- Email: [email protected]
Progress in AMS measurement of U isotope ratios in nanogram U samples
Nuclear Instruments and Methods in Physics Research B xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Progress in AMS measurement of U isotope ratios in nanogram U samples Dong Kejun a, He Ming a, Wang Chen a, Zhao Xinhong a, Li Lili a, Zhao Yonggang a, Wang Xianggao b, Shen Hongtao c, Wang Xiaoming a, Pang Fangfang a, Xu Yongning a, Zhao Qingzhang a, Dou Liang a, Yang Xuran a, Wu Shaoyong a, Lin Deyu a, Li Kangning a, You Qubo a, Bao Yiwen a, Hu Yueming a, Xia Qingliang a, Yin Xinyi a, Jiang Shan a,⇑ a
China Institute of Atomic Energy, P.O. Box 275(50), Beijing 102413, China College of Physics Science and Technology, Guangxi University, Nanning 530004, China c College of Physics and Technology, Guangxi Normal University, Guilin 541004, China b
a r t i c l e
i n f o
Article history: Received 6 November 2014 Received in revised form 27 April 2015 Accepted 30 April 2015 Available online xxxx Keywords: Accelerator Mass Spectrometry Nanogram uranium Isotope ratios
a b s t r a c t The determination of uranium isotopic composition in ultra-trace U samples is very important in different fields, especially for the nuclear forensics. A new Accelerator Mass Spectrometry (AMS) technique has been developed for the measurement of uranium isotopic ratios in ng level uranium samples at China Institute of Atomic Energy (CIAE). Recently, the method was further optimized and developed by using a series of blank and standard samples. The results show that the 236U at the femtogram level can be determined in nanogram U samples by the newly developed AMS technique at CIAE. The experimental setup, performances and results will be detailed in this contribution. Ó 2015 Published by Elsevier B.V.
1. Introduction Measurements of U isotopic composition in ultra-trace tiny uranium samples are extremely important in a variety of scientific and industrial areas, especially for the nuclear safeguards. For example, the ratio of 234U/238U may serve as an additional tool to indicate the origin of nuclear materials or the types of isotope enrichment processes, the ratio of 235U/238U can reveal the presence of undeclared nuclear activities (military or civilian), the measurement of the 236U/238U ratio is an important nuclear safeguards tool which can provide important information on nuclear activities in the region [1,2]. In recent years, a variety of methods have been applied to the analysis of the ultra-trace uranium samples for isotopic composition to provide information on enrichment, irradiation history and nuclear fuel burn-up. Especially for some methods such as fission track combined with thermal ionization mass spectrometry (FT + TIMS) [3], scanning electron microscope combined with secondary ion mass spectrometry (SEM + SIMS) [4,5] and various versions of inductively coupled-plasma mass-spectrometry (ICP-MS) [6], are being used for the measurements of isotopic ratios in ⇑ Corresponding author. Tel.: +86 1069358335; fax: +86 1069357787. E-mail addresses: [email protected] (K. Dong), [email protected] (S. Jiang).
uranium-containing particle samples for the nuclear forensics and safeguards. However, the measurement sensitivity especially for the ratio of 236U/238U was limited by some factors, such as nuclear reactor requirement, molecular background interferences and so on. In these methods, the SIMS technique is a method frequently used for analysis of micron-size U particles. Nevertheless, isobaric interferences caused by molecular ions pose major obstacles for accurate measurement of 234U/238U and 236 U/238U isotopic ratios by SIMS. Accelerator Mass Spectrometry (AMS) has advantages over conventional mass spectrometry in better suppression of molecular interferences leading to improved sensitivity and lower detection limits. In recent years, two new methods for the determination of U isotopic composition in nanogram U sample have been explored and developed using AMS technique at Australian Nuclear Science and Technology Organisation (ANSTO) [7] and China Institute of Atomic Energy (CIAE) [8]. A set of procedures for sample preparation and measurement of U isotopic ratios has been established at CIAE. Recently, this method was developed in an effort to further improve the AMS sensitivity for U isotopic ratio analysis in tiny U samples. As a result, a sensitivity of 10 6 for 236U/238U in nanogram U samples, that is femtogram level 236U can be detected by using AMS method. The newest experimental progress and the main results are presented in this paper.
http://dx.doi.org/10.1016/j.nimb.2015.04.079 0168-583X/Ó 2015 Published by Elsevier B.V.
Please cite this article in press as: K. Dong et al., Progress in AMS measurement of U isotope ratios in nanogram U samples, Nucl. Instr. Meth. B (2015), http://dx.doi.org/10.1016/j.nimb.2015.04.079
2
K. Dong et al. / Nuclear Instruments and Methods in Physics Research B xxx (2015) xxx–xxx
2. Experimental 2.1. Carrier background investigation AMS is one of the most promising methods to measure U isotope ratios, especially the ratio of 236U/238U. The tiny U samples have to be dispersed in a carrier, i.e. some kind of ultra purity conduct material, required by the CIAE AMS ion source. Therefore, one of the most severe interferences for AMS measurement of U isotopic ratios in nanogram U samples is from the U contained in the carrier material. In order to search for an ideal carrier, a series of high purity metals and compounds (Table 1) were measured by AMS for 238U. As presented in Table 1, Nb powder seems to be an ideal carrier for its minimum 238U count rate. However, U isotope extracting in ion source as high as possible is required for the measurement of ultra-trace samples in order to save experimental time and obtain higher counting statistics. As can be seen from Table 1, Fe powder carrier brought about far higher 238U count rate than those extracted from other carrier-uranyl nitrate mixtures, while contributed a moderate background. As a result, high purity Fe powder was finally selected as the carrier material.
