# Radiation Hardness of Oxide Thin Films Prepared by Magnetron Sputtering Deposition
## 1 Introduction  
The interaction of energetic heavy ions with materials is a complex process that evolves over several phases. The typical kinetic energy of such a projectile is between several and a few hundred MeV. By considering that such a projectile often consists of dozens of protons and neutrons, one can conclude that specific kinetic energy typically ranges between 100 keV/A and 10 MeV/A. Historically, projectiles having specific kinetic energy above 1 MeV/A have been called swift heavy ions. Recently, it was recognised that even lighter and less energetic ions can also introduce significant damage levels into materials [1, 2, 3, 4]. For this reason, a projectile that has a specific kinetic energy between 100 keV/A and 1 MeV/A is considered a high-energy heavy ion. Due to similarity with fission fragments, radiation hardness studies of materials exposed to high-energy heavy ion beams are of interest for nuclear energy applications [5]. Also, they are often used as a proxy to damage production by other radiation types such as alpha recoils, neutrons, and gamma-rays, since by adopting a self-irradiation approach, damage levels that are of interest can be accumulated in hours instead of months, without exposing oneself to any kind of radiation risk [6]. High-energy heavy ions travel at ~1--5% speed of light, and cover a distance of 1 nm in less than one femtosecond. Thus, dense electronic excitation along the ion trajectory, which is the initial excitation of the material, occurs on the femtosecond timescale. This type of material excitation is specific for high-energy heavy ions, because electronic stopping(i.e., numerous collisions with electrons) is the dominant process of their kinetic energy loss. On the other hand, nuclear stopping is negligible because direct collisions of the projectile with atomic nuclei occur only very rarely. The next stage of ion-matter interaction, which can cause significant material changes, occurs on the picosecond timescale when excited electrons heat the material via electron-phonon coupling, giving rise to a thermal spike. If the temperature rise is significant and melting of the material occurs, then in the last stage (which can extend up to the nanosecond timescale), permanent damage can be formed because rapid quenching of the melt yields a large number of defects along the ion trajectory. This permanent damage is also known as an ion track and was studied in many different materials, mostly crystalline insulators [1,7]. Ion tracks in crystals can be visualised easily because of the large contrast between the amorphous ion track and the surrounding crystal matrix. Thus, many experimental techniques are at disposal for ion track studies, like transmission electron microscopy (TEM) and Rutherford backscattering spectrometry in channelling (RBS/c) [1,8, 9, 10]. Previously, we have studied ion tracks in crystalline oxides produced by high-energy ion beams available at the accelerator facility at the Ruder Boskovic Institute (RBI). Of interest to the present study, ion tracks were studied by TEM, RBS/c and atomic force microscopy (AFM) in quartz SiO\({}_{2}\)[11], and in Al\({}_{2}\)O\({}_{3}\) and MgO single crystals [12,13]. Ion tracks in amorphous materials can be produced easily, and therefore non-standard parameters of the thermal spike models (describing ion track formation) have to be invoked to explain observed ion track sizes [14]. At the same time, it is much more difficult to observe them because the contrast between the ion track and the surrounding material is very weak. For this reason, ion tracks in amorphous materials have been studied to a much lesser extent and thoroughly just in the case of thermally grown a-SiO\({}_{2}\)[15, 16, 17, 18, 19, 20, 21, 22], where infrared (IR) spectroscopy and small-angle X-ray scattering (SAXS) were found to be effective for the determination of ion track radii.