As humanity prepares to return to the Moon, understanding its complex radiation environment becomes increasingly critical. Radiation hardness assurance is essential to ensure that the payload remains operational and reliable under prolonged exposure to harsh space conditions. This thesis investigates the performance of the Floating Gate Dosimeter (FGDOS) as part of the Lunar Zebro nano-rover payload developed at TU Delft. The sensor’s response was evaluated through a series of irradiation campaigns: low-dose gamma radiation at the Reactor Institute Delft (RID), proton irradiation at HollandPTC, and mixed-field exposure at CERN’s CHARM facility. While the FGDOS demonstrated linear dose-response behavior and potential for spaceborne dosimetry, several limitations were identified, including reduced sensitivity, temperature dependence, and a firmware-related recharging issue.

A preliminary investigation into the use of a boron carbide layer showed a measurable increase in neutron sensitivity, suggesting its relevance for shielding strategies in mixed-field environments. Although the current payload is not yet flight-ready, it provides a robust foundation for further development. Future work should focus on high-flux gamma testing and refinement of both hardware and firmware to improve measurement reliability and system resilience in the lunar radiation environment.

Photochromic materials, which exhibit reversible changes in transparency upon light exposure, have gained significant interest for their potential applications in smart window technologies. Among these materials, yttrium oxyhydride (YOH) stands out due to its stability and promising photochromic properties. A critical challenge in developing practical photochromic windows lies in balancing two key properties: high photochromic contrast (∆T) and short bleaching time (τB). These properties are strongly influenced by the pressure during the thin film’s deposition. This study investigates how thicker YOH films deposited at higher pressures (0.8Pa) affect photochromic performance and explores the impact of combining two layers in double-layer configurations to optimize the Photochromic properties.

YOH thin films were prepared using reactive DC magnetron sputtering at two deposition pressures: 0.5Pa and 0.8Pa. Single-layer samples were deposited at 0.8Pa with varying thicknesses (400nm, 600nm, and 800nm), while double-layer samples were created by stacking layers deposited at different pressures in two configurations: substrate/0.8Pa/0.5Pa (DL-1) and substrate/0.5Pa/0.8Pa (DL-2). The optical and structural properties of these films were characterized using a combination of transmission and reflection measurements, spectroscopic ellipsometry, X-ray diffraction (XRD), and Doppler Broadening Positron Annihilation Spectroscopy (DB-PAS). These techniques provided insights into the relationship between the material’s oxidation state, atomic structure, and photochromic response.

The results revealed that single-layer films deposited at 0.8Pa exhibited lower photochromic contrast (11.4 ± 0.4% for 400nm) but faster bleaching time constants (0.6 hours) compared to those deposited at 0.5Pa, which achieved higher contrast (53.0 ± 0.4%) but slower bleaching time constants (5.1 hours). This trade-off aligns with previous findings, confirming the inverse relationship between deposition pressure and photochromic performance. While thicker films (600nm and 800nm) at 0.8Pa showed increased photochromic contrast, the 800nm sample suffered from inhomogeneity and a lower-than expected contrast, highlighting the challenges of scaling up film thickness for practical applications.

In double layer configurations, the properties of the bottom layer significantly influenced the overall photochromic performance. The DL-1 sample, with a 0.8Pa bottom layer, exhibited increased oxidation and inhomogeneity in the 0.5Pa top layer, resulting in a reduced photochromic contrast of 29.7 ± 0.4% and significantly lower transparent transmission (31.6 ± 0.2%). In contrast, the DL-2 sample, with a 0.5Pa bottom layer, allowed the 0.8Pa top layer to retain its intrinsic photochromic properties, achieving a contrast of 55.2 ± 0.4%, closely matching theoretical predictions of ≈52%. XRD measurements further supported these findings, showing a lattice constant increase of 0.026 ± 0.005Å in the DL-1 sample compared to 0.5Pa single-layer thin films, indicative of higher oxygen incorporation in the top layer. Additionally, both double layer samples exhibited longer bleaching time constants compared to their respective bottom layers, suggesting that the capping of the bottom layer may influence hydrogen mobility within the film.