2.2. Sample description and preparation Six Certified Reference Material (CRM) of CRM0002, CRM030, CRM005A, CRM010, CRM100, CRM200 in the uranium-containing form of particle and uranyl nitrate solution respectively, one carrier blank (C_Blank, untreated), one flow blank (F_Blank, dissolved in nitric acid and dried), and two natural blanks (N_Blank 1, natural uranium particle dissolved in nitric acid, and N_Blank 2, uranyl nitrate solution, respectively dispersed in Fe powder carrier matrix) were used as test samples for AMS measurement of U isotopic ratios in tiny U samples. The uranium-containing CRM particles with size of about 2 lm were selected by scanning electron microscope (SEM) and dissolved in HNO3. The F_Blank sample was used for carrier background subtraction. The N_Blank samples containing about 5 ng natural uranium dissolved in HNO3, were used for the verify sample preparation and stability of measurement system. An example of particle isolation process for the simulated sample is shown in Fig. 1. The uranium-containing particles were collected onto a glassy carbon planchet with a diameter of 25 mm and a thickness of 3 mm (Hitachi Chemical Co., Ltd., Japan) by an impaction method [9]. Si chips (5 mm 5 mm, Semitec Co., Ltd., Japan) were cleaned, in sequence, with 40%, 2% high purity HNO3 and deionized Milli-Q water (18 MX). The carbon planchet containing uranium particles and the Si chip were introduced into a chamber of a scanning electron microscope (SEM, JSM-6700F, Jeol Co., Ltd., Japan) simultaneously. Individual uranium particles on the carbon planchet were observed with SEM and identified with energy dispersive X-ray (EDX) analysis. Single uranium particle was manipulated with a tungsten needle attached to a manipulator and transferred onto the center of Si chip.
A polytetrafluoroethylene (PFA) bottle with a volume of 8 mL were cleaned with 4% HNO3 and deionized Milli-Q water. The Si chip containing transferred uranium particle was removed from the chamber of the SEM instrument and introduced into the PFA bottle, and 250 lL sub-boiling distilled HNO3 was added. The PFA bottle was then heated on a heating plate for 15 h at 100 °C to dissolve the particle. About 1 mL deionized Milli-Q water and 5 mg ultra purity Fe powder were added into the PFA bottle, respectively, after particle dissolution. Finally, the mixture was evaporated to dryness in an oven at 150 °C for 5 h. All samples, except for C_Blank and F_Blank, were dispersed in ultra purity Fe powder (5 mg) matrix. The resulting samples of CRM standards and blanks, was each pressed firmly into an Al-target holder of a 40 position NEC Multi-Cathode Source of Negative Ions by Cesium Sputtering (MC-SNICS) source. 2.3. AMS analysis The measurement of U isotopic ratios in ultra-trace uranium samples was performed on the HI-13 tandem AMS system at CIAE [10]. The 208Pb16O2 beam current from high purity PbO2 alone was used to simulate the transport of uranium isotopes to avoid the ionizer poisoning for the MC-SNICS ion source and cross-contamination caused by thorium element and constant uranium, respectively. The transport and measurement procedures were detailed in a separate paper [8]. Briefly, PbO2 ions were extracted from the PbO2 sample in ion source, 208Pb16O2 ions were selected by an AMS-dedicated injection system. The 208Pb16O2 ions were accelerated by the tandem terminal voltage set at appropriate value. After carbon foil stripping, 208Pb10+ ions with the same momentum as the uranium isotopic ions under the corresponding terminal voltages (Table 2), were selected by a 90° double focusing High Energy Analyzing Magnet (HEAM). By means of the switching magnet the selected ions were transported further to the AMS beam line and guided to a surface barrier detector (SBD) by the AMS ElectroStatic Analyzer (ESA). The parameters of HEAM and switching magnet were optimized and confirmed by 208Pb10+ ions transmission, and kept for the overall experimental procedure. The interference from the nuclides with neighboring masses can be suppressed effectively at the detector position of the AMS system by using CIAE-AMS dedicated injection system [11], and different uranium isotopes was respectively recorded on the SBD by properly setting the accelerator terminal voltage, and AMS ESA voltage. 3. Results and discussion The results of 235,236U/238U ratios were obtained by absolute measurements, based on the count rates after F_Blank background subtractions of 235,236,238U11+ ions recorded by the SBD detector. Preliminary measurement results for these CRM reference standards are presented in Table 3. From the results, it can be seen that: (1) majority data of measured 235,236U/238U ratio vs certified one for a series of CRM
Table 1 U count rates in a series of carriers and carrier-U mixtures measured by AMS.
238
Materials
Count rate
Untreated carriera
Treated carrierb
Carrier + UTB750c
Fe
Nb
Ag
Al
Fe2O3
Fe
Nb
Ag
Al
Fe2O3
Fe
Nb
Ag
Al
Fe2O3
8.96
2.55
7.70
16.89
11.62
10.13
9.26
9.49
10.52
10.28
124.85
18.97
35.63
39.40
30.27
a
Commercial products, without chemical treatment. b Commercial products, dissolved in nitric acid and dried. c Laboratory U reference standard containing about 5 ng uranium with materials and dried.
