  Many characteristics of Silicon Photomultipliers can be tuned with
temperature and operation voltage. We present preliminary results of a study of
the effect of these two operation parameters on the time resolution of large
active area Multi-Pixel Photon Counters (MPPCs) manufactured by Hamamatsu.
Measurements at -10 C, 0 C, and 10 C at different bias voltages were performed.
The time resolution is best at low temperature and high over-voltage. Most
significant improvements can be achieved in cases with low number of fired
pixels (<10 pixels). Between the worst and best case among the considered
conditions a factor of 5 improvement was observed. In cases with large number
of fired pixels (>40 pixels) the effect of temperature and operation voltage
becomes smaller. The timing performance still improves with decreasing
temperature (~ factor of 2) but it hardly depends on the operation voltage. The
study shows, that especially in applications where only few photons are
available for detection a careful optimization of temperature and operation
voltage are advisable to obtain optimum timing results with the MPPC.
