Fast Charged-Particle Irradiation of Solids: Excitation of Secondary Electrons and Related Energy Deposition Function in SiO2

The secondary electrons excited during fast charged particle irradiation of solids play a relevant role in track formation which are variously attributed to: 1) inhomogeneous charge distribution leading to ion-explosion, 2) thermal-spike effects which stem from energy dissipation of the secondary electrons, and 3) self-trapping of an exciton, which occurs when the exciton-lattice coupling overcomes the freedom of translational motion of the exciton thus localizing the electronic excitation energy by introducing local lattice distortion. Here, we analyse the problem of the excitation of secondary electrons in atoms and solids when the electronic stopping power of the projectile prevails over the nuclear stopping power. Neither energy nor emission angle of the excited electrons are well established quantities even if some trends in experimental results may be recognized. What is important is that the spread of energy is between a few eV to some KeV: this is a relevant point when one tries to quantify the effects of secondary electrons in solids. The emission angle, on the contrary, seems not to be a main problem because the elastic collisions of the emitted electrons randomize their trajectories. We quantitatively analyse the problem of the energy deposition function of the secondary electrons excited in SiO2 during fast charged-particle irradiation. Monte Carlo calculations are performed to evaluate the energy deposition function of electrons in SiO2. In so doing, we face the problem of the relevance of the thermal-spike model, widely utilized in the current literature, showing that it is basically unjustified