ALETHEIA (a liquid hElium time projection cHambEr In dark matter) project is an originally creative dark matter experiment aiming to search for low-mass (100 MeV/c
2–10 GeV/c
2) WIMPs. While there have existed more than ten experiments doing research on low-mass WIMPs, the ALETHEIA is supposed to grow up to be a leading project worldwide due to many unique advantages, including but are not limited to extremely low intrinsic backgrounds, easy purification , and strong potential capability of signal/background discrimination. Owing to the project’s original creativity, there has existed 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.
An incident particle that hits a liquid helium detector would generate 80-nm-long scintillation. There are currently no commercially available photon detectors capable of efficiently detecting the scintillation light and a wavelength converter must be used to convert the 80-nm-long scintillator into visible light. Silicon photomultipliers (SiPMs) can then be implemented to detect the 450-nm-wavelength light. The TPB (Tetraphenyl Butadiene, 1, 1,4, 4-tetraphenyl-1, 3-butadiene) is widely used for realizing the conversion. Although in thedark matter experiment using argon pulse-shape discrimination (DEAP) , 2.3-μm-thick TPB is successfully coated on the inner wall of the sphere with a radius of 85 cm, we cannot mimic the whole process in our experiment directly out of the two following reasons: (a) our detector shape is cylindrical, not spherical, and (b) the diameter of the current detector prototype is only 10 cm, while the one of the DEAP detectors is as large as 1.7-meter. Consequently, we must design and build an appropriate coating apparatus suitable for our detector. Owing to the existence of necessary auxiliary parts (such as cables for heating and temperature sensors), on which some vapored TPB molecules would be deposited 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-long 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. In 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 the TPB powder is heated andevaporated into the gas, the TPB molecules should move slowly enough to ensure that they scatter from each other long enough within the source before randomly finding a hole to escape. As a result, the 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 on the inner wall is in a range between 1.50 and 3.02 μm, which corresponds to the thinnest and thickest TPB plate, 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-long scintillation is almost the same as that for the TPB thickness in a range from 0.7 to 3.7 μm.
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 the comparison of thickness among coating plates from three independent methods. The whole process addressed in this paper is expected to provide a valuable reference for other experiments with similar requirements.
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