  We have characterized in depth the time response of three detectors equipped
with cylindrical LaBr$_{3}$ (Ce) crystals with dimensions of 1-in. in height
and 1-in. in diameter, and having nominal Ce doping concentration of 5%, 8% and
10%. Measurements were performed at $^{60}$Co and $^{22}$Na {\gamma}-ray
energies against a fast BaF$_{2}$ reference detector. The time resolution was
optimized by the choice of the photomultiplier bias voltage and the fine tuning
of the parameters of the constant fraction discriminator, namely the
zero-crossing and the external delay. We report here on the optimal time
resolution of the three crystals. It is observed that timing properties are
influenced by the amount of Ce doping and the crystal homogeneity. For the
crystal with 8% of Ce doping the use of the ORTEC 935 CFD at very shorts delays
in addition to the Hamamatsu R9779 PMT has made it possible to improve the
LaBr$_{3}$(Ce) time resolution from the best literature value at 60Co photon
energies to below 100 ps.
