Ultra-trace determination of Am-241 by SF-ICP-MS combined with a high efficiency sample introduction system

Americium is the third element in the transuranium element series. The long-lived isotopes of americium are 243Am (T1/2 = 7400 y), 241Am (T1/2 = 433 y) and 242mAm (T1/2 = 152 y), among them, 241Am is the most important isotope in environmental study due to its utilization in industrial applications and its existence in the environment. 241Am is an alpha and gamma-emitter (α: 5.49 MeV (85%), 5.44 MeV (13%); γ: 0.06 MeV (36%)). 241Am was introduced into the environment by human nuclear activities, such as nuclear weapon tests, nuclear fuel reprocessing discharge and nuclear accidents. In recent years, there has been an increasing demand of ultra-trace determination of 241Am isotopes in environmental samples in studies of radiation protection, safeguards and nuclear forensics, and in environmental studies, such as tracing the sources of radioactive contamination, estimating the potential radiation exposure to man via soil to crop transfer, and in study of the ongoing environmental changes, for instance, soil erosion and recent sedimentation dating technique development. Conventionally, alpha spectrometry has usually been used for 241Am measurement in various environmental samples. This method, however, normally required a tedious chemical separation procedure and long counting time (days to weeks). In contrast, mass spectrometry, especially ICP-MS, as an atom-counting approach that counts the atoms themselves, irrespective of their decay mode, specific activity, and half-life, is gradually replacing/has replaced alpha spectrometry as a main-stream technique for transuranium isotope analysis. Due to the strength of argon plasma source of ICP-MS, extremely high, almost 100% ionization efficiency can be achieved for Am isotopes with a first ionization potential of 5.97 eV as calculated from the Saha equation, thus, highly sensitive determination of 241Am isotopes can be realized. Recent development of ICP-MS techniques, especially the sector-field ICP-MS, has pushed the detection limit for analysis of 241Am down to sub-fg level, comparable to that of alpha spectrometry. A new sector-field ICP-MS, Element XR equipped with the JET interface, was first installed for actinides analysis in Japan two years ago at National Institute of Radiological Sciences (NIRS). Following our investigation on the determination of Pu isotopes last year, here we report a detailed investigation on its analytical potential for Am isotopes. Combining with high efficiency sample introduction system Aridus II), exceptionally high sensitivity was obtained, achieving a detection limit at attogram (10-18 g) level, which is comparable to that of the accelerator mass spectrometry (AMS). This new sector-field ICP-MS now has been used for the determination of 241Am isotopes in Japanese soil and fallout samples. \nAcknowledgements: This work was partially supported by the Agency for Natural Resources and Energy, the Ministry of Economy, trade and Industry (METI), Japan.第１７回「環境放射能」研究