Darkside ExPeriment

DarkSide-50 is a dual-phase argon Time Projection Chamber (TPC), developed for use in a search for direct evidence of dark matter. The experiment is hosted in Hall C of the Laboratori Nazionali del Gran Sasso, underneath the Gran Sasso e Monti della Laga National Park, and has been collecting data since October 2013.

DS-20k is a two-phase LAr detector with a 50 tonnes active volume. It will be installed in LNGS Hall-C in 2022 and will either detect WIMP dark matter or reach a 90% exclusion sensitivity to WIMP-nucleon cross sections of 7.4x10^-48cm² at the mass of 1TeV/c².

DarkSide-50 is a dual-phase argon TPC. Thirty-eight 3" Hamamatsu low-background R11065 PMTs (photomultiplier tubes), 19 each on the top and the bottom, view the active region through fused silica windows. The windows are coated on both sides with Indium Tin Oxide, ITO, a transparent conductor. This allows the inner window surfaces to serve as the grounded anode (top) and -60 KV cathode (bottom) of the TPC while maintaining their outer surfaces near the -1.5 KV PMT photocathode potential.

A gas layer for production of the electroluminescence signature is provided by a cylindrical rim on the fused silica anode window, which extends downward to form a “diving bell” containing the 2 cm-thick argon vapor layer (“gas pocket”) above the TPC drift volume. The cylindrical vessel containing the active region is made of PTFE, treated to be highly reflective at visible wavelengths. The entire inner surface of the active volume is coated with the wavelength shifter TetraPhenylButadiene (TPB) to convert the 128 nm argon scintillation into the wavelength range detectable by the PMTs. The drift field is produced by system consisting of the ITO cathode and anode planes, a field cage, and a grid that separates the drift and electron extraction regions.

The DarkSide-50 TPC is deployed within a borated liquid scintillator-based neutron veto, which is in turn inside a water Cherenkov muon veto. This not only gave DarkSide-50 the benefits of active background suppression, but also provided the direct experience in operating a low background experiment in these active shields, in preparation for future upgrades.

DS-20k detector consists of two nested detectors housed within a ProtoDUNE-style membrane cryostat. It is foreseen to begin operations in 2023 and will either detect WIMP dark matter or exclude a large fraction of favored WIMP parameter space, to WIMP-nucleon cross sections of 7.4x10^-48cm² for a 1TeV/c² mass.

The inner detector is a dual-phase argon time projection chamber (LAr TPC) contained within a vessel made from ultra-pure acrylic (PMMA) filled with ultra low-radioactivity Argon from Underground sources (UAr) and readout by 8280 SiPM-based PhotoDetector Modules (PDM) arrays. The height of the TPC is 350 cm and the total mass of UAr in the active volume is 49.7 t.

The outer veto detector is made of a passive Gd-loaded PMMA shell surrounding the inner detector, and of an outer copper faraday cage. As a result, the volume contained in the ProtoDUNE-style cryostat is subdivided in three different volumes, requiring a fill of 760 tonnes of liquid Argon from the Atmosphere (AAr). All three volumes are equipped with PDMs and will be operated as anticoincidence veto detectors, reducing at once background from cosmic rays and from neutrons and gamma-rays sourced by the construction materials of the detector.

DarkSide-20k is designed to operate with zero backgrounds, meaning that all sources of instrumental background are reduced to <0.1 events over a 200 t yr exposure. All background from minimum-ionizing radiation sources will be completely removed thanks to the combined action of the Pulse Shape Discrimination (PSD) of the primary scintillation and comparison of the primary and secondary scintillation. This outstanding sensitivity to coherent nuclear recoils will enable DarkSide-20k to detect a supernova neutrino burst coming from anywhere in the Milky Way Galaxy and, for a majority of the galaxy, clearly identify the neutronization burst, thus performing a flavor-blind measurement of the total neutrino flux and average energy.