Argon is used as the active medium in DarkSide because of its scintillation and ionization characteristics and the relative ease with which it can be chemically purified to extraordinary levels. Atmospheric Argon contains the 39Ar isotope (β–, half-life = 269 yr, Q = 565 eV) at the level of 1 part in 10^15, which produces an activity of 1 Bq/kg.
The scintillation pulse shape discrimination in liquid argon would reject events from 39-Ar decays by a factor of about 108. However, pileup during the drift time in a LAr-TPC would still limit the active mass of a detector filled with atmospheric argon to less than 1 ton. But “G2” detectors with active mass much greater than a ton are essential to detect or rule out WIMP Dark Matter in the theoretically favored cross section range.
DarkSide collaborators have sought and found a way to extend the reach of LAr TPC’s into the G2 region. Deep underground sources of argon have been discovered and a method developed to extract and purify detector grade argon from these sources. This “UAr” (underground argon) has been shielded from cosmic radiation for many half-lives of 39-Ar and therefore could be expected to be very low in the isotope. We currently have a separation plant in operation at the Kinder Morgan Doe Canyon CO2 facility in southwestern Colorado, which has extracted over 150 kg of UAr to date. The extracted gas is then further purified at Fermilab before it is used in DarkSide. The extraction methods are described in arXiv 1204.6024 and arXiv 1204.6061.
We have also developed a technique to make a sensitive measurement of 39-Ar content in this material. So far an upper limit of 0.08% of the atmospheric 39-Ar abundance has been established (arXiv 1510.00702). These results exceed the design specifications for DarkSide 50, and allow the extremely powerful electron recoil rejection capabilities intrinsic to LAr-TPC’s to be employed in detectors with multiple tons of active mass.
[ to be updated soon, c/o H. O. Back ]