**1. Introduction**

Ionizing radiation is known to have the ability to drastically alter material microstructure and performance primarily through the displacement of constituent atoms from their lattice sites, resulting in the generation of damage and point defects [1, 2]. These point defects tend to diffuse and coalesce into larger, ordered defect structures such as dislocation loops, cavities, and stacking faults, typically resulting in deleterious effects in structural materials such as radiation-induced hardening and embrittlement or void swelling [1, 2]. Irradiation can also cause solute redistribution in alloys and composite materials, encouraging the precipitation of secondary phases or promoting localized corrosion, which has been utilized in metal, ceramics, and polymers to provide added functionality not previously possible [3–8]. The fundamental mechanisms of radiation damage and the subsequent effects on materials properties and performance have been and continue to be heavily studied in commonly used and candidate materials for nuclear reactor and extraterrestrial applications.

While the potential effects of ionizing radiation environments are not commonly considered when discussing nanoparticle performance, there are a subset of both current and potential future applications for gold nanoparticles that

necessitate investigation and understanding of how their structure and properties change when exposed to energetic photons and particles. Radiation therapy is likely one of the most notable examples, in which gold nanoparticle injections in the vicinity of a tumor or conjugation to antibodies such that they preferentially bind to cancer cells have been shown to increase local radiation dose during X-ray irradiation through secondary photon interactions, thus lessening the absorbed dose in surrounding healthy tissue [9–13]. A similar local dose enhancement effect has been shown for proton and heavy ion irradiation therapies, which are being pursued to further reduce dose to healthy tissue as a consequence of how energetic charged particles deposit a majority of their energy at the end of their range [14–16]. Furthermore, potential applications in satellites or space electronics would involve bombardment with high-energy cosmic radiation [17, 18].

The mechanism of radiation damage in materials with limited dimensions has been shown to be significantly different from bulk material behavior [19–23]. Due to the prodigious surface-to-volume ratio of nanoparticles compared to even thin films, they have emerged as an attractive material choice for many applications. However, this same surface-to-volume ratio results in sputtering and free surface effects ultimately dominating the radiation response [20, 21]. Put simply, instead of diffusing into organized defect structures, the generated point defects tend to annihilate at the particle surface, or material is ejected from the particle volume, as a result of the energetic collision [24, 25]. This can drastically alter the shape of individual particles and cause agglomeration of closely spaced particle groupings [26–28]. Such morphological changes have the potential to adversely affect their efficacy in applications where the high surface-to-volume ratio and local structure are essential for performance.

This chapter attempts to summarize the current body of work relating to interactions of ionizing radiation with gold nanoparticles. Broader reviews of the interaction of nanostructured materials can be found in the detailed reviews by A.V. Krasheninnikov and K. Nordlund titled "Ion and electron irradiation-induced effects in nanostructured materials" [21] and the more recent review by X. Zhang et al. titled "Radiation damage in nanostructured materials" [29]. As one might expect, the interactions of radiation with elemental gold is highly dependent on the species of the incident radiation. Both the length scales of interaction and energy transferred per interaction event can vary widely due to the energy, mass, and charge of incident radiation, as highlighted in **Figures 1** and **2**. As such, this chapter

#### **Figure 1.**

*Illustration of the relative average interaction length scales for various types of radiation at an energy of 1 MeV incident on gold. Dotted lines indicate the mean free path of the incident particles. Positive and negative signs indicate ionization events, while blue highlighted areas denote displacement damage cascades.*

**55**

**Figure 2.**

*various particle types with a gold atom.*

*Evolution of Gold Nanoparticles in Radiation Environments*

is organized according to the type of radiation environment a gold nanoparticle might encounter, and the subsequent sections highlight experimental studies of how the structure and properties of gold nanoparticles are altered by bombardment with specific types of energetic radiation. Section 2 discusses ionizing photon interactions, while Section 3 considers neutron environments. Sections 4–6 focus on different types of charged particle irradiation: beta particles (electrons/positrons), light ions (protons through alpha particles), and heavy ions. Section 6 also explores potential applications of ion beam modification in both freestanding and embedded gold nanoparticles from heavy ion bombardment. Finally, Section 7 discusses future experimental and modeling work needed to enhance our understanding of radiation effects in gold nanoparticles and speculates as to potential future applications

*Plot showing the theoretical maximum fraction of incident particle energy transferred via a ballistic collision of* 

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

combining gold nanoparticles with radiation environments.

