Brookhaven National Laboratory (BNL) is developing water-based liquid scintillator (WbLS) for deployment in the next generation of large-scale, underground physics detectors. WbLS may also be used as a detection medium for the dosimetric quality assurance of radiation therapy. At BNL, WbLS is made primarily of pure water with varying concentrations of liquid scintillator (LS) and surfactant. Metallic isotopes can be added depending on the intended application. Compared to pure LS, WbLS can be cheaper, more stable, simpler to handle, and have a longer attenuation length, with the added benefit of Cherenkov radiation seen in water to provide directionality to the physics experiments.

In order to better understand the optical properties of WbLS, I conducted a preliminary study of its fluorescence quantum yield (QY), defined as the fraction of photons absorbed by a material that are re-emitted as light. The QY is a key measure of the scintillator’s performance; a QY of at least 0.9 is considered a good light yield. I measured the absolute QY using an integrating sphere setup. Corrections were applied for reabsorption effects and wavelength dependent sensitivities of the instruments. The system calibration using eight QY standards is yet incomplete. The values obtained show a negative bias. The preliminary result of 0.84 ± 0.08 for 4% linear alkyl sulfonate in water is nonetheless encouraging, since it suggests that WbLS has a favorable light yield. Obtaining a more accurate QY measurement will be beneficial for ongoing studies modeling WbLS performance in its intended applications.