The discovery of argon from underground sources with significantly less 39Ar than atmospheric argon was an important step in the development of direct-detection dark matter experiments using argon as the active target. We report on the design and operation of a low background detector with a single phase liquid argon target that was built to study the 39Ar content of the underground argon. Underground argon from the Kinder Morgan CO2 plant in Cortez, Colorado was determined to have less than 0.65% of the 39Ar activity in atmospheric argon.
@article{arxiv.1204.6011,
title = {A Study of the Residual 39Ar Content in Argon from Underground Sources},
author = {J. Xu and F. Calaprice and C. Galbiati and A. Goretti and G. Guray and T. Hohman and D. Holtz and A. Ianni and M. Laubenstein and B. Loer and C. Love and C. J. Martoff and D. Montanari and S. Mukhopadhyay and A. Nelson and S. D. Rountree and R. B. Vogelaar and A. Wright},
journal= {arXiv preprint arXiv:1204.6011},
year = {2016}
}