14C的紧凑型加速器质谱测量技术研究

Research on Compact Accelerator Mass Spectrometry Measurement Technique for 14C

  • 摘要: 作为重要的环境示踪核素,14C广泛参与地球系统的物质循环过程。本研究依托中国原子能科学研究院自主研制的紧凑型加速器质谱(AMS)装置,系统开展了14C的测量技术。通过碳的离子源引出、AMS系统束流光学优化、粒子电荷态选择、剥离气压设定、本底排除技术等研究,实现了14C的AMS系统高效传输与高灵敏分析,传输效率达45%,测量灵敏度达14C/12C=1.6×10−15。另外,还改进了样品石墨化制备流程,建立了高效的样品制备方法,提高了束流强度。本研究建立了从样品制备到高灵敏分析的完整 14C 加速器质谱测量技术体系,为其在环境科学、地质年代学及生命科学等领域的广泛应用奠定了基础。

     

    Abstract: Carbon-14 (14C) is a key environmental tracer nuclide that plays an essential role in the biogeochemical cycling of the Earth system and has been widely applied in environmental science, geochronology, and life sciences. Accelerator mass spectrometry (AMS) is currently the most sensitive analytical technique for 14C detection, and its development has progressively trended toward low-energy, compact, and highly integrated systems. In this work, comprehensive research on compact AMS measurement techniques for 14C was conducted using a compact multi-nuclide AMS system independently developed by the China Institute of Atomic Energy (CIAE). Systematic investigations were carried out on critical aspects affecting 14C measurement performance, including negative ion extraction, beam optics optimization, charge state selection, stripping gas species and pressure, background suppression, and ion detection. An online Zn-Fe graphitization sample preparation system was designed and built to improve sample preparation efficiency, safety, and controllability. By optimizing the reduction tube structure and reaction parameters, the graphitization time was shortened from 10 h to 7 h, while stable graphite samples with 12C ion beam currents of 50-60 μA were routinely obtained, thus providing high-quality ion beams for AMS analysis. Beam transmission optimization was initially performed using 13C as a surrogate isotope. Helium was adopted as the stripping gas owing to its low atomic number and favorable ionization properties, which minimizes angular and energy straggling during the stripping process. By optimizing the stripping gas pressure and accelerator terminal voltage (165 kV), a maximum transmission efficiency of 45% for 13C+ was achieved, a value comparable to those of internationally similar compact AMS facilities. After beam tuning, 14C+ ions were stably delivered to the detector by scaling the magnetic field strengths according to the mass-to-charge ratios. To achieve high-sensitivity 14C measurements, molecular background interference originating from ions such as 13CH+ was systematically investigated. Gas stripping was adopted as the primary method for molecular dissociation, and the optimal stripping pressure was determined to be 3.74×10−3 Pa, at which the molecular background was suppressed to the 10−15 level. Furthermore, a background correction method based on monitoring the correlated 13C+(H) signal was established to further eliminate residual molecular interference. Consequently, the compact AMS system achieved an abundance sensitivity of 14C/12C=1.6×10−15 following background correction. These results demonstrate that the developed compact AMS system is capable of rapid, stable, and highly sensitive 14C measurements under low terminal voltage conditions, with performance reaching the internationally advanced level. This work provides a solid technical foundation for future applications of compact AMS in environmental tracing, nuclear safety, and multi-nuclide analysis.

     

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