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He/
He ratios in natural fluidsBajo, Kenichi*; Sumino, Hirochika*; Toyoda, Michisato*; Okazaki, Ryuji*; Osawa, Takahito; Ishihara, Morio*; Katakuse, Itsuo*; Notsu, Kenji*; Igarashi, Joji*; Nagao, Keisuke*
Mass Spectrometry (Internet), 1(2), p.A0009_1 - A0009_10, 2012/11
The construction of a small-size, magnetic sector, single focusing mass spectrometer (He-MS) for the continuous, on-site monitoring of the He isotope ratio (He/He) is described. The instrument is capable of measuring He/
Ne ratios dissolved in several different types of natural fluids of geochemical interest, such as groundwater and gas from hot springs, volcanoes and gas well fields. Ion optics of the He-MS was designed by using the ion trajectory simulation program TRIO so as to measure He and He simultaneously with a double collector system under mass resolution power of 
500. Presently attained specifications of the He-MS are; (1) mass resolving power of ca. 490, enough to separate He
from interfering ions, HD and H
, (2) ultra-high vacuum condition, and (3) high sensitivity enough to detect He amount 3000000 atoms).
Nagao, Keisuke*; Okazaki, Ryuji*; Nakamura, Tomoki*; Miura, Yayoi*; Osawa, Takahito; Bajo, Kenichi*; Matsuda, Shintaro*; Ebihara, Mitsuru*; Ireland, T.*; Kitajima, Fumio*; et al.
Science, 333(6046), p.1128 - 1131, 2011/08
Times Cited Count：130 Percentile：95.22(Multidisciplinary Sciences)A steroid surface materials record regolith processes and a history of cosmic-ray irradiation. Noble gas isotopes in three rocky grains from Itokawa have been determined. High concentrations of solar He, Ne, and Ar, as high as those in lunar soils, are released at variable temperatures from each sample. The isotopic compositions are essentially identical to those of solar wind but distinguishable in He relative abundance. These noble gas characteristics can be explained by repeated implantation and preferential loss of solar He by removal of weathered He-rich rim on the grain surface through friction among regolith grains on Itokawa. Residence time of regolith materials on Itokawa is alculated to be shorter than 10 Myr, suggesting that regolith materials of small asteroids would escape easily to space.
