  The direct response of Silicon PhotoMultipliers being traversed by a MIP
charged particle have been studied in a systematic way for the first time.
Using beam test data, time resolution and the crosstalk probability have been
measured. A characterization of the SiPM by means of a laser beam is also
reported. The results obtained for different sensors indicate a measured time
resolution around 40-70 ps. Although particles are expected to traverse only
one SPAD per event, crosstalk measurements on different sensors indicate an
unexpected higher value with respect to the one related to the sensor noise.
