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}) 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 -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) photoelectrons/photon and g_2 = (20.0 pm 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 investigated 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}, 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.
Highlights
Experiments searching for weakly interacting massive particles (WIMPs) play a central role in the multifaceted effort aiming to shed light on the nature and properties of dark matter in the Universe
DarkSide-20k will be the first large-scale experiment to employ 1) argon extracted from underground reservoirs and 2) a readout system based on Silicon Photomultipliers (SiPMs), fulfilling two key ingredients of the GADMC program
An additional asset to the GADMC program would be to demonstrate the directional sensitivity of argon-based Time Projection Chamber (TPC) technology, since directional information provides a unique handle for discriminating against otherwise-irreducible backgrounds and is an essential requisite for correlating a candidate signal with an astrophysical phenomenon in the celestial sky [5]
Summary
Experiments searching for weakly interacting massive particles (WIMPs) play a central role in the multifaceted effort aiming to shed light on the nature and properties of dark matter in the Universe. The acceleration of electrons by the electric field in the gas pocket produces a secondary signal composed of electroluminescent light [7] and referred to as S2, whose amplitude is proportional to the number of electrons escaping recombination and whose delay with respect to S1 is equal to the time needed for the electrons to drift through the liquid. Both S1 and S2 signals are detected by photosensors externally viewing the active volume. Since Eex and Eel are determined mostly by the anode voltage and the thickness of the gas pocket, the difference in the calculated values between the simplified and full simulations was found to be below 3%
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