Applications of accelerator mass spectrometry in nuclear science

Jiang Shan, Yang Xuran, He Ming, Dong Kejun, and Dou Liang

Department of Nuclear Physics, China Institute of Atomic Energy, Beijing 102413, P.R.China



【Physics / 物理】

Applications of accelerator mass spectrometry in nuclear science

Jiang Shan†, Yang Xuran, He Ming, Dong Kejun, and Dou Liang†

Department of Nuclear Physics, China Institute of Atomic Energy, Beijing 102413, P.R.China

Belonging to the category of isotope mass spectrometry(MS), accelerator mass spectrometry(AMS)is a high-energy mass spectrometry based on accelerators and ion detectors．AMS overcomes the molecular and isobaric background interferences extant in conventional MS, and therefore has an extremely high isotopic abundance sensitivity, which reaches 10-15(isotopic abundance sensitivity of conventional MS is 10-8at highest) for measurement of nuclides such as14C(T1/2=5 730 a),10Be(T1/2=1.5×106a) and36Cl(T1/2=3.0×105a). Accordingly, AMS has extremely broad application prospects. This paper introduces the principle, technique and development status of AMS and focuses on the introduction of CIAE’s AMS technique and research advances in its application in nuclear science and technology, such as studies on measurements of the half-life of long-lived nuclides(79Se), small nuclear reaction cross sections(238U(n,3n)236U) and long-lived fission product nucleus in valley areas, as well as AMS measurements of 129I as the new approach for nuclear facility monitoring, nuclear environment and emergency detection.

particle physics and nuclear physics; accelerator mass spectrometry; half-life; nuclear reaction cross sections; fission product nucleus; nuclear energy and nuclear safety

Accelerator mass spectrometry (AMS) technique[1]is a measurement approach for detection of trace nuclides and analysis of rare particles, which was developed in the late 1970s. It belongs to the category of nuclear analytical techniques in the nuclear technology. Compared with the conventional long-life radioisotope detection techniques (liquid scintillation counting, LC-MS/hyphenated MS, etc.), this technique is free from the influences of composite structure and matrix effect of substances being detected, and does not require specific extraction and chromatographic separation of the detected substances[2]. It can also very effectively remove various background interferences (molecular and isobaric backgrounds), thus can greatly improve the measuring sensitivity (isotopic abundance reaching 10-16orders of magnitude). Currently, sample amount in AMS measurements can reach ng scale, and the corresponding minimum detection limit is 104atoms, which is a nuclide measurement method with the highest sensitivity among all nuclear analytical techniques so far, with extremely broad application prospects．

1 Introduction of AMS measurement technique

AMS belongs to the category of isotope mass spectrometry with molecular and isobaric background eliminating capabilities[3]. Its basic structure is shown in Fig.1．

Fig.1 Schematic diagram of AMS图1 加速器质谱原理图

Its basic measuring process is as follows:

1) After the negative ions are extracted from the ion source, they are then selected by the injection system and injected into the accelerator.

2) The negative molecular ions are accelerated by the accelerator. Stripping foil (marked with an arrow) is employed to break up the molecular ions and to produce atomic ions with high charge states.

3) The energy, mass and charge state of ions are selected by high-energy magnetic analyzing magnet, and then electrostatic analyzer is employed to select the energy and charge state of ions.

4) Ions are identified and measured by detector.

The current international status for measurements of some long-lived radioactive tracer isotopes by AMS measurement technique are shown in table 1, from which we can see that AMS measurement technique can completely meet the measurement demand of ultra-trace long-lived radionuclides, which have a high practical value in the field of nuclear science and technology application.

China Institute of Atomic Energy (CIAE) had the AMS test capability as early as in 1989. Through a series of improvements in the last two decades, HI-13 tandem AMS system has been built. So far, CIAE has successively carried out measurements for nuclides such as10Be,26Al,36Cl,41Ca,79Se,93Zr,92Nb ,99Tc,121Sn ,126Sn ,129I,151Sm,182Hf and236U, which reach the international advanced level, and has conducted application research in the aspects of nuclear physics, astrophysics, geology, environmental science and life science.

