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

Abstract

We measure the liquid argon scintillation response to electronic recoils in the energy range of $2.82$ to $1274.6\mathrm{keV}$ at null electric field. The single-phase detector with a large optical coverage used in this measurement yields $12.8\pm 0.3(11.2\pm 0.3)photoelectron/keV$ for $511.0$-$\mathrm{keV}$ $\gamma$-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 ${}^{22}\mathrm{Na}$ and ${}^{241}\mathrm{Am}$ $\gamma$-ray emitters, and a ${}^{252}\mathrm{Cf}$ fast neutron emitter that induces quasimonoenergetic $\gamma$ rays through a $(n,n^{\prime }\gamma )$ reaction with ${}^{19}\mathrm{F}$ in polytetrafluoroethylene. In addition, the high light detection efficiency of the detector enables identification of the $2.82$-$\mathrm{keV}$ 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 $511.0$-$\mathrm{keV}$ line. The Thomas-Imel box model with its constant parameter $\varsigma =0.033_{-0.008}^{+0.012}$ is found to explain the result. For liquid argon, this is the first measurement on the energy-dependent scintillation yield down to a few $\mathrm{keV}$ at null field and provides essential inputs for tuning the argon response model to be used for physics experiments.

11 pages, 13 figures