PSI - Issue 78

Michele Angiolilli et al. / Procedia Structural Integrity 78 (2026) 1807–1814

1808

demanding challenges from the engineering perspective, requiring the design, and integration of an array of advanced cryogenic, mechanical, structural, and radiopurity-critical systems - all within the seismically active environment of the Central Apennines. At the heart of the detector lies a ∼ 50-ton active low-radioactivity underground argon (UAr) mass (while the fiducial volume for Weakly Interacting Massive Particle - WIMP - searches is ∼ 20 tons) housed in a ∼ 600m 3 cryostat derived from Liquified Natural Gas-based storage technologies, adapted for underground operation and enhanced radiopurity constraints. The cryostat design, derived from experience in the DUNE project (B. Abi et al. (2020)) and operating at ∼ 87K( ∼ 186 ◦ C), demands rigorous control over thermal contraction, structural stability, and outgassing, and integrates hybrid insulation systems and ultra-clean internal components. DS-20k was developed following the success of the prototype experiment DarkSide-50, which employed a dual-phase LAr-TPC with an active volume of 50 kg. Hence, the new detector represents a thousand-fold scale-up compared to its predecessor, reflecting a bold scientific ambition and a major technological upgrade beyond conventional incremental improvements. A distinctive engineering feature of DS-20k project is the full-scale infrastructure, well described in A. Zani (2024), developed to enable the procurement and ultra-purification of the UAr, which is essential to suppress the background from cosmogenically produced isotopes. Indeed, beyond the detector itself, DS-20k relies on a complex supply chain and infrastructure to procure UAr, including: the URANIA extraction plant in Colorado (USA), capable of delivering 250 kg / day of UAr; the ARIA cryogenic distillation column in Sardinia E. Aaron, et al. (2023) for further purification and isotopic separation of the extracted UAr; the DArTinArDM facility at the Canfranc Underground Laboratory to precisely quantify residual radioactive content, and Nuova O ffi cina Assergi (NOA) facility at LNGS L. Consiglio (2024), a class ISO 6 clean room dedicated to photo-detectors production and to large-volume detector assembly. These facilities represent a dedicated global-scale engineering pipeline for the procurement, transportation, and verification of detector-grade argon, as well as for the production of cryogenic photo-detectors. The structural elements of the cryostat are composed of I-beams (IPEV600 steel profiles) that form a lattice system through M36 10.9 grade bolted connections. A 10 mm-thick steel plate, reinforced with sti ff ening ribs, constitutes the so-called tertiary membrane, placed just outside the insulation (described in §2.2). This will provide an e ff ective gas enclosure, allowing the insulation volume to be filled with nitrogen gas. The material used for beams and plates is S460ML. To facilitate the installation of the tertiary membrane, optimize welding operations, and mitigate potential second-order e ff ects—such as thermal stresses and geometric imperfections—proper connections with S355NL steel clamps were adopted at tertiary membrane-beams interface. Each clamp is designed with two M12 bolts and one M16 bolt to ensure the connection to both the ribs of the tertiary membrane and the flanges of the IPE160V beams, respectively. The nominal outer dimensions of the cryostat are 11.41 m × 11.41 m in plan and 10.76 m in height, while the nominal inner dimensions are 8.548 m × 8.548m × 7.900 m. These dimensions are determined by several constraints, including the required active volume of liquid argon, the layout of service penetrations, installation requirements, clearances between the active volume and the inner cryostat walls, and the insulation thickness. The cryostat rests on a 280 mm thick reinforced concrete sub-foundation of class C50 / 60. The mechanical structure simply sits on an electrical insulation system, which consists, from top to bottom, of: 0.5 mm of G10 fiberglass insulator, 5 mm of steel, a 25 mm elastomer pad, and another 5 mm of steel. This multilayer system also serves indirectly as a modal decoupling layer within the seismic isolation scheme. The entire assembly rests on the sub foundation floor without any anchorage, and is laterally restrained by unidirectional constraints provided by S460 steel devices (“brackets”). Note that a small gap of 1 to 5mm exists with respect to the IPE600V beams to put vertical layers of electrical insulation. Figures 1.a–g illustrate the key stages of the cryostat outer structure assembly — without the roof components, as described in § 2.2 — which was completed in October 2023. The outer cryostat is designed to withstand the hydrostatic pressure of the liquid argon (density of 1380 kg / m 3 ), the pressure of the gaseous volume (design value due to opening safety valves of 350 mBarg), the e ff ect of the fully assembled detector hanging on the roof (about 30 tons), and potential seismic actions. Regarding the latter, the 2. DS-20k cryostat 2.1. Cryostat outer structure