2 Application of AMS measurement technique in nuclear science and technology

2.1 Measurements of half-lives of long-lived nuclides

After the determination of activityAandnumberofatomsNoflong-livedradionuclides,thehalf-livesofradionuclidescanbecalculatedaccordingtotherelationbetweenradioactivityofnuclidesandtimeA=-λN,whereλ=ln 2/T1/2.Asthenumberofatomsisusuallyrelativelysmallforlong-livedradionuclides,theycannotbemeasuredwithordinaryMSbecauseofitsstrongbackgroundinterference.AMShasmolecularandisobaricbackgroundeliminatingcapabilities,thusitcanbeemployedforthemeasurementsofhalf-livesoflong-livedradionuclideswithrelativelysmallnumberofdecayatoms.

Table 1 AMS measurements of major nucleus 表1 AMS测量的主要核素

Taking79Se for an example, as a fission product of235U, one can find that its half-life is very long (105to 106a), which is a major issue in the field of landfill waste treatment. Since 1948, the half-life of79Se has been measured repeatedly, and the difference among the measured values are evident(table 2). In 2002, Jiang Songsheng et al[21]measured the half-life of79Se to be (2.95±0.38)×105a based on CIAE-AMS system using radiochemical separation+liquid scintillation+AMS method. The data were adopted by the U.S. National Nuclear Data Center and the IAEA Nuclear Data Center, as the reference data for half-life of79Se. Currently, we are improving the AMS measurement method and using absolute measurement method to measure the number of79Se atoms in the test sample, thereby accurately measuring the half-life of79Se. After a series of improvements, we obtained that the half-life of79Se was (2.79±0.16)×105a, within the error being range of the results obtained in 2002, and with lower relative uncertainty from 12.9% to 5.7%. Thus it is a big step forward in the measurements of half-life of79Se.

Table 2 Measurements of half-life of 79Se[22]表2 79Se半衰期的测量

2.2 Measurements of small nuclear reaction cross sections

Measurement of small nuclear reaction cross section is an important research direction of experimental nuclear physics. It can reveal the interaction mechanism between incident particles and target nuclei, which is conducive to deepen the understanding of nuclear force and structure, and is the fundamental basis for testing nuclear theory. In addition, nuclear cross section data are also the basis for the application of nuclear science and technology, which are of importance especially to the building and improvement of theoretical models of nuclear reactions, design of fusion reactor, building of nuclear data libraries, as well as nuclear astrophysics.

Due to the extremely high sensitivity (minimum detection limit of 104atoms) of AMS in measurements of the nuclear reaction products, i.e. nuclides, AMS can achieve the measurements of some small nuclear reaction cross sections. The AMS measurements of nuclear reaction cross sections are based on the formulaσ=n/(IN),inwhichnisthenumberofatomsproducedbyreactionandmeasuredbyAMS,Iistheintensityofincidentbeam,andNisthenumberoftargetnuclei.

Thecrosssectionsof238U(n，3n)236Unuclearfissionreactioninducedby14MeVneutronsareimportantnucleardata.Iftheemittedneutronsaremeasuredonlineusingdirectmeasurementmethod,theaccuracywillnotbehighduetotheinfluencesfrommultipleaspectssuchaspromptandscatteredneutronsduringthefissionprocess.Iftheneutronsaremeasuredbyneutronactivation,thelonghalf-lifeandlowgenerationlevelofresidualnucleus236U(T1/2=2.34×107a) will make it very difficult to be measured. If the residual nucleus236U is measured by ordinary MS, the ordinary MS will be subject to molecular background interferences (such as235UH+), and its maximum measurement sensitivity of236U is236U/238U≈1×10-6. In comparison, AMS lifts the restriction of isobaric and molecular background interferences existing in ordinary MS, which is a good option for analyzing the trace of236U(236U/238U～10-8≈10-14). In the actual test, to measure the cross section of 14 MeV neutron induced reaction238U(n，3n)236U, we only need to measure236U/238U, i.e. the ratio of atom numbers between daughter nucleus236U and mother nucleus238U, as well as neutron fluence.

