- Email: [email protected]
Standardization of 68(Ge+Ga)
ARTICLE IN PRESS
Applied Radiation and Isotopes 60 (2004) 429–431
68
Standardization of
(Ge+Ga)
E.L. Grigorescua,*, C.D. Neguta, A. Lucaa, Anamaria Cristina Razdolescua, Mioara Tanaseb a
National Institute of R&D for Physics and Nuclear Engineering ‘‘Horia Hulubei’’- IFIN-HH, Atomistilor str. 407, POB MG-6, Bucharest-Magurele 76900, Romania b CS ISPAT SIDEX, Galati, Romania
Abstract The radionuclide 68Ga is mainly a positron emitter (89.2%), with a half-life of 67.7 min. It is used in nuclear medicine, being chemically extracted from the mixture 68(Ge+Ga); its precursor, 68Ge, disintegrates 100% by electron capture, with a half-life of 270.8 d (Table of radionuclides, comments and Evaluation). A 4pb–g coincidence method was used for standardization, with a 4p proportional b-detector and a NaI(Tl) g detector. Registration of the capture radiations was avoided using foil absorption and a high b threshold. Using supplementary foils for positron absorption, extrapolation graphs were obtained, with a mean slope of 4.4%. Care was taken to compensate for the loss of 68Ge during the preparation of solid sources for measurement. A combined uncertainty of 1.1% was estimated. r 2003 Elsevier Ltd. All rights reserved. Keywords: Absolute activity measurement; Coincidence method;
1. Introduction
68
Ge;
*Corresponding author. Tel.: +40-1-404-2350; fax: +40-1423-1701. E-mail address: [email protected] (E.L. Grigorescu).
Ga
2. Method
68
Ge disintegrates 100% by electron capture, with a half-life of 270.8 days (B!e et al., 1999), producing X-rays and Auger electrons with energies less than 10 keV. The resulting 68Ga is mainly a positron emitter with a half-life of 67.7 min (B!e et al., 1999), used in nuclear medicine; it is chemically extracted from the mixture 68(Ge+Ga). A simplified decay scheme of the pair is represented in Fig. 1 (B!e et al., 1999). The 4pb–g coincidence method was used for standardization. The capture radiations were not registered using foil absorption or a high b threshold. So, coincidences were measured between positrons and the 511 keV annihilation radiations, with a very small interference of the 1077 keV photons on the gamma channel. One determines in an absolute manner the activity of 68Ga, which is virtually the same for 68Ge . (Smith and Williams, 1971; Schonfeld et al., 1994; Piepke and Cook, 1997).
68
The coincidence equations may be written as Nb ¼ ðb1 þ b2 Þ½eb þ ð1 eb Þ2ebg N0 1 eb ¼ ðb1 þ b2 Þeb 1 þ 2ebg ; eb Ng ¼ ðb1 þ b2 Þ2eg þ b3 eg1077 N0 b3 eg1077 ; ¼ ðb1 þ b2 Þ2eg 1 þ b1 þ b2 2eg Nc ¼ ðb1 þ b2 Þ½eb 2eg þ ð1 eb Þ2ebg ðeg þ egc Þ N0 1 eb egc ebg 1 þ ; ¼ ðb1 þ b2 Þeb 2eg 1 þ eg eb
ð1Þ
where b1 ; b2 ; and b3 are branching ratios; eb is the positron efficiency (supposed equal for the two b1, b2 b-branches, as b2 is only 1.2%); ebg is the efficiency of the 4p proportional counter for the 511 keV photons (o1%); eg is the efficiency of the gamma detector for those photons; egc is the efficiency of the gamma detector
0969-8043/$ - see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2003.11.054
ARTICLE IN PRESS E.L. Grigorescu et al. / Applied Radiation and Isotopes 60 (2004) 429–431
430
were determined by gamma comparison with a drop of solution deposited in a small cavity on a support and immediately sealed with Al foil. To avoid differences in absorption and self-absorption, the 1077 keV photons were used for comparison. The loss of Ge for different sources was of 20–26%. Registration of the capture radiations was avoided using foil absorption and a high b threshold. For the extrapolation procedure, Al foils were used; the range of eb was 58–81%. A graph was obtained for every source (Fig. 2) by plotting Nb Ng =Nc as a function of ðð1 eb Þ=eb Þ: The value of eb is well approximated by
68 32 Ge
68 31
EC 1.8%
1077 keV 3.2% (b3)
Ga
822 keV 1.2% (b2)
EC 8.7%
1899 keV 87.9% (b1)
Nc eb D : Ng
68 30 Zn
Fig. 1. Simplified decay scheme of
68
(Ge+Ga) mixture.
