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1. Flow and thermal modelling of the argon volume in the DarkSide-20k TPC

   https://doi.org/10.1088/1748-0221/20/06/P06046  

   Acerbi, F; Adhikari, P; Agnes, P; Ahmad, I; Albergo, S; Albuquerque, IF; Alexander, T; Alton, AK; Amaudruz, P; Angiolilli, M; et al (June 2025, Journal of Instrumentation)

   Abstract The DarkSide-20k dark matter experiment, currently under construction at LNGS, features a dual-phase time projection chamber (TPC) with a ∼ 50 t argon target from an underground well. At this scale, it is crucial to optimise the argon flow pattern for efficient target purification and for fast distribution of internal gaseous calibration sources with lifetimes of the order of hours. To this end, we have performed computational fluid dynamics simulations and heat transfer calculations. The residence time distribution shows that the detector is well-mixed on time-scales of the turnover time (∼ 40 d). Notably, simulations show that despite a two-order-of-magnitude difference between the turnover time and the half-life of^83mKr of 1.83 h, source atoms have the highest probability to reach the centre of the TPC 13 min after their injection, allowing for a homogeneous distribution before undergoing radioactive decay. We further analyse the thermal aspects of dual-phase operation and define the requirements for the formation of a stable gas pocket on top of the liquid. We find a best-estimate value for the heat transfer rate at the liquid-gas interface of 62 W with an upper limit of 144 W and a minimum gas pocket inlet temperature of 89 K to avoid condensation on the acrylic anode. This study also informs the placement of liquid inlets and outlets in the TPC. The presented techniques are widely applicable to other large-scale, noble-liquid detectors. 

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2. The underground argon project: procurement and purification of argon for dark matter searches and beyond

   https://doi.org/10.3389/fphy.2024.1387069  

   Agnes, P; Back, H O; Bonivento, W; Boulay, M G; Canci, N; Caravati, M; Cebrian, S; Cocco, V; Diaz_Mairena, D; Franco, D; et al (December 2024, Frontiers in Physics)

   The existence of dark matter in the universe is inferred from abundant astrophysical and cosmological observations. The Global Argon Dark Matter Collaboration (GADMC) aims to perform the searches for dark matter in the form of weakly interacting massive particles (WIMPs), whose collisions with argon nuclei would produce nuclear recoils with tens of keV energy. Argon has been considered an excellent medium for the direct detection of WIMPs as argon-based scintillation detectors can make use of pulse shape discrimination (PSD) to separate WIMP-induced nuclear recoil signals from electron recoil backgrounds with extremely high efficiency. However, argon-based direct dark matter searches must confront the presence of intrinsic³⁹Ar as the predominant source of electron recoil backgrounds (it is a beta-emitter with an endpoint energy of 565 keV and half-life of 269 years). Even with PSD, the³⁹Ar activity in atmospheric argon (AAr), mainly produced and maintained by cosmic ray-induced nuclear reactions, limits the ultimate size of argon-based detectors and restricts their ability to probe very-low-energy events. The discovery of argon from deep underground wells with significantly less³⁹Ar than that in AAr was an important step in the development of direct dark matter detection experiments using argon as the active target. Thanks to pioneering research and successful R&D, in 2012, the first 160 kg batch of underground argon (UAr) was extracted from a CO₂well in Cortez, Colorado. The DarkSide-50 experiment at the Gran Sasso National Laboratory (LNGS) in Italy, the first liquid argon detector ever operated with a UAr target, demonstrated a ∼ 1,400 suppression of the³⁹Ar activity with respect to the atmospheric argon. An even larger suppression is expected for⁴²Ar (another intrinsic beta-emitter with the⁴²K daughter isotope, also a beta-emitter) as its production is expected mainly in the upper atmosphere. Following the results of DarkSide-50, the GADMC initiated the UAr project for extraction from underground and cryogenic purification of 100 t of argon to be used as a target in the next-generation experiment DarkSide-20k. This paper contains a description of the Urania Plant in Cortez, Colorado, where UAr is extracted; the Aria Plant in Sardinia, Italy, an industrial-scale plant comprising a 350-m state-of-the-art cryogenic isotopic distillation column, designed for further purification of the extracted argon and further reduction of the isotopic abundance of³⁹Ar; and DArT, a facility for UAr radiopurity qualification at the Canfranc Underground Laboratory (LSC), Spain. Moreover, the high radiopurity of UAr leads to other possible applications, for instance, for those neutrinoless double-beta decay experiments using argon as shielding material or, more generally, for all those activities on argon-based detectors in high-energy physics or nuclear physics, which will be briefly discussed. 

