Performance evaluation of LaBr3(Ce) crystal geometries designed for fast timing applications

LaBr3(Ce) crystals unite high gamma detection efficiency, good energy resolution and fast response, and have become the standard in many gamma-ray measurements. Nowadays they are the key ingredient in the application of Ultra Fast Timing method allowing the experimental determination of nuclear lifetimes of excited nuclear states in the range of few nanoseconds down to tens of picoseconds. The size and shape of the crystals have a strong impact on the time resolution. We have designed special geometries aimed at improving the light collection and thus the time response, and also enhancing the packing factor for the construction ring arrays around a fixed target. In this work we have assess the performance of two specially-designed LaBr
₃
(Ce) crystals, a truncated cone shape (with 1 in. height and bases of 1.5 in. and 1 in.) and a tapered crystal, and compared them to a standard 1-inch cylindrical one. Analogue signal processing via CFD and TAC is used for the timing. The external CFD delay, the CFD zero-crossing value (Z), and the high voltage have been optimised, leading to excellent time resolutions at
⁶⁰
Co and
²²
Na comparable to those of the smaller 1-inch cylindrical LaBr
₃
(Ce) crystal. Monte Carlo simulations of optical photons have been performed using the Geant4 toolkit in order to test the effect of the geometry and the doping in the LaBr3(Ce) crystals, and to compare to the experimental results. These simulations are in reasonable agreement with the experiment and allow a deeper understanding of the most influential factors affecting the time and energy resolution of the detectors.