Preliminary studies on TPB coating on inner walls of ten-cm-scale cylindrical PTFE detector

Abstract

The ALETHEIA (A Liquid hElium Time projection cHambEr In dArk matter) project is an originally creative dark matter experiment aiming to hunt for low-mass (100 MeV/c^2~10 GeV/C ^2) WIMPs. While there exist more than ten experiments doing research on low-mass WIMPs, ALETHEIA is supposed to grow up to be a leading project worldwide thanks to many unique advantages, including but are not limited to:extremely low intrinsic backgrounds, easy to be purified, and strong potential capability of signal/background discrimination. Due to the project's original creativity, there exists no direct experience of building such a detector yet; consequently, we have to launch a set of R&D programs from scratch, including the TPB coating process conveyed in this paper.<br>An incident particle that hits a liquid helium detector would generate 80 nm scintillation. Given that there are no commercially available photon detectors capable of detecting the scintillation light with high efficiency, a wavelength shifter is a must to convert the 80 nm scintillator into visible light. SiPMs (Silicon Photomultipliers) can then be implemented to detect the 450 nm light. TPB (Tetraphenyl Butadiene,1, 1,4, 4-tetraphenyl-1, 3-butadiene) is widely used for the conversion. Although the DEAP (Dark matter Experiment using Argon Pulse-shape discrimination) experiment coated 2.3 μ m thickness TPB successfully on the inner wall of the sphere with a radius of 85 centimeters, we can't mimic the whole process in our experiment directly since (a). Our detector shape is cylindrical, not a sphere, and (b) the diameter of the current detector prototype is only 10 cm, while the one of the DEAP detector is as large as 1.7-meter. Consequently, we must design and build the appropriate coating apparatuses to adapt to our detector. Due to the existence of necessary auxiliary parts (such as cables for heating and temperature sensors), on which some vapored TPB molecules would deposit when the coating is in progress. As a result, a blind spot on the inner wall always exists that cannot be fully coated; the blind spot area will affect the visible light yield of 80 nm scintillation. To solve the problem, we split the coating process into two steps:coating the curved surface and one base together in the first step and coating another base in the second step. This way, the cylindrical detector's whole inner wall (the curved surface and the two bases) will be coated. Another key technology is to design an appropriate source sphere containing TPB powder. There are 20 holes evenly distributed on the surface of the sphere. After heating TPB powder to vaporize into the gas, TPB molecules should move slowly enough to ensure they scatter each other sufficient times inside the source before randomly finding a hole to escape. As a result, TPB molecules come out of the source in an isotropic way then adhere to the inner surfaces of a cylindrical detector (diameter and height are both 10 cm) with nearly the same thickness. The TPB coating thickness of the inner wall is between 1.50 to 3.02 μm, which corresponds to the thinnest and thickest TPB plates, respectively. The variation mainly comes from the different distances from the coating place to the source, which lies at the center of the PTFE cylinder. The thickness difference will not bother us because the conversion efficiency for 80 nm scintillation is almost the same as long as the TPB thickness is 0.7 to 3.7 μm.<br>In addition to introducing the ALETHEIA project briefly at the beginning, we mainly address several aspects of TPB coating:coating principle, source design, coating process, coating thickness monitoring, and coating plates' thickness comparison with three independent methods. The whole process we addressed in this paper is supposed to be a valuable reference for other experiments with similar demands.