  A liquid scintillator (LS) is developed for the Taishan Antineutrino
Observatory (TAO), a ton-level neutrino detector to measure the reactor
antineutrino spectrum with sub-percent energy resolution by adopting Silicon
Photomultipliers (SiPMs) as photosensor. To reduce the dark noise of SiPMs to
an acceptable level, the LS has to work at -50 degree or lower. A customized
apparatus based on a charge-coupled device (CCD) is developed to study the
transparency of the liquid samples in a cryostat. We find that the water
content in LS results in transparency degradation at low temperature, which can
be cured by bubbling dry nitrogen to remove water. Adding 0.05% ethanol as
co-solvent cures the solubility decrease problem of the fluors PPO and bis-MSB
at low temperature. Finally, a Gadoliniumdoped liquid scintillator (GdLS), with
0.1% Gd by weight, 2 g/L PPO, 1 mg/L bis-MSB, and 0.05% ethanol by weight in
the solvent LAB, shows good transparency at -50 degree and also good light
yield.