for the Compton radiations produced by the 511 keV photons registered in the 4p detector; and eg1077 is the efficiency of the gamma detector for the 1077 keV photons. In the third equation, coincidences produced by the 1077 keV photons were neglected, as b2 is only 1.2% and the ratio egc /eg was estimated from the previous gamma measurement to have a value of 0.15. In principle, the gamma efficiencies for the 511 keV photons produced in the walls of the 4p detector and in the source covered with foils must be different; as the Ng count rate remain unchanged during the extrapolation procedure used, the two efficiencies were considered equal in the equation. Similar considerations are valid for ebg : From system (1), one may obtain Nb Ng 1 þ ðð1 eb Þ=eb Þ2ebg ¼ ðb1 þ b2 Þ Nc N0 1 þ ðð1 eb Þ=eb Þebg ð1 þ egc =eg Þ b3 eg1077 1þ b1 þ b2 2eg 1 eb egc ebg 1 D ðb1 þ b2 Þ 1 þ eb eg b3 eg1077 1þ : b1 þ b2 2eg
The resulting straight line had a mean slope of (4.470.5)%; it intercepts the ordinate axes at the b3 eg1077 : ð4Þ N0 ðb1 þ b2 Þ 1 þ b1 þ b2 2eg The negative sign of the slope is due to the large g window of 100–600 keV in the g channel, which allowed for egc > eg : The extrapolation procedure justifies the supposition of equal b efficiency for the b1 ; b2 branches. The mean value of the massic activity was 36.1 kBq g1. The small activities of the sources allowed for very small corrections for dead time and resolution time. An important correction was due to the g background. Corrections for the decay scheme parameters are included in expression (4). The main uncertainty components are * * * *
ð2Þ
ð3Þ
counting 0.4% ‘‘equivalent’’ masses determination 0.7% approximations in formulae 0.5% extrapolation and decay scheme parameters 0.5%.
The combined uncertainty for a coverage factor k ¼ 1 is 1.1% which is an acceptable result. So, the final result for massic activity is 36.170.4 kBq g1.
The approximation is justified by the small values, o1% of the term of the type ðð1 eb Þ=eb Þebg :
450
Four solid sources were prepared from the (Ge+Ga) solution to be measured by the 4pb–g method. Masses of the sources were between 10 and 20 mg. Germanium was precipitated as sulphide in drops deposited on plastic supports. After drying, the sources were sandwiched between two plastic foils, 0.45-mm thick. As some Ge was lost during drying (a well-known behaviour of Ge), the ‘‘equivalent’’ masses of sources 68
(Nβ Nγ) / Nc
445
3. Results
440 435 430 425 420 0.0
0.2
0.4 Nγ / Nc - 1
0.6
Fig. 2. Extrapolation graph for source no. 3.
0.8
ARTICLE IN PRESS E.L. Grigorescu et al. / Applied Radiation and Isotopes 60 (2004) 429–431
4. Conclusions *
*
*
*
*
*
A 68(Ge+Ga) solution was standardized by a coincidence method. Capture radiations were not registered using absorbing foils as a high b threshold. Coincidences between positrons and 511 keV photons were registered. The obtained extrapolation graphs provided interceptions with the ordinate axis containing the corrections for the decay scheme parameters (expression (4)). To compensate for the loss of Ge during the preparation of sources, ‘‘equivalent’’ masses were determined. A final combined uncertainty of 1.1% was obtained, which is an acceptable result.
Acknowledgements are due to our chemist colleagues, Zoltan Szucs, Catalina Campeanu and the staff of IFIN-HH Cyclotron. The work was done in the frame
431
of the Contract with European Commission ICA1CT-2000-70023/12.02.2001-Center of Excellence IDRANAP.
References B!e, M.M., Duchemin, B., Browne, E., Chechev, V., Helmer, R.G., Schonfeld, E., 1999. Table de Radionucleides: Comments on Evaluations, ISBN 2 7272 0211 3, CEA/ DIMRI/LNHB, France. Piepke, A., Cook, B., 1997. A method to calibrate a neutrino detector using the positron emitter 68Ge. Nucl. Instrum. Methods A 385, 85–90. . Schonfeld, E., et al., 1994. Standardization and decay data of 68 Ge/68Ga. Appl. Radiat. Isot. 45 (9), 955–961. Smith, D., Williams, A., 1971. A 4pb–g coincidence method for mixed electron-capture/positron emitters: the absolute measurement of 68Ga. Int. J. Appl. Radiat. Isot. 22, 615–623.
