Study of light collection process from cylindrical scintillators

Abstract

The time spread and the efficiency of the light collected from cylindrical samples of a fixed 2.5 cm diameter and different heights made from perspex were experimentally studied placing the XP 22 light emitting diode in a small hole at the centre of the top of the samples. The samples, coated with NE 560 reflecting paint, aluminum foil and just polished ones were optically coupled to the photocathode of a C 31024 photomultiplier. The overall time resolution of the system, measured with single photoelectrons, was 300 ps for 2.5 cm diameter of the central part of the photocathode. The performed study showed that the light collection process introduces a large time spread which limits very seriously the time resolution obtainable with scintillation counters. The standard deviation of the photons transit time in the 2.5 × 2.5 cm 2 sample, coated with NE 560 paint, was measured to be equal to 0.5 ns, thus significantly larger than the time jitter of fast photomultipliers. The obtained data on the light collection process in the samples coated with NE 560 paint confirmed Nutt's predictions that the probability density function of the transit time of photons is represented well by a single exponential function. The standard deviation σ L is proportional to the height of the samples for a fixed diameter and depends on the quality of reflecting surfaces. To interpret larger values of σ L found in the experiment compared to calculated ones, the back reflections from the boundary between the glass window of the photomultiplier and the photocathode itself are postulated. It was shown that, in the samples coated with the reflecting paint, light is mainly collected due to randomly distributed reflections from the paint. Thus the measured characteristics for the point light source may be also representative for the light generated in the whole volume of the sample. The light collection process in the sample coated with aluminum foil is more complex and the presented data show mainly the transmission properties of these samples. But it was shown that for that purpose the polished samples introduce the lowest time jitter.