In the Materials and Life science experimental Facility (MLF) at the Japan Proton Accelerator Research Complex (J-PARC), a unified mercury radioactivity monitor (UHAM) was installed to detect a sign of failure of the mercury target, the mercury loop system and the gas supply system by detecting radioactive material leaking from the system using a γ-ray energy analysis with germanium semi-conductor detectors (Ge detectors). UHAM is composed of three separate monitors: “the mercury (Hg) activity monitor” (HAM) for the interstitial helium gas layer between the mercury vessel and the surrounding water shroud, “hot-cell activity monitor” (CAM) for the atmosphere in the hot cell where the mercury loop is operated, and “the helium vessel activity monitor” (VAM) for helium gas in the helium vessel where the target vessel is installed.

When two incidents occurred in 2015, the UHAM was activated. HAM and VAM detected an abnormal increase in radioactive material, demonstrating that the incidents were triggered by smidge water coolant leaks.