Applied Radiation and Isotopes 60 (2004) 429–431
68
Standardization of
(Ge+Ga)
E.L. Grigorescua,*, C.D. Neguta, A. Lucaa, Anamaria Cristina Razdolescua, Mioara Tanaseb a
National Institute of R&D for Physics and Nuclear Engineering ‘‘Horia Hulubei’’- IFIN-HH, Atomistilor str. 407, POB MG-6, Bucharest-Magurele 76900, Romania b CS ISPAT SIDEX, Galati, Romania
Abstract The radionuclide 68Ga is mainly a positron emitter (89.2%), with a half-life of 67.7 min. It is used in nuclear medicine, being chemically extracted from the mixture 68(Ge+Ga); its precursor, 68Ge, disintegrates 100% by electron capture, with a half-life of 270.8 d (Table of radionuclides, comments and Evaluation). A 4pb–g coincidence method was used for standardization, with a 4p proportional b-detector and a NaI(Tl) g detector. Registration of the capture radiations was avoided using foil absorption and a high b threshold. Using supplementary foils for positron absorption, extrapolation graphs were obtained, with a mean slope of 4.4%. Care was taken to compensate for the loss of 68Ge during the preparation of solid sources for measurement. A combined uncertainty of 1.1% was estimated. r 2003 Elsevier Ltd. All rights reserved. Keywords: Absolute activity measurement; Coincidence method;
1. Introduction
68
Ge;
*Corresponding author. Tel.: +40-1-404-2350; fax: +40-1423-1701. E-mail address: [email protected] (E.L. Grigorescu).
Ga
2. Method
68
Ge disintegrates 100% by electron capture, with a half-life of 270.8 days (B!e et al., 1999), producing X-rays and Auger electrons with energies less than 10 keV. The resulting 68Ga is mainly a positron emitter with a half-life of 67.7 min (B!e et al., 1999), used in nuclear medicine; it is chemically extracted from the mixture 68(Ge+Ga). A simplified decay scheme of the pair is represented in Fig. 1 (B!e et al., 1999). The 4pb–g coincidence method was used for standardization. The capture radiations were not registered using foil absorption or a high b threshold. So, coincidences were measured between positrons and the 511 keV annihilation radiations, with a very small interference of the 1077 keV photons on the gamma channel. One determines in an absolute manner the activity of 68Ga, which is virtually the same for 68Ge . (Smith and Williams, 1971; Schonfeld et al., 1994; Piepke and Cook, 1997).
68
The coincidence equations may be written as Nb ¼ ðb1 þ b2 Þ½eb þ ð1 eb Þ2ebg N0 1 eb ¼ ðb1 þ b2 Þeb 1 þ 2ebg ; eb Ng ¼ ðb1 þ b2 Þ2eg þ b3 eg1077 N0 b3 eg1077 ; ¼ ðb1 þ b2 Þ2eg 1 þ b1 þ b2 2eg Nc ¼ ðb1 þ b2 Þ½eb 2eg þ ð1 eb Þ2ebg ðeg þ egc Þ N0 1 eb egc ebg 1 þ ; ¼ ðb1 þ b2 Þeb 2eg 1 þ eg eb
ð1Þ
where b1 ; b2 ; and b3 are branching ratios; eb is the positron efficiency (supposed equal for the two b1, b2 b-branches, as b2 is only 1.2%); ebg is the efficiency of the 4p proportional counter for the 511 keV photons (o1%); eg is the efficiency of the gamma detector for those photons; egc is the efficiency of the gamma detector
0969-8043/$ - see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2003.11.054
ARTICLE IN PRESS E.L. Grigorescu et al. / Applied Radiation and Isotopes 60 (2004) 429–431
430
were determined by gamma comparison with a drop of solution deposited in a small cavity on a support and immediately sealed with Al foil. To avoid differences in absorption and self-absorption, the 1077 keV photons were used for comparison. The loss of Ge for different sources was of 20–26%. Registration of the capture radiations was avoided using foil absorption and a high b threshold. For the extrapolation procedure, Al foils were used; the range of eb was 58–81%. A graph was obtained for every source (Fig. 2) by plotting Nb Ng =Nc as a function of ðð1 eb Þ=eb Þ: The value of eb is well approximated by
68 32 Ge
68 31
EC 1.8%
1077 keV 3.2% (b3)
Ga
822 keV 1.2% (b2)
EC 8.7%
1899 keV 87.9% (b1)
Nc eb D : Ng
68 30 Zn
Fig. 1. Simplified decay scheme of
68
(Ge+Ga) mixture.
