionuclides 54Mn, 57Co, 65Zn, 85Sr and 125I with small uncertainties. r 2006 Elsevier Ltd. All rights reserved. Keywords: Liquid scintillation counting; Secondary activity standardization
1. Introduction Liquid scintillation counting (LSC) in combination with the CIEMAT/NIST method (Coursey et al., 1986) is a widely used procedure for reliable activity determinations of a great number of radionuclides. An important advantage of this method is that, particularly for highenergy beta emitters, small uncertainties can be obtained. Moreover, the method furnished good results for some radionuclides decaying by electron capture (see e.g. Grau Carles et al., 1994). The uncertainties are, however, clearly larger than in the case of other methods, and for some nuclides discrepancies up to a few per cent are observed. Beyond that, the CIEMAT/NIST method cannot easily be applied to radionuclides which include excited levels in their decay scheme with half-lives of a few nanoseconds up to a few microseconds. For activity determinations of the two groups of radionuclides mentioned, a new secondary standard measuring procedure can be used which is described in this article. In this procedure, the efficiency curve (the counting efficiency of the nuclide to be investigated as a Tel.: +49 531 592 6311; fax: +49 531 592 6305.
E-mail address: [email protected]. 0969-8043/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2006.02.060
function of the tritium counting efficiency), is—in contrast to the CIEMAT/NIST method—not calculated but determined by experiment. For the measurements, activity standards were used which were calibrated by other methods. As the nuclide counting efficiency is always traced back to measurements with a tritium activity standard, conservation over many years is possible. The method was successfully used to calibrate solutions of the radionuclides 54Mn, 57Co, 65Zn, 85Sr and 125I with small uncertainties. The results were compared with those obtained by absolute measurements or measurements with calibrated ionization chambers. In addition to an example with a detailed uncertainty analysis, possible future applications are presented. These also comprise a possible use of the method within the scope of the extended international reference system (extended SIR). 2. Method The new method is illustrated in Fig. 1. The first step covers the measurement of the counting efficiency of the nuclide under study as a function of a quench-indicating parameter QIP. To this end, a set with a few vials, each containing the scintillation cocktail and a well-known amount of an activity standard solution, is prepared. Since
ARTICLE IN PRESS 1460
K. Kossert / Applied Radiation and Isotopes 64 (2006) 1459–1464
activity standards of tracer and nuclide + measurement
εtracer (QIP ) εnuclide (εtracer) εnuclide (QIP )
activity standard + measurement of the tracer
measurement
εtracer (QIP )
Rnuclide, (QIP )
εnuclide
anuclide =
Rnuclide εnuclide m
Fig. 1. Illustration of the secondary standard measuring procedure. R denotes the counting rate, QIP the quench indicating parameter, e the counting efficiency, anuclide is the solution activity concentration and m is the source mass.
the activity of the samples is known, the measured net counting rates R yield the counting efficiency enuclide. From the second sample onwards, increasing amounts of a quenching agent are used to decrease the counting efficiency. In the same way the counting efficiency etracer of the tritium tracer is measured as a function of the quench-indicating parameter. The quench-indicating parameter QIP, which is automatically measured by an external standard source such as 133Ba, 137Cs, 226Ra and 152Eu, can be assumed to be independent of the radionuclide in the sample. Therefore the QIP allows one to get a link between the counting efficiencies of both radionuclides, and consequently to obtain the counting efficiency enuclide of the nuclide under study as a function of the counting efficiency etracer of the tracer. This function is called the ‘‘efficiency curve’’. This procedure replaces the CIEMAT/ NIST calculations, where the counting efficiencies of both the nuclide and 3H are calculated as a function of a free parameter M. The link is then obtained via this parameter M for a given quench state. In the next step which may occur a few years later, the 3 H calibration curve (the counting efficiency etracer of the tracer expressed as a function of the quench-indicating parameter QIP), is measured again. The newly measured calibration curve is required to account for instability of the apparatus, in particular to allow for changes of the spectra and the system response. Finally, a sample series is prepared with a solution of the nuclide under study. Again, the counting rate and the quenching indicator QIP are measured. This indicator allows one to find the corresponding tracer efficiency from the 3H calibration curve. With the known tracer efficiency, the corresponding nuclide efficiency can be obtained by using the measured efficiency curve. In conclusion, there are all the ingredients
to calculate the source activity or the activity concentration of the solution. It should be noted that the sample composition and the measuring geometry must be very similar for all measurements and that the period between the measurements with the tracer and the nuclide under study should be small. On the other hand, the duration between the determination of the efficiency curve and new measurements can in principle be unlimited, since the measured efficiency curve is time independent. This efficiency curve is even valid if another counter is used. These are the main benefits of the method in comparison with less robust secondary standardization techniques, where enuclide is measured only as a function of the QIP. 3. Applications The measurements were performed in a two-photomultiplier tube Wallac 1414 GuardianTM liquid scintillation spectrometer at 20 1C. The quench-indicating parameter SQP(E) was measured by means of an external source of 152 Eu. The samples were prepared with 15 ml of Ultima GoldTM scintillator and 1 ml of distilled water in 20 ml lowpotassium borosilicate glass vials. Nitromethane (CH3NO2) was used as quenching agent to vary the counting efficiency. To obtain the calibration curve, e (3H) vs. SQP(E), weighed aliquots of a standard solution of 3H (tritiated water) were used to prepare a set of eleven samples. The activity of the 3H standard solution is traceable to the national German measurement standard at the PTB and was verified by other national metrology institutes within the scope of EUROMET and ICRM comparisons (Makepeace et al., 1994).