238
U isotopic abundance of 92.89% in the form of uranyl nitrate solution, dispersed in carrier
Please cite this article in press as: K. Dong et al., Progress in AMS measurement of U isotope ratios in nanogram U samples, Nucl. Instr. Meth. B (2015), http://dx.doi.org/10.1016/j.nimb.2015.04.079
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K. Dong et al. / Nuclear Instruments and Methods in Physics Research B xxx (2015) xxx–xxx
Fig. 1. Example of the SEM images of uranium particle and transfer process.
Table 2 Optimized parameters in uranium isotopic ratio measurements. Extracted ions
Terminal voltage (MV)
Stripping efficiency (%)
Charge state
Energy after stripping (MeV)
HEA magnetic field (kG)
Switching magnet current (A)
ESA voltage (kV)
208
7.457 7.179 7.240 7.271
18.706 12.171 12.580 12.786
10+ 11+ 11+ 11+
81.132 85.804 86.528 86.891
14.626
132.530
±135.3 ±130.1 ±131.2 ±131.8
PbO2 UO UO 235 UO 238 236
Note: the beam-line transport efficiency for the
Table 3 Results of
Ions measured Tuning U U 235 U 238 236
208
PbO2 pilot beam is 0.152%.
235,236
Samples
U/238U ratios for blanks and references standards.
U
C_Blank F_Blank N_Blank-1 N_Blank-2 CRM0002-1 CRM0002-2 CRM030-1 CRM030-2 CRM005A-1 CRM005A-2 CRM010-1 CRM010-2 CRM100-1 CRM100-2 CRM200-1 CRM200-2
Certifieda
Counting rate (cps) 238
34.46 9.09 142.50 231.84 4505.00 2643.00 23.89 25.59 59.43 88.91 165.63 115.13 317.67 40.58 21.11 34.23
236
U (10
ND ND ND ND 0.33 ND 0.00 0.00 1.00 1.71 29.20 17.80 717.00 12.99 28.00 57.20
3
)
235
U
0.25 0.08 1.04 1.67 0.38 0.32 0.61 0.39 0.27 0.33 3.15 1.42 47.69 5.83 3.07 5.69
236
U/
238
Measuredb 235
U
U/
238
7.20 10
<1.00 10
7
3
(1.76 ± 0.005) 10
(6.18 ± 0.052) 10 (1.19 ± 0.01) 10
236
U
6
(6.88 ± 0.071) 10 (4.23 ± 0.011) 10
(5.09 ± 0.003) 10 5
(1.01 ± 0.001) 10
4
(2.66 ± 0.0076) 10
4
2
3
2
(1.14 ± 0.0011) 10 3
235
U/238U
ND
(3.14 ± 0.0017) 10
5
U/238U
(2.51 ± 0.0026) 10
1
1
(7.41 ± 7.41) 10 ND <3.19 10 5 <3.32 10 5 (1.15 ± 0.81) 10 (1.15 ± 0.47) 10 (6.91 ± 0.57) 10 (6.96 ± 1.02) 10 (4.18 ± 0.16) 10 (4.16 ± 1.16) 10 (2.72 ± 0.33) 10 (2.60 ± 0.11) 10
8
5 5 5 5 4 4 3 3
(7.35 ± 0.73) 10 (7.15 ± 0.69) 10 (7.26 ± 0.51) 10 (7.20 ± 0.39) 10 (1.83 ± 0.12) 10 (1.78 ± 0.16) 10 (3.03 ± 0.15) 10 (3.09 ± 0.25) 10 (5.13 ± 0.35) 10 (5.05 ± 0.49) 10 (1.06 ± 0.06) 10 (1.09 ± 0.09) 10 (1.11 ± 0.01) 10 (1.26.±0.02) 10 (2.81 ± 0.15) 10 (2.58 ± 0.24) 10
3 3 3 3 4 4 2 2 3 3 2 2 1 1 1 1
1 and 2 represent samples prepared with uranyl nitrate solution and particles dissolved in nitric acid, respectively, before dispersed in ultra purity Fe powder matrix. ND is not detected. a The uncertainties include uncertainties in inhomogeneity and analysis. b The uncertainties include counting statistics only.
standards shows a good coincidence within uncertainties especially for 235U/238U ratios; (2) uranium extracting in F_Blank sample was suppressed effectively from the comparison of C_Blank
and F_Blank; (3) although the results of the samples prepared from solution seems little bit better than those prepared from particles that compared with the counting rates between the
Please cite this article in press as: K. Dong et al., Progress in AMS measurement of U isotope ratios in nanogram U samples, Nucl. Instr. Meth. B (2015), http://dx.doi.org/10.1016/j.nimb.2015.04.079
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K. Dong et al. / Nuclear Instruments and Methods in Physics Research B xxx (2015) xxx–xxx
samples-1 and samples-2, it is still needed to further verify by experiments. However, the results are still not satisfied for the insufficient counting statistics of U isotopes. In principle, the counting rate of 238 U should several thousand per second for every sample according to the beam-line transport efficiency (0.152%) of the 208PbO2 pilot beam and the content of 238U, however, the counting rates of all samples lower about several decades times than the expected values, except for CRM0002-1 and CRM0002-2. This could be due to the samples extraction efficiency in ion source was affected by the inhomogeneous distribution of U in Fe powder matrix, or the change of the beam optical system during measurement, such as caused by the accelerator terminal voltage extremely small drift, and so on. In general, the stability of AMS system and uniformity of samples is the prerequisite for accurate measurement and analysis of nanogram level U samples.
