Increasing light collection efficiency of liquid argon detector for low mass WIMP search

Liquid argon (LAr) is known as an excellent target material for weakly interacting massive particle (WIMP) dark matter direct-detection search experiment. In WIMP searches, distinguishing signal events from electronic recoil (ER) backgrounds is essential. The reduction of ER power using pulse shape discrimination (PSD) improves as light collection efficiency increases. However photosensors, which have to be operational in a cryogenic environment, have no high quantum efficiency (QE) at the 128 nm VUV scintillation light of argon. Therefore, a wavelength shifter, i.e. tetraphenyl butadiene (TPB), is evaporated on the inner surface of the detector to convert VUV to visible light that can be detected by photosensors. Thus, we constructed a well-controlled TPB evaporation system to ensure reproducibility and measured the amount of TPB on the surface using a quartz crystal microbalance. We obtained the world's highest light yield (similar to 11.5 PE/keVee) by optimizing the amount of evaporated TPB. We describe our efforts to improve light collection efficiency.