Positron annihilation processes in a liquid scintillator to be used for the detection of electron antineutrinos in the KamLAND project were studied. The liquid scintillator was a mixture of isoparaffin, pseudocumene and 2, 5-diphenyloxazole. We measured positron annihilation lifetime (PAL) spectra which contained important timing information on identification of the electron antineutrinos. The PAL spectra of a nitrogen-saturated liquid scintillator consisted of three lifetime components. Two fast components with the lifetime of 0.19 ± 0.05 ns and 0.48 ± 0.05 ns correspond to the mixture of para-positronium annihilation and free positron annihilation. The delayed component (3.41 ± 0.02 ns) with the intensity of 48.9 ± 0.3 % is ascribed to pick-off annihilation of ortho-positronium (ο-Ps) annihilation. To understand the 2γ annihilation processes of ο-Ps in the liquid, we examined the effects of dissolved gases (nitrogen, oxygen, helium and argon) and mixing ratio of isoparaffin and pseudocumene which were the main components of the liquid scintillator. The lifetime of ο-Ps was drastically shortened in the presence of oxygen, but not for other gases. The reaction rate constant for quenching of ο-Ps by oxygen was obtained from the O2 concentration dependence of ο-Ps lifetime component. The pick-off annihilation of ο-Ps with the liquid scintillator is explained based on the bubble model. We discuss a possible microscopic picture of the surroundings of Ps in the liquid scintillator.
The Japan Society of Nuclear and Radiochemical Sciences