- Email: [email protected]
Half-life measurement of 95mTc—refinement in the uncertainty value
ARTICLE IN PRESS
Applied Radiation and Isotopes 64 (2006) 1425–1427 www.elsevier.com/locate/apradiso
Half-life measurement of
95m
Tc—refinement in the uncertainty value
Timothy Catterson, Daniel James Van Dalsem Isotope Products Laboratories, Radioassay Department, 24937 Avenue Tibbitts, Valencia, CA 91355-3427, USA
Abstract Measurements at Isotope Products Laboratories (IPL) carried out with a hyper-pure germanium (HPGe) spectrometer have sought to improve upon the accepted half-life value for 95mTc. The uncertainty associated with the currently accepted value of (6172) days allows for potentially significant improvement. Through a series of 65 measurements over the course of nearly 2 years, using multiple source-todetector counting distances and multiple reference photopeaks, the authors have determined a half-life that agrees with the current value while reducing the uncertainty of the value by 25-fold. r 2006 Elsevier Ltd. All rights reserved. Keywords: Half-life; Gamma spectrometry
1. Introduction Refinement in the uncertainty of the half-life value for Tc is reported as performed at Isotope Products Laboratories (IPL) using a hyper-pure germanium (HPGe) spectrometer system. Unik and Rasmussen (1959) had previously reported a half-life value for 95mTc of (6172) days, which was thought by the authors to provide opportunity for improvement. The methodology used was observing the 95mTc decaying away and utilizing graphical techniques to determine the half-life, natural logarithm of the counts per second (cps) versus time.
95m
2. Data collection and analysis The work performed began with approximately 1.2 MBq (32.4 mCi) of 95mTc in the form of carrier-free NH95m 4 TcO4 in 0.5 M NH4OH, sealed in a 1-mL V-vial, which remained sealed throughout the entire series of measurements lasting approximately 10 half-lives. Measurements were made using a reverse electrode closed-end coaxial HPGe detector system with a relative efficiency of 22.6% and a full-width at half-maximum (FWHM) value of 1.8 keV for the Corresponding author. Tel.: +1 661 309 1038; fax: +1 661 309 1098.
E-mail address: [email protected] (D.J. Van Dalsem). 0969-8043/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2006.02.055
1332 keV photopeak of 60Co in which the region of interest for the three photopeaks measured (204.1, 582.1 and 835.1 keV) remained constant via manual integration of the photopeak areas. The signal from the detector was fed through a digital signal processor (Canberra Inspector 2000) and loaded into a commercially available multichannel analyzer program (Canberra Genie2 K, version 2.0). Initially, a source-to-detector counting distance of 390 mm was used to make the measurements. As the 95m Tc activity value dropped to a point where the sourceto-detector counting distance of 390 mm no later allowed for a sufficient number of counts to be recorded for adequate counting statistics, the source-to-detector counting distance was decreased to 110 mm and when this counting distance proved too large to acquire counting statistics with a sufficient degree of precision, the source-todetector counting distance was decreased again to 45 mm. The source-to-detector counting distances were held constant from measurement to measurement by using customconstructed acrylic cylindrical tubes that fit directly over the top of the HPGe detector end-cap on one end and proved a flat surface upon which to place the 95mTc source on the other end. Thus, the source-to-detector counting distance for each counting position was replicated with a high degree of precision each time. Each counting stand was marked with an indelible marker before the initial
ARTICLE IN PRESS T. Catterson, D.J. Van Dalsem / Applied Radiation and Isotopes 64 (2006) 1425–1427
measurement was made with that particular stand to facilitate as great as possible reproducibility for source placement relative to the center of axis for each stand and thus the detector system itself. During the measurements, the count times employed ranged from 6.5 103 to 4.24 105 s as dictated by the source-to-detector counting distance, 95mTc activity present at the time of the measurement and availability of the HPGe detector system needed to make the measurement. Since the detector system used to make the measurements was a system used for routine calibration measurements on a daily basis, the 95mTc measurements were most often confined to taking place overnight or during a weekend. At the time each measurement was made, the spectroscopy software produced a cps value for the source by dividing the total counts recorded by the length of the counting period. This preliminary cps value was corrected for decay during count to produce a cps value at the start of count for each measurement. The start-of-count cps value for each measurement was plotted against the start time for that measurement after the start-of-count cps value was transformed into its natural logarithmic value. The natural logarithmic value plotted versus date, including time of day, generated a linear relationship between the two variables through which a least-squares fit regression was performed to produce a value for the slope of the fitted line. The slope of such a line is mathematically related to the half-life of the radionuclide measured by the equation
0.5 0.3 0.2 0.1 0 -0.1 -0.2 -0.3 -0.4 -0.5 18-Feb-03 30-Jun-03 9-Nov-03 20-Mar-04 30-Jul-04 9-Dec-04 Date Fig. 1. Percent residual from the nine linear regression curves (i.e., three photopeaks each from three source-to-detector distances).
