  In this work, we will present a physical model and measurements of the
transport of small charge packets in the bulk of thick high resistivity CCD
before being collected by the pixel potential wells. A new technique to measure
the lateral spread of the charge as a function of the ionization depth in the
bulk is presented. Results from measurements on CCD currently in use for
several scientific instruments are shown and validated with a new mathematical
algorithm to extend the current modeling based only on the diffusion of the
charge in silicon.

Instrumentation and Detectors;High Energy Physics - Experiment;Nuclear Experiment