**3. Radiation environment on the Moon and its effect on materials**

### **3.1 Radiation environment on the lunar surface**

The radiation environment on the Moon is constituted, apart from solar electromagnetic radiation, by three radiation "populations"—the constant solar wind, the intense but sporadic Solar Energetic Particles (SEPs), and the constant background of Galactic Cosmic Rays (GCRs). A summary of the radiation environment on the lunar surface is given in **Table 1**.

The solar wind is a constant flux of plasma from the upper atmosphere of the Sun. It consists mainly of ionized hydrogen (protons and electrons), a small percentage of *α*-particles, and trace amounts of heavier ions, with kinetic energy between 0.5 and 2 keV/nucleon [75]. The solar wind flux, temperature, density, and speed vary over time and solar longitude and latitude. The lunar surface is under continuous bombardment by the solar wind, as the Moon does not have a significant global geomagnetic field that could deflect solar particles. Particles penetrate the surface and undergo collisions with the ions of the lunar regolith. Their penetration depth depends on the impact energy, angle of incidence, and composition of the target surface. For a proton with a nominal energy of 1 keV, the penetration depth is typically about 20 nm [79]. The implanted protons diffuse and chemically combine with the regolith atoms, such as oxygen, or become trapped in physical defects. Recent studies have suggested that the implantation of solar wind protons in the lunar regolith is a major source of hydrogen in the formation of OH/H2O [80, 81], whose presence is confirmed by experimental measurements [82].

SEPs originate from solar transient events, such as coronal mass ejections or flares, and consist in a sudden intense flux of high-energy protons and electrons (and a small amount of *α*-particles and heavier ions) [76, 78]. Typical energies of SEPs range from ten to hundreds of MeV. Such transient events have a higher occurrence probability during solar maximum, but they may also occur during solar minimum. Studies have shown that the lunar surface can charge to a high negative potential up to a few kV during SEP events [83, 84]. Such values of the potentials are much higher than the typical night-side potentials of a few hundred volts negative and may increase the risk of electrostatic discharge. The latter represents an additional hazard to the already dangerous radiation environment on the lunar surface.

GCRs constitute the slowly varying, low-intensity (few particles/cm<sup>2</sup> (m2 ) per second), highly-energetic radiation background in space. They are mainly


#### **Table 1.**

*Radiation particle types, their flux, and energies on the lunar surface [72–78].*

*Modeling Radiation Damage in Materials Relevant for Exploration and Settlement on the Moon DOI: http://dx.doi.org/10.5772/intechopen.102808*

associated with supernova explosions in the galaxy, but extra-galactic contributions also exist. GCRs are constituted by 87% of hydrogen ions (protons), 12% of *α*particles, 1–2% of high-energy and highly charged ions (high-charge Z and energy (HZE)-particles), and 1% of electrons and positrons [85]. The energy spectrum of GCRs covers a wide range, extending roughly up to 10<sup>18</sup> eV, with higher energies (up to 10<sup>21</sup> eV) being associated with ultrahigh-energy GCRs originating from extra-galactic sources. GCRs are modulated by the heliospheric field linked to solar activity. At solar maximum, the solar magnetic field increases, shielding the heliosphere from the lowest energy component of GCRs [86], thus decreasing the overall GCRs flux. At the solar minimum, the reduced solar magnetic field leads to a more intense GCRs flux in our interplanetary space [87, 88].

The annual exposure caused by GCRs on the lunar surface is 380 mSv during solar minimum and 110 mSv during solar maximum, as compared to the annual dose of natural ionizing radiation of 2.4 mSv on Earth [89] (1 Sv—1 Sievert, represents the equivalent biological effect of the deposit of a Joule of radiation energy in a kilogram of human tissue). The worst-case scenario studies suggest that SEPs may lead to a much higher exposure of 1 Sv or even reach > 2 Sv per event [90]. Studies of the radiation dose of GCRs and SEPs at the lunar surface and in a lava tube [90, 91] have shown that the exposure may be reduced to values similar to Earth in horizontal lava tubes.

