**3. Neutron interaction with gold nanoparticles**

To understand the interactions that can possibly occur between an energetic neutron and a gold nanoparticle, one must first understand the well-studied interaction of a neutron and a gold atom. As gold is monoisotopic, Au-197 is assumed as a target for the purposes of gauging probable interactions. Thus, neutron interactions with gold atoms are dominated either by radiative capture (n,γ) reactions or by scattering. Capture reactions are most prevalent for thermal neutron spectra (E < 1 eV), though there is a notable resonance absorption peak for energies near 4.9 eV and several other resonances between 60 eV and 2 keV [38]. This absorption reaction coincides with emission of a gamma ray with an energy between 4.78 and 6.52 MeV [39] and results in an Au-198 nucleus that subsequently decays to stable Hg-198 via beta particle emission with an energy release of 1.37 MeV and a half-life of 2.7 days. The reaction can also potentially result in two metastable states of Au-198 that emit 312 and 811 keV gamma rays with half-lives of 124 ns and 2.3 days, respectively.

Scattering interactions are more common for higher energy neutron spectra. Due to the large difference in mass between neutrons and gold nuclei, neutrons undergoing elastic scattering interactions (i.e., ballistic collisions) only transfer approximately 1% of their energy to the nucleus [1]. Inelastic scattering interactions, though less common, can transfer significantly more energy, leaving the nucleus in an excited state that results in gamma ray production. With sufficiently energetic incident neutrons, both scattering mechanisms are capable of generating gold primary knock-on atoms (PKAs) and generating displacement damage in the material microstructure.

No evidence of experimental work investigating neutron effects on individual particle structure or morphology was found, likely due to the challenges associated with working with neutron beams or research reactors and the high fluences required to induce appreciable damage. The mean free path (average distance traveled before interaction) of a neutron is much higher than that of a charged particle due to the lack of Coulombic interaction (see **Figure 2**), such that a vast majority of neutrons incident on a gold nanoparticle are not expected to result in a damage event. However, local injection of gold nanoparticles has recently been shown to enhance the effectiveness of neutron radiation therapies [40].
