【摘要】 <正> 一、前言地球上氦同位素组成有三个层次:地幔氦、地壳氦和大气氦。地幔氦是地球生成时保留下来的原生氦,³He含量高,³He/⁴He比值约10^-5;地壳氦是地壳中铀、钍等放射性元素蜕变过程中生成的,³He/⁴He比值约10^-7～10^-8;大气氦是在地幔氦与地壳氦不断地向更多还原

【Abstract】 To measure the isotopic abundance ratio of ³He and ⁴He in samples by massspectrometers is an important detection mean for helium isotope geochemistryresearch. A symmetrically arranged tandem ³He/⁴He mass spectrometer is describedin the paper. The front stage, used to analyse ³He （including HD and H₃） and⁴He, is a homogeneous analysing magnet with a bending angle of 90° and abending radius of 6 cm. The end stage, used to analyse ³He, HD and H₃, is anon-homogeneous analysing magnet with a bending angle of 180°, a bending radiusof 15 cm and a magnetic field gradient of 0.75. Because of the use of the non-zero second order coefficient β and curved entrance face of the later magnet foreliminating second order aberrations, the resolving power of the system is notablyimprbved, and theoretically reaches about 3800.更多还原