Real-time monitoring method for gadolinium concentration in a water Cherenkov detector

Time-resolved laser-induced luminescence spectroscopy is useful for real-time measurement of lanthanide ion concentrations in aqueous solution. Gadolinium ions (Gd3+\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$<^>{3+}$$\end{document}), in particular, have a long (similar to\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\sim$$\end{document}ms) emission lifetime, so that the ion emission can be easily distinguished from scattering of the excitation pulsed laser without the need for a monochromator. In this work, we have developed a real-time monitoring method for Gd concentration in water, aiming at application to the Super-Kamiokande (SK) water Cherenkov detector in which 0.03% Gd is currently dissolved in the form of sulfate for the observation of supernova relic neutrino events. The basic concept is to install a tube to run a portion of the water sample through a quartz cell (2 cm on each side), where a ns-pulsed laser at 266 nm is irradiated to excite Gd3+\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$<^>{3+}$$\end{document} ions. The generated Gd3+\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$<^>{3+}$$\end{document} ion emission at 312 nm is collimated by a lens, transmitted through a bandpass filter, and then detected by a photomultiplier tube placed about 10 cm away from the quartz cell. While lower Gd concentration and higher pulsed laser energy resulted in shorter Gd3+\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$<^>{3+}$$\end{document} emission lifetime, good linearity was confirmed between Gd concentration and normalized peak emission voltage in the wide range of 1-1000 ppm (0.1%) Gd in ultrapure water. The detection limit, defined as three times the standard deviation of the background level, was determined to be similar to\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\sim$$\end{document}60 ppb for Gd sulfate in ultrapure water. This value is about two orders of magnitude better than the reported value using laser-induced breakdown spectroscopy, and is close to that using inductively coupled plasma optical emission spectrometry which requires sample introduction into the spectrometer. Sulfate ions in aqueous solution appear to have a smaller quenching effect than O-H vibrations of water molecules coordinated to the cation. By confirming a detection sensitivity below the ppm-level, this method could be effective for monitoring of drainage water from the SK detector tank as well. Our real-time monitoring method is expected to support the long-term operation of the SK-Gd project.