A highly sensitive SF$_6$-based leak test system for JUNO 3-inch PMT underwater electronics boxes

TL;DR

The paper presents a highly sensitive SF$_6$-based leak detection system for underwater electronics boxes of PMTs at JUNO, offering an alternative to helium-based systems with improved sensitivity and reliability.

Contribution

It introduces a novel SF$_6$-based leak test system that surpasses helium-based methods in sensitivity for underwater electronics boxes in neutrino observatories.

Findings

Achieved a leak detection limit of 2.3×10⁻⁹ Pa·m³/s SF₆

Corresponds to 1.65×10⁻⁸ Pa·m³/s helium equivalent

Enabled early detection and replacement of leaking boxes

Abstract

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 SF$_6$-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 SF$_6$ from the underwater boxes, a leak rate detection limit of $2.3\times{10}^{-9}$~Pa$\cdot$m$^3$/s in terms of SF$_6$ was achieved, corresponding to $1.65\times{10}^{-8}$~Pa$\cdot$ m$^3$/s helium equivalent. This meets the sensitivity requirement of 1$\times$10$^{-7}$~Pa$\cdot$m$^3$/s. This system was critical in identifying and replacing a few cases of leaking underwater boxes before installation.