ARTICLE IN PRESS K. Kossert / Applied Radiation and Isotopes 64 (2006) 1459–1464
For all nuclides studied the experimental data was fitted with a fourth-order polynomial so as to obtain the working efficiency curve. The polynomial coefficients pi are listed in Table 1. The counting efficiency of the nuclide corresponding to a specific counting efficiency of tritium is then given by efit ¼ enuclide ðetracer Þ ¼
4 X
pi ðetracer Þi :
(1)
i¼0
1461
The residuals D ¼ ðei 2efit Þ=efit of the individual measured efficiencies ei and Eq. (1) are shown in Fig. 2. Before starting the LSC measurements, impurity studies by Ge-detector spectrometry were carried out in the energy range from 10 keV to 2.7 MeV. No photon-emitting impurity could be found in the solutions of 54Mn, 109Cd and 125I. Some of the 85Sr solutions used for the measurements contained small amounts of 59Fe, 84Rb
Table 1 Polynomial coefficients for the calculation of efficiency curves and relative standard uncertainties uðaÞ=a of the activity concentration a of the reference solutions used for the measurements (k ¼ 1) Radionuclide
P0
p1
p2
p3
p4
u(a)/a of standard in %
54
0.08312699 0.1265048 0.3208553 0.7208347 0.2240818 0.4640571 1.237731 0.1683856 0.2299414
2.422951 2.486873 5.137528 3.143484 0.8228705 8.938517 0.9669007 3.494121 8.281299
8.162614 6.463662 15.48763 20.36633 2.556439 30.28222 8.676466 13.89836 31.71742
17.43643 12.08019 27.88327 41.87398 6.864565 49.22582 22.63171 26.42405 59.48567
12.17397 8.458781 18.88876 31.55018 3.796875 30.88905 20.69645 18.81830 41.45826
0.16 0.3 0.2–0.41 1 0.5 0.5 0.25 1 0.42
The efficiencies of
57
67
Co,
Ga and
85
Sr depend on the counter system (see text for explanation).
54
57
Mn
1 (εi−εfit)/εfit in %
0.5 0 -0.5
1
0.5 0 -0.5
0 -0.5
-1
-1
-1
-1.5
-1.5
0.25 0.3 0.35 0.4 0.45 εtracer
(εi−εfit)/εfit in %
1
0.5 0 -0.5
88
Sr
1 (εi−εfit)/εfit in %
85
Ga
0.5 0 -0.5
0 -0.5
-1
-1
-1.5
-1.5
-1.5
1
1.5
109
Cd
1 (εi−εfit)/εfit in %
1.5
0.5 0 -0.5
0.25 0.3 0.35 0.4 0.45 εtracer
In
0.5 0 -0.5
-1.5
1.5
111
1
0.25 0.3 0.35 0.4 0.45 εtracer
125
I
0.5 0 -0.5 -1
-1
-1 -1.5
0.25 0.3 0.35 0.4 0.45 εtracer
Y
0.5
-1 0.25 0.3 0.35 0.4 0.45 εtracer
0.25 0.3 0.35 0.4 0.45 εtracer
1.5
1.5 67
1
0.25 0.3 0.35 0.4 0.45 εtracer
Zn
0.5
-1.5
1.5 (εi−εfit)/εfit in %
65
Co
(εi−εfit)/εfit in %
(εi−εfit)/εfit in %
1
1.5
1.5
1.5
(εi−εfit)/εfit in %
a
(εi−εfit)/εfit in %
Mn Coa 65 Zn 67 Gaa 85 a Sr 88 Y 109 Cd 111 In 125 I 57
0.25 0.3 0.35 0.4 0.45 εtracer
-1.5
0.25 0.3 0.35 0.4 0.45 εtracer
Fig. 2. Residuals D ¼ ðei 2efit Þ=efit of the individual measured efficiencies ei and Eq. (1).
ARTICLE IN PRESS K. Kossert / Applied Radiation and Isotopes 64 (2006) 1459–1464
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0.800
Table 2 Standard uncertainty components for the activity concentration of a solution of 125I measured by the liquid scintillation secondary counting technique
125
I
εnuclide
0.775 0.750
Component
0.725
Statistical component 0.05 Weighing 0.05 Starting time and duration o0.01 Dead time 0.1 Background 0.03 Adsorption o0.01 Impurities (no impurities detected) o0.01 Half life (Dt ¼ 310 d) 0.11 Efficiency curve (fitting) 0.05 125 I activity standard 0.42 Calibration (experimental determination of the efficiency curve) 0.2 Sample stability o0.05
0.700 0.675 0.650 0.625 0.25
Fig. 3. Efficiency curve for
0.30
0.35 εtracer
0.40
0.45
I in 15 ml Ultima GoldTM plus 1 ml water.