#### **3.2 Radiation-induced effects in materials**

The effects of radiation on materials and devices can be cumulative (long term) and noncumulative (caused even by a single particle). The so-called Single Event Effects (SEEs) can occur when an ionizing particle passing through an electronic device carries a charge large enough to affect the device's performance. SEEs in aerospace technology can lead to errors, corrupt the data, create noise, reset the device, or even cause fatal part failure [92–95]. Cumulative radiation damage, on the other hand, occurs through continuous radiation exposure or exposure to intense flux due to SEPs events and can lead to the degradation of optical components and solar cells, eventually causing permanent damage. The total ionizing dose experienced by an electronic device can cause variations in threshold voltage or leakage current.

Cumulative non-ionizing damage in materials due to protons, electrons, and neutrons (originating from the interaction of energetic protons and electrons with the lunar surface) leads to defect formation (displacement damage) [94]. The types and sources of radiation, as well as the effects it can cause in materials, are summarized in **Table 2**.


#### **Table 2.**

*Sources and types of radiation and the effects it causes in materials and devices [96].*

Cumulative radiation damage is a multiscale process in terms of time and length. A schematic representation of the so-called displacement damage cascade is shown in **Figure 1**. At first, an energetic external particle approaches (**Figure 1**(1)) and enters the target (**Figure 1**(2)). As the particle passes through the material, it first transfers its kinetic energy to electronic degrees of freedom of the target (electronic stopping) (**Figure 1**(3)). Electronic excitations happen at a very short time scale (100 as). After the particle has been slowed down by the target's electrons, it undergoes nuclear elastic collisions, displacing atoms in the target (Primary Knock-on Atoms, PKAs) that constitute themselves additional projectiles (**Figure 1**(4)). The PKA collides with other atoms creating a cascade of collisions [97] (**Figure 1**(5)). Atomic displacements induce the creation of different types of point defects, such as vacancies and interstitials (Frenkel pairs) and defect clusters (**Figure 1**(6)) and happen on a much longer time scale (up to ns). Eventually, many defects are healed due to the thermal motion of atoms (annealing stage, **Figure 1**(7)), leaving a finite number of defects in the structure (**Figure 1**(8)).

Atomic displacements described above lead to defect clustering and eventual amorphization in crystalline materials. Consequently, mechanical, physical, and other properties of the irradiated material can be significantly altered. The scale of the changes depends on the energy of incoming particles and the actual number and spatial distribution of survived defects after eventual self-healing [98].

The radiation-induced effects after atomic displacements strongly depend on the type of material. For metals and metallic alloys, the main effect of radiation is the generation of dislocation loops and point defects which cause significant radiationinduced strengthening or hardening. As a result, the ductility and fracture toughness of the metals (alloys) can be reduced, leading to brittle behavior [99]. Ductileto-brittle transition is especially pronounced at low temperatures at which the defect mobility, and consequently the annealing of defects, is reduced.

As to other materials, such as semiconductors in solar cells, cumulative exposure to space radiation or high SEPs fluxes can strongly affect the performance of MJ solar cells [100]. Moreover, the impacting radiation can reduce the transmittance of the protective SiO2 cover-glass on top of MJ cells by inducing color centers in the oxide material. The color centers appear when electrons excited by radiation become trapped by impurities in the oxide to form stable defect complexes. On the other hand, the radiation which is not blocked by the cover-glass causes damage in the functional layers of MJ solar cells by displacing atoms. Different energy levels can be created within the bandgap as a consequence of such structural defects. Such

*Modeling Radiation Damage in Materials Relevant for Exploration and Settlement on the Moon DOI: http://dx.doi.org/10.5772/intechopen.102808*

electronic defect levels affect the electrical performance of MJ solar cells acting as traps, recombination centers, or carrier removal sites which reduce free carrier concentration [100, 101].

Below, we will present different methods used to describe radiation-induced effects in materials focusing on the description of cumulative effects related to atomic displacements.
