||Advance of ICP-MS for ultra-trace Pu isotope measurements in environmental samples
鄭, 建 ,
ト, 文庭 ,
田上, 恵子内田, 滋夫
Proceedings of the 16th Workshop on Environmental Radioactivity
144 , 2015-11 , High Energy Accelerator Research Organization (KEK)
In recent years, there has been an increasing demand of ultra-trace determination of Pu isotopes in studies of radiation protection, safeguards and nuclear forensics, and in environmental studies, such as tracing the sources of radioactive contamination, revealing the transport process of aerosols in the atmosphere, and in study of the ongoing environmental changes, for instance, soil erosion and desertification. Conventionally, alpha spectrometry has usually been used for Pu measurement in various environmental samples. This method, however, normally required a tedious chemical separation procedure and long counting times (days to weeks). Moreover, alpha spectrometry cannot distinguish 239Pu and 240Pu due to the close alpha radiation energies (5.16 MeV and 5.17 MeV for 239Pu and 240Pu, respectively), thus cannot provide isotopic composition information. In contrast, mass spectrometry, especially ICP-MS, as an atom-counting approach that counts the atoms themselves, irrespective of their decay mode, specific activity, or half-life, is gradually replacing/has replaced alpha spectrometry as a main-stream technique for Pu isotopic analysis. Due to the strength of argon plasma source of ICP-MS, extremely high, almost 100% ionization efficiency can be achieved for Pu isotopes with a first ionization potential of 6.03 eV as calculated from the Saha equation, thus, highly sensitive determination of Pu isotopes can be realized. Recent development of ICP-MS techniques, especially the sector-field ICP-MS, has pushed the detection limit for analysis of Pu isotopes down to sub-fg level. A new sector-field ICP-MS, Element XR equipped with the JET interface (Thermo Scientific, Germany) was installed in Research Center for Radiation Protection, National Institute of Radiological Sciences (NIRS) in March 2014. As this Element XR is first installed for actinides analysis in Japan, we made an investigation in detail on its analytical potential for Pu isotopes. Combining with high efficiency sample introduction systems (APEX-Q and Aridus II), exceptionally high sensitivity was obtained, achieving a detection limit at attogram (10-18 g) level, which is comparable to or even better than that of the accelerator mass spectrometry (AMS). This new sector-field ICP-MS now has been used for the determination of Pu isotopes in various environmental samples, such as sediments, seawater, soil and agricultural crops.