**5. Light ion interaction with gold nanoparticles**

For the purpose of this chapter, light ions will be defined as energetic particles as light as proton or as heavy as helium ions (alpha particles). Alpha particles are common products of radioactive decay for actinides and other heavy radioactive isotopes, but, along with protons, deuterons, and tritons, they can also result from and induce a number of different nuclear reactions. For example, irradiation of Au-197 with protons with energies of 4.5 MeV or higher can cause a (p,n) reaction resulting in the production of metastable Hg-197m [46]. However, threshold energies of these reactions for gold are, in most cases, sufficiently high and reaction cross sections sufficiently low that these types of interactions rarely occur in practical applications. More often, light ions will interact via ionization and through Coulombic forces. Similar to the other types of interactions discussed, ionization has the potential to result in the emission of characteristic X-rays and other secondary radiations. Coulombic interactions with other atomic nuclei can tend to cause displacement damage, usually in the form of Frenkel pairs or small, isolated cascades.

Proton irradiation experiments have commonly been conducted in the context of increasing local dose for proton therapy-based applications [16, 47, 48]. Again, the primary mechanism for this dose enhancement comes from local energy deposition from ionization and secondary radiation that is produced. He irradiation experiments are not typically performed, but the effect can be assumed to be similar. Evidence of the effects of proton and He irradiation on individual gold nanoparticle structure is lacking in the literature.