make up for the current disadvantage of SIMS in the determination of 236U at extremely low level. Generally, this work provided a new detection technique for AMS measurement of U isotopes and laid a good foundation for accurate determination of plutonium and other indicator nuclides in trace samples for nuclear safeguard and forensics. Although the intrinsically disadvantage in the lower transmission efficiency of our large accelerator system still constitutes a major obstacle in further lowering the detection limit and reducing the uncertainty of ultra-trace U isotopes determination, we have no reason to doubt that the potential of AMS in specialized analytical applications such as nuclear safeguards and forensics for its intrinsic high sensitivity in isotope abundance ratios of uranium-bearing particles with the development of AMS technique and instrument. Acknowledgements
4. Summary In this work, the AMS measurement technique of U isotope ratios in nanogram U samples has been extended and improved by comparing with different chemical forms of sample materials, using a series of CRM standard and blank samples at CIAE. Preliminary results indicate that a lowest detectable 236U at the femtogram level in nanogram level U samples can be achieved based on current analysis method. Currently, SIMS is also a powerful tool to measure isotope abundance ratios of uranium-bearing particles [12,5]. The detection limits is estimated that the 235U amount of 4.5 fg is sufficient to obtain the 235U/238U ratio with the relative standard deviation (RSD) within 5.0%, the 234U amount of 0.42 fg and the 236U amount of 1.1 fg are sufficient to obtain the 234U/238U and 236U/238U ratios with the RSD within 20%, respectively [13]. In recent years, some combination methods for improving the measurement precision and detection limit of uranium isotope abundance ratios [5,14,15]. Although these developed techniques enabled to acquire stable signals needed for isotope ratio measurement, there is a difficulty in analyzing 234U/238U and 236U/238U isotope ratios accurately because of isobaric interferences caused by molecular ions of other elements. AMS has advantages in better suppression of molecular interferences leading to improved sensitivity and lower detection limits. Although our present results did not demonstrate the obvious advantage of AMS over SIMS in terms of measurement uncertainty and 235U/238U ratio at present, this method can at least
The authors gratefully acknowledge Professor Tian Weizhi from Department of Nuclear Physics, CIAE for helpful discussions, as well as to the technical staff at CIAE for smooth accelerator operation. This work was mainly supported by the National Natural Science Foundations of China (NSFC), under Grant Nos. 11075221. References [1] M.A.C. Hotchkis, D. Child, D. Fink, G.E. Jacobsen, P.J. Lee, N. Mino, A.M. Smith, C. Tuniz, Nucl. Instr. Meth. B 172 (2000) 659. [2] S.F. Boulyga, K.G. Heumann, J. Environ. Radioact. 88 (2006) 1. [3] C.G. Lee, K. Iguchi, F. Esaka, M. Magara, S. Sakurai, K. Watanabe, S. Usuda, Jpn. J. Appl. Phys. 45 (2006) L294. [4] N. Erdmann, M. Betti, O. Stetzer, G. Tamborini, J.V. Kratz, N. Trautmann, Spectrochim. Acta B 55 (2000) 1565. [5] F. Esaka, K.T. Esaka, C.G. Lee, M. Magara, S. Sakurai, S. Usuda, K. Watanabe, Talanta 71 (2007) 1011. [6] F. Esaka, M. Magara, C.G. Lee, S. Sakurai, S. Usuda, N. Shinohara, Talanta 78 (2009) 290. [7] M.A.C. Hotchkis, D.P. Child, B. Zorko, Nucl. Instr. Meth. B 268 (2010) 1257. [8] K.J. Dong, C. Wang, M. He, et al., Chin. Phys. Lett. 31 (1) (2014) 010701-1. [9] F. Esaka, K. Watanabe, H. Fukuyama, T. Onodera, K.T. Esaka, M. Magara, S. Sakurai, S. Usuda, J. Nucl. Sci. Technol. 41 (2004) 1027. [10] S. Jiang, M. He, S.S. Jiang, et al., Nucl. Instr. Meth. B 172 (2000) 87. [11] S. Jiang, M. He, K.J. Dong, et al., Nucl. Instr. Meth. B 294 (2013) 39. [12] G. Tamborini, M. Betti, V. Forcina, T. Hiernaut, B. Giovannone, L. Koch, Spectrochim. Acta B 53 (1998) 1289. [13] F. Esaka, K. Watanabe, T. Onodera, C.G. Lee, M. Magara, S. Sakurai, S. Usuda, Appl. Surf. Sci. 255 (2008) 1512. [14] F. Esaka, C.G. Lee, M. Magara, T. Kimura, Anal. Chim. Acta 721 (2012) 122. [15] F. Esaka, M. Magara, Talanta 120 (2014) 349.
