-77], [Lera-89a], [Peyr-91]. For time scales of the order of irradiation experiments, [Wino-94] suggests to use an activation energy of about 0.4 eV. III-22 It must be noted that such polaron behavior has been investigated for electrons, but no evidence has been found [Othm-80]. R.C. Hughes made thorough investigations on SiO2 photocurrents, and mesured the dependence of electron drift velocity versus electric field (fig. 16). It is similar with other type of semiconductors, except that .02 cm the critical field is higher (200 kV/cm). This 3-.04 cm 1-.10 cm is attributed to the high value of the optical phonons in this material. Ferry calculated the theoretical value of the saturation velocity (2 107 cm s-1 [Ferr], which is in good agreement with the experimental value of Hughes [Hugh-73, Hugh-78]. E (v/cm) Figure 16. Drift velocity of electrons in SiO2 versus electric field for three thicknesses of Suprasil-II glass [Hugh-75]. The slope is the electron mobility of 21+2 cm2 V-Superscript(1) 3.6.3. MODELING OF THE EARLY REGIME OF MOTION AND THE SELF TRAPPING OF HOLES For "free" holes (called also "dry holes" as opposed to "self-trapped" holes or "polaron"), Hughes introduced and indirectly measured a lifetime p0 before it becomes self trapped. For this to achieve, he measures the product p0 1.4 10-12 cm2 V-1 by integrating the current released in thermal silicon dioxide irradiated by 3 nanosecond pulses [Hugh-78]. This value is also found in other type of experiments [Curt-75, Boes-76, Srou-76, Srou-77a]. Hughes derived a value for p0 by theoretical estimation of , based on the value of the mass attributed to the free holes. With =1 cm2 it gives p0 1.4 10-12 S which is a value compatible with the self-trapping phenomenology [Hugh-98]. So, if we note Po and Jp0 the "dry" hole density and current: In steady state, the first term can be neglected (it requires that the dose rate does not vary much in time comparable with p0, which might be questionable in case of heavy ion strike, for instance). If we neglect diffusio