We measure the liquid argon scintillation response to electronic recoils in the energy range of 2.82 to 1274.6 keV at null electric field. The single-phase detector with a large optical coverage used in this measurement yields 12.8±0.3(11.2±0.3) photoelectron/keV for 511.0-keV γ-ray events based on a photomultiplier tube single photoelectron response modeling with a Gaussian plus an additional exponential term (with only a Gaussian term). It is exposed to a variety of calibration sources such as Na22 and Am241 γ-ray emitters, and a Cf252 fast neutron emitter that induces quasimonoenergetic γ rays through a (n,n′γ) reaction with F19 in polytetrafluoroethylene. In addition, the high light detection efficiency of the detector enables identification of the 2.82-keV peak of Ar37, a cosmogenic isotope in atmospheric argon. The observed light yield and energy resolution of the detector are obtained by the full-absorption peaks. We find up to approximately 25% shift in the scintillation yield across the energy range and 3% of the energy resolution for the 511.0-keV line. The Thomas-Imel box model with its constant parameter ς=0.033-0.008+0.012 is found to explain the result. For liquid argon, this is the first measurement on the energy-dependent scintillation yield down to a few keV at null field and provides essential inputs for tuning the argon response model to be used for physics experiments.
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