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

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

#### **Figure 2.**

*Gold Nanoparticles - Reaching New Heights*

necessitate investigation and understanding of how their structure and properties change when exposed to energetic photons and particles. Radiation therapy is likely one of the most notable examples, in which gold nanoparticle injections in the vicinity of a tumor or conjugation to antibodies such that they preferentially bind to cancer cells have been shown to increase local radiation dose during X-ray irradiation through secondary photon interactions, thus lessening the absorbed dose in surrounding healthy tissue [9–13]. A similar local dose enhancement effect has been shown for proton and heavy ion irradiation therapies, which are being pursued to further reduce dose to healthy tissue as a consequence of how energetic charged particles deposit a majority of their energy at the end of their range [14–16]. Furthermore, potential applications in satellites or space electronics would involve

The mechanism of radiation damage in materials with limited dimensions has been shown to be significantly different from bulk material behavior [19–23]. Due to the prodigious surface-to-volume ratio of nanoparticles compared to even thin films, they have emerged as an attractive material choice for many applications. However, this same surface-to-volume ratio results in sputtering and free surface effects

ultimately dominating the radiation response [20, 21]. Put simply, instead of diffusing into organized defect structures, the generated point defects tend to annihilate at the particle surface, or material is ejected from the particle volume, as a result of the energetic collision [24, 25]. This can drastically alter the shape of individual particles and cause agglomeration of closely spaced particle groupings [26–28]. Such morphological changes have the potential to adversely affect their efficacy in applications where the high surface-to-volume ratio and local structure are essential for performance. This chapter attempts to summarize the current body of work relating to interactions of ionizing radiation with gold nanoparticles. Broader reviews of the interaction of nanostructured materials can be found in the detailed reviews by A.V. Krasheninnikov and K. Nordlund titled "Ion and electron irradiation-induced effects in nanostructured materials" [21] and the more recent review by X. Zhang et al. titled "Radiation damage in nanostructured materials" [29]. As one might expect, the interactions of radiation with elemental gold is highly dependent on the species of the incident radiation. Both the length scales of interaction and energy transferred per interaction event can vary widely due to the energy, mass, and charge of incident radiation, as highlighted in **Figures 1** and **2**. As such, this chapter

*Illustration of the relative average interaction length scales for various types of radiation at an energy of 1 MeV incident on gold. Dotted lines indicate the mean free path of the incident particles. Positive and negative signs* 

*indicate ionization events, while blue highlighted areas denote displacement damage cascades.*

bombardment with high-energy cosmic radiation [17, 18].

**54**

**Figure 1.**

*Plot showing the theoretical maximum fraction of incident particle energy transferred via a ballistic collision of various particle types with a gold atom.*

is organized according to the type of radiation environment a gold nanoparticle might encounter, and the subsequent sections highlight experimental studies of how the structure and properties of gold nanoparticles are altered by bombardment with specific types of energetic radiation. Section 2 discusses ionizing photon interactions, while Section 3 considers neutron environments. Sections 4–6 focus on different types of charged particle irradiation: beta particles (electrons/positrons), light ions (protons through alpha particles), and heavy ions. Section 6 also explores potential applications of ion beam modification in both freestanding and embedded gold nanoparticles from heavy ion bombardment. Finally, Section 7 discusses future experimental and modeling work needed to enhance our understanding of radiation effects in gold nanoparticles and speculates as to potential future applications combining gold nanoparticles with radiation environments.
