- Email: [email protected]
Standardization and nuclear decay data of 65Zn
ARTICLE IN PRESS
Applied Radiation and Isotopes 64 (2006) 1420–1424 www.elsevier.com/locate/apradiso
Standardization and nuclear decay data of
65
Zn
K. Kossert, H. JanXen, R. Klein, M. Schneider, H. Schrader Physikalisch-Technische Bundesanstalt (PTB), Department 6.1, Bundesallee 100, D-38116 Braunschweig, Germany
Abstract Standardization by means of 4pb–g coincidence counting was applied to calibrate a 65Zn solution within the scope of EUROMET project no. 721. The activity result was combined with gamma-ray spectrometry measurements to investigate the photon emission probabilities. The half-life of 65Zn was measured with improved accuracy by means of 4p ionization chamber measuring systems that were also used for secondary activity determination. In addition, we tested a new secondary standardization procedure by means of liquid scintillation counting. r 2006 Elsevier Ltd. All rights reserved. Keywords:
65
Zn; Photon emission probability; Half-life; Activity standardization
1. Introduction 65
Zn decays by electron capture (EC) to the 1115-keV excited level and by EC and a b+ transition to the ground state of 65Cu. Activity standards of 65Zn are frequently used for detector calibrations in terms of activity, such as ionization chambers and energy-selective photon spectrometers. However, response functions of such systems as a function of photon energy show inconsistencies for the 1115-keV line in the order of 1%, indicating that the available decay data are not reliable. For this reason and in the framework of the EUROMET project no. 721 (Be´ et al., 2005), new measurements were carried out at PTB to determine the photon emission probabilities per decay of 65 Zn with high accuracy. 2. Experimental An ampoule containing 65Zn solution with carrier concentration of 405-mg/g ZnCl2 in 0.1 M HCl, as stated by the supplier, was provided by the Laboratoire National Henri Becquerel (LNHB). The original ampoule was used for preliminary activity determinations by means of ionization chamber measurements. This ampoule was subsequently opened and the solution was transferred into Corresponding author. Tel. +49 531 592 6311; fax: +49 531 592 6305.
E-mail address: [email protected] (K. Kossert). 0969-8043/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2006.02.054
two PTB-type glass ampoules. In addition, we prepared sources for gamma-spectrometry as well as for the primary activity standardization. A portion of the original solution was diluted with an inactive carrier solution to get a second solution appropriate for liquid scintillation counting (LSC) sample preparation. The masses of all samples as well as the dilution factor were determined gravimetrically using two Mettler balances traceable to the German national mass standard. 2.1. 4p(EC+b+)-g coincidence counting Eight sources were prepared from the 65Zn solution using VYNS foils mounted on aluminium rings. The foils were coated with thin layers of a gold–palladium alloy (approx. 80/20) on both sides of the backing and pretreated by electro-spraying with silica gel. The sources were measured in a 4pb–g coincidence system equipped with a pressurized (0.9 MPa) pillbox-type proportional counter working with a mixture of argon (90%) and methane (10%). The photons were counted by means of a NaI(Tl) detector (70 70 mm2) with an aluminium window. Corrections for accidental coincidences were taken into account (Smith, 1978, 1994). The EC+b+ efficiency was reduced from about 0.72–0.17 by changing the discriminator adjustment of the beta channel. Then, the ratio nbng/nc was plotted as a function of x ¼ ð1 eb Þ=eb . Finally, this curve was
ARTICLE IN PRESS K. Kossert et al. / Applied Radiation and Isotopes 64 (2006) 1420–1424
extrapolated to eb ¼ 1 with a straight line function f(x). The residuals D ¼ ðnb ng =nc f ðxÞÞ=f ðxÞ are shown in Fig. 1. 2.2. g-spectrometry
Counting statistics Weighing Dead time Background Resolving time Gandy effect Counting time Quenching Half-life (T 1=2 ¼ 244:06ð10Þ d; Dt ¼ 63 d for 4pb–g- and 127 d for LSC) Extrapolation of efficiency curve Dilution (weighing) LS source stability 65 Zn standard LSC calibration (three measurements to obtain the efficiency curve and a calibration curve) Square root of the sum of quadratic components (correlation coefficients are taken into account in the summation)
u(a)/a in % 4pb–g CC
LSC
0.10 0.19 o0.02 0.02 o0.02 0.01 o0.001 – 0.01
0.050 0.048 0.1 0.03 – – o0.01 0.05 0.015
0.01 – – – –
– 0.042 0.2 0.25 0.2
0.20
0.4
-1.5
The annihilation radiation was measured after covering the sources on both sides with aluminium disks of 1 mm thickness. The 511-keV photon peaks were corrected for the annihilation in flight effect (Kantele and Valkonen, 1973). The spectrometer was also used to look for radioactive gamma-ray emitting impurities. A small impurity of 60Co was found in the solution, with an activity ratio Að60 CoÞ=Að65 ZnÞ ¼ 4:7ð4Þ 105 that was considered to be negligible. In addition, some efforts were made to measure the X-ray emission probabilities of Cu using a Si(Li) spectrometer. However, we found a low reproducibility as well as a low energy resolution for all sources prepared by different techniques, such as drop deposition and electrolytic segregation. As a consequence, the peak analysis gave high uncertainties and we have to suppose systematic errors. A possible explanation for our observations might be the absorption due to the relatively high carrier concentration of the solution used for this exercise.