for the Compton radiations produced by the 511 keV photons registered in the 4p detector; and eg1077 is the efficiency of the gamma detector for the 1077 keV photons. In the third equation, coincidences produced by the 1077 keV photons were neglected, as b2 is only 1.2% and the ratio egc /eg was estimated from the previous gamma measurement to have a value of 0.15. In principle, the gamma efficiencies for the 511 keV photons produced in the walls of the 4p detector and in the source covered with foils must be different; as the Ng count rate remain unchanged during the extrapolation procedure used, the two efficiencies were considered equal in the equation. Similar considerations are valid for ebg : From system (1), one may obtain Nb Ng 1 þ ðð1 eb Þ=eb Þ2ebg ¼ ðb1 þ b2 Þ Nc N0 1 þ ðð1 eb Þ=eb Þebg ð1 þ egc =eg Þ b3 eg1077 1þ b1 þ b2 2eg 1 eb egc ebg 1 D ðb1 þ b2 Þ 1 þ eb eg b3 eg1077 1þ : b1 þ b2 2eg
The resulting straight line had a mean slope of (4.470.5)%; it intercepts the ordinate axes at the b3 eg1077 : ð4Þ N0 ðb1 þ b2 Þ 1 þ b1 þ b2 2eg The negative sign of the slope is due to the large g window of 100–600 keV in the g channel, which allowed for egc > eg : The extrapolation procedure justifies the supposition of equal b efficiency for the b1 ; b2 branches. The mean value of the massic activity was 36.1 kBq g1. The small activities of the sources allowed for very small corrections for dead time and resolution time. An important correction was due to the g background. Corrections for the decay scheme parameters are included in expression (4). The main uncertainty components are * * * *
ð2Þ
ð3Þ
counting 0.4% ‘‘equivalent’’ masses determination 0.7% approximations in formulae 0.5% extrapolation and decay scheme parameters 0.5%.
The combined uncertainty for a coverage factor k ¼ 1 is 1.1% which is an acceptable result. So, the final result for massic activity is 36.170.4 kBq g1.
The approximation is justified by the small values, o1% of the term of the type ðð1 eb Þ=eb Þebg :
450
Four solid sources were prepared from the (Ge+Ga) solution to be measured by the 4pb–g method. Masses of the sources were between 10 and 20 mg. Germanium was precipitated as sulphide in drops deposited on plastic supports. After drying, the sources were sandwiched between two plastic foils, 0.45-mm thick. As some Ge was lost during drying (a well-known behaviour of Ge), the ‘‘equivalent’’ masses of sources 68
(Nβ Nγ) / Nc
445
3. Results
440 435 430 425 420 0.0
0.2
0.4 Nγ / Nc - 1
0.6
Fig. 2. Extrapolation graph for source no. 3.
0.8
ARTICLE IN PRESS E.L. Grigorescu et al. / Applied Radiation and Isotopes 60 (2004) 429–431
4. Conclusions *
*
*
*
*
*
A 68(Ge+Ga) solution was standardized by a coincidence method. Capture radiations were not registered using absorbing foils as a high b threshold. Coincidences between positrons and 511 keV photons were registered. The obtained extrapolation graphs provided interceptions with the ordinate axis containing the corrections for the decay scheme parameters (expression (4)). To compensate for the loss of Ge during the preparation of sources, ‘‘equivalent’’ masses were determined. A final combined uncertainty of 1.1% was obtained, which is an acceptable result.
Acknowledgements are due to our chemist colleagues, Zoltan Szucs, Catalina Campeanu and the staff of IFIN-HH Cyclotron. The work was done in the frame
431
of the Contract with European Commission ICA1CT-2000-70023/12.02.2001-Center of Excellence IDRANAP.
References B!e, M.M., Duchemin, B., Browne, E., Chechev, V., Helmer, R.G., Schonfeld, E., 1999. Table de Radionucleides: Comments on Evaluations, ISBN 2 7272 0211 3, CEA/ DIMRI/LNHB, France. Piepke, A., Cook, B., 1997. A method to calibrate a neutrino detector using the positron emitter 68Ge. Nucl. Instrum. Methods A 385, 85–90. . Schonfeld, E., et al., 1994. Standardization and decay data of 68 Ge/68Ga. Appl. Radiat. Isot. 45 (9), 955–961. Smith, D., Williams, A., 1971. A 4pb–g coincidence method for mixed electron-capture/positron emitters: the absolute measurement of 68Ga. Int. J. Appl. Radiat. Isot. 22, 615–623.