Timing properties of scintillation counters

Abstract

The study of time resolution as a function of the triggering fraction C/ R for NE111 and Naton 136 scintillators of different dimensions and XP1021 photomultipliers showed that the shape of C/ R curves and in particular the optimum of C/ R values depend strongly on the type and dimension of scintillators. Including a new description of the light pulse from a scintillator as a convolution of an exponential function with a Gaussian function to the Hyman theory gave a quantitative agreement of experimental and theoretical C/ R curves. The values of parameters describing the timing properties of the photomultiplier XP1021 and the scintillators were found. The previously reported disagreement between the measured time resolution versus absorbed energy and the predictions of Gatti and Svelto, and of Hyman, is shown to be removed when the experiments are performed with a realistic choice of energy window settings. The study of C/ R curves with constant fraction timing shows a shift of the optimum to the higher value of C/ R. The time resolution for 60Co is ≈ 30% better than with leading-edge timing even for 20% pulse height selection. The measurements show that the new NE111 scintillator is faster than Naton 136. For NE111, the present limitation in the obtainable time resolution appears to be the time jitter of the photomultiplier while in the case of Naton 136 the limitation is given by a time spread of light pulse generation. A time resolution of 132 ps for 60Co and 216 ps for 22Na was obtained using NE111 scintillators with 2.5 cm diameter and 1.0 cm height.