Gadolinium-loaded liquid scintillator for high-precision measurements of antineutrino oscillations and the mixing angle, θ13

The use of multiple, identical liquid scintillator (LS) detectors at distances between 0.1 and 2 km from a nuclear reactor has the potential to detect antineutrino oscillations and to measure the value of the unknown neutrino-mixing angle θ13. The nuclear reaction of interest in the LS is the inverse β-decay reaction on protons, with the coincidence tag between the emitted prompt positron and the delayed neutron-capture providing a clear signature of the antineutrino capture. The neutron-capture signal is greatly enhanced by loading gadolinium into the LS, because of the large (n, γ) cross-sections of the Gd isotopes (49,000 barns for natural abundance Gd), and the ∼8 MeV of emitted γ-rays. We have developed procedures at BNL to synthesize high-quality Gd-loaded LS (Gd-LS) in a simplified, consistent, and reproducible way, using the method of pH-controlled, solvent–solvent extraction. This Gd-LS has long attenuation length (>15 m), high-light output (∼90% of pseudocumene), and long-term stability (>1.5 years to date). The preparation and characteristics of this Gd-LS are discussed in this paper.