Comparison between photon detection efficiency and tetraphenyl-butadiene coating stability of photomultiplier tubes immersed in liquid argon

Liquid argon detectors are an interesting option for neutrino experiments. The high density of liquid argon allows using it directly as target for neutrino interactions. The relatively large abundance of argon in the atmosphere makes it a cost-effective medium, allowing the construction of detectors of several hundred tons, such as ICARUS, and even several kton detectors are foreseen for the next generation of experiments, such as DUNE . Besides being suitable to act as the target, liquid argon also serves as the detection medium for the charged particles coming out of the interaction vertex. The interaction time, t0, can be determined by detecting the photons which are produced together with the electrons in the ionization process. To detect them, the integration of an efficient photon detection system into the liquid argon detector is necessary. One difficulty here is related to the fact that the emitted photons have a wavelength in the VUV range (128 nm). The classical approach for the photon detection system is, therefore, the usage of photomultiplier tubes coated with tetraphenyl-butadiene which shifts the VUV photons to about 430 nm, a wavelength to which the photomultiplier tubes are directly sensitive to. While the basic concept is well established and has been used in several experiments such as ICARUS, ArDM and DarkSide, some aspects of the coating are not well understood such as the preparation of the photomultiplier tube surface which can be either polished or sandblasted. To better understand the effect on the overall photon detection efficiency on one hand and the tetraphenyl-butadiene stability during the cooling down, on the other hand, the quantum efficiencies of sandblasted and polished photomultiplier tubes, coated following the same procedure, were measured with a setup at the Istituto Nazionale di Fisica Nucleare (INFN), in Pavia. In addition, the immersion of the photomultiplier tubes with different soak timings was tested, showing a significantly different behavior for fast and slow immersion for the polished photomultiplier tubes.