22.—Organic Scintillators with Improved Timing Characteristics.

SynopsisTheoretical and experimental studies have been made of the scintillation pulse shapes of organic scintillators. In a unitary crystal the pulse shape is characterised by the scintillation decay time τs. In solution scintillators additional parameters are required to describe the pulse shape: Δt, the pulse width (FWHM); tm, the time at which the intensity reaches a maximumpmax; and t½, the time at which the intensity reaches ½pmax. General relations are derived for τs, Δτ, τm, τ½, and pmax for binary liquid solutions in terms of molecular parameters, and the corresponding expression is given for the scintillation pulse shape of a binary plastic solution.The scintillation pulse shapes of 4 binary liquid solutions and 12 binary and 15 ternary plastic solutions were measured using a single-photon sampling fluorometer. The results confirm the theoretical analysis. A new plastic scintillator (NE 111) with improved timing characteristics has thus been developed, and some of its applications in nuclear physics are briefly discussed. The time association of nuclear events can be measured to within 130 ps with this scintillator, the limiting factor being the time jitter of the photomultiplier itself.