Liquid argon scintillation response to electronic recoils between 2.8-1275 keV in a high light yield single-phase detector LIQUID ARGON SCINTILLATION RESPONSE to ELECTRONIC ... KIMURA M., AOYAMA K., TANAKA M., and YORITA K.

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.