The source of the delayed fluorescence by triplet–triplet interactions is discussed as the basis of the technique of differentiating between particles of different specific ionizations. The variation in the relative intensity of delayed fluorescence (called the slow component of scintillation) is correlated with the type of ionizing radiation. The decay times of the prompt and slow components do not depend upon the type of ionizing particle. The slow component in liquid scintillator solutions free of dissolved gases (especially oxygen) has a decay time of about 250 × 10<sup>−9</sup> sec. Liquid scintillator solutions with pulse shape discrimination properties have been used to measure neutrons (proton recoils) in the presence of gamma rays (Compton scattered electrons). They have also been demonstrated as able to measure the relative activities of alpha particles and fission fragments from a fission source in the presence of gamma and beta background.

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