CIAE-AMS group was the first one in the world to establish the method for AMS measurements of 14 MeV neutron238U(n，3n)236U reaction cross section[50], demonstrating that the AMS method is the best option for measurements of long-lived nuclide generating fission nuclide reaction cross sections. The method has laid the foundation for successful measurements of significant reaction cross sections of other fission nuclides (Pu，Am). The study has initially given the reaction cross section data of238U(n，3n)236U. The reaction cross sections with neutron energies (14.65±0.40) MeV (0° irradiated samples) and (14.18±0.30) MeV (85° irradiated samples) are (556.7±43.4) mb and (489.3±54.3) mb, respectively.

Some of the nuclear reaction cross sections by AMS method currently in the world are listed in table 3.

Table 3 Measurements of cross section by AMS 表3 AMS方法测量的部分核反应截面

2.3 Measurements of long-lived fission product nucleus in valley area

Since the 1990s, the experimental measurements of peak-to-valley ratio of long-lived fission products have been promoted because of the proposals of the clean nuclear energy system separation, the transmutation plan and the deepening of research on potential long-term risks in deep geological disposal of high level radioactive waste. As the fission products of uranium and other fissile nuclei,121mSn and126Sn are two long-lived radionuclides whose peak-to-valley ratio determination is relatively sensitive. Their half-lives are (43.9±5.0)a and (2.30±0.14)×105a(National Nuclear Data Center), respectively. With the proceeding of human activities, large amount of artificial121mSn and126Sn are introduced into the environment, which results in significantly higher121mSn and126Sn contents in some areas. The nuclides121mSn and126Sn have become important indicator nuclides for long-term contamination in nuclear engineering environment.

The establishment of highly sensitive measurement method for fission product nuclides121mSn and126Sn can lay the foundation for the application of peak-to-valley ratio as well as the research of environmental safety. However, due to long half-life, weak radioactivity and very low content of121mSn and126Sn in samples, measurements can hardly be achieved by conventional decay counting because of too large demand for samples. Nor can the ordinary MS achieve high sensitivity measurements due to the limits in measurement sensitivity.

CIAE-AMS group has successfully established a highly sensitive method for AMS measurements of121mSn and126Sn based on HI-13 tandem AMS[51], with the measurement sensitivity better than (4.2±0.1)×10-10and (5.2±2.3)×10-12, respectively.

2.4 Application of AMS measurements in nuclear energy and nuclear safety

Both3H and129I are important nuclides in the field of nuclear energy and nuclear safety application. With the continuous application and development of fission reactors, fusion reactors and nuclear materials, tritium has been gaining increasing attention as one of the important nuclides. Monitoring of tritium content in surrounding air, primary loop water (heavy water) and structural components (zirconium alloy) is an important role. The sensitivities of common tritium monitoring techniques are around 0.1 Bq at present, which limits the progress of research work. AMS measurement technique can improve tritium measurement sensitivity at least by one order of magnitude[52], which has great development potential in the field of tritium application.

129I has a half-life of 1.6×107a, and a fission yield of 0.8%, which is a long-lived fission nuclide produced by spontaneous fission of238U or neutron-induced fission of235U. No matter whether it is during reactor leak or spent fuel reprocessing,129I will inevitably release into the environment and accumulate in the environment to cause long-term contamination. At present, AMS is the most powerful means for measuring the trace of129I in the environment, with isotope abundance ratio being reaching 10-13to 10-15. AMS measurement of129I in environmental samples has become an important means of nuclear inspection internationally.