125
and 110mAg. A small impurity of 114mIn could be detected in the 111In solution. The impurities found in other solutions were negligibly small. 3.1.
125
I
The liquid scintillation measurements were carried out with an 125I solution, which was calibrated by photon– photon coincidence counting (Schrader and Walz, 1987) within the scope of an international comparison organized in 2004 by the Bureau International des Poids et Mesures (BIPM). The efficiencies determined are shown as open circles in Fig. 3. The efficiency curve was then obtained by fitting the fourth-order polynomial, which is also shown in Fig. 3. The efficiency curve was used to calibrate another solution of 125I which was also calibrated by coincidence counting. The liquid scintillation counting result was only lower by 0.03% than the result obtained by absolute standardization. The relevant uncertainty components are summarized in Table 2. It is important to note that the uncertainty of the tracer activity cancels out if the aliquots used for the determination of the efficiency curve and the calibration curve stem from the same standard solution. In this case the corresponding uncertainty u(atracer) of the 3H activity standard does not contribute to the total uncertainty. This characteristic of the method can easily be demonstrated by altering the value of the activity concentration atracer of the 3H activity standard, e.g. if atracer is arbitrarily increased by 20% one gets a completely different efficiency curve, but the result for anuclide would be the same if this curve and the same (wrong) value atracer is used again for the final analysis. Concluding, the method can even be applied if the true value of the activity concentration atracer is unknown. Since the activity of the second 125I solution was also known from an alternative technique, the efficiency was determined again. The corresponding results are shown as solid triangles in Fig. 3.
u(a)/a in %
Square root of the sum of quadratic components
3.2.
65
0.50
Zn
The efficiency curve of 65Zn was determined by measuring a solution which had been calibrated by 4pb–g coincidence counting (Kossert et al., 2006). The result is traceable to an international comparison organized by the BIPM in 2002. One and a half years later the efficiency curve was used to analyze new measurement data within the scope of the EUROMET exercise no. 721 (Be´ et al., 2005). The solution of this exercise was also calibrated by 4pb–g coincidence counting with a total relative standard uncertainty of 0.2%. The uncertainty of the LSC result was found to be 0.4%, and the result was only lower than the result from the primary standardization method by 0.05%. In the case of 65Zn 1 ml of the widely used complexing agent EDTA (0.05 mol/l) was used instead of distilled water in order to improve the sample stability. However, it is important to mention that the long-term sample stability was low and the experimentally determined counting efficiency decreased slowly. The corresponding uncertainty was reduced by using a similar time of a few days between sample preparations and counting for all measurements. More details about the 65Zn standardization and the liquid scintillation counting measurements are given by Kossert et al. (2006) in the same issue of this journal. 3.3.
54
Mn,
88
Y,
109
Cd and
111
In
The method was also applied to 54Mn, 88Y, 109Cd and In. The corresponding polynomial coefficients to calculate the efficiency curve as well as the relative standard uncertainties u(a)/a of the reference solutions used are listed in Table 1. In the case of 88Y, the samples were prepared with 15 ml Ultima GoldTM scintillator, 0.8 ml of distilled water and 0.2 ml of the complexing agent HDEHP. The measured efficiencies varied considerably from one measurement to
111
ARTICLE IN PRESS K. Kossert / Applied Radiation and Isotopes 64 (2006) 1459–1464
another for the same sources. At present the author does not have any explanation for this poor reproducibility of the results. 3.4.
