Speaker
Description
The direct search for dark matter in the form of Weakly Interacting Massive Particles (WIMPs) employs several experimental techniques, including noble-gas Time Projection Chambers (TPCs). In these detectors, a WIMP may interact via elastic scattering off target nuclei, producing low-energy nuclear recoils. For WIMPs with masses of a few GeV, recoil signals of only a few keV are expected. In this energy regime, the scintillation component in liquid argon becomes extremely weak, making the study of the ionization signal essential.
To bridge the gap in direct measurements of the argon ionization yield (defined as the number of electrons produced per unit of recoil energy, $Q_y$) of nuclear recoils energy below 7 keV, the Recoil Directionality (ReD) experiment was designed and operated at the Istituto Nazionale di Fisica Nucleare (INFN) Division of Catania. The experimental setup was conceived to produce nuclear recoils in argon in the 2–10 keV range through elastic scattering of neutrons provided by a ²⁵²Cf fission source. This was housed inside a boron-loaded polyethylene shielding together with two $BaF_2$ scintillator crystals used as fission-event taggers.
After interacting inside the TPC, neutrons were detected by a spectrometer consisting of 18 EJ-276 plastic scintillators arranged in 3×3 matrices to preserve up/down symmetry and provide the stop signal for neutron time-of-flight (ToF) measurement, from which the kinetic energy was evaluated.
The measured ionization yield is consistent with existing results above 7 keV and extends the experimental characterization down to 2 keV. Indeed, the region below 6.7 KeV is probed for the first time using a direct and model-independent approach and the Red data indicate an increase of the $Q_y$ at lower recoil energies.
Building on these encouraging results, a new configuration, ReD+, is currently in progress to extend measurements to even lower recoil energies. In the new setup the addition of two 3×3 matrices of plastic scintillators to cover the left–right configuration is foreseen. A new measurement campaign is planned for early 2026 at the INFN – Laboratori Nazionali del Sud using the current ReD TPC. This will be followed by a second campaign with an optimized TPC designed to reach recoil energies down to 0.5 keV. In the final phase, the ²⁵²Cf source will be replaced by a deuterium-deuterium neutron generator from the Universidade de São Paulo, enabling measurements of recoil energies as low as approximately 0.2 keV.