Please cite this article in press as: K. Dong et al., Progress in AMS measurement of U isotope ratios in nanogram U samples, Nucl. Instr. Meth. B (2015), http://dx.doi.org/10.1016/j.nimb.2015.04.079
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Progress in AMS measurement of U isotope ratios in nanogram U samples Dong Kejun a, He Ming a, Wang Chen a, Zhao Xinhong a, Li Lili a, Zhao Yonggang a, Wang Xianggao b, Shen Hongtao c, Wang Xiaoming a, Pang Fangfang a, Xu Yongning a, Zhao Qingzhang a, Dou Liang a, Yang Xuran a, Wu Shaoyong a, Lin Deyu a, Li Kangning a, You Qubo a, Bao Yiwen a, Hu Yueming a, Xia Qingliang a, Yin Xinyi a, Jiang Shan a,⇑ a
China Institute of Atomic Energy, P.O. Box 275(50), Beijing 102413, China College of Physics Science and Technology, Guangxi University, Nanning 530004, China c College of Physics and Technology, Guangxi Normal University, Guilin 541004, China b
a r t i c l e
i n f o
Article history: Received 6 November 2014 Received in revised form 27 April 2015 Accepted 30 April 2015 Available online xxxx Keywords: Accelerator Mass Spectrometry Nanogram uranium Isotope ratios
a b s t r a c t The determination of uranium isotopic composition in ultra-trace U samples is very important in different fields, especially for the nuclear forensics. A new Accelerator Mass Spectrometry (AMS) technique has been developed for the measurement of uranium isotopic ratios in ng level uranium samples at China Institute of Atomic Energy (CIAE). Recently, the method was further optimized and developed by using a series of blank and standard samples. The results show that the 236U at the femtogram level can be determined in nanogram U samples by the newly developed AMS technique at CIAE. The experimental setup, performances and results will be detailed in this contribution. Ó 2015 Published by Elsevier B.V.
1. Introduction Measurements of U isotopic composition in ultra-trace tiny uranium samples are extremely important in a variety of scientific and industrial areas, especially for the nuclear safeguards. For example, the ratio of 234U/238U may serve as an additional tool to indicate the origin of nuclear materials or the types of isotope enrichment processes, the ratio of 235U/238U can reveal the presence of undeclared nuclear activities (military or civilian), the measurement of the 236U/238U ratio is an important nuclear safeguards tool which can provide important information on nuclear activities in the region [1,2]. In recent years, a variety of methods have been applied to the analysis of the ultra-trace uranium samples for isotopic composition to provide information on enrichment, irradiation history and nuclear fuel burn-up. Especially for some methods such as fission track combined with thermal ionization mass spectrometry (FT + TIMS) [3], scanning electron microscope combined with secondary ion mass spectrometry (SEM + SIMS) [4,5] and various versions of inductively coupled-plasma mass-spectrometry (ICP-MS) [6], are being used for the measurements of isotopic ratios in ⇑ Corresponding author. Tel.: +86 1069358335; fax: +86 1069357787. E-mail addresses: [email protected] (K. Dong), [email protected] (S. Jiang).
uranium-containing particle samples for the nuclear forensics and safeguards. However, the measurement sensitivity especially for the ratio of 236U/238U was limited by some factors, such as nuclear reactor requirement, molecular background interferences and so on. In these methods, the SIMS technique is a method frequently used for analysis of micron-size U particles. Nevertheless, isobaric interferences caused by molecular ions pose major obstacles for accurate measurement of 234U/238U and 236 U/238U isotopic ratios by SIMS. Accelerator Mass Spectrometry (AMS) has advantages over conventional mass spectrometry in better suppression of molecular interferences leading to improved sensitivity and lower detection limits. In recent years, two new methods for the determination of U isotopic composition in nanogram U sample have been explored and developed using AMS technique at Australian Nuclear Science and Technology Organisation (ANSTO) [7] and China Institute of Atomic Energy (CIAE) [8]. A set of procedures for sample preparation and measurement of U isotopic ratios has been established at CIAE. Recently, this method was developed in an effort to further improve the AMS sensitivity for U isotopic ratio analysis in tiny U samples. As a result, a sensitivity of 10 6 for 236U/238U in nanogram U samples, that is femtogram level 236U can be detected by using AMS method. The newest experimental progress and the main results are presented in this paper.
http://dx.doi.org/10.1016/j.nimb.2015.04.079 0168-583X/Ó 2015 Published by Elsevier B.V.
Please cite this article in press as: K. Dong et al., Progress in AMS measurement of U isotope ratios in nanogram U samples, Nucl. Instr. Meth. B (2015), http://dx.doi.org/10.1016/j.nimb.2015.04.079
2
K. Dong et al. / Nuclear Instruments and Methods in Physics Research B xxx (2015) xxx–xxx
2. Experimental 2.1. Carrier background investigation AMS is one of the most promising methods to measure U isotope ratios, especially the ratio of 236U/238U. The tiny U samples have to be dispersed in a carrier, i.e. some kind of ultra purity conduct material, required by the CIAE AMS ion source. Therefore, one of the most severe interferences for AMS measurement of U isotopic ratios in nanogram U samples is from the U contained in the carrier material. In order to search for an ideal carrier, a series of high purity metals and compounds (Table 1) were measured by AMS for 238U. As presented in Table 1, Nb powder seems to be an ideal carrier for its minimum 238U count rate. However, U isotope extracting in ion source as high as possible is required for the measurement of ultra-trace samples in order to save experimental time and obtain higher counting statistics. As can be seen from Table 1, Fe powder carrier brought about far higher 238U count rate than those extracted from other carrier-uranyl nitrate mixtures, while contributed a moderate background. As a result, high purity Fe powder was finally selected as the carrier material.
