Figure 2. Comparison of track structure of ionising events either on the surface (solid lines) or in the bulk (dashed lines) of a 20 nm spherical gold nanoparticle, plotted both in 3D (plot a), and as a 2D projection (plot b).

Here, an incident 50 keV photon (green tracks) interacts with the gold nanoparticle and ejects a number of electrons (red tracks). For the event which occurs in the bulk, the majority of low-energy electrons are stopped immediately in the nanoparticle, allowing only the most energetic and sparsely ionising electrons to escape. By contrast, the surface event also produces a very large shower of low energy electrons who deposit their energy very densely in the vicinity of the nanoparticle, leading to high doses and many ionising events in a small volume.