**2. Ionizing photon interaction with gold nanoparticles**

The three primary interactions of photons with matter include the photoelectric effect, Compton scattering, and pair production [30]. In a photoelectric event, a photon completely transfers its energy to an orbital electron, ejecting it from its shell and ionizing the atom. This is the dominant interaction mechanism for lowenergy (E < ≈0.5 MeV) photons. In Compton scattering, an incident photon transfers a portion of its energy to an orbital electron, and both the electron and photon continue on, typically with different trajectories when compared to the incoming photon. This will be the dominant interaction mechanism for incident photons of intermediate energies (≈0.5 MeV < E < ≈2 MeV). Finally, pair production dominates for high-energy photons and can only occur for incident photons greater than 1.022 MeV. In pair production, a photon interacts with the electromagnetic field surrounding the atomic nucleus and is converted into an electron and a positron, with the total photon energy less the rest energy of the two particles (0.511 MeV each) being shared between them. It should be noted that for high photon energies

(>≈8 MeV), photodisintegration can also occur, in which the incident photon causes the atom to emit one or more neutrons, potentially resulting in a radioactive isotope of gold [31].

Since incident photons interact primarily with the orbital electrons, the high atomic number (79) and density (19.32 g/cc) of gold significantly increase the likelihood of interaction compared to most other materials. The creation of secondary ionizing radiation through these photon interactions is the primary reason for the attractiveness of gold nanoparticles in radiation therapy applications [10]. Electrons and positrons resulting from the described interactions will lose their energy as Bremsstrahlung radiation (also known as braking radiation) as they slow down, and X-rays and Auger electrons will be produced as electrons shift from higher-energy orbitals to replace those ionized from lower-energy orbital positions. Furthermore, positrons generated from pair production will eventually annihilate with another electron, resulting in the generation of two 0.511 MeV photons. An illustration of these potential interaction mechanisms and resulting secondary radiation effects is shown in **Figure 3** [10]. This secondary radiation will typically deposit its energy in the vicinity of the gold particles, which can be taken advantage of to increase dose to cancerous cells [10, 11].

Photon irradiation experiments involving gold nanoparticles are prevalent, as X-ray therapies for cancer treatment are a fairly common practice [32–34]. While these studies are predominantly focused on the biological effects in the vicinity of cancerous tissue, they highlight how photons interact with these high atomic number (high-Z) nanoparticles to deposit their energy locally. High-energy photons are also expected to cause Frenkel pair displacement damage (i.e., vacancy and interstitial defect pairs), primarily through the energetic electrons that they tend to generate. However, the effects of these displacement effects on the long-term stability of nanoparticle structures, if any, have not been studied to our knowledge.

#### **Figure 3.**

*Schematic illustrating potential interactions of incident photons with a gold atom or other high-Z materials [10]. Source: http://tcr.amegroups.com/article/view/1550/html. AME Publishing Company. Republished with permission of Pioneer Bioscience Publishing Company, from D. Kwatra et al., Translational Cancer Research, 2(4), pp. 332, 2013; permission conveyed through Copyright Clearance Center, Inc.*

**57**

*Evolution of Gold Nanoparticles in Radiation Environments*

**3. Neutron interaction with gold nanoparticles**

In addition to understanding the role of radiation on the nanoparticle and the surrounding material, a few research groups have shown that gamma and other forms of ionizing photons can even be used to induce nucleation and growth of nanoparticles out of solution including far-from-equilibrium structures that might be difficult to obtain through other more classical chemical synthesis routes [35, 36]. Most studies and reviews of the response of gold nanoparticles to various stimuli limit themselves to ionizing radiation produced by photons [37]. To expand these previous reviews and studies, the remainder of this chapter will focus on neutron, beta, alpha, and heavier charged particle irradiation effects on gold

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

Charged particles are common by-products of radioactive decay and nuclear reactions and are a primary component of cosmic radiation. Accelerators and ion beams are also common ion sources used in both research and industry. For example, ions are frequently used in radiation damage experiments to simulate material microstructures resulting from neutron radiation exposure in nuclear

enhance the effectiveness of neutron radiation therapies [40].

**4. Beta particle interaction with gold nanoparticles**

*DOI: http://dx.doi.org/10.5772/intechopen.80366*

nanoparticles.

*Evolution of Gold Nanoparticles in Radiation Environments DOI: http://dx.doi.org/10.5772/intechopen.80366*

*Gold Nanoparticles - Reaching New Heights*

of gold [31].

to cancerous cells [10, 11].

(>≈8 MeV), photodisintegration can also occur, in which the incident photon causes the atom to emit one or more neutrons, potentially resulting in a radioactive isotope

Since incident photons interact primarily with the orbital electrons, the high atomic number (79) and density (19.32 g/cc) of gold significantly increase the likelihood of interaction compared to most other materials. The creation of secondary ionizing radiation through these photon interactions is the primary reason for the attractiveness of gold nanoparticles in radiation therapy applications [10]. Electrons and positrons resulting from the described interactions will lose their energy as Bremsstrahlung radiation (also known as braking radiation) as they slow down, and X-rays and Auger electrons will be produced as electrons shift from higher-energy orbitals to replace those ionized from lower-energy orbital positions. Furthermore, positrons generated from pair production will eventually annihilate with another electron, resulting in the generation of two 0.511 MeV photons. An illustration of these potential interaction mechanisms and resulting secondary radiation effects is shown in **Figure 3** [10]. This secondary radiation will typically deposit its energy in the vicinity of the gold particles, which can be taken advantage of to increase dose

Photon irradiation experiments involving gold nanoparticles are prevalent, as X-ray therapies for cancer treatment are a fairly common practice [32–34]. While these studies are predominantly focused on the biological effects in the vicinity of cancerous tissue, they highlight how photons interact with these high atomic number (high-Z) nanoparticles to deposit their energy locally. High-energy photons are also expected to cause Frenkel pair displacement damage (i.e., vacancy and interstitial defect pairs), primarily through the energetic electrons that they tend to generate. However, the effects of these displacement effects on the long-term stability of nanoparticle structures, if any, have not been studied to our knowledge.

*Schematic illustrating potential interactions of incident photons with a gold atom or other high-Z materials [10]. Source: http://tcr.amegroups.com/article/view/1550/html. AME Publishing Company. Republished with permission of Pioneer Bioscience Publishing Company, from D. Kwatra et al., Translational Cancer Research, 2(4), pp. 332,* 

*2013; permission conveyed through Copyright Clearance Center, Inc.*

**56**

**Figure 3.**

In addition to understanding the role of radiation on the nanoparticle and the surrounding material, a few research groups have shown that gamma and other forms of ionizing photons can even be used to induce nucleation and growth of nanoparticles out of solution including far-from-equilibrium structures that might be difficult to obtain through other more classical chemical synthesis routes [35, 36]. Most studies and reviews of the response of gold nanoparticles to various stimuli limit themselves to ionizing radiation produced by photons [37]. To expand these previous reviews and studies, the remainder of this chapter will focus on neutron, beta, alpha, and heavier charged particle irradiation effects on gold nanoparticles.
