MEASUREMENT OF THE UNDERGROUND ARGON RADIOPURITY FOR DARK MATTER DIRECT SEARCHES Ludovico Luzzi (CIEMAT) on behalf of DarkSide Collaboration • C.E. Aalseth et al. 2020 JINST 15 P02024 (Feb. 2020) LOW RADIOACTIVITY UAr FOR DARKSIDE-20K DARKSIDE-20K EXPERIMENT AND UNDERGROUND ARGON DS-20k aims to <0.1 background events except coherent neutrino scatters after all TPC & veto event selection criteria in ROI (20-200 keVnr) within 20.2 t fiducial volume during 10 y run. Liquid argon is used as active medium because of its scintillation and ionization characteristics. DarkSide-20k (DS-20k) is a two- phase liquid argon detector, under construction at LNGS. DArT is a small low-background detector designed to measure the 𝟑𝟗𝐀𝐫 depletion factor of different UAr batches (URANIA + ARIA): OFHC Cu cylinder, 1 l active volume, PMMA support structure with TPB coating, Readout two PDMs (1 cm² SiPMs) from DS-20k (DArTeyes). Located at LSC (1400 m.w.e.) will be installed inside ArDM single-phase: • 13 PMTs will see ~1 tonne AAr buffer used as shield and veto • New Pb-shield attached to existing Polyethilene shield • Substitution of AAr bath by pressurized LN2 @ 85 K. No veto • Installation in an ad-hoc Pb-castle flushed with Rn-free air (undergorund only) DArT IN ArDM: PRESENT AND FUTURE DArT: FROM CIEMAT TO LSC Why Argon? Underground argon (UAr), depleted of 𝟑𝟗𝐀𝐫 Future Ar-based experiments (DarkSide-20k, Argo) will use UAr. Main limitation Intrinsic activity of 𝟑𝟗𝐀𝐫in atmospheric argon (via spallation of cosmic rays on 𝟒𝟎𝐀𝐫): • beta decay with Qβ = 565 keV • t1/2 = 269 y • ~1 Bq/kg • Procurement of 50 tonnes of UAr from the Kinder- Morgan facility in Cortez (Colorado, USA) • Extraction of 330 kg/day, with 99.9% purity • UAr transported to Sardinia for final chemical purification at Aria The Aria and Urania projects are two fundamental steps in order to provide tonnes of detector-grade UAr. • Big cryogenic distillation column in Seruci, Sardinia • Final chemical purification of the UAr • Can process O(1 tonne/day) with 10³ reduction of all chemical impurities • Ultimate goal is to isotopically separate 𝟑𝟗𝐀𝐫from 𝟒𝟎𝐀𝐫 (at the rate of 10 kg/day in Seruci II) Urania Aria 350 m Seruci-I Seruci-II Specific setup for tests in surface (CIEMAT) and undeground (LSC): With all that it is clearly seen the 39Ar on top of the background, assuming a featureless línear background and considering a threshold of 33 keV it is possible to measure 1.0 cps. No uncertainty is quoted here because an understanding of the systematics is still ongoing. Bibliography Two events from the present setup of DArT are showed. From the difference between α decay (top) and β decay (bottom) Pulse Shape Discrimination (PSD) is performed and electron recoil events can be isolated. Comparing everything from surface to underground with lead shield, a reduction of the background is observed. Once adding the lead shield (black) the background is significantly reduced and the 39Ar β spectrum for low • arXiv:1707.08145v1 [physics.ins-det] 25 Jul 2017 PRELIMINARY For tests only AAr is used. The triplet lifetime is very long (1230 ns) because of the slower SiPM response: 𝐟𝟔𝟒𝟎is defined so the signal is integrated in the first 640 ns to define the fast component of the pulse energies is visible.