Dark Matter
Many astrophysical and cosmological evidences suggest that about the 27 % of all the Universe (or rather, about 85% of all the matter) consists of matter that does not emit light, or rather matter that cannot be associated with the ordinary one that we experience in everyday life, consisting of protons, neutrons and electrons. One of the best candidates for Dark Matter are the Weakly Interacting Massive Particles (WIMPs). WIMPs would have been formed in the early universe and gravitationally clustered (in a bottom-up structure) together with baryonic matter. The motion of galactic halo WIMPs relative to a detector on Earth could result in WIMP-nucleus elastic collisions, nuclear recoils, detectable by a low-background (the collision rate are extremely low), low-threshold detector (the nuclear recoils are very low-energy events, <100 keV). The goal of the DarkSide experiment is to make a direct detection of Dark Matter.
The Working Principle
The DarkSide detection principle is based on liquid Argon depleted in 39Ar, used both as target and as a detector. In particular, a dual-phase Ar Time-Projection Chamber (TPC) has been chosen to be capable of unambiguously identifying a small number the nuclear recoils in a very large exposure.
In the video, the working principle of a dual-phase TPC is schematically explained. The recoils of the Ar nucleus excites and ionizes the argon atoms. The excited atoms lead to a (prompt) scintillation light, S1, which is detected from the photosensor, placed at the top and the bottom of the TPC. The electrons from the ionization are instead immediately drifted (to reduce the recombination with positive ions) to the top of the TPC thanks to an electric field applied; at the TPC top, a much higher electric field extracts the electrons into the gas pocket above the liquid and accelerate them enough to perform a secondary scintillation light signal (electroluminescence), S2, proportional to the ionization charge. The S2 is then detected both from top and bottom (not shown in the video for simplicity) photosensors. In this way you can have a 3D position reconstruction of the event: the horizontal coordinates are measured from the S2 pattern in the top photosensor while the vertical coordinates from the drift time between S1 and S2.
Due to the nature of the Ar scintillation light (in the ultraviolet range, ~ 128 nm), all the surfaces of the TPC must be covered by a wavelength shifter (like TPB) to convert the photons to a wavelength efficiently detectable by a photosensor (TPB: ~420 nm).
The 3D reconstruction position is fundamental to reduce the radioactive material background performing fiducial cuts in the volume. Moreover, the DarkSide exploit one of the key feature of the detector: the Pulse Shape Discrimination (PSD) power of an Argon target. Indeed the typical Argon scintillation, S1, contains two different components with different decay time constants: a prompt one and a delayed one. In the Argon they can be easily distinguished and their amount depends strictly on the interaction type if it's a nuclear recoil (usually due to neutrons or WIMP-like particles) or an electron recoil (usually due to photons, electrons or neutrinos). This PSD can bring to a rejection power of electron recoils of about 108.
The DarkSide Experiment
The DarkSide collaboration is nowadays taking data thanks to a 50 kg detector, DS-50, operating at Laboratori Nazionali del Gran Sasso (LNGS). A deep underground laboratory like the LNGS is fundamental for a rare events experiment like DarkSide since it strongly reduces the background from cosmic rays. DS-50 is taking data since December 2014. In the video above, the TPC represented is a DS-50 like TPC, with a total of 38 photosensors (PMs), 19 in the top array and 19 in the bottom array, and a diameter of about 36 cm. Since October 2015, DS-50 is producing the first WIMP search results using low-radioactivity Underground Ar (UAr). This UAr permitted a depletion in 39Ar of a factor 1400 and consequently a reduction of the background rate.
The DarkSide collaboration plans to increase the detector and in particular the fiducial mass of the liquid argon TPC up to 20 tonnes. One of the key parameters is light detection: DS-20k will use the Silicon PhotoMultipliers (SIPMs), instead of the standard PhotoMultipliers (PMs).
To validate DS-20k in mechanical and functional aspect, two prototypes will be assembled and tested at CERN: first a 10 kg detector, DS-Proto-0, to optimise S2 using the SiPMs technology, and then a 175 kg detector, DS-Proto-1, a scaled-down version of DS-20k that will prove the principle. The former will have a total of 50 channels (1200 SiPMs) and very first tests have been already taken in July 2019 at CERN (and will continue in 2020). The second one, DS-Proto-1, will follow the DS-Proto-0 and will have a total of 250 channels (6000 SiPMs) and will operate at CERN or LNGS in 2021.
DarkSide-20k
DarkSide-20k (DS-20k) will be characterised by an ultra-low background level to achieve sensitivity to a WIMP-nucleon cross section of 7.4 10−48 cm2 (6.9 10−47 cm2) for 1 TeV/c2 (10 TeV/c2) WIMPs. It will operate at LNGS from 2023. The inner detector is a dual-phase Ar TPC of 350 cm of height and a distance between parallel walls of 355 cm. Two identical arrays of SiPMs are placed on the top and bottom of the TPC.
The DS-20k crucial key technologies that will permit these results are:
- the proto-Dune liquid Argon Cryostat: The DS-20k TPC will be hosted in a (scalable) Proto-DUNE-like cryostat (represented in orange in the figure below) that will eliminate the need of a cryostat in the immediate proximity of the TPC; this permits the use of an ultra-pure vessel (PMMA) for the TPC. These will greatly reduce the background rate due to the material radioactivity.
- the depleted (UAr) and purified Argon used: to reduce the background rate DS-20k not only will make use of the UAr (depleted in Ar39), thanks to Urania plant (Colorado, USA), but also will purify the UAr and isotropically separate the Argon from other elements, for a reduction of 103. This will be thanks to the Aria plant (Sardegna, Italy), a distillation column 350 m tall.
- the photodetectors modules (based on SiPMs instead of the standard PMs): DS-20k photosensor unit will be a Photo Detector Module (PDM), consisting of a tile of 24 SiPMs of the dimension of 11.7 mm x 7.9 mm covering a total area of 5 x 5 cm2 operating as a single detector and working as a single channel (Figure in the left). After extensive R&D performed by DarkSide collaboration and FBK, the PDM performance has reached all the DS-20k requirements, thanks to a new SiPM technology: SiPM NUV-HD-Cryo. Besides the tile, each PDM includes also a front-end board made of a TransImpedence Amplifier (TIA) developed and optimised for cryogenic operations and the mechanical structure (plastic cage) required to assemble all components. In the final detector, there will be in total about 8200 PDMs (channels) which is equal to 196800 cryogenic SiPMs. The PDMs are assembled in a Photo Detector Unit (PDU, Figure in the middle), an array of 25 PDMs for a total of 344 PDUs, covering the top and bottom of the TPC (Figure in the right).
In the exclusion plot below are reported all the results for the spin-independent cross section of dark matter WIMPs on nucleons that have been reached until now (continues lines) compared with sensitivities of the future experiment (dotted lines), among which DS-20k. As you can notice, at low mass (below 3-4 GeV/c2) DS-50 is still the best results. (DARWIN and Argo are the very next stage evolution of Xenon and DarkSide collaborations respectively).