The effect of temperature of fluorescence for liquid scintillators and their solvents

The pulse-height distributions for 241Am and 131mXe in PPO solutions of the aromatic hydrocarbons, i.e. benzene, o-xylene, m-xylene, ethylbenzene and cumene are found to shift toward higher pulse-heights with decreasing temperature. Under u.v. excitation, the fluorescence intensity of these pure aromatic hydrocarbons increases markedly and the fluorescence maximum is found to shift to longer wavelengths with decreasing temperature. In conjunction with observations of the pulse-height shift in liquid scintillators and the fluorescence emission from the pure solvents, the pulse-height shifts observed are interpreted in terms of excimer formation in the aromatic hydrocarbons.