Table 1 Individual results for the nine detector-to-source distances Position
Energy (keV)
Value (days)
A-5
204.1 582.1 835.1 204.1 582.1 835.1 204.1 582.1 835.1 Mean
62.0470.03 61.9070.04 62.0770.14 61.9770.01 61.9870.02 61.9070.01 61.9870.02 61.9570.03 61.8070.04 61.9570.08
B-5
D-5
ln A ¼ ln A0 lt; where l is the slope of the line for the equation and l¼
ln 2 T 1=2
with T1/2 ¼ half-life of the radionuclide as described in numerous publications such as Ehmann and Vance (1991). Data reduction was performed using Microsoft Excel 2002 and Table Curve 2D, version 2.03. MS Excel was used to calculate the start-of-count cps values and Table Curve 2D was used to perform the linear regression analysis in which each start-of-count cps value was weighted individually based on the uncertainty introduced into that value by such contributing factors as instrument stability, peak fit, source position, count time and decay during count. Verification of the linear regression fit, including calculation of the uncertainty in the value of the slope by Table Curve 2D was performed manually in MS Excel using the equations for linear least-squares fit of data with uncertainty values as described by Irvin and Quickenden (1983). Fig. 1 shows the percent residual values from all nine sets of measurements made (three source-to-detector distances, three photopeaks for each source-to-detector distance), where the percent residual value was calculated by dividing the mean value of the natural logarithm of the
204.1 keV 582.1 keV 835.1 keV
0.4
Residual (%)
1426
start-of-count cps values into the residual value and multiplying by 100. Table 1 lists the results from the linear regression analysis on each of the nine individual sets of measurements along with the weighted mean of the individual values, the newly proposed half-life value for 95m Tc, including the calculated uncertainty for the new half-life value: 61.9570.08 days. 3. Conclusions The half-life value of 95mTc was experimentally determined using graphical methods and found to be in agreement with the previously published value. The uncertainty of the newly determined half-life value represents the majority of the improvement in the value and is 25-fold smaller than the previous uncertainty value, 0.08 days currently versus 2 days previously. Acknowledgements The authors wish to acknowledge the support received from the top-level management at Isotope Products Laboratories that allowed them to perform this work.
ARTICLE IN PRESS T. Catterson, D.J. Van Dalsem / Applied Radiation and Isotopes 64 (2006) 1425–1427
References Ehmann, W.D., Vance, D.E., 1991. Radiochemistry and Nuclear Methods of Analysis. Wiley, USA, p. 124.
1427
Irvin, J.A., Quickenden, T.I., 1983. Linear least squares treatment when there are errors in both X and Y. J. Chem. Educ. 60 (9), 711–712. Unik, J.P., Rasmussen, J.O., 1959. Decay schemes of the isomers of Tc95 and Tc97. Phys. Rev. 115 (6), 1687–1692.
Applied Radiation and Isotopes 64 (2006) 1425–1427 www.elsevier.com/locate/apradiso
Half-life measurement of
95m
Tc—refinement in the uncertainty value
Timothy Catterson, Daniel James Van Dalsem Isotope Products Laboratories, Radioassay Department, 24937 Avenue Tibbitts, Valencia, CA 91355-3427, USA
Abstract Measurements at Isotope Products Laboratories (IPL) carried out with a hyper-pure germanium (HPGe) spectrometer have sought to improve upon the accepted half-life value for 95mTc. The uncertainty associated with the currently accepted value of (6172) days allows for potentially significant improvement. Through a series of 65 measurements over the course of nearly 2 years, using multiple source-todetector counting distances and multiple reference photopeaks, the authors have determined a half-life that agrees with the current value while reducing the uncertainty of the value by 25-fold. r 2006 Elsevier Ltd. All rights reserved. Keywords: Half-life; Gamma spectrometry
1. Introduction Refinement in the uncertainty of the half-life value for Tc is reported as performed at Isotope Products Laboratories (IPL) using a hyper-pure germanium (HPGe) spectrometer system. Unik and Rasmussen (1959) had previously reported a half-life value for 95mTc of (6172) days, which was thought by the authors to provide opportunity for improvement. The methodology used was observing the 95mTc decaying away and utilizing graphical techniques to determine the half-life, natural logarithm of the counts per second (cps) versus time.
95m
2. Data collection and analysis The work performed began with approximately 1.2 MBq (32.4 mCi) of 95mTc in the form of carrier-free NH95m 4 TcO4 in 0.5 M NH4OH, sealed in a 1-mL V-vial, which remained sealed throughout the entire series of measurements lasting approximately 10 half-lives. Measurements were made using a reverse electrode closed-end coaxial HPGe detector system with a relative efficiency of 22.6% and a full-width at half-maximum (FWHM) value of 1.8 keV for the Corresponding author. Tel.: +1 661 309 1038; fax: +1 661 309 1098.
