## 2 Degradation of Materials in Space--Stability Issues on Mirrors
### Charged Particles
In fact, highly energetic photons as well as MeV particles can easily penetrate mm thicknesses of materials, undergoing a deceleration in the case of particles and,in general, producing secondary photon and particle emissions. By their nature, secondary particles have to be analyzed on a case-by-case basis through Monte-Carlo simulation in order to obtain global information that can be used during the design and testing procedures. For this reason, spacecraft requirements always include a total ionizing dose (TID) specification (expressed in krad), a value that corresponds to the total energy deposited in matter by ionizing radiation per unit of mass. By definition, TID is an integral dose, and therefore it takes into account the cumulative effect due to particles of different energies. The ground validation of the spacecraft components is then usually performed by evaluating the effect given by a specific TID, deposited via acceleration facilities. Although this approach is suitable for testing the radiation hardness of an electronic component or investigating the degradation of the opto-mechanical properties of bulk materials, it becomes inappropriate for the optical coatings because the effects occurring in the thin films strongly depend on the proton energy and therefore the implantation depth of particles. High-energy particles penetrate deeper in the optical components, in the order of tens of \(\upmu\)m or more, interacting very little with the nanometric coatings and depositing all the energy in the substrate. In contrast, keV ions implant within the coatings with a profile highly dependent on their density, potentially inducing changes of their optical, structural, and morphological properties. As a general rule, we can affirm that thin films in the nanometric scale are mostly affected by low energy particles, that implant in the coating itself, but not by MeV particles, which overcome the structure, eventually reaching the substrate. For instance, considering the typical materials employed for a mirror coating, protons at 1 keV have a penetration peak at a depth of 20-30 nm; at 16 keV, the implantation peak is at 150-200 nm; at 50 keV, it is at 300-500 nm; and at 100 keV, it is around 700-900 nm. In contrast, protons at 1 MeV already penetrate the coating for different \(\upmu\)m. Experiments with MeV electrons and protons with typical fluences faced in the space environment (i.e., \(<\)10\({}^{12}\) ions/cm\({}^{2}\)) have proven negligible degradation effects on optical coatings having a total thickness lower than few microns in the visible-UV [104; 105; 106], in FUV [93] and even in the EUV [107]. Visible multilayer filters irradiated with protons at 4, 18, and 30 MeV [106; 104] and electrons at 50 MeV [105] showed no changes after irradiation. [PERSON] et al. [93] irradiated Al/MgF\({}_{2}\) mirrors with 1 MeV electrons and 5 MeV protons. No effect on the reflectance at 121.6 nm was observed. [PERSON] and [PERSON] [66] reported also the effect of energetic electrons and protons on Al/MgF\({}_{2}\) coatings. The investigation of the effects induced on optical mirrors by low-energy particles and ions are typically performed by using terrestrial facilities based on ion implanters and accelerators. However, the simulation of the space environment exposure is extremely challenging, since it is extremely challenging to approach the irradiation conditions occurring in space. For example, while the exposure in space usually lasts for several years, a ground-based experiment needs high particle flux rates in order to reach the mission life-time expected fluences in a reasonable amount of time. Moreover, during the accelerating tests can arise potential synergistic effects, not present in space, such as thermal effects related to the high flux and surface contaminations due to the contaminants present in the employed vacuum chamber [108]. Moreover, irradiation experiments are also highly influenced by practical reasons, such as the availability of a facility able to provide the desired ion species, energy, and flux.