-2.0
2.3. Measurements with ionization chambers (IC)
2.5 2.0 1.5 1.0 Residuals ∆ in %
Table 1 Standard uncertainty components of the activity divided by mass of a 65Zn solution measured by 4pb–g coincidence counting and LSC—undertaken as part of EUROMET project no. 721 (components with u(a)/ao0.001% are not listed) Component
The measurements were carried out with an n-type coaxial high-purity germanium detector (volume 127 cm3) with a Be entrance window of 0.05 cm thickness. The spectrometer was calibrated in the energy region from 6 keV to 2 MeV using the photon emission of more than 20 different radionuclides calibrated at PTB. The efficiency curve was obtained by linear least-squares fitting of a cubic spline function (JanXen, 1990, 1994). The number of counts in the gamma-ray peaks was determined by fitting a Gaussian and appropriate background functions (Scho¨tzig et al., 1973). Losses due to pile-up and dead-time effects were corrected applying the pulser method described by Debertin and Scho¨tzig (1977). The coincidence summing effects were taken into account by calculating corrections with the KORSUM program (Debertin and Scho¨tzig, 1979). The gamma-ray emission probability is given by p ¼ R/ A, where R is the source emission rate and A the activity of the sample. The measurements of R were performed with the same samples as for the coincidence counting. Thus, we are able to set aside the uncertainty assigned to the drop mass which is the dominant contribution to the combined uncertainty of the activity divided by mass of solution (see Table 1). However, since the uncertainty of an individual source is somewhat higher than the uncertainty for the averaged value, the benefit of this procedure becomes negligible.
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0.5 0.0 -0.5 -1.0
-2.5 0.0
1.0
2.0 x =(1-εβ) / εβ
3.0
4.0
5.0
Fig. 1. Residual D ¼ ðnb ng =nc f ðxÞÞ=f ðxÞ measured with a pressurized proportional counter as a function of x ¼ ðð1 eb Þ=eb Þ; function f(x) is a straight line which was fitted to the experimental data, and the bars are the statistical uncertainty of the individual data (coverage factor k ¼ 1).
Three different solutions which were standardized by 4pb–g coincidence counting were used for ionization chamber measurements. The first solution was calibrated within the scope of an international comparison organized in 2002 by the Bureau International des Poids et Mesures (BIPM). The comparison result was conserved
ARTICLE IN PRESS K. Kossert et al. / Applied Radiation and Isotopes 64 (2006) 1420–1424
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as a calibration factor in our ionization chambers. The second solution was used for new half-life measurements as well as to reduce the uncertainty of the calibration factor. Finally, the calibration factor was used for secondary activity determination of the solution measured within EUROMET project no. 721. The half-life measurement was made with a highpressure 4p-ionization chamber IG12/A20, Centronic 20th Century Electronics Ltd., filled with argon to a pressure of 2 MPa, with an iron entrance window approximately 3-mm thick to the well and connected to a commercially available Keithley electrometer model 6517A. The radioactive source was a 65Zn solution of about 2-ml volume in PTB standard ampoule geometry. No photon-emitting impurity was detected by germanium detector spectrometry in this solution. Further details of the measurement techniques and data treatment have been described by Schrader (2004). The residuals of a least-squares fit to the measured data are shown in Fig. 2, relative to those of a 226Ra reference source and both sets of data corrected for background. 2.4. Liquid scintillation counting (LSC) The CIEMAT/NIST method (Coursey et al., 1986) is known to be a reliable tool for the determination of the counting efficiency of beta and beta-gamma emitters. However, in the case of low-Z electron-capture nuclides, the method failed, since both the simple KLM (Grau Carles et al., 1994) as well as the more sophisticated KL1L2L3M (Grau Malonda et al., 1999) atomic rearrangement models overestimate the counting efficiency up to a few per cent. The model was recently extended by Grau Carles (2006) with an improved consideration of the interaction of low-energy X-rays with the scintillation cocktail. A comparison of calculated counting efficiencies 0.40 0.30
Residuals ∆ in %
0.20 0.10 -0.00 -0.10 -0.20 -0.30 -0.40
0
100
200
300 400 Time in d
500
600
700
800
Fig. 2. Residuals D from a least-squares fit of the half-life measurement of 65 Zn by an ionization chamber of type IG12/A20 (Centronic 20th Century Electronics Ltd.); vertical lines indicate (from left to right) the start in March 2003, 1 January 2004 and 2005, and the end of the measurements in April 2005.