57
Co,
67
Ga and
85
Sr
The decay schemes of 57Co, 67Ga and 85Sr contain excited levels with half-lives of a few nanoseconds up to a few microseconds. The decays of these states cannot be treated as coincident transitions because the coincidence time of commercial counters is usually a few nanoseconds. On the other hand these transitions cannot be treated as independent decays due to the counter dead time, which usually amounts to a few microseconds. Therefore, the CIEMAT/NIST method cannot be applied for such nuclides without complicated corrections. The specific activities of the solution of the three nuclides mentioned were determined at PTB by measurements with calibrated 4p ionization chambers. Since the liquid scintillation counting efficiencies depend on the coincidence time and dead time of the counter system, the efficiency curves determined can probably not be transferred to other counters. The residual plots of 57Co and 85Sr in Fig. 2 contain several points which deviate considerably from zero. A possible explanation for this is the—in some cases— relatively large period between the 3H calibration and the nuclide calibration. It was ascertained that the method is also capable of calibrating solutions of 57Co and 85Sr with an activity concentration of about 35 and 10 Bq/g, respectively. The determination of such low activities by other methods is hardly possible if the time for the measurements is limited. Finally, the efficiency curve of 67Ga was also determined. The corresponding polynomial coefficients are listed in Table 1. 4. Possible applications for international comparisons As explained above, the method can even be applied if the activity of the 3H solution is not known, provided the same 3H standard solution is used for both the determination of the efficiency curve and the measurement of an updated calibration curve that is used for the final activity determination. Therefore, the method can be used for comparisons. Two examples of such possible procedures are explained briefly in the following. To keep the examples simple, it is assumed that the two laboratories A and B participate in a bilateral comparison of their 63Ni activity standards of known activity concentration. To this end, both laboratories determine the efficiency curve efit as given in Eq. (1). The participants have to use the same sample composition (scintillation cocktail plus other ingredients), aliquots of the same 3H solution and the same value for the tracer activity atracer so that a tracer calibration curve can be determined. The counting efficiencies efit,A and efit,B can finally be compared. A mean value of the two results efit,A and efit,B could be
1463
considered as a reference value. Of course, this would become more meaningful if there would be more participants in such a comparison. Alternatively, the two laboratories may send standardized solutions to a neutral laboratory which then determines the efficiency curves, the 3H calibration curve and the counting efficiencies efit,A and efit,B using the values for the activity concentrations communicated by the participants. This procedure could even be applied to the nuclides 57Co, 67Ga and 85Sr since only one liquid scintillation counter is used. Such a comparison procedure would be similar to the international reference system (SIR) which makes use of 4p ionization chambers (IC) instead of a liquid scintillation counter. The LS counting efficiencies efit are comparable to calibration factors of ionization chambers, although it is to be noted that efit is a function whereas a calibration factor is a number. In addition, it must be considered that the LS results are traced back to a tritium standard, whereas IC measurements are usually compared with standard sources of 226 Ra. Thus, the half-life of 3H must be taken into account and if comparison exercises take place over several decades, the decay correction would be a somewhat greater problem. For both examples it would be desirable to have an international 3H activity standard as well as a recipe for the production of a standardized scintillation cocktail. 5. Summary and outlook The new method presented in this paper was successfully applied to calibrate solutions of radionuclides decaying by electron-capture. This also comprises radionuclides like 57 Co and 85Sr, which cannot easily be calibrated by the CIEMAT/NIST method. The method could become an important secondary standardization method in particular due to a better control of the apparatus stability. It has also been demonstrated that the method is suitable for low activities and for nuclides having low detection efficiencies in 4p ionization chambers. Thus, this liquid scintillation secondary counting technique could supplement measurements by calibrated ionization chambers and the procedure could also become a powerful tool for international comparisons. However, the applicability for comparisons has still to be investigated. Acknowledgements The author wishes to thank E. Gu¨nther for his kind encouragement as well as R. Klein and H. Schrader for their measurements. References Be´, M.-M., Amiot, M.-N., Bobin, C., Le´py, M.-C., Plagnard, J., Lee, J.-M., Lee, K.B., Park, T.S., Luca, A., Sahagia, M., Razdolescu, A.-M.,
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Grigorescu, L., Sato, Y., Hino, Y., Kossert, K., Klein, R., Schneider, M.K.H., Schrader, H., Dryak, P., Sochorova, J., Kovar, P., Auerbach, P., Havelka, M., Altzitzoglou, T., Iwahara, A., da Silva, M.A.L., Delegado, J.U., da Silva, C.J., Johansson, L., Collins, S., Stroak, A., 2005. Activity measurements and gamma emission intensities determination in the decay of 65Zn. Rapport CEA-R-6081, CEA Saclay, Gifsur-Yvette, France. Coursey, B.M., Mann, W.B., Grau Malonda, A., Garcia-Toran˜o, E., Los Arcos, J.M., Gibson, J.A.B., Reher, D., 1986. Standardization of carbon-14 by 4pb liquid scintillation efficiency tracing with hydrogen3. Appl. Radiat. Isot. 37, 403.
Grau Carles, A., Grau Malonda, A., Grau Carles, P., 1994. EMI, the counting efficiency for electron capture, electron capture-gamma and isomeric transitions. Comput. Phys. Comm. 79, 115. Kossert, K., Janben, H., Klein, R., Schneider, M., Schrader, H., 2006. Standardization and nuclear decay data of Zn-65. Appl. Radiat. Isot. in press, doi:10.1016/j.apradiso.2006.02.054. Makepeace, J.L., Clark, F.E., Picolo, J.L., Coursol, N., Gu¨nther, E., Unterweger, M.P., 1994. Intercomparison of internal proportional gas counting of 85Kr and 3H. Nucl. Instrum. Meth. A 339, 343. Schrader, H., Walz, K.F., 1987. Standardization of 125I by photon–photon coincidence counting and efficiency extrapolation. Appl. Radiat. Isot. 38, 763.
