Backgrounds and pulse shape discrimination in the ArDM liquid argon TPC

The ArDM experiment completed a single-phase commissioning run (ArDM Run I) with an active liquid argon target of nearly one tonne in mass. The analysis of the data and comparison to predictions from full detector simulations allowed extraction of the detector properties and an assessment of the low background conditions. The ³⁹Ar specific activity from the employed atmospheric argon is measured to be (0.95±0.05) Bq/kg. The cosmic muon flux at the Canfranc underground site was determined to be in the range (2–3.5)× 10⁻³m⁻²s⁻¹. The statistical rejection power for electronic recoil events using the pulse shape discrimination method was estimated using a ²⁵²Cf neutron calibration source. Electronic and nuclear recoil band profiles were found to be well described by Gaussian distributions. Employing such a model we derive values for the electronic recoil statistical rejection power of more than 10⁸ in the tonne-scale liquid argon target for events with more than 50 detected photons at a 50% acceptance for nuclear recoils. The ²²²Rn emanation rate of the ArDM cryostat at room temperature was found to be (65.6±0.4) μHz/l. These results represent an important physics milestone for the next run in the double-phase mode and in the context of foreseen developments towards the use of depleted argon targets.