  One of the most powerful techniques for direct detection of dark matter via
elastic scattering of galactic WIMPs is the use of liquid argon time projection
chambers. Atmospheric argon (AAr) has a naturally occurring radioactive
isotope, \ar{}, of cosmogenic origin. The use of argon extracted from
underground wells, deprived of \ar{}, is key to the physics potential of these
experiments. The DarkSide-20k (DS-20k) dark matter search experiment will
operate with 50 tonnes of radio-pure underground argon (UAr), extracted from
the Urania plant in Cortez (USA) and purified in the Aria distillation plant
(Sardinia, Italy). Assessing the radio-purity of UAr in terms of \ar{} is
crucial for the success of DS-20k, as well as for future experiments of the
Global Argon Dark Matter Collaboration (GADMC), and will be done with the
experiment named \dia{}. DArT is a small chamber that will contain the argon
under test. The detector will be immersed in the LAr active volume of the ArDM
detector, located at the Canfranc Underground Laboratory (LSC) in Spain, which
will act as active veto for background events coming from photons from detector
materials and surrounding rock radioactivity.
