Liquid argon scintillation response to electronic recoils between $2.8$--$1275~{\rm keV}$ in a high light yield single-phase detector

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 gamma-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 Na-22 and Am-241 gamma-ray emitters, and a Cf-252 fast neutron emitter that induces quasimonoenergetic gamma rays through a (n, n'gamma) reaction with F-19 in polytetrafluoroethylene. In addition, the high light detection efficiency of the detector enables identification of the 2.82-keV peak of Ar-37, 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 zeta = 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.