ABSTRACT
We demonstrate fast analysis of 39Ar/Ar at the 10-16 level using a mass spectrometer for isotope pre-enrichment and an atom trap for counting. An argon gas sample first passes through a dipole mass separator that reduces the dominant isotope 40Ar by two orders of magnitude while preserving both the rare tracer isotope 39Ar and a minor stable isotope 38Ar for control purposes. Measurements of both natural and enriched samples with atom trap trace analysis demonstrate that the 39Ar/38Ar ratios change less than 10%, while the overall count rates of 39Ar are increased by one order of magnitude. By overcoming the analysis-speed bottleneck, this advance will benefit large-scale applications of 39Ar dating in the earth sciences, particularly for mapping ocean circulation.
ABSTRACT
Cosmogenic 39Ar dating is an emerging technique in dating mountain glacier ice, mapping ocean circulation, and tracing groundwater flow. We have realized an atom-trap system for the analysis of the radioactive isotope 39Ar (half-life = 269 years) in environmental samples. The system is capable of analyzing small (1-5 kg) environmental water or ice samples and achieves a count rate of 10 atoms/h for 39Ar at the modern isotopic abundance level of 8 × 10-16. By switching frequently between counting 39Ar atoms and measuring the stable and abundant isotope 38Ar, drift effects in the trapping efficiency are largely suppressed, leading to a more precise measurement of the isotope ratio 39Ar/38Ar. Moreover, cleaning techniques are developed to alleviate cross-sample contamination, reducing the background 39Ar count rate down to <0.5 atoms/h. These advances allow us to determine the 39Ar age in the range of 250-1300 years with precisions of <20%.
ABSTRACT
An isotope enrichment system for 39Ar has been developed at the Institute of Modern Physics, which is designed to increase the abundance of 39Ar in the incident sample gas. With intense Ar+ beams produced by a 2.45 GHz electron cyclotron resonance ion source and a high mass resolution spectrometer system, Ar isotopes are evidently separated on the target plane and selectively collected by an Al target. The separated Ar isotopes have been identified on the target plane, which is consistent with the simulations. According to the recent cross-checked results with atom trap trace analysis, a high enrichment factor of 39Ar has been successfully achieved. This paper will present the design and test results of this system.
