Instrumental background

Static vacuum noble gas mass spectrometer type VG-5400 with sample inlet line and gas cleanup system

The VG-5400 mass spectrometer is suitable for determination of isotope ratios of all noble gases. This is the first noble gas mass spectrometer in Hungary which has the sensitivity, accuracy and resolution required by various important fields of research, like environmental research, nuclear fission safety, waste disposal and geochronology.

The sample inlet line is equipped with pneumatically activated NUPRO valves SS-4-BK, controlled by a computer. The line is made of stainless steel with UHV connections (VCR) with copper or aluminium gaskets. Four samples can be analyzed in a row without any manual intervention. The line includes two vacuum units: one ion pump for the inlet section, one turbomolecular pump with its rotary pump for evacuating the purification section. This provides a high vacuum in the system (<10-8 torr). The pressure is controlled manually and via the computer at different stages in the system with Pirani and Penning gauges connected to the computer through an HP-IB interface.

The purification section consists of an unfilled trap and a charcoal trap, placed in the same Dewar filled with liquid nitrogen, the level of which is regulated automatically by thermal sensors and an electrically activated valve connected to a liquid nitrogen tank. These two traps are followed by a SAES getter pump for removing the hydrogen.

The mass spectrometer has a magnet of 90o deflection and 27 cm deflection radius with an extended geometry. The sensitivity is 1x10-3 A/mbar for Ar and 2x10-4 A/mbar for He. The resolution is adjusted to 600 which is sufficient for the resolution of 3He+, H3+ and HD+ beams. The abundance sensitivity measured as contribution from mass 40 at mass 39 is less than 0.5 ppm at 10-7 torr. The static vacuum level in the 1.3 l volume deflection chamber is 1x10-12 cm3 STP/min at mass 40.

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