  Large liquid argon detectors offer one of the best avenues for the detection
of galactic weakly interacting massive particles (WIMPs) via their scattering
on atomic nuclei. The liquid argon target allows exquisite discrimination
between nuclear and electron recoil signals via pulse-shape discrimination of
the scintillation signals. Atmospheric argon (AAr), however, has a naturally
occurring radioactive isotope, $^{39}$Ar, a $\beta$ emitter of cosmogenic
origin. For large detectors, the atmospheric $^{39}$Ar activity poses pile-up
concerns. The use of argon extracted from underground wells, deprived of
$^{39}$Ar, is key to the physics potential of these experiments. The
DarkSide-20k dark matter search experiment will operate a dual-phase time
projection chamber with 50 tonnes of radio-pure underground argon (UAr), that
was shown to be depleted of $^{39}$Ar with respect to AAr by a factor larger
than 1400. Assessing the $^{39}$Ar content of the UAr during extraction is
crucial for the success of DarkSide-20k, as well as for future experiments of
the Global Argon Dark Matter Collaboration (GADMC). This will be carried out by
the DArT in ArDM experiment, a small chamber made with extremely radio-pure
materials that will be placed at the centre of the ArDM detector, in the
Canfranc Underground Laboratory (LSC) in Spain. The ArDM LAr volume acts as an
active veto for background radioactivity, mostly $\gamma$-rays from the ArDM
detector materials and the surrounding rock. This article describes the DArT in
ArDM project, including the chamber design and construction, and reviews the
background required to achieve the expected performance of the detector.