of 54Mn, 55Fe and 65Zn with experimental results is discussed by Kossert and Grau Carles (2006). In the present work we applied a new secondary standardization method as described by Kossert (2006). The efficiency curve in this method is defined as the counting efficiency enuclide of the nuclide under study as a function of the counting efficiency etracer of tritium (3H), and is not calculated but measured. The counting efficiency enuclide of 65Zn as a function of the quench-indicating parameter SQP(E) was obtained by measuring a sample series with a radioactive solution standardized by 4pb–g coincidence counting (u(a)/a ¼ 0.25%) traceable to the BIPM comparison in 2002. We measured the counting efficiency of tritium etracer as a function of the quenching indicator SQP(E) in the same way by using a 3H standard solution (tritiated water) calibrated with the aid of the internal gas counting facility at PTB. The parameter SQP(E) is assumed to be independent of the radionuclide in the sample. Therefore, the parameter allows us to establish a link between the counting efficiencies of both radionuclides, and consequently we obtain the counting efficiency enuclide of 65Zn as a function of the counting efficiency etracer of tritium. About one and a half years later, the experimentally determined efficiency curve was used to calibrate a 65Zn solution within EUROMET project no. 721. A new calibration curve defined as the counting efficiency of tritium etracer as a function of the quenching indicator SQP(E) was measured. It is important to note that the uncertainty assigned to the tracer activity cancels out if the aliquots used for the determination of the efficiency curve and the calibration curve stem from the same 3H standard solution. In this case the corresponding uncertainty u(atracer) of the 3H activity standard does not contribute to the total uncertainty. The LSC measurements were performed in a twophotomultiplier 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 152Eu. The samples were prepared with 15 ml of Ultima GoldTM scintillator and 1 ml of distilled water in 20 ml low-potassium borosilicate glass vials. Nitromethane (CH3NO2) was used as quenching agent to vary the counting efficiency. The 65Zn samples were prepared with the same recipe, but with 1-ml 0.05 mol/l EDTA instead of water. However, it is important to emphasize that the longterm stability of the 65Zn samples 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 preparation and counting for all measurements.
3. Analysis and results Applying the three different methods for the determination of the activity divided by mass of solution a at the
ARTICLE IN PRESS K. Kossert et al. / Applied Radiation and Isotopes 64 (2006) 1420–1424
reference date of 15 January 2004 (0 h UTC), we found the following results: 4p(EC+b+)-g coincidence: calibrated IC: LSC (secondary standardization)
a ¼ 860:1 1:8 kBq=g; a ¼ 859:9 2:6 kBq=g; a ¼ 859:5 3:5 kBq=g.
The uncertainty budgets for coincidence counting and LSC are listed in Table 1. The combined relative standard uncertainty was found to be 0.2% and 0.4% for coincidence counting and LSC, respectively. The LSC activity result was only 0.05% lower than the result from the primary standardization method. The IC result was in even better agreement and the associated relative standard uncertainty was estimated to be 0.3%. This result verifies the excellent long-term stability of the IC system. The relevant uncertainty components for the half-life determination are summarized in Table 2, and result in a square root of the sum of quadratic terms of 0.092 d or 0.038% in comparison to the uncertainty of fit of only 0.0056 d or 0.0023%. Similar components were observed in measurements of other long-lived radionuclides (Schrader, 2004). A half-life was determined for 65Zn of 243.61670.092 d. This value agrees within a standard deviation with the two earlier determined PTB values, measured independently with different equipments and under different measuring conditions; 243.970.8 d from Walz et al. (1983) and 243.6670.09 d from Schrader (2004). The newly measured half-life also shows a deviation within a limit of 0.16% from a recently evaluated value of 244.0170.09 d (Be´ et al., 2005). Combining the results of the activity determination with those from gamma-ray spectrometry, we obtain emission probabilities with well-defined uncertainty budgets for the 511- and 1115-keV photons (Table 3). Upper limits were also determined for the two possible photon emissions at 344 and 770 keV in terms of detection and decision limits. The two emission probabilities determined in this work are correlated due to uncertainty components which are common to both quantities. Firstly, the efficiency curve of
1423
the photon spectrometer is based on measurements on sources that have been produced and calibrated in our laboratory. The same balance has been used in source preparation to determine the droplet masses of the sources. The absolute calibrations of the master solutions used for source preparation have mostly been performed by coincidence counting with the same equipment. Although a detailed covariance analysis of the input data could not be performed due to the lack of dedicated information, the uncertainty band assigned to the efficiency curve should cover about 70% of the data points. This leads to a common contribution of 0.4% to the relative standard uncertainty of the efficiency curve in the two energy regions in question. Secondly, the relative standard uncertainty of 0.2% assigned to the source activities is an additional cause of correlation. If we assume that these two quantities are not correlated, we get uðe AÞ=ðe AÞ ¼
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 0:0042 þ 0:0022 ¼ 0:45%.