Applied Radiation and Isotopes 64 (2006) 1459–1464 www.elsevier.com/locate/apradiso
A new method for secondary standard measurements with the aid of liquid scintillation counting K. Kossert Physikalisch-Technische Bundesanstalt (PTB), Department 6.1, Bundesallee 100, D-38116 Braunschweig, Germany
Abstract A new secondary standard measuring procedure based on liquid scintillation counting has been developed at PTB. In this procedure, the efficiency curve, i.e. the counting efficiency of the nuclide to be investigated as a function of the tritium counting efficiency, is—in contrast to the CIEMAT/NIST method—not calculated but determined by experiment. For the measurements, gamma-emitting activity standards are used which were calibrated by other methods. The procedure was successfully used to calibrate solutions of the radionuclides 54Mn, 57Co, 65Zn, 85Sr and 125I with small uncertainties. r 2006 Elsevier Ltd. All rights reserved. Keywords: Liquid scintillation counting; Secondary activity standardization
1. Introduction Liquid scintillation counting (LSC) in combination with the CIEMAT/NIST method (Coursey et al., 1986) is a widely used procedure for reliable activity determinations of a great number of radionuclides. An important advantage of this method is that, particularly for highenergy beta emitters, small uncertainties can be obtained. Moreover, the method furnished good results for some radionuclides decaying by electron capture (see e.g. Grau Carles et al., 1994). The uncertainties are, however, clearly larger than in the case of other methods, and for some nuclides discrepancies up to a few per cent are observed. Beyond that, the CIEMAT/NIST method cannot easily be applied to radionuclides which include excited levels in their decay scheme with half-lives of a few nanoseconds up to a few microseconds. For activity determinations of the two groups of radionuclides mentioned, a new secondary standard measuring procedure can be used which is described in this article. In this procedure, the efficiency curve (the counting efficiency of the nuclide to be investigated as a Tel.: +49 531 592 6311; fax: +49 531 592 6305.
E-mail address: [email protected]. 0969-8043/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2006.02.060
function of the tritium counting efficiency), is—in contrast to the CIEMAT/NIST method—not calculated but determined by experiment. For the measurements, activity standards were used which were calibrated by other methods. As the nuclide counting efficiency is always traced back to measurements with a tritium activity standard, conservation over many years is possible. The method was successfully used to calibrate solutions of the radionuclides 54Mn, 57Co, 65Zn, 85Sr and 125I with small uncertainties. The results were compared with those obtained by absolute measurements or measurements with calibrated ionization chambers. In addition to an example with a detailed uncertainty analysis, possible future applications are presented. These also comprise a possible use of the method within the scope of the extended international reference system (extended SIR). 2. Method The new method is illustrated in Fig. 1. The first step covers the measurement of the counting efficiency of the nuclide under study as a function of a quench-indicating parameter QIP. To this end, a set with a few vials, each containing the scintillation cocktail and a well-known amount of an activity standard solution, is prepared. Since
ARTICLE IN PRESS 1460
K. Kossert / Applied Radiation and Isotopes 64 (2006) 1459–1464
activity standards of tracer and nuclide + measurement
εtracer (QIP ) εnuclide (εtracer) εnuclide (QIP )
activity standard + measurement of the tracer
measurement
εtracer (QIP )
Rnuclide, (QIP )
εnuclide
anuclide =
Rnuclide εnuclide m
Fig. 1. Illustration of the secondary standard measuring procedure. R denotes the counting rate, QIP the quench indicating parameter, e the counting efficiency, anuclide is the solution activity concentration and m is the source mass.
the activity of the samples is known, the measured net counting rates R yield the counting efficiency enuclide. From the second sample onwards, increasing amounts of a quenching agent are used to decrease the counting efficiency. In the same way the counting efficiency etracer of the tritium tracer is measured as a function of the quench-indicating parameter. The quench-indicating parameter QIP, which is automatically measured by an external standard source such as 133Ba, 137Cs, 226Ra and 152Eu, can be assumed to be independent of the radionuclide in the sample. Therefore the QIP allows one to get a link between the counting efficiencies of both radionuclides, and consequently to obtain the counting efficiency enuclide of the nuclide under study as a function of the counting efficiency etracer of the tracer. This function is called the ‘‘efficiency curve’’. This procedure replaces the CIEMAT/ NIST calculations, where the counting efficiencies of both the nuclide and 3H are calculated as a function of a free parameter M. The link is then obtained via this parameter M for a given quench state. In the next step which may occur a few years later, the 3 H calibration curve (the counting efficiency etracer of the tracer expressed as a function of the quench-indicating parameter QIP), is measured again. The newly measured calibration curve is required to account for instability of the apparatus, in particular to allow for changes of the spectra and the system response. Finally, a sample series is prepared with a solution of the nuclide under study. Again, the counting rate and the quenching indicator QIP are measured. This indicator allows one to find the corresponding tracer efficiency from the 3H calibration curve. With the known tracer efficiency, the corresponding nuclide efficiency can be obtained by using the measured efficiency curve. In conclusion, there are all the ingredients
to calculate the source activity or the activity concentration of the solution. It should be noted that the sample composition and the measuring geometry must be very similar for all measurements and that the period between the measurements with the tracer and the nuclide under study should be small. On the other hand, the duration between the determination of the efficiency curve and new measurements can in principle be unlimited, since the measured efficiency curve is time independent. This efficiency curve is even valid if another counter is used. These are the main benefits of the method in comparison with less robust secondary standardization techniques, where enuclide is measured only as a function of the QIP. 3. Applications The measurements were performed in a two-photomultiplier tube Wallac 1414 GuardianTM liquid scintillation spectrometer at 20 1C. The quench-indicating parameter SQP(E) was measured by means of an external source of 152 Eu. The samples were prepared with 15 ml of Ultima GoldTM scintillator and 1 ml of distilled water in 20 ml lowpotassium borosilicate glass vials. Nitromethane (CH3NO2) was used as quenching agent to vary the counting efficiency. To obtain the calibration curve, e (3H) vs. SQP(E), weighed aliquots of a standard solution of 3H (tritiated water) were used to prepare a set of eleven samples. The activity of the 3H standard solution is traceable to the national German measurement standard at the PTB and was verified by other national metrology institutes within the scope of EUROMET and ICRM comparisons (Makepeace et al., 1994).
