作者简介: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
中文责编:英 子; 英文责编:木 南
Jiang Shan, Yang Xuran, He Ming, Dong Kejun, and Dou LiangDepartment of Nuclear Physics, China Institute of Atomic Energy, Beijing 102413, P.R.China
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=1.5×106 a)和36Cl(T1/2=3.0×105 a)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.
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 10-8 at highest)for measurement of nuclides such as 14C(T1/2=5 730 a), 10Be(T1/2=1.5×106 a)and 36Cl(T1/2=3.0×105 a). 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.
Accelerator mass spectrometry(AMS)technique 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. It can also very effectively remove various background interferences(molecular and isobaric backgrounds), thus can greatly improve the measuring sensitivity(isotopic abundance reaching 10-16 orders of magnitude). Currently, sample amount in AMS measurements can reach ng scale, and the corresponding minimum detection limit is 104 atoms, which is a nuclide measurement method with the highest sensitivity among all nuclear analytical techniques so far, with extremely broad application prospects.
AMS belongs to the category of isotope mass spectrometry with molecular and isobaric background eliminating capabilities. Its basic structure is shown in Fig.1.
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 as 10Be, 26Al, 36Cl, 41Ca, 79Se, 93Zr, 92Nb, 99Tc, 121Sn, 126Sn, 129I, 151Sm, 182Hf and 236U, which reach the international advanced level, and has conducted application research in the aspects of nuclear physics, astrophysics, geology, environmental science and life science.
After the determination of activity A and number of atoms N of long-lived radionuclides, the half-lives of radionuclides can be calculated according to the relation between radioactivity of nuclides and time A=-λN, where λ=ln 2/T1/2. As the number of atoms is usually relatively small for long-lived radionuclides, they cannot be measured with ordinary MS because of its strong background interference. AMS has molecular and isobaric background eliminating capabilities, thus it can be employed for the measurements of half-lives of long-lived radionuclides with relatively small number of decay atoms.
Taking 79Se for an example, as a fission product of 235U, one can find that its half-life is very long(105 to 106 a), which is a major issue in the field of landfill waste treatment. Since 1948, the half-life of 79Se has been measured repeatedly, and the difference among the measured values are evident(table 2). In 2002, Jiang Songsheng et al measured the half-life of 79Se to be(2.95±0.38)×105 a 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 of 79Se. Currently, we are improving the AMS measurement method and using absolute measurement method to measure the number of 79Se atoms in the test sample, thereby accurately measuring the half-life of 79Se. After a series of improvements, we obtained that the half-life of 79Se was(2.79±0.16)×105 a, 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 of 79Se.
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 104 atoms)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 formula σ=n/(IN), in which n is the number of atoms produced by reaction and measured by AMS, I is the intensity of incident beam, and N is the number of target nuclei.
The cross sections of 238U(n,3n)236U nuclear fission reaction induced by 14 MeV neutrons are important nuclear data. If the emitted neutrons are measured online using direct measurement method, the accuracy will not be high due to the influences from multiple aspects such as prompt and scattered neutrons during the fission process. If the neutrons are measured by neutron activation, the long half-life and low generation level of residual nucleus 236U(T1/2=2.34×107 a)will make it very difficult to be measured. If the residual nucleus 236U is measured by ordinary MS, the ordinary MS will be subject to molecular background interferences(such as 235UH+), and its maximum measurement sensitivity of 236U is 236U/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 of 236U(236U/238U～10-8≈10-14). In the actual test, to measure the cross section of 14 MeV neutron induced reaction 238U(n,3n)236U, we only need to measure 236U/238U, i.e. the ratio of atom numbers between daughter nucleus 236U and mother nucleus 238U, 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 neutron 238U(n,3n)236U reaction cross section, 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 of 238U(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.
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 and 126Sn 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)×105 a(National Nuclear Data Center), respectively. With the proceeding of human activities, large amount of artificial 121mSn and 126Sn are introduced into the environment, which results in significantly higher 121mSn and 126Sn contents in some areas. The nuclides 121mSn and 126Sn have become important indicator nuclides for long-term contamination in nuclear engineering environment.
The establishment of highly sensitive measurement method for fission product nuclides 121mSn and 126Sn 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 of 121mSn and 126Sn 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 of 121mSn and 126Sn based on HI-13 tandem AMS, with the measurement sensitivity better than(4.2±0.1)×10-10 and(5.2±2.3)×10-12, respectively.
Both 3H and 129I 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, which has great development potential in the field of tritium application.
129I has a half-life of 1.6×107 a, and a fission yield of 0.8%, which is a long-lived fission nuclide produced by spontaneous fission of 238U or neutron-induced fission of 235U. 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 of 129I in the environment, with isotope abundance ratio being reaching 10-13 to 10-15. AMS measurement of 129I in environmental samples has become an important means of nuclear inspection internationally.
CIAE-AMS group measured 129I content in the aerosol samples in Beijing during the Fukushima Nuclear Power Plant(FNPP)accident, and compared it with the 131I content in the aerosol samples of the same period(Fig.2). 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 the 129I 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 of 129I, 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 the 131I 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 of 129I by AMS has great advantages in terms of nuclear accident warning and detection, because during the early stage of nuclear accident, the amount of 129I released is 1 to 2 orders of magnitude higher than that of 131I. And the data analysis results can be more reasonable and credible because much 129I can be obtained in a short time. On-machine measurement time of 129I 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, although 131I samples do not require preprocessing, too much time needed for sample collection. The total demand for 129I samples is relatively simplenss. All of these provide more favorable evidences for the monitoring of early stage nuclear leak.
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 arious 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.