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⁴⁰Ar/³⁹Ar Laboratory


A state of the art ⁴⁰Ar/³⁹Ar laboratory, featuring three fully automated gas extraction-mass spectrometry systems, is used for dating rocks and minerals by methods based on the natural radioactive decay of potassium. These systems can be used to date samples as young as 2,000 years, and as old as the Earth itself (4.6 billion years), depending on the nature of the sample.

System 1 features an MAP 215C mass spectrometer with a Nier source, retractable Faraday and stationary Balzers electron multiplier detectors, and adjustable collector slit. The mass spectrometer is mated on-line to a fully automated extraction line with two C-50 getters and a cryogenic condensation trap. Samples are loaded into a UHV chamber mounted on an automated translating x-y stage and degassed with a 40 W CO2 laser. A video camera and monitor allow magnified observation of the sample chamber. An automated air pipette system allows periodic monitoring of sensitivity and mass discrimination. All aspects of sample degassing, gettering, and mass spectrometry are controlled by software developed by BGC, and run on a Macintosh computer.

System 2 features a custom-designed MAP 215-50 mass spectrometer with dual electron multipliers (permitting simultaneous measurement of masses 36 and 40) in addition to a retractable Faraday cup. The mass spectrometer is mated to a highly flexible extraction line dubbed "Nexus", with a CO2 laser. Twin sample chambers allow bake-out of one while the other is engaged in analysis. Other features (e.g., video monitoring, automated stage translation, automated air-pipette) are analogous to those on System 1. 


System 3 features a multi-collector noble gas mass spectrometer (Noblesse, from Nu Instruments) and is primarily used for archeometric research. This instrument is committed to high-sensitivity ⁴⁰Ar/³⁹Ar work as required for small, young, or low-K materials, and features four ion-counters and an axial Faraday detector for simultaneous ion counting measurement of ³⁶Ar, ³⁷Ar, ³⁹Ar, and ⁴⁰Ar. ³⁸Ar is also measured on an ion counter, by peakhopping. This system includes an automated extraction line with two sample chambers, a CO2 laser, and an automated cocktail pipette system delivering prescribed mixtures of ³⁶Ar, ³⁹Ar, and ⁴⁰Ar for intercalibrating detectors and determining discrimination and spectrometer sensitivity. Other features (e.g., video monitoring, automated stage translation, automated air-pipette) are analogous to those on System 1.

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