2.2. Sample description and preparation Six Certified Reference Material (CRM) of CRM0002, CRM030, CRM005A, CRM010, CRM100, CRM200 in the uranium-containing form of particle and uranyl nitrate solution respectively, one carrier blank (C_Blank, untreated), one flow blank (F_Blank, dissolved in nitric acid and dried), and two natural blanks (N_Blank 1, natural uranium particle dissolved in nitric acid, and N_Blank 2, uranyl nitrate solution, respectively dispersed in Fe powder carrier matrix) were used as test samples for AMS measurement of U isotopic ratios in tiny U samples. The uranium-containing CRM particles with size of about 2 lm were selected by scanning electron microscope (SEM) and dissolved in HNO3. The F_Blank sample was used for carrier background subtraction. The N_Blank samples containing about 5 ng natural uranium dissolved in HNO3, were used for the verify sample preparation and stability of measurement system. An example of particle isolation process for the simulated sample is shown in Fig. 1. The uranium-containing particles were collected onto a glassy carbon planchet with a diameter of 25 mm and a thickness of 3 mm (Hitachi Chemical Co., Ltd., Japan) by an impaction method [9]. Si chips (5 mm 5 mm, Semitec Co., Ltd., Japan) were cleaned, in sequence, with 40%, 2% high purity HNO3 and deionized Milli-Q water (18 MX). The carbon planchet containing uranium particles and the Si chip were introduced into a chamber of a scanning electron microscope (SEM, JSM-6700F, Jeol Co., Ltd., Japan) simultaneously. Individual uranium particles on the carbon planchet were observed with SEM and identified with energy dispersive X-ray (EDX) analysis. Single uranium particle was manipulated with a tungsten needle attached to a manipulator and transferred onto the center of Si chip.
A polytetrafluoroethylene (PFA) bottle with a volume of 8 mL were cleaned with 4% HNO3 and deionized Milli-Q water. The Si chip containing transferred uranium particle was removed from the chamber of the SEM instrument and introduced into the PFA bottle, and 250 lL sub-boiling distilled HNO3 was added. The PFA bottle was then heated on a heating plate for 15 h at 100 °C to dissolve the particle. About 1 mL deionized Milli-Q water and 5 mg ultra purity Fe powder were added into the PFA bottle, respectively, after particle dissolution. Finally, the mixture was evaporated to dryness in an oven at 150 °C for 5 h. All samples, except for C_Blank and F_Blank, were dispersed in ultra purity Fe powder (5 mg) matrix. The resulting samples of CRM standards and blanks, was each pressed firmly into an Al-target holder of a 40 position NEC Multi-Cathode Source of Negative Ions by Cesium Sputtering (MC-SNICS) source. 2.3. AMS analysis The measurement of U isotopic ratios in ultra-trace uranium samples was performed on the HI-13 tandem AMS system at CIAE [10]. The 208Pb16O2 beam current from high purity PbO2 alone was used to simulate the transport of uranium isotopes to avoid the ionizer poisoning for the MC-SNICS ion source and cross-contamination caused by thorium element and constant uranium, respectively. The transport and measurement procedures were detailed in a separate paper [8]. Briefly, PbO2 ions were extracted from the PbO2 sample in ion source, 208Pb16O2 ions were selected by an AMS-dedicated injection system. The 208Pb16O2 ions were accelerated by the tandem terminal voltage set at appropriate value. After carbon foil stripping, 208Pb10+ ions with the same momentum as the uranium isotopic ions under the corresponding terminal voltages (Table 2), were selected by a 90° double focusing High Energy Analyzing Magnet (HEAM). By means of the switching magnet the selected ions were transported further to the AMS beam line and guided to a surface barrier detector (SBD) by the AMS ElectroStatic Analyzer (ESA). The parameters of HEAM and switching magnet were optimized and confirmed by 208Pb10+ ions transmission, and kept for the overall experimental procedure. The interference from the nuclides with neighboring masses can be suppressed effectively at the detector position of the AMS system by using CIAE-AMS dedicated injection system [11], and different uranium isotopes was respectively recorded on the SBD by properly setting the accelerator terminal voltage, and AMS ESA voltage. 3. Results and discussion The results of 235,236U/238U ratios were obtained by absolute measurements, based on the count rates after F_Blank background subtractions of 235,236,238U11+ ions recorded by the SBD detector. Preliminary measurement results for these CRM reference standards are presented in Table 3. From the results, it can be seen that: (1) majority data of measured 235,236U/238U ratio vs certified one for a series of CRM
Table 1 U count rates in a series of carriers and carrier-U mixtures measured by AMS.
238
Materials
Count rate
Untreated carriera
Treated carrierb
Carrier + UTB750c
Fe
Nb
Ag
Al
Fe2O3
Fe
Nb
Ag
Al
Fe2O3
Fe
Nb
Ag
Al
Fe2O3
8.96
2.55
7.70
16.89
11.62
10.13
9.26
9.49
10.52
10.28
124.85
18.97
35.63
39.40
30.27
a
Commercial products, without chemical treatment. b Commercial products, dissolved in nitric acid and dried. c Laboratory U reference standard containing about 5 ng uranium with materials and dried.
