Construction of a newly designed small-size mass spectrometer for helium isotope analysis; Toward the continuous monitoring of He/He ratios in natural fluids

Bajo, Kenichi*; Sumino, Hirochika*; Toyoda, Michisato*; Okazaki, Ryuji*; Osawa, Takahito  ; Ishihara, Morio*; Katakuse, Itsuo*; Notsu, Kenji*; Igarashi, Joji*; Nagao, Keisuke*

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).