日本地球惑星科学連合2015年大会

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セッション記号 S (固体地球科学) » S-GC 固体地球化学

[S-GC51] 希ガス同位体地球惑星科学の最前線

2015年5月24日(日) 16:15 〜 18:00 A04 (アパホテル&リゾート 東京ベイ幕張)

コンビーナ:*兵藤 博信(岡山理科大学自然科学研究所)、角野 浩史(東京大学大学院理学系研究科附属地殻化学実験施設)、田上 高広(京都大学大学院理学研究科)、座長:兵藤 博信(岡山理科大学自然科学研究所)、角野 浩史(東京大学大学院理学系研究科附属地殻化学実験施設)

17:51 〜 17:54

[SGC51-P03] Giese型イオン源を搭載した高感度希ガス質量分析計

ポスター講演3分口頭発表枠

*角野 浩史1長尾 敬介1 (1.東京大学理学系研究科地殻化学実験施設)

キーワード:希ガス, 質量分析計, 四重極レンズ, イオン源

Although noble gas isotopes are powerful tracers in geosciences, their extremely low abundances in mineral and rock samples make their analysis quite difficult. For example, concentration of 3He, which is a good indicator of mantle-derived component because of its primordial origin, is as high as 0.01 parts-per-trillion in volcanic rocks and mantle-derived materials. Such scarce noble gas isotopes are detected with a special mass spectrometer operated in static mode. We have made it possible to detect 103 to 104 atoms of noble gas isotopes by modifying a commercial sector-type single focusing noble gas mass spectrometer (VG5400), which is equipped with a double collector system to detect 3He and 4He simultaneously with a secondary electron multiplier and Faraday cup, respectively [1]. Here we report an attempt of further improvement of sensitivity of the mass spectrometer by installation of a new ion source (Giese-type source).
The Giese-type electron ionization (EI) ion source is equipped with two electrostatic quadrupole lenses [2]. This source has been reported to have up to two orders of magnitude higher sensitivity than conventional Nier-type EI source because of the absence of a beam defining slit to collimate the ion beam and thus high transmission [3]. We designed a Giese-type source to have an adequate resolution to separate 3He+ from HD+ and H3+, to have the source housing volume as small as possible, and to be bankable at up to 300℃ to reduce outgas from the source materials. The ion and electron optics were based on a calculation by Lu and Carr [4] and refined using SIMION-3D software [5]. Prior to the installation on the mass spectrometer, the ion beam profile emitted from the source was monitored by a microchannel plate and phosphor screen to optimize the configuration of the quadrupole lens.
A sufficient mass resolution over 500 essential for 3He/4He analysis has been achieved with an improved sensitivity approximately three times higher than the previous condition. The amount of helium required to obtain a precision with 3He/4He ratio is two orders of magnitude smaller than that with the condition installed by the manufacture. However, total ion transmission is estimated to be about 30%, suggesting further refinement of the source condition is required to obtain the maximum sensitivity.

References: [1] H. Sumino et al., J. Mass Spectrom. Soc. Jpn., 49, 61-68 (2001). [2] C.F. Giese, Rev. Sci. Instrum., 30, 260-261 (1959). [3] E.T. Kinzer and H. Carr, Rev. Sci. Instrum., 30, 1132 (1959). [4] C.-S. Lu and H.E Carr, Rev. Sci. Instrum., 33, 823-824 (1962). [5] D.A. Dahl, Int. J. Mass Spectrom., 200, 3-25 (2000).