238
U isotopic abundance of 92.89% in the form of uranyl nitrate solution, dispersed in carrier
Please cite this article in press as: K. Dong et al., Progress in AMS measurement of U isotope ratios in nanogram U samples, Nucl. Instr. Meth. B (2015), http://dx.doi.org/10.1016/j.nimb.2015.04.079
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K. Dong et al. / Nuclear Instruments and Methods in Physics Research B xxx (2015) xxx–xxx
Fig. 1. Example of the SEM images of uranium particle and transfer process.
Table 2 Optimized parameters in uranium isotopic ratio measurements. Extracted ions
Terminal voltage (MV)
Stripping efficiency (%)
Charge state
Energy after stripping (MeV)
HEA magnetic field (kG)
Switching magnet current (A)
ESA voltage (kV)
208
7.457 7.179 7.240 7.271
18.706 12.171 12.580 12.786
10+ 11+ 11+ 11+
81.132 85.804 86.528 86.891
14.626
132.530
±135.3 ±130.1 ±131.2 ±131.8
PbO2 UO UO 235 UO 238 236
Note: the beam-line transport efficiency for the
Table 3 Results of
Ions measured Tuning U U 235 U 238 236
208
PbO2 pilot beam is 0.152%.
235,236
Samples
U/238U ratios for blanks and references standards.
U
C_Blank F_Blank N_Blank-1 N_Blank-2 CRM0002-1 CRM0002-2 CRM030-1 CRM030-2 CRM005A-1 CRM005A-2 CRM010-1 CRM010-2 CRM100-1 CRM100-2 CRM200-1 CRM200-2
Certifieda
Counting rate (cps) 238
34.46 9.09 142.50 231.84 4505.00 2643.00 23.89 25.59 59.43 88.91 165.63 115.13 317.67 40.58 21.11 34.23
236
U (10
ND ND ND ND 0.33 ND 0.00 0.00 1.00 1.71 29.20 17.80 717.00 12.99 28.00 57.20
3
)
235
U
0.25 0.08 1.04 1.67 0.38 0.32 0.61 0.39 0.27 0.33 3.15 1.42 47.69 5.83 3.07 5.69
236
U/
238
Measuredb 235
U
U/
238
7.20 10
<1.00 10
7
3
(1.76 ± 0.005) 10
(6.18 ± 0.052) 10 (1.19 ± 0.01) 10
236
U
6
(6.88 ± 0.071) 10 (4.23 ± 0.011) 10
(5.09 ± 0.003) 10 5
(1.01 ± 0.001) 10
4
(2.66 ± 0.0076) 10
4
2
3
2
(1.14 ± 0.0011) 10 3
235
U/238U
ND
(3.14 ± 0.0017) 10
5
U/238U
(2.51 ± 0.0026) 10
1
1
(7.41 ± 7.41) 10 ND <3.19 10 5 <3.32 10 5 (1.15 ± 0.81) 10 (1.15 ± 0.47) 10 (6.91 ± 0.57) 10 (6.96 ± 1.02) 10 (4.18 ± 0.16) 10 (4.16 ± 1.16) 10 (2.72 ± 0.33) 10 (2.60 ± 0.11) 10
8
5 5 5 5 4 4 3 3
(7.35 ± 0.73) 10 (7.15 ± 0.69) 10 (7.26 ± 0.51) 10 (7.20 ± 0.39) 10 (1.83 ± 0.12) 10 (1.78 ± 0.16) 10 (3.03 ± 0.15) 10 (3.09 ± 0.25) 10 (5.13 ± 0.35) 10 (5.05 ± 0.49) 10 (1.06 ± 0.06) 10 (1.09 ± 0.09) 10 (1.11 ± 0.01) 10 (1.26.±0.02) 10 (2.81 ± 0.15) 10 (2.58 ± 0.24) 10
3 3 3 3 4 4 2 2 3 3 2 2 1 1 1 1
1 and 2 represent samples prepared with uranyl nitrate solution and particles dissolved in nitric acid, respectively, before dispersed in ultra purity Fe powder matrix. ND is not detected. a The uncertainties include uncertainties in inhomogeneity and analysis. b The uncertainties include counting statistics only.
standards shows a good coincidence within uncertainties especially for 235U/238U ratios; (2) uranium extracting in F_Blank sample was suppressed effectively from the comparison of C_Blank
and F_Blank; (3) although the results of the samples prepared from solution seems little bit better than those prepared from particles that compared with the counting rates between the
Please cite this article in press as: K. Dong et al., Progress in AMS measurement of U isotope ratios in nanogram U samples, Nucl. Instr. Meth. B (2015), http://dx.doi.org/10.1016/j.nimb.2015.04.079
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K. Dong et al. / Nuclear Instruments and Methods in Physics Research B xxx (2015) xxx–xxx
samples-1 and samples-2, it is still needed to further verify by experiments. However, the results are still not satisfied for the insufficient counting statistics of U isotopes. In principle, the counting rate of 238 U should several thousand per second for every sample according to the beam-line transport efficiency (0.152%) of the 208PbO2 pilot beam and the content of 238U, however, the counting rates of all samples lower about several decades times than the expected values, except for CRM0002-1 and CRM0002-2. This could be due to the samples extraction efficiency in ion source was affected by the inhomogeneous distribution of U in Fe powder matrix, or the change of the beam optical system during measurement, such as caused by the accelerator terminal voltage extremely small drift, and so on. In general, the stability of AMS system and uniformity of samples is the prerequisite for accurate measurement and analysis of nanogram level U samples.
