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抄録
一般に表面電離固体用質量分析計による同位体比測定において,コレクターとして電子増倍管を用いた場合,同位体比は二つの同位体の質量比の平方根(√<m_1/m_2>)で補正すればよいという報告もある.しかし著者らは鉛同位体比を精度良く測定した結果,機器固有の多様な原因による補正項が無視できないことを確認した.本論文では東京国立文化財研究所の日本電子社製05RB型表面電離固体用質量分析計で電子増倍管をコレクターとして用い,標準試料を精密に測定した結果,正しい値を得るためには√<m_1/m_2>より大きな補正項が必要であることを報告する.
To determine the mass correction factor(MCF) which is a factor to convert the observed raw ratio to the absolute value, CIT Pb shelf standard of California Institute of Technology and SRM-981 Pb standard of US National Bureau of Standards were measured with a 05RB type, surface ionization solid source mass spectrometer (ion beam radius 30 cm, 90° sector, max. acceleration voltage 10 kV) of JEOL Co. Ltd. installed in Tokyo National Research Institute of Cultural Properties. Using an electron multiplier as ion beam collector, our measurement showed larger correction factor than the factor of root mass (√<m_1/m_2> hitherto used by some authors. Our MCFs were 1.01024 (±0.1 %), 1.00411 (±0.05%) and 1.01022 (±0.05 %) for the ratios of ^<206>Pb/^<204>Pb, ^<207>Pb/^<206>Pb and ^<208>Pb/^<206>Pb respectively. These factors were obtained by comparing observed raw values with the values by Catanzaro(1967) and Catanzaro et al. (1968) for the CIT Pb and SRM-981 respectively. The lead isotope ratios of four galena samples corrected with these factors were in excellent agreement with the reported values for these samples.