CIAE-AMS group measured129I content in the aerosol samples in Beijing during the Fukushima Nuclear Power Plant (FNPP) accident, and compared it with the131I content in the aerosol samples of the same period(Fig.2)[53]. The results showed that around 2011-03-26,129I concentration in the air started to be higher than the background level of March 20, indicating that the129I produced in the accident had arrived in Beijing. In comparison,131I content in the same sample did not change significantly. Due to the cumulative effect of129I, to 2011-03-28,129I concentration in the air had been significantly elevated, which was 3 to 4 times higher than the background level, while the131I content in the same sample still had no significant change. Therefore, in the early stage of nuclear leak accident, conventional radioactivity measurement approaches for short-lived nuclides often have the shortcomings of low sensitivity and long measurement time. The measurement of129I by AMS has great advantages in terms of nuclear accident warning and detection, because during the early stage of nuclear accident, the amount of129I released is 1 to 2 orders of magnitude higher than that of131I. And the data analysis results can be more reasonable and credible because much129I can be obtained in a short time. On-machine measurement time of129I is only 2 to 3 h by AMS, while the on-machine measurement of conventional γ spectrometry takes 20 h or longer.129I samples require relatively short preprocessing time, although131I samples do not require preprocessing, too much time needed for sample collection. The total demand for129I samples is relatively simplenss. All of these provide more favorable evidences for the monitoring of early stage nuclear leak.

Fig.2 (Color online) 129I and 131I contents in the aerosol samples in Beijing during FNPP accident in 2011[53]图2 2011年福岛事故期间北京地区空气样品中129I浓度与 131I浓度[53]

3 Conclusion and outlook

In summary, AMS measurement technique is closely related to the application of nuclear science and technology. With people’s increasing degree of emphasis on nuclear energy and nuclear safety, the research on distribution and application of various trace elements in nuclear materials will also continue to be deepened. In terms of sensitivity, radiation safety and economic efficiency, AMS measurement technique is one of the most significant techniques among accelerator-based techniques in the field of isotope analysis, which has been widely used in various fields of nuclear science and technology application. Furthermore, AMS measurement has extremely high sensitivity. With continuous maturation and development of AMS technique, it will definitely play an increasingly important role in the field of nuclear science and technology application.

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【中文责编：英 子；英文责编：木 南】

加速器质谱在核科学中的应用

姜 山，杨旭冉，何 明，董克君，窦 亮

中国原子能科学研究院核物理研究所, 北京 102413

加速器质谱(accelerator mass spectrometry，AMS)是基于加速器和离子探测器的一种高能质谱，属于一种同位素质谱(mass spectroscopy, MS)，它克服了传统MS存在的分子本底和同量异位素本底干扰，因此同位素丰度灵敏度很高，对14C(T1/2=5 730 a)、10Be(T1/2=1.5×106a)和36Cl(T1/2=3.0×105a)等核素测量的丰度灵敏度均达10-15(传统MS的同位素丰度灵敏度最高为10-8)．因此，AMS具有极其广泛的应用前景．简述AMS原理、技术和发展现状，介绍中国原子能科学研究院的AMS技术，及该技术在核科学与技术中的应用研究进展，包括长寿命核素半衰期的测量(如79Se)，核反应微小截面的测量(如238U(n,3n)236U)，长寿命谷区裂变产物核测量以及129I的AMS测量作为核设施监测、核环境与核应急检测的新方法等.

粒子物理与原子核物理；加速器质谱；半衰期；核反应截面；裂变产物核；核能与核安全

国家自然科学基金资助项目(11375272)

姜 山(1956—)，男(蒙古族)，河北省青龙县人，中国原子能科学研究院研究员、博士生导师.E-mail：jiangs @ciae.ac.cn

/ References：

：Jiang Shan, Yang Xuran, He Ming，et al，et al．Applications of accelerator mass spectrometry in nuclear science[J]. Journal of Shenzhen University Science and Engineering, 2015, 32(1): 8-16.

O 571.1； O 657.63 Document code：A

10.3724/SP.J.1249.2015.01008

Received：2014-09-02；Accepted：2014-12-09

Foundation：National Natural Science Foundation of China (11375272)

† Corresponding author：Professor Jiang Shan．E-mail：Jiangs@cise.ac.cn；Research associate Dou Liang. E-mail:douer-2@163.com

引 文：姜 山，杨旭冉，何 明，等．加速器质谱在核科学中的应用[J]. 深圳大学学报理工版，2015，32(1)：8-16.(英文版)