ARTICLE IN PRESS K. Kossert / Applied Radiation and Isotopes 64 (2006) 1459–1464
For all nuclides studied the experimental data was fitted with a fourth-order polynomial so as to obtain the working efficiency curve. The polynomial coefficients pi are listed in Table 1. The counting efficiency of the nuclide corresponding to a specific counting efficiency of tritium is then given by efit ¼ enuclide ðetracer Þ ¼
4 X
pi ðetracer Þi :
(1)
i¼0
1461
The residuals D ¼ ðei 2efit Þ=efit of the individual measured efficiencies ei and Eq. (1) are shown in Fig. 2. Before starting the LSC measurements, impurity studies by Ge-detector spectrometry were carried out in the energy range from 10 keV to 2.7 MeV. No photon-emitting impurity could be found in the solutions of 54Mn, 109Cd and 125I. Some of the 85Sr solutions used for the measurements contained small amounts of 59Fe, 84Rb
Table 1 Polynomial coefficients for the calculation of efficiency curves and relative standard uncertainties uðaÞ=a of the activity concentration a of the reference solutions used for the measurements (k ¼ 1) Radionuclide
P0
p1
p2
p3
p4
u(a)/a of standard in %
54
0.08312699 0.1265048 0.3208553 0.7208347 0.2240818 0.4640571 1.237731 0.1683856 0.2299414
2.422951 2.486873 5.137528 3.143484 0.8228705 8.938517 0.9669007 3.494121 8.281299
8.162614 6.463662 15.48763 20.36633 2.556439 30.28222 8.676466 13.89836 31.71742
17.43643 12.08019 27.88327 41.87398 6.864565 49.22582 22.63171 26.42405 59.48567
12.17397 8.458781 18.88876 31.55018 3.796875 30.88905 20.69645 18.81830 41.45826
0.16 0.3 0.2–0.41 1 0.5 0.5 0.25 1 0.42
The efficiencies of
57
67
Co,
Ga and
85
Sr depend on the counter system (see text for explanation).
54
57
Mn
1 (εi−εfit)/εfit in %
0.5 0 -0.5
1
0.5 0 -0.5
0 -0.5
-1
-1
-1
-1.5
-1.5
0.25 0.3 0.35 0.4 0.45 εtracer
(εi−εfit)/εfit in %
1
0.5 0 -0.5
88
Sr
1 (εi−εfit)/εfit in %
85
Ga
0.5 0 -0.5
0 -0.5
-1
-1
-1.5
-1.5
-1.5
1
1.5
109
Cd
1 (εi−εfit)/εfit in %
1.5
0.5 0 -0.5
0.25 0.3 0.35 0.4 0.45 εtracer
In
0.5 0 -0.5
-1.5
1.5
111
1
0.25 0.3 0.35 0.4 0.45 εtracer
125
I
0.5 0 -0.5 -1
-1
-1 -1.5
0.25 0.3 0.35 0.4 0.45 εtracer
Y
0.5
-1 0.25 0.3 0.35 0.4 0.45 εtracer
0.25 0.3 0.35 0.4 0.45 εtracer
1.5
1.5 67
1
0.25 0.3 0.35 0.4 0.45 εtracer
Zn
0.5
-1.5
1.5 (εi−εfit)/εfit in %
65
Co
(εi−εfit)/εfit in %
(εi−εfit)/εfit in %
1
1.5
1.5
1.5
(εi−εfit)/εfit in %
a
(εi−εfit)/εfit in %
Mn Coa 65 Zn 67 Gaa 85 a Sr 88 Y 109 Cd 111 In 125 I 57
0.25 0.3 0.35 0.4 0.45 εtracer
-1.5
0.25 0.3 0.35 0.4 0.45 εtracer
Fig. 2. Residuals D ¼ ðei 2efit Þ=efit of the individual measured efficiencies ei and Eq. (1).