make up for the current disadvantage of SIMS in the determination of 236U at extremely low level. Generally, this work provided a new detection technique for AMS measurement of U isotopes and laid a good foundation for accurate determination of plutonium and other indicator nuclides in trace samples for nuclear safeguard and forensics. Although the intrinsically disadvantage in the lower transmission efficiency of our large accelerator system still constitutes a major obstacle in further lowering the detection limit and reducing the uncertainty of ultra-trace U isotopes determination, we have no reason to doubt that the potential of AMS in specialized analytical applications such as nuclear safeguards and forensics for its intrinsic high sensitivity in isotope abundance ratios of uranium-bearing particles with the development of AMS technique and instrument. Acknowledgements
4. Summary In this work, the AMS measurement technique of U isotope ratios in nanogram U samples has been extended and improved by comparing with different chemical forms of sample materials, using a series of CRM standard and blank samples at CIAE. Preliminary results indicate that a lowest detectable 236U at the femtogram level in nanogram level U samples can be achieved based on current analysis method. Currently, SIMS is also a powerful tool to measure isotope abundance ratios of uranium-bearing particles [12,5]. The detection limits is estimated that the 235U amount of 4.5 fg is sufficient to obtain the 235U/238U ratio with the relative standard deviation (RSD) within 5.0%, the 234U amount of 0.42 fg and the 236U amount of 1.1 fg are sufficient to obtain the 234U/238U and 236U/238U ratios with the RSD within 20%, respectively [13]. In recent years, some combination methods for improving the measurement precision and detection limit of uranium isotope abundance ratios [5,14,15]. Although these developed techniques enabled to acquire stable signals needed for isotope ratio measurement, there is a difficulty in analyzing 234U/238U and 236U/238U isotope ratios accurately because of isobaric interferences caused by molecular ions of other elements. AMS has advantages in better suppression of molecular interferences leading to improved sensitivity and lower detection limits. Although our present results did not demonstrate the obvious advantage of AMS over SIMS in terms of measurement uncertainty and 235U/238U ratio at present, this method can at least
The authors gratefully acknowledge Professor Tian Weizhi from Department of Nuclear Physics, CIAE for helpful discussions, as well as to the technical staff at CIAE for smooth accelerator operation. This work was mainly supported by the National Natural Science Foundations of China (NSFC), under Grant Nos. 11075221. References [1] M.A.C. Hotchkis, D. Child, D. Fink, G.E. Jacobsen, P.J. Lee, N. Mino, A.M. Smith, C. Tuniz, Nucl. Instr. Meth. B 172 (2000) 659. [2] S.F. Boulyga, K.G. Heumann, J. Environ. Radioact. 88 (2006) 1. [3] C.G. Lee, K. Iguchi, F. Esaka, M. Magara, S. Sakurai, K. Watanabe, S. Usuda, Jpn. J. Appl. Phys. 45 (2006) L294. [4] N. Erdmann, M. Betti, O. Stetzer, G. Tamborini, J.V. Kratz, N. Trautmann, Spectrochim. Acta B 55 (2000) 1565. [5] F. Esaka, K.T. Esaka, C.G. Lee, M. Magara, S. Sakurai, S. Usuda, K. Watanabe, Talanta 71 (2007) 1011. [6] F. Esaka, M. Magara, C.G. Lee, S. Sakurai, S. Usuda, N. Shinohara, Talanta 78 (2009) 290. [7] M.A.C. Hotchkis, D.P. Child, B. Zorko, Nucl. Instr. Meth. B 268 (2010) 1257. [8] K.J. Dong, C. Wang, M. He, et al., Chin. Phys. Lett. 31 (1) (2014) 010701-1. [9] F. Esaka, K. Watanabe, H. Fukuyama, T. Onodera, K.T. Esaka, M. Magara, S. Sakurai, S. Usuda, J. Nucl. Sci. Technol. 41 (2004) 1027. [10] S. Jiang, M. He, S.S. Jiang, et al., Nucl. Instr. Meth. B 172 (2000) 87. [11] S. Jiang, M. He, K.J. Dong, et al., Nucl. Instr. Meth. B 294 (2013) 39. [12] G. Tamborini, M. Betti, V. Forcina, T. Hiernaut, B. Giovannone, L. Koch, Spectrochim. Acta B 53 (1998) 1289. [13] F. Esaka, K. Watanabe, T. Onodera, C.G. Lee, M. Magara, S. Sakurai, S. Usuda, Appl. Surf. Sci. 255 (2008) 1512. [14] F. Esaka, C.G. Lee, M. Magara, T. Kimura, Anal. Chim. Acta 721 (2012) 122. [15] F. Esaka, M. Magara, Talanta 120 (2014) 349.
Please cite this article in press as: K. Dong et al., Progress in AMS measurement of U isotope ratios in nanogram U samples, Nucl. Instr. Meth. B (2015), http://dx.doi.org/10.1016/j.nimb.2015.04.079