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3. Quality assurance and quality control of the $$26~\text {m}^2$$ SiPM production for the DarkSide-20k dark matter experiment

   https://doi.org/10.1140/epjc/s10052-025-14196-9  

   Acerbi, F; Adhikari, P; Agnes, P; Ahmad, I; Albergo, S; Albuquerque, I F; Alexander, T; Alton, A K; Amaudruz, P; Angiolilli, M; et al (May 2025, The European Physical Journal C)

   Abstract DarkSide-20k is a novel liquid argon dark matter detector currently under construction at the Laboratori Nazionali del Gran Sasso (LNGS) of the Istituto Nazionale di Fisica Nucleare (INFN) that will push the sensitivity for Weakly Interacting Massive Particle (WIMP) detection into the neutrino fog. The core of the apparatus is a dual-phase Time Projection Chamber (TPC), filled with 50 tonnes of low radioactivity underground argon (UAr) acting as the WIMP target. NUV-HD-cryo Silicon Photomultipliers (SiPM)s designed by Fondazione Bruno Kessler (FBK) (Trento, Italy) were selected as the photon sensors covering two$$10.5~\text {m}^2$$ $10.5{\text{m}}^2$Optical Planes, one at each end of the TPC, and a total of$$5~\text {m}^2$$ $5{\text{m}}^2$photosensitive surface for the liquid argon veto detectors. This paper describes the Quality Assurance and Quality Control (QA/QC) plan and procedures accompanying the production of FBK NUV-HD-cryo SiPM wafers manufactured by LFoundry s.r.l. (Avezzano, AQ, Italy). SiPM characteristics are measured at 77 K at the wafer level with a custom-designed probe station. As of March 2025, 1314 of the 1400 production wafers (94% of the total) for DarkSide-20k were tested. The wafer yield is$$93.2\pm 2.5$$ $93.2\pm 2.5$%, which exceeds the 80% specification defined in the original DarkSide-20k production plan. 

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4. Production, quality assurance and quality control of the SiPM Tiles for the DarkSide-20k Time Projection Chamber

   https://doi.org/10.1140/epjc/s10052-025-14940-1  

   Acerbi, F; Adhikari, P; Agnes, P; Ahmad, I; Albergo, S; Albuquerque, I F; Alexander, T; Alton, A K; Amaudruz, P; Angiolilli, M; et al (November 2025, The European Physical Journal C)

   Abstract The DarkSide-20k dark matter direct detection experiment will employ a$${21}\,\hbox {m}^{2}$$ $21{\text{m}}^2$silicon photomultiplier (SiPM) array, instrumenting a dual-phase 50 tonnes liquid argon Time Projection Chamber (TPC). SiPMs are arranged into modular photosensors calledTiles, each integrating 24 SiPMs onto a printed circuit board (PCB) that provides signal amplification, power distribution, and a single-ended output for simplified readout.$$16$$ $16$Tiles are further grouped intoPhoto-Detector Units(PDUs). This paper details the production of the Tiles and the Quality Assurance and Quality Control (QA-QC) protocol established to ensure their performance and uniformity. The production and QA-QC of the Tiles are carried out at Nuova Officina Assergi (NOA), an ISO-6 clean room facility at LNGS. This process includes wafer-level cryogenic characterisation, precision die attaching, wire bonding, and extensive electrical and optical validation of each Tile. The overall production yield exceeds 83.5%, matching the requirements of the DarkSide-20k production plan. These results validate the robustness of the Tile design and its suitability for operation in a cryogenic environment. 

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5. Experimental Characterization of the Transient Response of Air/Water Crossflow Heat Exchangers for Data Center Cooling Systems

   https://doi.org/10.1115/IPACK2015-48375  

   del Valle, Marcelo; Ortega, Alfonso (July 2015, ASME 2015 International Technical Conference and Exhibition on Packaging and Integration of Electronic and Photonic Microsystems collocated with the ASME 2015 13th International Conference on Nanochannels, Microchannels, and Minichannels)

   Data Center hybrid air/liquid cooling systems such as rear door heat exchangers, overhead and in row cooling systems enable localized, on-demand cooling, or “smart cooling.” At the heart of all hybrid cooling systems is an air to liquid cross flow heat exchanger that regulates the amount of cooling delivered by the system by modulating the liquid or air flows and/or temperatures. Due the central role that the heat exchanger plays in the system response, understanding the transient response of the heat exchanger is crucial for the precise control of hybrid cooling system. This paper reports on the transient experimental characterization of heat exchangers used in data centers applications. An experimental rig designed to introduce controlled transient perturbations in temperature and flow on the inlet air and liquid flow streams of a 12 in. × 12 in. heat exchanger test core is discussed. The conditioned air is delivered to the test core by a suction wind tunnel with upstream air heaters and a frequency variable axial blower to allow the control of air flow rate and bulk temperature. The conditioned water is delivered to the test core by a water delivery system consisting of two separate water circuits, one delivering cold water, and the other hot water. By switching from one circuit to the other or mixing water from both circuits, the rig is capable of generating step, ramp and frequency perturbations in water temperature at constant flow or step, ramp or frequency perturbations in water flow at constant temperature or combinations of temperature and water flow perturbations. Experimental data are presented for a 12×12 heat exchanger core with a single liquid pass under different transient perturbations 

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