The Project
ARIS is a small scale dual-phase liquid argon (LAr) Time Projection Chamber (TPC) designed to characterize LAr response to nuclear recoils. The TPC is exposed to a highly collimated neutron beam, produced by the 1H(7Li,n)7Be Licorne source at the ALTO facility. ARIS will impact on direct dark matter experiments, based on LAr, by constraining the scintillation and ionization energy scales, the nuclear quenching factor, the recombination probability, and the time response. In addition, the liquid argon response may depend on the nuclear recoil direction with respect to the electric field. In case this effect will be observed, liquid argon based experiments will benefit from an unambiguous signature for the direct dark matter detection.
TPC
ARIS is a dual-phase (gas/liquid) argon Time Projection Chamber (TPC). TPCs are incredibly versatile and useful apparatus for particle detection. A particle interaction in the liquid volume liberates electrons from argon atoms, which are then drifted upward, toward the gas region, by an applied electric field. The initial particle interaction in the liquid generates light (or scintillation), which we call S1. As they enter the gas region, the drifting electrons electroluminesce, generating a second signal, which we call S2.
The time profiles of the S1 and S2 pulses (and the ratio of their integrals) allow us to determine the species of particle (electron, neutron, or other) which interacted in the liquid argon. We can pinpoint the interaction location by utilizing the time delay between S1 and S2 (to find the interaction depth), and the distribution of S2 light across our light-sensing photomultiplier tubes (PMTs). All of this allows for a position-sensitive detector, with powerful particle discrimination capabilities, within a small 8cm diameter package.
LICORNE
LICORNE is a directional neutron source based at the Tandem accelerator of the IPN Orsay which produces high-flux, kinematically focused, monoenergetic beams of neutrons between 0.5 MeV to 4 MeV. Neutron production is achieved via the reaction p(7Li,n)7Be by the bombardment of a hydrogen-rich target with the 7Li beam (13-17 MeV) generated by Tandem. Currently, fluxes of up to 10e8 neutrons/second/steradian are achievable. The resulting neutron beam is highly collimated, with cone opening angles of 0-25 degrees.
LICORNE offers both high neutron flux and a kinematically-restricted neutron directionality, increasing event rate and reducing coincident background, relative to other high-intensity neutron sources. For ARIS, several neutron detectors will be placed around the TPC, but outside the path of the beam. Requiring a coincident signal in the TPC and a single one of these neutron detectors insures that we're seeing the same neutron interact in the TPC, recoil, and then interact again. Knowing the initial energy of the neutron, and the recoil angle from the TPC, we can precisely calculate the energy deposited within the liquid argon.
GOALS
ARIS has four primary goals:
1. Precision measurement of the relative scintillation efficiency of nuclear recoils (NRs) as function of the energy, in the range 16 keV to 130 keV. Results present in literature are controversial, especially below 50 keV.
2. Map of the Pulse Shape Discrimination (PSD) estimator, f90, as function of the energy and the drift field. f90 is defined as the number of detected primary scintillation photons in the first 90 ns with respect to the total number of detected photons in S1. This parameter is used to discriminate the electron recoil (ER) background in WIMP search experiments.
3. Measurement of recombination probability as a function of the energy and drift field. We will derive its behavior by combining S1 and S2 scintillation signals. The result will be compared with a model developed at APC for the DarkSide dark matter detection experiment, working for ERs but never tested on a NR data set.
4. Measurement of recoil directionality. We will measure the scintillation and ionization yields, at fixed NR energies, for perpendicular and (almost) parallel NR directions with respect to the drift field. This measurement has the potential to deeply impact the future of direct dark matter experiments, extending their sensitivities to extremely small WIMP elastic scattering cross sections, where also astrophysical neutrinos are expected.
