  To increase the light yield of a liquid Ar (LAr) detector, we optimized the
evaporation technique of tetraphenyl butadiene (TPB) on the detector surface
and tested the operability of a silicon photomultiplier (SiPM), namely, the
multi-pixel photon counter with through-silicon-via (TSV-MPPC, Hamamatsu
Photonics K.K.) at LAr temperature. TPB converts the LAr scintillations (vacuum
ultraviolet light) to visible light, which can be detected by high-sensitivity
photosensors. Because the light yield 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 $\mu g/cm^2$ on the photosensor window and 40 $\mu g/cm^2$ on the
detector wall), the light yield was 12.8 $\pm$ 0.3 p.e./keVee using PMTs with a
quantum efficiency of approximately 30% for TPB-converted light. We also tested
the low-temperature tolerance of TSV-MPPC, which has a high photon-detection
efficiency, in the LAr environment. The TSV-MPPC detected the LAr
scintillations converted by TPB with a photon-detection efficiency exceeding
50%.
