Phenylxylylethane (PXE): a high-density, high-flashpoint organic liquid scintillator for applications in low-energy particle and astrophysics experiments

We report on the study of a new liquid scintillator target for neutrino interactions in the framework of the research and development program of the BOREXINO solar neutrino experiment. The scintillator consists of 1,2-dimethyl-4-(1-phenylethyl)-benzene (phenyl-o-xylylethane, PXE) as solvent and 1,4-diphenylbenzene (para-Terphenyl, p-Tp) as primary and 1,4-bis(2-methylstyryl)benzene (bis-MSB) as secondary solute. The density close to that of water and the high flash point makes it an attractive option for large scintillation detectors in general. The study focused on optical properties, radioactive trace impurities and novel purification techniques of the scintillator. Attenuation lengths of the scintillator mixture of 12 m at 430 nm were achieved after purification with an alumina column. A radio carbon isotopic ratio of C-14/C-12 = 9.1 * 10^{-18}$ has been measured in the scintillator. Initial trace impurities, e.g. U-238 at 3.2 * 10^{-14} g/g could be purified to levels below 10^{-17} g/g by silica gel solid column purification.