LAUSR

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 $12.8\ifmmode\pm\else\textpm\fi{}0.3(11.2\ifmmode\pm\else\textpm\fi{}0.3)\text{ }\text{ }\text{photoelectron}/\mathrm{keV}$ for 511.0-keV $\ensuremath{\gamma}$-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 $^{22}\mathrm{Na}$ and $^{241}\mathrm{Am}$ $\ensuremath{\gamma}$-ray emitters, and a $^{252}\mathrm{Cf}$ fast neutron emitter that induces quasimonoenergetic $\ensuremath{\gamma}$ rays through a $(n,{n}^{\ensuremath{'}}\ensuremath{\gamma})$ reaction with $^{19}\mathrm{F}$ in polytetrafluoroethylene. In addition, the high light detection efficiency of the detector enables identification of the 2.82-keV peak of $^{37}\mathrm{Ar}$, 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 $\ensuremath{\varsigma}=0.03{3}_{\ensuremath{-}0.008}^{+0.012}$ 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.