  During the test measurements of the PMTs to be used in the future Borexino
experiment, the data from 2200 PMT were accumulated. The unique timing
characteristics of the apparatus used and the large statistics acquired, permit
resololution of the fine structure of the ETL9351 PMT time response.
  A method to obtain the probability density function of the single
photoelectron counting from the experimental data is proposed and applied to
derive the PMT average characteristics. For the first time, an analytical model
of the single photoelectron PMT time response is proposed, describing all the
features of the single photoelectron time arrival. The origin of the small
amplitude pulses is explained.