Neglecting all further correlation it follows: Covðpð511Þ; pð1115ÞÞ ¼ pð511Þ pð1115Þ ðuðe AÞ=ðe AÞÞ2 2:85 107
Table 3 Standard uncertainty components for the photon emission probabilities of 65 Zn u(a)/a in %
Component
Counting statistics Peak area Efficiency Correction factors Annihilation in flight Activity Square root of the sum of quadratic components Correlation coefficient
511 keV
1115 keV
0.2 0.6 0.52 0.03 0.2 0.2 0.87
0.1 0.18 0.47 0.03 – 0.2 0.56 0.4
Table 2 Standard uncertainty components of the half-life of 65Zn measured with an ionization chamber IG12/A20 of Centronic; measurement sequence repeated approximately once per week, and yielding an averaged data point with several repetitions (X15 per run) as carried out for long-lived radionuclides Component
u(a)/a in %
Statistic component of a data point (number of repetitions X15) Ionization current measurement, including system linearity and switching Time measurements; starting time and duration for a data point Long-term instrument stability, not affected by reference source measurements Dis-equilibrium of 226Ra reference source Long-term measurement geometry changes, including source stability Radionuclidic impurity correction (detection limit) Least-squares fitting procedure for half-life Square root of the sum of quadratic components
0.02 0.025 0.01 0.005 0.015 0.005 o0.001 o0.0023 0.038
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K. Kossert et al. / Applied Radiation and Isotopes 64 (2006) 1420–1424
Table 4 Photon emission probabilities determined in this work compared to other selected references Reference
p(1115 keV)
p(511 keV)
p(344 keV)
p(770 keV)
De Roost et al. (1972) Scho¨tzig (1990) Be´ et al. (2005) (evaluation)a This work
0.5075(10) 0.502(4) 0.5022(11) 0.5015(28)
0.0286(6) 0.0284(4) 0.02842(13) 0.0281(3)
– –
– –
0.0000254(18) o0.000091b o0.00014c
0.0000269(22) o0.00015b o0.00023c
a
Results of this work were taken into account for this evaluation. Detection limit. c Decision limit. b
and we obtain a correlation coefficient rðpð511Þ; pð1115ÞÞ Covðpð511Þ; pð1115ÞÞ=ðuðpð511ÞÞ uðpð1115ÞÞÞ 0:4, which should be taken into account in all further evaluations of published photon emission probabilities. Table 4 contains the emission probabilities determined in this work and those obtained by other selected references. Our results are in excellent agreement with those previously published by Scho¨tzig (1990) and Be´ et al. (2005). The latter work is an evaluation that already includes consideration of the results of this work. In conclusion, the result of this work clearly indicates that p(1115 keV) was considerably overestimated in some previous data evaluations and experiments. In particular, the uncertainty of the result determined by de Roost et al. (1972) is to be questioned. Acknowledgements We wish to thank M.-M. Be´ from LNHB for the coordination of EUROMET project no. 721, and for providing us with one of the 65Zn solutions used for this work. References Be´, M.-M., et al., 2005. Activity measurements and gamma emission intensities determination in the decay of 65Zn. CEA Report CEA-R6081, CEA Saclay, Gif-sur-Yvette, France. Coursey, B.M., et al., 1986. Standardization of carbon-14 by 4pb liquid scintillation efficiency tracing with hydrogen-3. Appl. Radiat. Isot. 37, 403. Debertin, K., Scho¨tzig, U., 1977. Limitations of the pulser method for pile-up corrections in Ge(Li) spectrometry. Nucl. Instrum. Methods 140, 337.
Debertin, K., Scho¨tzig, U., 1979. Coincidence summing corrections in Ge(Li) spectrometry at low source-to-detector distances. Nucl. Instrum. Methods 158, 471. De Roost, E., et al., 1972. The decay of 65Zn. Z. Phys. 250, 395. Grau Carles, A., 2006. EMILIA, the LS counting efficiency for electroncapture and capture-gamma emitters. Comput. Phys. Commun. 174, 35. 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. Commun. 79, 115. Grau Malonda, A., Grau Carles, A., Grau Carles, P., Galiano Casas, G., 1999. EMI2, the counting efficiency for electron capture by KL1L2L3 M model. Comput. Phys. Commun. 123, 114. JanXen, H., 1990. SPLINE techniques for fitting efficiency curves in gamma-ray spectrometry. Nucl. Instrum. Meth. Phys. Res. A 286, 398. JanXen, H., 1994. BSPLINE—Ein Dialogprogramm fu¨r lineare Ausgleichsrechnungen mit Spline-Funktionen. PTB Laboratory Report PTB6.31-94-4 (in German), PTB Braunschweig, Germany. Kantele, J., Valkonen, M., 1973. Corrections for position annihilation in flight in nuclear spectrometry. Nucl. Instrum. Methods 140, 501. Kossert, K., 2006. A new method for secondary standard measurements with the aid of liquid scintillation counting. Appl. Radiat. Isot., in press, DOI:10.1016/j.apradiso.2006.02.060. Kossert, K., Grau Carles, A., 2006. The LSC efficiency for low-Z electroncapture nuclides. Appl. Radiat. Isot., in press, DOI:10.1016/j.apradiso. 2006.02.059. Scho¨tzig, U., 1990. Photon emission probabilities of 44Ti, 65Zn, 88Y, 89Sr, 147 Pm, 204Tl and 210Pb. Nucl. Instrum. Methods Phys. Res. A 286, 523. Scho¨tzig, U., Debertin, K., WeiX, H., 1973. Bestimmung von Gammastrahlen-Emissionswahrscheinlichkeiten mit einem Ge(Li)-Spektrometer, PTB-Mitteilungen 83, pp. 307–312, PTB Braunschweig, Germany. Schrader, H., 2004. Half-life measurements with ionization chambers—A study of systematic effects and results. Appl. Radiat. Isot. 60, 317. Smith, D., 1978. Improved correction formulae for coincidence counting. Nucl. Instrum. Methods 152, 505. Smith, D., 1994. Particle counting in radioactivity measurements. ICRU Report 52, p. 34. Walz, K.F., Debertin, K., Schrader, H., 1983. Half-life measurements at the PTB. Int. J. Appl. Radiat. Isot. 34, 1191.
