Japan Geoscience Union Meeting 2016

Presentation information


Symbol M (Multidisciplinary and Interdisciplinary) » M-TT Technology & Techniques

[M-TT28] Frontiers in Geochemistry : Prospect for geochemistry and cosmochemistry in future

Sun. May 22, 2016 9:00 AM - 10:30 AM A04 (APA HOTEL&RESORT TOKYO BAY MAKUHARI)

Convener:*Hajime Obata(Marine inorganic chemistry division, Atmosphere and Ocean Research Institute, University of Tokyo), Hirochika Sumino(Department of Basic Science, Graduate School of Arts and Sciences, The University of Tokyo), Tetsuya Yokoyama(Department of Earth and Planetary Sciences, Graduate School of Science and Engineering, Tokyo Institute of Technology), Takafumi Hirata(Graduate School of Science, Kyoto University), Urumu Tsunogai(Graduate School of Environmental Studies, Nagoya University), Yoshio Takahashi(Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo), Shogo Tachibana(Department of Natural History Scieces, Hokkaido University), Katsuhiko Suzuki(Research and Development Center for Submarine Resources, Japan Agency for Marine-Earth Science and Technology), Gen Shimoda(Geological Survey of Japan, AIST), Hiroyuki Kagi(Geochemical Research Center, Graduate School of Science, University of Tokyo), Yusuke Yokoyama(Atmosphere and Ocean Research Institute, University of Tokyo), Chair:Hirochika Sumino(Department of Basic Science, Graduate School of Arts and Sciences, The University of Tokyo), Tetsuya Yokoyama, Hajime Obata(Marine inorganic chemistry division, Atmosphere and Ocean Research Institute, University of Tokyo)

9:15 AM - 9:30 AM

[MTT28-02] Determination of absolute 3He/4He ratio of He Standard of Japan

*Kenji Mishima1, Hirochika Sumino2, Takahito Yamada3, Sei Ieki3, Naoki Nagakura3, Hidetoshi Otono4 (1.KEK, 2.Dept. of Basic Science, University of Tokyo-Komaba, 3.Grad. School of Sci., University of Tokyo, 4.RCAPP, Kyushu University)

Keywords:Isotope ratio analysis, 3He/4He ratio, Mass spectrometry, Big bang nucleosynthesis, Neutron beta decay

3He/4He ratios in terrestrial samples vary more than three orders of magnitude, because primordial helium with 3He/4He of (1.4–4.6) x 10–4 has been diluted by addition of radiogenic 4He produced by decay of U- and Th-series elements in different degrees depending on 3He/(U+Th) ratio of each reservoir. This feature makes 3He/4He ratio a powerful tracer in geochemistry and cosmochemistry. Though atmospheric helium with 3He/4He ratio of 1.4 x 10–6 is used to calibrate 3He/4He measurement with a noble gas mass spectrometer, relatively low concentration and 3He/4He ratio of the atmospheric helium cause many difficulties to use it as a working standard for daily measurements. Thus noble gas laboratories often use their own working standards prepared from a natural gas sample with high 3He/4He ratio or by mixing of isotopically pure 3He and 4He. "He Standard of Japan" (HESJ) is one of the latter originally prepared by four noble gas laboratories in Japan [1] and now distributed worldwide as an interlaboratory standard [1,2]. However, 3He/4He ratio of HESJ was determined by comparison with that of atmospheric helium, i.e., absolute 3He/4He ratio has not been determined yet and the accuracy of the value still rely on the early determinations of absolute 3He/4He ratio of atmospheric helium [3].
As long as 3He/4He ratio is used to compare relative contributions of primordial and radiogenic in each geochemical reservoir, absolute 3He/4He value of atmospheric helium or HESJ is less important. However, it is a critical issue in some applications of helium isotopes, such as tritium-3He dating and an experimental project to measure the neutron lifetime with total uncertainty of 1 sec (0.1%) using pulsed neutron source at J-PARC [4].
A neutron decays into a proton, an electron, and an anti-neutrino with a lifetime of 880.3 ± 1.1 sec [5]. The lifetime is an important constant in the Big Bang nucleosynthesis (BBN) that controls amounts of primordial elements in our universe. In this experiment, the incident neutron flux is measured by counting 3He(n,p)3H reaction in a time projection chamber detector filled with 3He, 4He and CO2. To determine neutron lifetime with uncertainty less than 0.1%, 3He number density in the detector must be accurately known with even smaller uncertainty. As a part of this experiment, we are developing a gas handling system to control 3He number density with uncertainty of 0.1%. The 3He gas is mixed with research grade He in a vessel with measuring pressures of these gases precisely using a calibrated piezoresistive transducer.
We fabricated control samples of known 3He/4He ratio using the gas handling system and measured the ratio using a sector type single focusing noble gas mass spectrometer with double collector system [6] at Dept. of Basic Sci., the Univ. of Tokyo by referring to HESJ. The results will contribute to determine the absolute 3He/4He value of HESJ, and that of atmospheric helium also [6].
[1] J. Matsuda et al., Geochem. J. 36, 191 (2002).
[2] Y. Sano, T. Tokutake, and N. Takahata, Anal. Sci. 24, 521 (2008).
[3] Y. Sano, B. Marty and P. Burnard, “The Noble Gases as Geochemical Tracers”, Chapter 2. “Noble gases in the atmosphere”, Springer (2013).
[4] Y. Arimoto, et al, Nucl. Inst. Meth. Phys. Res. A 799, 187–196, (2015).
[5] K.A. Olive et al. (Particle Data Group), Chin. Phys. C, 38, 090001 (2014) and 2015 update.
[6] H. Sumino et al., J. Mass Spectrom. Soc. Jpn. 49, 61 (2001).