ARTICLE IN PRESS K. Kossert / Applied Radiation and Isotopes 64 (2006) 1459–1464
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0.800
Table 2 Standard uncertainty components for the activity concentration of a solution of 125I measured by the liquid scintillation secondary counting technique
125
I
εnuclide
0.775 0.750
Component
0.725
Statistical component 0.05 Weighing 0.05 Starting time and duration o0.01 Dead time 0.1 Background 0.03 Adsorption o0.01 Impurities (no impurities detected) o0.01 Half life (Dt ¼ 310 d) 0.11 Efficiency curve (fitting) 0.05 125 I activity standard 0.42 Calibration (experimental determination of the efficiency curve) 0.2 Sample stability o0.05
0.700 0.675 0.650 0.625 0.25
Fig. 3. Efficiency curve for
0.30
0.35 εtracer
0.40
0.45
I in 15 ml Ultima GoldTM plus 1 ml water.
125
and 110mAg. A small impurity of 114mIn could be detected in the 111In solution. The impurities found in other solutions were negligibly small. 3.1.
125
I
The liquid scintillation measurements were carried out with an 125I solution, which was calibrated by photon– photon coincidence counting (Schrader and Walz, 1987) within the scope of an international comparison organized in 2004 by the Bureau International des Poids et Mesures (BIPM). The efficiencies determined are shown as open circles in Fig. 3. The efficiency curve was then obtained by fitting the fourth-order polynomial, which is also shown in Fig. 3. The efficiency curve was used to calibrate another solution of 125I which was also calibrated by coincidence counting. The liquid scintillation counting result was only lower by 0.03% than the result obtained by absolute standardization. The relevant uncertainty components are summarized in Table 2. It is important to note that the uncertainty of the tracer activity cancels out if the aliquots used for the determination of the efficiency curve and the calibration curve stem from the same standard solution. In this case the corresponding uncertainty u(atracer) of the 3H activity standard does not contribute to the total uncertainty. This characteristic of the method can easily be demonstrated by altering the value of the activity concentration atracer of the 3H activity standard, e.g. if atracer is arbitrarily increased by 20% one gets a completely different efficiency curve, but the result for anuclide would be the same if this curve and the same (wrong) value atracer is used again for the final analysis. Concluding, the method can even be applied if the true value of the activity concentration atracer is unknown. Since the activity of the second 125I solution was also known from an alternative technique, the efficiency was determined again. The corresponding results are shown as solid triangles in Fig. 3.
u(a)/a in %
Square root of the sum of quadratic components
3.2.
65
0.50
Zn
The efficiency curve of 65Zn was determined by measuring a solution which had been calibrated by 4pb–g coincidence counting (Kossert et al., 2006). The result is traceable to an international comparison organized by the BIPM in 2002. One and a half years later the efficiency curve was used to analyze new measurement data within the scope of the EUROMET exercise no. 721 (Be´ et al., 2005). The solution of this exercise was also calibrated by 4pb–g coincidence counting with a total relative standard uncertainty of 0.2%. The uncertainty of the LSC result was found to be 0.4%, and the result was only lower than the result from the primary standardization method by 0.05%. In the case of 65Zn 1 ml of the widely used complexing agent EDTA (0.05 mol/l) was used instead of distilled water in order to improve the sample stability. However, it is important to mention that the long-term sample stability was low and the experimentally determined counting efficiency decreased slowly. The corresponding uncertainty was reduced by using a similar time of a few days between sample preparations and counting for all measurements. More details about the 65Zn standardization and the liquid scintillation counting measurements are given by Kossert et al. (2006) in the same issue of this journal. 3.3.
54
Mn,
88
Y,
109
Cd and
111
In
The method was also applied to 54Mn, 88Y, 109Cd and In. The corresponding polynomial coefficients to calculate the efficiency curve as well as the relative standard uncertainties u(a)/a of the reference solutions used are listed in Table 1. In the case of 88Y, the samples were prepared with 15 ml Ultima GoldTM scintillator, 0.8 ml of distilled water and 0.2 ml of the complexing agent HDEHP. The measured efficiencies varied considerably from one measurement to
111
ARTICLE IN PRESS K. Kossert / Applied Radiation and Isotopes 64 (2006) 1459–1464
another for the same sources. At present the author does not have any explanation for this poor reproducibility of the results. 3.4.
