A zero-cross timing discriminator for pure CsI calorimeter

A new type of timing discriminator module was designed for use with phototube signals from a pure CsI calorimeter. The crucial point is the handling of the slow component in pure CsI signals at a high repetition rate. The zero-crossing method was used for time pick-off to circumvent the pile-up effect due to slow signals. In order to achieve the best performance, a circuit was designed in such a way that the comparator works in the optimum way. The performance of the discriminator was tested and compared with a commercially available CFT (constant-fraction timing) discriminator. It was found that the time walk was less than a few hundred picosecond for a pulse height range from 20 mV to 4 V and that the discriminator works better than the CFT discriminator. The delay time variation was less than 300 ps and less than 100 ps for repetition rate variation and baseline shift, respectively