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  5. Abstract A double-phase argon Time Projection Chamber (TPC), with an active mass of 185 g, has been designed and constructed for the Recoil Directionality (ReD) experiment. The aim of the ReD project is to investigate the directional sensitivity of argon-based TPCs via columnar recombination to nuclear recoils in the energy range of interest (20– $$200\,\hbox {keV}_{nr}$$ 200 keV nr ) for direct dark matter searches. The key novel feature of the ReD TPC is a readout system based on cryogenic Silicon Photomultipliers (SiPMs), which are employed and operated continuously for the first time in an argon TPC. Over the course of 6 months, the ReD TPC was commissioned and characterised under various operating conditions using $$\gamma $$ γ -ray and neutron sources, demonstrating remarkable stability of the optical sensors and reproducibility of the results. The scintillation gain and ionisation amplification of the TPC were measured to be $$g_1 = (0.194 \pm 0.013)$$ g 1 = ( 0.194 ± 0.013 ) photoelectrons/photon and $$g_2 = (20.0 \pm 0.9)$$ g 2 = ( 20.0 ± 0.9 ) photoelectrons/electron, respectively. The ratio of the ionisation to scintillation signals (S2/S1), instrumental for the positive identification of a candidate directional signal induced by WIMPs, has been investigatedmore »for both nuclear and electron recoils. At a drift field of 183 V/cm, an S2/S1 dispersion of 12% was measured for nuclear recoils of approximately 60– $$90\,\hbox {keV}_{nr}$$ 90 keV nr , as compared to 18% for electron recoils depositing 60 keV of energy. The detector performance reported here meets the requirements needed to achieve the principal scientific goals of the ReD experiment in the search for a directional effect due to columnar recombination. A phenomenological parameterisation of the recombination probability in LAr is presented and employed for modeling the dependence of scintillation quenching and charge yield on the drift field for electron recoils between 50–500 keV and fields up to 1000 V/cm.« less
  6. Finding unambiguous evidence of dark matter interactions in a particle detector is a main objective of physics research. The liquid argon time projection chamber technique for the detection of Weakly Interacting Massive Particles (WIMP) allows sensitivities down to the so-called neutrino floor for high and low WIMP masses. Based on the successful operation of the DarkSide-50 detector, a new and more sensitive experiment, DarkSide-20k, was designed and is now under construction. A thorough understanding of the DarkSide-50 detector response to events classified as dark matter as well as all other interactions is essential for an optimal design of the new experiment. In this paper, we report on a particular set of events, for which scintillation-ionization signals are observed in association with signals from single or few isolated electrons. We identified and provided an interpretation for two event types in which electrons are produced via photoelectric effect on the cathode electrode and in the bulk liquid. Events with photoelectric emissions are observed in association with most interactions with large energy depositions in the detector. From the measured rate of these events, we determine the photo-ionization probability, or photoelectric quantum efficiency, of tetraphenyl butadiene (TPB) at wavelengths around 128 nm.
  7. DarkSide-50 has demonstrated the high potential of dual-phase liquid argon time projection chambers in exploring interactions of WIMPs in the GeV/c2 mass range. The technique, based on the detection of the ionization signal amplified via electroluminescence in the gas phase, allows to explore recoil energies down to the sub-keV range. We report here on the DarkSide-50 measurement of the ionization yield of electronic recoils down to about 180 eVer, exploiting 37Ar and 39Ar decays, and extrapolated to a few ionization electrons with the Thomas-Imel box model. Moreover, we present the determination of the ionization response to nuclear recoils down to ∼ 500 eVnr , the lowest ever achieved in liquid argon, using in situ neutron calibration sources and external datasets from neutron beam experiments.