The ReD Experiment
Since 2018 the Recoil Directionality (ReD) project has operated a miniaturized dual phase Argon TPC readout by Silicon Photomultipliers (SiPM). The main goal of the project is to study the response of the detector to nuclear recoils, as expected from WIMP interactions.
The ReD TPC
At the heart of ReD is a dual-phase liquid argon Time Projection Chamber (TPC)—a compact detector measuring just 5 × 5 × 6 cm, essentially a miniature version of the DarkSide-20k detector. The chamber is enclosed by transparent acrylic windows on the top and bottom and acrylic walls on the sides. The windows are coated with a thin, transparent conductive layer called indium tin oxide (ITO), which acts as electrodes to generate the electric field inside the detector.
Like all dual-phase TPCs, ReD contains liquid argon beneath a thin layer of argon gas. When a particle interacts in the liquid, it produces an immediate flash of light known as the S1 (scintillation) signal. The interaction also releases electrons, which are guided upward by the electric field to the liquid-gas interface. There, they are extracted into the gas, where they generate a second burst of light called the S2 (electroluminescence) signal. The time between the S1 and S2 signals reveals the depth at which the interaction occurred inside the detector.
Both S1 and S2 are detected by two arrays of silicon photomultipliers (SiPMs) positioned above and below the chamber. Each array contains 24 SiPMs covering about 1 cm² each. The top array reads out every sensor individually, allowing scientists to reconstruct the interaction's position from the S2 light pattern. The sensors in the bottom array are read out in groups of six.
Before starting the experimental program, the ReD TPC was studied and characterized in depth at INFN Napoli, in order to ensure that the performance met the requirements necessary to achieve the scientific goals of the subsequent phases.
the ReD TPC detector
The array "tiles" of SiPMs
Directionality Studies
One of the most powerful ways to confirm a dark matter signal is to determine the direction from which it originated. As the Solar System moves through the Milky Way, Earth is expected to encounter a steady "wind" of dark matter particles (WIMPs) that appears to come from the direction of the Cygnus constellation. If the direction of nuclear recoils inside a detector aligns with this predicted galactic wind, it would provide compelling evidence for the direct detection of dark matter.
Results from the SCENE experiment suggested that dual-phase liquid argon Time Projection Chambers (TPCs) may be sensitive to the direction of nuclear recoils, producing slightly different signals for interactions of the same energy depending on their direction. ReD was designed to investigate this possibility by creating nuclear recoils with known energies and directions inside its detector. To do this, the experiment uses neutrons, which interact with argon nuclei in much the same way as WIMPs are expected to.
The neutrons are produced using a lithium-7 beam from the TANDEM accelerator at INFN Laboratori Nazionali del Sud, directed onto a polyethylene (CH₂) target. This reaction produces neutrons along with beryllium-7 nuclei, which serve as a tag for each event. After scattering from argon nuclei inside the ReD detector, the neutrons are detected by an array of liquid scintillators, allowing researchers to precisely reconstruct each interaction and study the detector's directional response.
Diagram of neutron beam experiment
Due to the two-body kinematics, the detection of the scattered neutron at a given angle is sufficient to infer the energy and direction of the nuclear recoil in the TPC.
The beam run took place in February 2020: the configuration was set up to tag nuclear recoils of about 70 keV in the TPC, emitted in different directions with respect to the electric field, in order to test a potential directional sensitivity. Data were analyzed according to a dedicated theoretical model: no evidence of directional sensitivity was found and an upper limit on the extent of the effect could be set.
Measuring the TPC Response
One of the most powerful ways to confirm a dark matter signal is to determine the direction from which it originated. As the Solar System moves through the Milky Way, Earth is expected to encounter a steady "wind" of dark matter particles (WIMPs) that appears to come from the direction of the Cygnus constellation. If the direction of nuclear recoils inside a detector aligns with this predicted galactic wind, it would provide compelling evidence for the direct detection of dark matter.
Results from the SCENE experiment suggested that dual-phase liquid argon Time Projection Chambers (TPCs) may be sensitive to the direction of nuclear recoils, producing slightly different signals for interactions of the same energy depending on their direction. ReD was designed to investigate this possibility by creating nuclear recoils with known energies and directions inside its detector. To do this, the experiment uses neutrons, which interact with argon nuclei in much the same way as WIMPs are expected to.
Between January and March 2023, ReD collected data at the INFN Sezione di Catania. Subsequent analysis confirmed that the detector successfully measured and characterized nuclear recoils down to 2 keV, demonstrating the experiment's ability to achieve its key performance objective.
What's Next
Building on the success of ReD, the next phase of the project—ReD+—is funded by a PRIN grant from the Italian Ministry of Research. ReD+ will build on the original detector design with an upgraded experimental setup, extending its sensitivity to nuclear recoils as low as 0.5 keV.
A key component of ReD+ is a new deuterium-deuterium (DD) neutron generator, currently being commissioned at the University of São Paulo Physics Institute. The generator produces a highly collimated, monoenergetic neutron beam, enabling researchers to study even lower-energy nuclear recoils with greater precision and further investigate the detector's potential directional sensitivity.
The DarkSide Collaboration aims to unveil the nature of dark matter through liquid argon detector technology and innovative underground experiments. It is located at the Laboratori Nazional del Gran Sasso in L’Aquila, Italy.
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