Applied Radiation and Isotopes 64 (2006) 1420–1424 www.elsevier.com/locate/apradiso
Standardization and nuclear decay data of
65
Zn
K. Kossert, H. JanXen, R. Klein, M. Schneider, H. Schrader Physikalisch-Technische Bundesanstalt (PTB), Department 6.1, Bundesallee 100, D-38116 Braunschweig, Germany
Abstract Standardization by means of 4pb–g coincidence counting was applied to calibrate a 65Zn solution within the scope of EUROMET project no. 721. The activity result was combined with gamma-ray spectrometry measurements to investigate the photon emission probabilities. The half-life of 65Zn was measured with improved accuracy by means of 4p ionization chamber measuring systems that were also used for secondary activity determination. In addition, we tested a new secondary standardization procedure by means of liquid scintillation counting. r 2006 Elsevier Ltd. All rights reserved. Keywords:
65
Zn; Photon emission probability; Half-life; Activity standardization
1. Introduction 65
Zn decays by electron capture (EC) to the 1115-keV excited level and by EC and a b+ transition to the ground state of 65Cu. Activity standards of 65Zn are frequently used for detector calibrations in terms of activity, such as ionization chambers and energy-selective photon spectrometers. However, response functions of such systems as a function of photon energy show inconsistencies for the 1115-keV line in the order of 1%, indicating that the available decay data are not reliable. For this reason and in the framework of the EUROMET project no. 721 (Be´ et al., 2005), new measurements were carried out at PTB to determine the photon emission probabilities per decay of 65 Zn with high accuracy. 2. Experimental An ampoule containing 65Zn solution with carrier concentration of 405-mg/g ZnCl2 in 0.1 M HCl, as stated by the supplier, was provided by the Laboratoire National Henri Becquerel (LNHB). The original ampoule was used for preliminary activity determinations by means of ionization chamber measurements. This ampoule was subsequently opened and the solution was transferred into Corresponding author. Tel. +49 531 592 6311; fax: +49 531 592 6305.
E-mail address: [email protected] (K. Kossert). 0969-8043/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2006.02.054
two PTB-type glass ampoules. In addition, we prepared sources for gamma-spectrometry as well as for the primary activity standardization. A portion of the original solution was diluted with an inactive carrier solution to get a second solution appropriate for liquid scintillation counting (LSC) sample preparation. The masses of all samples as well as the dilution factor were determined gravimetrically using two Mettler balances traceable to the German national mass standard. 2.1. 4p(EC+b+)-g coincidence counting Eight sources were prepared from the 65Zn solution using VYNS foils mounted on aluminium rings. The foils were coated with thin layers of a gold–palladium alloy (approx. 80/20) on both sides of the backing and pretreated by electro-spraying with silica gel. The sources were measured in a 4pb–g coincidence system equipped with a pressurized (0.9 MPa) pillbox-type proportional counter working with a mixture of argon (90%) and methane (10%). The photons were counted by means of a NaI(Tl) detector (70 70 mm2) with an aluminium window. Corrections for accidental coincidences were taken into account (Smith, 1978, 1994). The EC+b+ efficiency was reduced from about 0.72–0.17 by changing the discriminator adjustment of the beta channel. Then, the ratio nbng/nc was plotted as a function of x ¼ ð1 eb Þ=eb . Finally, this curve was
ARTICLE IN PRESS K. Kossert et al. / Applied Radiation and Isotopes 64 (2006) 1420–1424
extrapolated to eb ¼ 1 with a straight line function f(x). The residuals D ¼ ðnb ng =nc f ðxÞÞ=f ðxÞ are shown in Fig. 1. 2.2. g-spectrometry
Counting statistics Weighing Dead time Background Resolving time Gandy effect Counting time Quenching Half-life (T 1=2 ¼ 244:06ð10Þ d; Dt ¼ 63 d for 4pb–g- and 127 d for LSC) Extrapolation of efficiency curve Dilution (weighing) LS source stability 65 Zn standard LSC calibration (three measurements to obtain the efficiency curve and a calibration curve) Square root of the sum of quadratic components (correlation coefficients are taken into account in the summation)
u(a)/a in % 4pb–g CC
LSC
0.10 0.19 o0.02 0.02 o0.02 0.01 o0.001 – 0.01
0.050 0.048 0.1 0.03 – – o0.01 0.05 0.015
0.01 – – – –
– 0.042 0.2 0.25 0.2
0.20
0.4
-1.5
The annihilation radiation was measured after covering the sources on both sides with aluminium disks of 1 mm thickness. The 511-keV photon peaks were corrected for the annihilation in flight effect (Kantele and Valkonen, 1973). The spectrometer was also used to look for radioactive gamma-ray emitting impurities. A small impurity of 60Co was found in the solution, with an activity ratio Að60 CoÞ=Að65 ZnÞ ¼ 4:7ð4Þ 105 that was considered to be negligible. In addition, some efforts were made to measure the X-ray emission probabilities of Cu using a Si(Li) spectrometer. However, we found a low reproducibility as well as a low energy resolution for all sources prepared by different techniques, such as drop deposition and electrolytic segregation. As a consequence, the peak analysis gave high uncertainties and we have to suppose systematic errors. A possible explanation for our observations might be the absorption due to the relatively high carrier concentration of the solution used for this exercise.
