抄録
A noble gas mass spectrometer equipped with a double collector system for helium isotopes was improved for the purpose of determining noble gas isotopic composition with extremely low concentration in terrestrial rock samples. Ion beams of 3He and 4He are measured at the same time with a Faraday cup and an ion-counting for 4He and 3He, respectively. As a result of changing detector to an ion-counting system and modification of a computer program for analysis, we can analyze the helium isotopic ratio (3He/4He) with a precision of ±10%, when partial pressure of 3He is in the range of 0.5-5×10-14 Torr (corresponds to 0.2-2×10-13 cm3 STP). The detection limit was highly improved to ca. 6×10-16 cm3 STP of helium as a counting rate of 0.04 cps, which was 1-2 orders of magnitude lower than that before modification: 4×10-14 cm3 STP. The diminution of detection limit enables us to measure other noble gas isotopes whose abundances are generally low in rock samples, such as neon, krypton and xenon with the mass spectrometer specialized to helium. In order to use HESJ (Helium Standard of Japan) as a working standard for helium isotopic measurement, we determined its 3He/4He ratio as 20.77±0.24 times the atmospheric ratio (=1.40×10-6). In addition, accuracy in helium isotopic ratio measurement with the modified mass spectrometer was checked by comparing them with results obtained by other mass spectrometers in our laboratory.