57
Co,
67
Ga and
85
Sr
The decay schemes of 57Co, 67Ga and 85Sr contain excited levels with half-lives of a few nanoseconds up to a few microseconds. The decays of these states cannot be treated as coincident transitions because the coincidence time of commercial counters is usually a few nanoseconds. On the other hand these transitions cannot be treated as independent decays due to the counter dead time, which usually amounts to a few microseconds. Therefore, the CIEMAT/NIST method cannot be applied for such nuclides without complicated corrections. The specific activities of the solution of the three nuclides mentioned were determined at PTB by measurements with calibrated 4p ionization chambers. Since the liquid scintillation counting efficiencies depend on the coincidence time and dead time of the counter system, the efficiency curves determined can probably not be transferred to other counters. The residual plots of 57Co and 85Sr in Fig. 2 contain several points which deviate considerably from zero. A possible explanation for this is the—in some cases— relatively large period between the 3H calibration and the nuclide calibration. It was ascertained that the method is also capable of calibrating solutions of 57Co and 85Sr with an activity concentration of about 35 and 10 Bq/g, respectively. The determination of such low activities by other methods is hardly possible if the time for the measurements is limited. Finally, the efficiency curve of 67Ga was also determined. The corresponding polynomial coefficients are listed in Table 1. 4. Possible applications for international comparisons As explained above, the method can even be applied if the activity of the 3H solution is not known, provided the same 3H standard solution is used for both the determination of the efficiency curve and the measurement of an updated calibration curve that is used for the final activity determination. Therefore, the method can be used for comparisons. Two examples of such possible procedures are explained briefly in the following. To keep the examples simple, it is assumed that the two laboratories A and B participate in a bilateral comparison of their 63Ni activity standards of known activity concentration. To this end, both laboratories determine the efficiency curve efit as given in Eq. (1). The participants have to use the same sample composition (scintillation cocktail plus other ingredients), aliquots of the same 3H solution and the same value for the tracer activity atracer so that a tracer calibration curve can be determined. The counting efficiencies efit,A and efit,B can finally be compared. A mean value of the two results efit,A and efit,B could be
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considered as a reference value. Of course, this would become more meaningful if there would be more participants in such a comparison. Alternatively, the two laboratories may send standardized solutions to a neutral laboratory which then determines the efficiency curves, the 3H calibration curve and the counting efficiencies efit,A and efit,B using the values for the activity concentrations communicated by the participants. This procedure could even be applied to the nuclides 57Co, 67Ga and 85Sr since only one liquid scintillation counter is used. Such a comparison procedure would be similar to the international reference system (SIR) which makes use of 4p ionization chambers (IC) instead of a liquid scintillation counter. The LS counting efficiencies efit are comparable to calibration factors of ionization chambers, although it is to be noted that efit is a function whereas a calibration factor is a number. In addition, it must be considered that the LS results are traced back to a tritium standard, whereas IC measurements are usually compared with standard sources of 226 Ra. Thus, the half-life of 3H must be taken into account and if comparison exercises take place over several decades, the decay correction would be a somewhat greater problem. For both examples it would be desirable to have an international 3H activity standard as well as a recipe for the production of a standardized scintillation cocktail. 5. Summary and outlook The new method presented in this paper was successfully applied to calibrate solutions of radionuclides decaying by electron-capture. This also comprises radionuclides like 57 Co and 85Sr, which cannot easily be calibrated by the CIEMAT/NIST method. The method could become an important secondary standardization method in particular due to a better control of the apparatus stability. It has also been demonstrated that the method is suitable for low activities and for nuclides having low detection efficiencies in 4p ionization chambers. Thus, this liquid scintillation secondary counting technique could supplement measurements by calibrated ionization chambers and the procedure could also become a powerful tool for international comparisons. However, the applicability for comparisons has still to be investigated. Acknowledgements The author wishes to thank E. Gu¨nther for his kind encouragement as well as R. Klein and H. Schrader for their measurements. References Be´, M.-M., Amiot, M.-N., Bobin, C., Le´py, M.-C., Plagnard, J., Lee, J.-M., Lee, K.B., Park, T.S., Luca, A., Sahagia, M., Razdolescu, A.-M.,
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Grigorescu, L., Sato, Y., Hino, Y., Kossert, K., Klein, R., Schneider, M.K.H., Schrader, H., Dryak, P., Sochorova, J., Kovar, P., Auerbach, P., Havelka, M., Altzitzoglou, T., Iwahara, A., da Silva, M.A.L., Delegado, J.U., da Silva, C.J., Johansson, L., Collins, S., Stroak, A., 2005. Activity measurements and gamma emission intensities determination in the decay of 65Zn. Rapport CEA-R-6081, CEA Saclay, Gifsur-Yvette, France. Coursey, B.M., Mann, W.B., Grau Malonda, A., Garcia-Toran˜o, E., Los Arcos, J.M., Gibson, J.A.B., Reher, D., 1986. Standardization of carbon-14 by 4pb liquid scintillation efficiency tracing with hydrogen3. Appl. Radiat. Isot. 37, 403.
Grau Carles, A., Grau Malonda, A., Grau Carles, P., 1994. EMI, the counting efficiency for electron capture, electron capture-gamma and isomeric transitions. Comput. Phys. Comm. 79, 115. Kossert, K., Janben, H., Klein, R., Schneider, M., Schrader, H., 2006. Standardization and nuclear decay data of Zn-65. Appl. Radiat. Isot. in press, doi:10.1016/j.apradiso.2006.02.054. Makepeace, J.L., Clark, F.E., Picolo, J.L., Coursol, N., Gu¨nther, E., Unterweger, M.P., 1994. Intercomparison of internal proportional gas counting of 85Kr and 3H. Nucl. Instrum. Meth. A 339, 343. Schrader, H., Walz, K.F., 1987. Standardization of 125I by photon–photon coincidence counting and efficiency extrapolation. Appl. Radiat. Isot. 38, 763.