-2.0
2.3. Measurements with ionization chambers (IC)
2.5 2.0 1.5 1.0 Residuals ∆ in %
Table 1 Standard uncertainty components of the activity divided by mass of a 65Zn solution measured by 4pb–g coincidence counting and LSC—undertaken as part of EUROMET project no. 721 (components with u(a)/ao0.001% are not listed) Component
The measurements were carried out with an n-type coaxial high-purity germanium detector (volume 127 cm3) with a Be entrance window of 0.05 cm thickness. The spectrometer was calibrated in the energy region from 6 keV to 2 MeV using the photon emission of more than 20 different radionuclides calibrated at PTB. The efficiency curve was obtained by linear least-squares fitting of a cubic spline function (JanXen, 1990, 1994). The number of counts in the gamma-ray peaks was determined by fitting a Gaussian and appropriate background functions (Scho¨tzig et al., 1973). Losses due to pile-up and dead-time effects were corrected applying the pulser method described by Debertin and Scho¨tzig (1977). The coincidence summing effects were taken into account by calculating corrections with the KORSUM program (Debertin and Scho¨tzig, 1979). The gamma-ray emission probability is given by p ¼ R/ A, where R is the source emission rate and A the activity of the sample. The measurements of R were performed with the same samples as for the coincidence counting. Thus, we are able to set aside the uncertainty assigned to the drop mass which is the dominant contribution to the combined uncertainty of the activity divided by mass of solution (see Table 1). However, since the uncertainty of an individual source is somewhat higher than the uncertainty for the averaged value, the benefit of this procedure becomes negligible.
1421
0.5 0.0 -0.5 -1.0
-2.5 0.0
1.0
2.0 x =(1-εβ) / εβ
3.0
4.0
5.0
Fig. 1. Residual D ¼ ðnb ng =nc f ðxÞÞ=f ðxÞ measured with a pressurized proportional counter as a function of x ¼ ðð1 eb Þ=eb Þ; function f(x) is a straight line which was fitted to the experimental data, and the bars are the statistical uncertainty of the individual data (coverage factor k ¼ 1).
Three different solutions which were standardized by 4pb–g coincidence counting were used for ionization chamber measurements. The first solution was calibrated within the scope of an international comparison organized in 2002 by the Bureau International des Poids et Mesures (BIPM). The comparison result was conserved
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as a calibration factor in our ionization chambers. The second solution was used for new half-life measurements as well as to reduce the uncertainty of the calibration factor. Finally, the calibration factor was used for secondary activity determination of the solution measured within EUROMET project no. 721. The half-life measurement was made with a highpressure 4p-ionization chamber IG12/A20, Centronic 20th Century Electronics Ltd., filled with argon to a pressure of 2 MPa, with an iron entrance window approximately 3-mm thick to the well and connected to a commercially available Keithley electrometer model 6517A. The radioactive source was a 65Zn solution of about 2-ml volume in PTB standard ampoule geometry. No photon-emitting impurity was detected by germanium detector spectrometry in this solution. Further details of the measurement techniques and data treatment have been described by Schrader (2004). The residuals of a least-squares fit to the measured data are shown in Fig. 2, relative to those of a 226Ra reference source and both sets of data corrected for background. 2.4. Liquid scintillation counting (LSC) The CIEMAT/NIST method (Coursey et al., 1986) is known to be a reliable tool for the determination of the counting efficiency of beta and beta-gamma emitters. However, in the case of low-Z electron-capture nuclides, the method failed, since both the simple KLM (Grau Carles et al., 1994) as well as the more sophisticated KL1L2L3M (Grau Malonda et al., 1999) atomic rearrangement models overestimate the counting efficiency up to a few per cent. The model was recently extended by Grau Carles (2006) with an improved consideration of the interaction of low-energy X-rays with the scintillation cocktail. A comparison of calculated counting efficiencies 0.40 0.30
Residuals ∆ in %
0.20 0.10 -0.00 -0.10 -0.20 -0.30 -0.40
0
100
200
300 400 Time in d
500
600
700
800
Fig. 2. Residuals D from a least-squares fit of the half-life measurement of 65 Zn by an ionization chamber of type IG12/A20 (Centronic 20th Century Electronics Ltd.); vertical lines indicate (from left to right) the start in March 2003, 1 January 2004 and 2005, and the end of the measurements in April 2005.
