著者

三島 賢二 角野 浩史 山田 崇人 家城 斉 長倉 直樹 音野 瑛俊

出版者

日本地球惑星科学連合

雑誌

日本地球惑星科学連合2016年大会

巻号頁・発行日

2016-03-10

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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).