Kazutaka Aoyama

To increase the light collection efficiency of a liquid argon (LAr) detector, we optimized the evaporation technique of tetraphenyl butadiene (TPB) on the detector surface and tested the operability of a silicon photomultiplier at LAr temperature. TPB converts LAr scintillations (vacuum ultraviolet light) to visible light, which can be detected using high-sensitivity photosensors. Because the light collection efficiency depends on the deposition mass of TPB on the inner surface of the detector, we constructed a well-controlled TPB evaporator to ensure reproducibility and measured the TPB deposition mass using a quartz crystal microbalance sensor. After optimizing the deposition mass of TPB (30 $\rm \mu g/cm^2$ on the photosensor window and 42 $\rm \mu g/cm^2$ on the detector wall), the observed light yield was 12.8 ± 0.3 p.e./keVee using photomultiplier tubes with a quantum efficiency of approximately 30% for TPB-converted light. We also tested the low-temperature tolerance of the silicon photomultiplier, which has a high photon detection efficiency, in a LAr environment. It detected the LAr scintillations converted by TPB with a photon detection efficiency exceeding 50%.

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 $12.8\pm 0.3(11.2\pm 0.3)\text{photoelectron}/\mathrm{keV}$ 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 ${}^{22}\mathrm{Na}$ 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-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 $\varsigma =0.033_{-0.008}^{+0.012}$ is found to explain the result. For liquid argon, this is the first measurement on the energy-dependent scintillation yield down to a few keV at null field and provides essential inputs for tuning the argon response model to be used for physics experiments.