of 54Mn, 55Fe and 65Zn with experimental results is discussed by Kossert and Grau Carles (2006). In the present work we applied a new secondary standardization method as described by Kossert (2006). The efficiency curve in this method is defined as the counting efficiency enuclide of the nuclide under study as a function of the counting efficiency etracer of tritium (3H), and is not calculated but measured. The counting efficiency enuclide of 65Zn as a function of the quench-indicating parameter SQP(E) was obtained by measuring a sample series with a radioactive solution standardized by 4pb–g coincidence counting (u(a)/a ¼ 0.25%) traceable to the BIPM comparison in 2002. We measured the counting efficiency of tritium etracer as a function of the quenching indicator SQP(E) in the same way by using a 3H standard solution (tritiated water) calibrated with the aid of the internal gas counting facility at PTB. The parameter SQP(E) is assumed to be independent of the radionuclide in the sample. Therefore, the parameter allows us to establish a link between the counting efficiencies of both radionuclides, and consequently we obtain the counting efficiency enuclide of 65Zn as a function of the counting efficiency etracer of tritium. About one and a half years later, the experimentally determined efficiency curve was used to calibrate a 65Zn solution within EUROMET project no. 721. A new calibration curve defined as the counting efficiency of tritium etracer as a function of the quenching indicator SQP(E) was measured. It is important to note that the uncertainty assigned to the tracer activity cancels out if the aliquots used for the determination of the efficiency curve and the calibration curve stem from the same 3H standard solution. In this case the corresponding uncertainty u(atracer) of the 3H activity standard does not contribute to the total uncertainty. The LSC measurements were performed in a twophotomultiplier 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 152Eu. The samples were prepared with 15 ml of Ultima GoldTM scintillator and 1 ml of distilled water in 20 ml low-potassium borosilicate glass vials. Nitromethane (CH3NO2) was used as quenching agent to vary the counting efficiency. The 65Zn samples were prepared with the same recipe, but with 1-ml 0.05 mol/l EDTA instead of water. However, it is important to emphasize that the longterm stability of the 65Zn samples 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 preparation and counting for all measurements.
3. Analysis and results Applying the three different methods for the determination of the activity divided by mass of solution a at the
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reference date of 15 January 2004 (0 h UTC), we found the following results: 4p(EC+b+)-g coincidence: calibrated IC: LSC (secondary standardization)
a ¼ 860:1 1:8 kBq=g; a ¼ 859:9 2:6 kBq=g; a ¼ 859:5 3:5 kBq=g.
The uncertainty budgets for coincidence counting and LSC are listed in Table 1. The combined relative standard uncertainty was found to be 0.2% and 0.4% for coincidence counting and LSC, respectively. The LSC activity result was only 0.05% lower than the result from the primary standardization method. The IC result was in even better agreement and the associated relative standard uncertainty was estimated to be 0.3%. This result verifies the excellent long-term stability of the IC system. The relevant uncertainty components for the half-life determination are summarized in Table 2, and result in a square root of the sum of quadratic terms of 0.092 d or 0.038% in comparison to the uncertainty of fit of only 0.0056 d or 0.0023%. Similar components were observed in measurements of other long-lived radionuclides (Schrader, 2004). A half-life was determined for 65Zn of 243.61670.092 d. This value agrees within a standard deviation with the two earlier determined PTB values, measured independently with different equipments and under different measuring conditions; 243.970.8 d from Walz et al. (1983) and 243.6670.09 d from Schrader (2004). The newly measured half-life also shows a deviation within a limit of 0.16% from a recently evaluated value of 244.0170.09 d (Be´ et al., 2005). Combining the results of the activity determination with those from gamma-ray spectrometry, we obtain emission probabilities with well-defined uncertainty budgets for the 511- and 1115-keV photons (Table 3). Upper limits were also determined for the two possible photon emissions at 344 and 770 keV in terms of detection and decision limits. The two emission probabilities determined in this work are correlated due to uncertainty components which are common to both quantities. Firstly, the efficiency curve of
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the photon spectrometer is based on measurements on sources that have been produced and calibrated in our laboratory. The same balance has been used in source preparation to determine the droplet masses of the sources. The absolute calibrations of the master solutions used for source preparation have mostly been performed by coincidence counting with the same equipment. Although a detailed covariance analysis of the input data could not be performed due to the lack of dedicated information, the uncertainty band assigned to the efficiency curve should cover about 70% of the data points. This leads to a common contribution of 0.4% to the relative standard uncertainty of the efficiency curve in the two energy regions in question. Secondly, the relative standard uncertainty of 0.2% assigned to the source activities is an additional cause of correlation. If we assume that these two quantities are not correlated, we get uðe AÞ=ðe AÞ ¼
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 0:0042 þ 0:0022 ¼ 0:45%.
