Measuring the Ionization Yield of Low-Energy Nuclear Recoils in Liquid Argon*

Liquid argon (LAr) has been proposed as a candidate target medium for the detection of coherent neutrino-nucleus scatter (CNNS). Design and deployment of a large ($\sim$10 kg active mass) dual-phase argon detector for the detection of CNNS at a nuclear power plant requires an understanding of the response of LAr to nuclear recoils from 0--6 keV. In this regime the prompt scintillation (S1) signal is below threshold and detection must rely solely on the ionization signal (S2) of the nuclear recoil. We briefly discuss the design and performance of a small prototype dual-phase argon detector and its calibration using $^{37}$Ar. We then report on our measurement of the ionization yield of nuclear recoils below 7 keV using this prototype detector with a filtered near-threshold $^7$Li(p,n) reaction as a neutron source.