Time-resolved laser-induced luminescence spectroscopy is useful for real-time measurement of lanthanide ion concentrations in aqueous solution. Gadolinium ions (Gd \(^{3+}\) ), in particular, have a long ( \(\sim\) 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 Gd \(^{3+}\) ions. The generated Gd \(^{3+}\) 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 Gd \(^{3+}\) 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 \(\sim\) 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.
