Unified mercury radioactivity monitoring system at J-PARC and its operation experiences

Harada, Masahide; Sekijima, Mitsuaki*; Morikawa, Noriyuki*; Masuda, Shiho; Kinoshita, Hidetaka; Sakai, Kenji; Kai, Tetsuya; Kasugai, Yoshimi; Muto, Giichi*; Suzuki, Akio*; et al.

JPS Conference Proceedings (Internet), 33, p.011099_1 - 011099_6, 2021/03

https://doi.org/10.7566/JPSCP.33.011099

In MLF at J-PARC, a unified mercury radioactivity monitor (UHAM) is installed to find an indication of failure of the mercury target and loop system by detecting radioactive materials leaked from the system with a -ray energy analysis with Germanium semi-conductor detectors (Ge detectors). It is composed of three units of sampling port and radiation monitors: (1) HAM for interstitial helium gas layer between the mercury vessel and surrounding water shroud of the mercury target, (2) CAM for atmosphere in the hot cell where the target loop is operated and (3) VAM for helium gas in the helium vessel where the target vessel is installed. Once any leakages of radioactive materials are detected, an alarm signal is issued immediately to the accelerator control system to stop beam operation. Software and hardware have been upgraded yearly. For example, two Ge detectors are used for HAM for redundancy, NaI Scintillation detectors are also used as supplemental for the Ge detector to keep availability of the system for high counting rate event. In April 2015, the UHAM activated when a small water coolant leakage from the water shroud of the mercury target occurred. VAM detected an abnormal increase of the counting rate in the helium vessel. It was also indicated that the measured radioactive nuclides were generated from the activation of the coolant (water) in the water shroud and not from the mercury.