Development of Gadolinium-Loaded Liquid Scintillators for 1{\%}-Precision Measurement at the Daya Bay Nuclear Reactors of the Neutrino Mixing Angle, $\theta _{13}$

The Daya Bay collaboration intends to use multiple organic-liquid-scintillator (LS) detectors placed at various distances between 0.3 and 2 km from the Daya Bay-Ling Ao nuclear power reactors to detect antineutrino oscillations and to determine the unknown neutrino-mixing angle, $\theta _{13}$. The nuclear reaction in the LS is inverse $\beta $-decay 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 enhanced by loading $\sim $0.1{\%} gadolinium into the liquid scintillator (Gd-LS), because of the 49000-barn (n,$\gamma )$ cross section of natural abundance Gd and the $\sim $8 MeV of emitted $\gamma $ rays. The Daya Bay plan is to use eight \textit{identical} antineutrino detectors, each containing 20 tons of Gd-LS. The BNL Nuclear Chemistry Group has developed chemical procedures to synthesize high-purity Gd-LS with long attenuation length ($>$15 m), high light output ($\sim $95{\%} of pseudocumene), and long-term stability ($>$1.5 years to date). Groups at IHEP in Beijing, China and JINR in Dubna, Russia are also doing Gd-LS R{\&}D. This paper discusses the properties of Gd-LS.