Abstract
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.
Highlights
The separation between the ER and NR bands, the capability of the PSD method to reject ER events is demonstrated
The four world-leading argon dark matter projects (ArDM, DarkSide-50, DEAP-3600 and MiniCLEAN) agreed on joining forces [7] to carry out a unified program for dark matter direct detection with 39Ar depleted liquid argon, in the framework of the DarkSide-20k (DS-20k) project [8]
The natural 39Ar depletion factor in gas extracted from the CO2 wells at Cortez, Colorado, showed a residual 39Ar specific activity of (0.73 ± 0.11) × 10−3 Bq/kg [9] to be e.g. compared to the activity in atmospheric argon of to be (0.95±0.05) Bq/kg reported in this paper
Summary
The Argon Dark Matter (ArDM) experiment is located at the Spanish underground site LSC (Laboratorio Subterraneo de Canfranc) [10]. After being extracted from the LAr to the gaseous argon (GAr) on top, these electrons are accelerated and the secondary scintillation light (S2), which is proportional to the amount of electrons extracted, is produced. The detector was commissioned first with a gaseous argon (GAr) target at room temperature, hereafter called warm gas. It was cooled down and was operated with a GAr target at LAr temperature, 87 K (cold gas). These were succeeded by the filling of the ArDM cryostat with a total of ∼2 tonnes of LAr. The detector was operated over six months in the full target mode. The calibration data collected with the full LAr target is summarized in table 2
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