Charge accumulation in an insulator, as achieved by electron irradiation in a SEM, is governed by complex phenomena. The understanding of the relevant processes (the generation of free carriers, transport and trapping/de-trapping) is a prerequisite for the design of materials having improved breakdown strengths. A simple modeling, which meets the planar geometry conditions, is proposed. The predictions of the evolution of secondary electron emission yield (as function of the quantity of trapped charges) agree well with the experimental results at room temperature, in pure amorphous SiO2. The agreement is obtained with trapping cross-sections near 10-10 cm2, which are much higher than the typical values (about 10-16 cm2). Trapping appears as an “effective” process that reflects the Brownian motion of secondary electrons.