Neglecting all further correlation it follows: Covðpð511Þ; pð1115ÞÞ ¼ pð511Þ pð1115Þ ðuðe AÞ=ðe AÞÞ2 2:85 107
Table 3 Standard uncertainty components for the photon emission probabilities of 65 Zn u(a)/a in %
Component
Counting statistics Peak area Efficiency Correction factors Annihilation in flight Activity Square root of the sum of quadratic components Correlation coefficient
511 keV
1115 keV
0.2 0.6 0.52 0.03 0.2 0.2 0.87
0.1 0.18 0.47 0.03 – 0.2 0.56 0.4
Table 2 Standard uncertainty components of the half-life of 65Zn measured with an ionization chamber IG12/A20 of Centronic; measurement sequence repeated approximately once per week, and yielding an averaged data point with several repetitions (X15 per run) as carried out for long-lived radionuclides Component
u(a)/a in %
Statistic component of a data point (number of repetitions X15) Ionization current measurement, including system linearity and switching Time measurements; starting time and duration for a data point Long-term instrument stability, not affected by reference source measurements Dis-equilibrium of 226Ra reference source Long-term measurement geometry changes, including source stability Radionuclidic impurity correction (detection limit) Least-squares fitting procedure for half-life Square root of the sum of quadratic components
0.02 0.025 0.01 0.005 0.015 0.005 o0.001 o0.0023 0.038
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Table 4 Photon emission probabilities determined in this work compared to other selected references Reference
p(1115 keV)
p(511 keV)
p(344 keV)
p(770 keV)
De Roost et al. (1972) Scho¨tzig (1990) Be´ et al. (2005) (evaluation)a This work
0.5075(10) 0.502(4) 0.5022(11) 0.5015(28)
0.0286(6) 0.0284(4) 0.02842(13) 0.0281(3)
– –
– –
0.0000254(18) o0.000091b o0.00014c
0.0000269(22) o0.00015b o0.00023c
a
Results of this work were taken into account for this evaluation. Detection limit. c Decision limit. b
and we obtain a correlation coefficient rðpð511Þ; pð1115ÞÞ Covðpð511Þ; pð1115ÞÞ=ðuðpð511ÞÞ uðpð1115ÞÞÞ 0:4, which should be taken into account in all further evaluations of published photon emission probabilities. Table 4 contains the emission probabilities determined in this work and those obtained by other selected references. Our results are in excellent agreement with those previously published by Scho¨tzig (1990) and Be´ et al. (2005). The latter work is an evaluation that already includes consideration of the results of this work. In conclusion, the result of this work clearly indicates that p(1115 keV) was considerably overestimated in some previous data evaluations and experiments. In particular, the uncertainty of the result determined by de Roost et al. (1972) is to be questioned. Acknowledgements We wish to thank M.-M. Be´ from LNHB for the coordination of EUROMET project no. 721, and for providing us with one of the 65Zn solutions used for this work. References Be´, M.-M., et al., 2005. Activity measurements and gamma emission intensities determination in the decay of 65Zn. CEA Report CEA-R6081, CEA Saclay, Gif-sur-Yvette, France. Coursey, B.M., et al., 1986. Standardization of carbon-14 by 4pb liquid scintillation efficiency tracing with hydrogen-3. Appl. Radiat. Isot. 37, 403. Debertin, K., Scho¨tzig, U., 1977. Limitations of the pulser method for pile-up corrections in Ge(Li) spectrometry. Nucl. Instrum. Methods 140, 337.
Debertin, K., Scho¨tzig, U., 1979. Coincidence summing corrections in Ge(Li) spectrometry at low source-to-detector distances. Nucl. Instrum. Methods 158, 471. De Roost, E., et al., 1972. The decay of 65Zn. Z. Phys. 250, 395. Grau Carles, A., 2006. EMILIA, the LS counting efficiency for electroncapture and capture-gamma emitters. Comput. Phys. Commun. 174, 35. 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. Commun. 79, 115. Grau Malonda, A., Grau Carles, A., Grau Carles, P., Galiano Casas, G., 1999. EMI2, the counting efficiency for electron capture by KL1L2L3 M model. Comput. Phys. Commun. 123, 114. JanXen, H., 1990. SPLINE techniques for fitting efficiency curves in gamma-ray spectrometry. Nucl. Instrum. Meth. Phys. Res. A 286, 398. JanXen, H., 1994. BSPLINE—Ein Dialogprogramm fu¨r lineare Ausgleichsrechnungen mit Spline-Funktionen. PTB Laboratory Report PTB6.31-94-4 (in German), PTB Braunschweig, Germany. Kantele, J., Valkonen, M., 1973. Corrections for position annihilation in flight in nuclear spectrometry. Nucl. Instrum. Methods 140, 501. Kossert, K., 2006. A new method for secondary standard measurements with the aid of liquid scintillation counting. Appl. Radiat. Isot., in press, DOI:10.1016/j.apradiso.2006.02.060. Kossert, K., Grau Carles, A., 2006. The LSC efficiency for low-Z electroncapture nuclides. Appl. Radiat. Isot., in press, DOI:10.1016/j.apradiso. 2006.02.059. Scho¨tzig, U., 1990. Photon emission probabilities of 44Ti, 65Zn, 88Y, 89Sr, 147 Pm, 204Tl and 210Pb. Nucl. Instrum. Methods Phys. Res. A 286, 523. Scho¨tzig, U., Debertin, K., WeiX, H., 1973. Bestimmung von Gammastrahlen-Emissionswahrscheinlichkeiten mit einem Ge(Li)-Spektrometer, PTB-Mitteilungen 83, pp. 307–312, PTB Braunschweig, Germany. Schrader, H., 2004. Half-life measurements with ionization chambers—A study of systematic effects and results. Appl. Radiat. Isot. 60, 317. Smith, D., 1978. Improved correction formulae for coincidence counting. Nucl. Instrum. Methods 152, 505. Smith, D., 1994. Particle counting in radioactivity measurements. ICRU Report 52, p. 34. Walz, K.F., Debertin, K., Schrader, H., 1983. Half-life measurements at the PTB. Int. J. Appl. Radiat. Isot. 34, 1191.