A total of 25600 3-inch photomultiplier tubes (PMTs), along with their corresponding frontend electronics, have been installed at the Jiangmen Underground Neutrino Observatory (JUNO). These electronics are housed in 200 stainless steel boxes that operate underwater. To verify the sealing integrity of the underwater boxes following integration, we developed an SF6-based leak test system, opting against the typical helium-based system due to helium's ability to penetrate the PMT glass. After a few hours of accumulating leaking SF6 from the underwater boxes, a leak rate detection limit of 2.3×10−9~Pa⋅m3/s in terms of SF6 was achieved, corresponding to 1.65×10−8~Pa⋅ m3/s helium equivalent. This meets the sensitivity requirement of 1×10−7~Pa⋅m3/s. This system was critical in identifying and replacing a few cases of leaking underwater boxes before installation.
@article{arxiv.2505.24142,
title = {A highly sensitive SF$_6$-based leak test system for JUNO 3-inch PMT underwater electronics boxes},
author = {Ziliang Chu and Diru Wu and Miao He and Jilei Xu and Xiaoping Jing and Jian Wang},
journal= {arXiv preprint arXiv:2505.24142},
year = {2025}
}