This study underscores the critical role of deposition pressure, film thickness, and layer sequencing in determining the photochromic properties of YOH thin films. Higher deposition pressures and thicker films lead to increased porosity and inhomogeneity, which can degrade photochromic performance. Additionally, careful control of layer stacking, particularly by using a less porous bottom layer (e.g., 0.5Pa), can result in a photochromic contrast similar to the stacking of layers. However, the bleaching time constant in negatively impacted.

The main goal of this thesis is to better understand the photochromic mechanism in rare-earth (RE) oxyhydride thin films. When illuminated, these materials have shown to darken, and bleach to a transparent state when the illumination source is removed. The RE oxyhydride thin films used in this thesis are deposited using magnetron sputtering and are YOxHy and GdOxHy films with different storage (air and vacuum) and oxidation (air and dry air) conditions. The films were probed using Doppler broadening positron annihilation spectroscopy, spectrophotometric (optical) measurements, X-ray diffraction and spectroscopic ellipsometry, measured before, during and after in or ex situ ultraviolet illumination.

Series of illumination and bleaching cycles have shown to permanently induce vacancy defects, which are most likely divacancies. The photochromic effect is seen as a reversible process, which leads us to believe that the changes seen in the defects are not directly responsible for the photochromic effect. During in-situ photodarkening and bleaching partially reversible changes are seen, whereby systematic shifts in the positron annihilation Doppler parameters are exhibited. This shift correlates with the photochromic effect, where the irreversible formation of vacancies is seen as a result of UV illumination. This leads us to the idea of the formation of metallic domains that may be attributed to H- ions that become mobile under illumination and may cluster to form metallic domains. Furthermore, the formation of the vacancy defects correlates with an increase in bleaching times seen from optical measurements, suggesting that they could play a role in the memory effect. Another optical property seen is a change in the band gap. We have seen that during photodarkening, the band gap tends to widen and decrease during bleaching. This phenomenon can be associated with the Burstein-Moss effect and can be used to estimate the photo-induced electron density in the conduction band. Despite the changes observed from positron annihilation techniques and optical measurements, no significant differences were observed by X-ray diffraction. The lattice constants of the film before and after illumination were shown to be constant.

There are two hypotheses explaining the photochromic mechanism: the segregation of metallic domains as a result of H- ion mobility and an Anderson-Mott insulator-to-metal transition as a result of overlapping electron wave functions. Using spectroscopic ellipsometry, these models were tested for YOxHy. It is shown that the mechanism leading to the darkening of the films is best described by the formation of Y or YHx metallic domains and voids. The percentage of metallic domains and voids grows with illumination time and starts stabilising after 90 minutes of illumination. The fraction of metallic domains (1.9-2.6% after 150 minutes of illumination) is similar in magnitude as seen in previous studies. Furthermore, the voids probed by ellipsometry are much larger than the vacancy-related defects probed by positrons. Such voids could give rise to the formation of positronium in the films, as could be examined in future positron annihilation lifetime spectroscopy (PALS) experiments.

Qualitatively GdOxHy and YOxHy behave similarly when it comes to optical measurements, XRD and DB-PAS data. For both systems, an increase in bleaching time as a function of illumination and bleaching cycles, a widening and shrinking of the band gap and a decrease in contrast are observed. However, there are quantitative differences between these films. The bleaching constant is shorter for GdOxHy and the contrast is higher, which makes it seem more attractive for potential applications. However, GdOxHy could not yet be satisfactorily modelled, in contrast to the case of YOxHy using spectroscopic ellipsometry. The GdOxHy film could not be described by a single homogeneous layer and therefore requires more complex models to describe the behaviour of the film in future work.