E-mail address: [email protected] (D.J. Van Dalsem). 0969-8043/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2006.02.055
1332 keV photopeak of 60Co in which the region of interest for the three photopeaks measured (204.1, 582.1 and 835.1 keV) remained constant via manual integration of the photopeak areas. The signal from the detector was fed through a digital signal processor (Canberra Inspector 2000) and loaded into a commercially available multichannel analyzer program (Canberra Genie2 K, version 2.0). Initially, a source-to-detector counting distance of 390 mm was used to make the measurements. As the 95m Tc activity value dropped to a point where the sourceto-detector counting distance of 390 mm no later allowed for a sufficient number of counts to be recorded for adequate counting statistics, the source-to-detector counting distance was decreased to 110 mm and when this counting distance proved too large to acquire counting statistics with a sufficient degree of precision, the source-todetector counting distance was decreased again to 45 mm. The source-to-detector counting distances were held constant from measurement to measurement by using customconstructed acrylic cylindrical tubes that fit directly over the top of the HPGe detector end-cap on one end and proved a flat surface upon which to place the 95mTc source on the other end. Thus, the source-to-detector counting distance for each counting position was replicated with a high degree of precision each time. Each counting stand was marked with an indelible marker before the initial
ARTICLE IN PRESS T. Catterson, D.J. Van Dalsem / Applied Radiation and Isotopes 64 (2006) 1425–1427
measurement was made with that particular stand to facilitate as great as possible reproducibility for source placement relative to the center of axis for each stand and thus the detector system itself. During the measurements, the count times employed ranged from 6.5 103 to 4.24 105 s as dictated by the source-to-detector counting distance, 95mTc activity present at the time of the measurement and availability of the HPGe detector system needed to make the measurement. Since the detector system used to make the measurements was a system used for routine calibration measurements on a daily basis, the 95mTc measurements were most often confined to taking place overnight or during a weekend. At the time each measurement was made, the spectroscopy software produced a cps value for the source by dividing the total counts recorded by the length of the counting period. This preliminary cps value was corrected for decay during count to produce a cps value at the start of count for each measurement. The start-of-count cps value for each measurement was plotted against the start time for that measurement after the start-of-count cps value was transformed into its natural logarithmic value. The natural logarithmic value plotted versus date, including time of day, generated a linear relationship between the two variables through which a least-squares fit regression was performed to produce a value for the slope of the fitted line. The slope of such a line is mathematically related to the half-life of the radionuclide measured by the equation
0.5 0.3 0.2 0.1 0 -0.1 -0.2 -0.3 -0.4 -0.5 18-Feb-03 30-Jun-03 9-Nov-03 20-Mar-04 30-Jul-04 9-Dec-04 Date Fig. 1. Percent residual from the nine linear regression curves (i.e., three photopeaks each from three source-to-detector distances).
Table 1 Individual results for the nine detector-to-source distances Position
Energy (keV)
Value (days)
A-5
204.1 582.1 835.1 204.1 582.1 835.1 204.1 582.1 835.1 Mean
62.0470.03 61.9070.04 62.0770.14 61.9770.01 61.9870.02 61.9070.01 61.9870.02 61.9570.03 61.8070.04 61.9570.08
B-5
D-5
ln A ¼ ln A0 lt; where l is the slope of the line for the equation and l¼
ln 2 T 1=2
with T1/2 ¼ half-life of the radionuclide as described in numerous publications such as Ehmann and Vance (1991). Data reduction was performed using Microsoft Excel 2002 and Table Curve 2D, version 2.03. MS Excel was used to calculate the start-of-count cps values and Table Curve 2D was used to perform the linear regression analysis in which each start-of-count cps value was weighted individually based on the uncertainty introduced into that value by such contributing factors as instrument stability, peak fit, source position, count time and decay during count. Verification of the linear regression fit, including calculation of the uncertainty in the value of the slope by Table Curve 2D was performed manually in MS Excel using the equations for linear least-squares fit of data with uncertainty values as described by Irvin and Quickenden (1983). Fig. 1 shows the percent residual values from all nine sets of measurements made (three source-to-detector distances, three photopeaks for each source-to-detector distance), where the percent residual value was calculated by dividing the mean value of the natural logarithm of the
204.1 keV 582.1 keV 835.1 keV
0.4
Residual (%)
1426
start-of-count cps values into the residual value and multiplying by 100. Table 1 lists the results from the linear regression analysis on each of the nine individual sets of measurements along with the weighted mean of the individual values, the newly proposed half-life value for 95m Tc, including the calculated uncertainty for the new half-life value: 61.9570.08 days. 3. Conclusions The half-life value of 95mTc was experimentally determined using graphical methods and found to be in agreement with the previously published value. The uncertainty of the newly determined half-life value represents the majority of the improvement in the value and is 25-fold smaller than the previous uncertainty value, 0.08 days currently versus 2 days previously. Acknowledgements The authors wish to acknowledge the support received from the top-level management at Isotope Products Laboratories that allowed them to perform this work.
ARTICLE IN PRESS T. Catterson, D.J. Van Dalsem / Applied Radiation and Isotopes 64 (2006) 1425–1427
References Ehmann, W.D., Vance, D.E., 1991. Radiochemistry and Nuclear Methods of Analysis. Wiley, USA, p. 124.
1427
Irvin, J.A., Quickenden, T.I., 1983. Linear least squares treatment when there are errors in both X and Y. J. Chem. Educ. 60 (9), 711–712. Unik, J.P., Rasmussen, J.O., 1959. Decay schemes of the isomers of Tc95 and Tc97. Phys. Rev. 115 (6), 1687–1692.