The DarkSide-20k underground argon procurement chain

Abstract

The DarkSide-20k experiment searches for dark matter by looking for interactions of WIMPs in a 50-tonnes target of liquid argon using double-phase time projection chamber (TPC) technology. The key component of the experiment is low radioactivity argon depleted in the isotope 39Ar. Unfortunately, 39Ar is naturally present in atmospheric argon and it is constantly produced due to the interaction with cosmic rays. Finding a source of depleted argon was the first step of the Collaboration when it started to size the DarkSide-20k detector. The procurement chain begins with the Urania plant in Colorado, which can produce argon with a purity of 99.99% from a crude CO2 stream extracted from a deep well, at a rate of about 250 kg per day. The plant has already been fabricated while the site is being prepared for installation. After the extraction and purification of 120 t of underground argon (UAr), it will be transported to Sardinia, Italy, where the ARIA plant, consisting of a 350 m cryogenic distillation column, will further remove impurities up to 2 orders of magnitude. The final purity is expected to be 99.9999%. After the ARIA purification stage, the ultra-pure UAr will be delivered to the Laboratori Nazionali del Gran Sasso, L'Aquila, Italy where it will fill the DarkSide-20k TPC. The ARIA plant has already been fully fabricated and is now in the installation phase in a mine shaft. A shorter version, about 26 m high, has been tested over the last three years with very positive results. We were able to measure the separation factor between two isotopes of nitrogen (29N2, 28N2) and three of argon (40Ar, 38Ar, 36Ar) thus validating the capability of performing isotopic distillation with ARIA.