Silica nanoparticle core structure examined by the E′Si_γ center ²⁹Si strong hyperfine interaction

β-Ray irradiation up to 1.2 GGy was employed to induce E′Si_γ defects and to study the structure of silica nanoparticles with diameters from 7 up to 20 nm. Defect concentration and their ²⁹Si strong hyperfine doublet were investigated through electron paramagnetic resonance measurements. Our data indicate that stable defects are located in the nanoparticle core. Furthermore, the E′Si_γ hyperfine interaction evidences that the core structure is denser than bulk silica and independent from the particle size. Finally, we put in evidence that the core structure is stable and unaffected by the irradiation in the investigated dose range maintaining the specific features of nanoparticles.