Commissioning and qualification of the Low-Energy X-ray Reectometer and its utilization in development of X-ray mirror coatings for astronomical instrumentation

With the continuous effort to study X-ray emission from astronomical objects, there is an ever increasing demand for observatories of high quality in terms of focusing properties of the energetic radiation and large photon collecting areas. The European Space Agency's next generation X-ray telescope, the Advanced Telescope for High Energy Astrophysics (Athena), is under development, and extensive studies are carried out to ensure that its scientific requirements will be met.

The main purpose of this project was the commissioning, qualification, and utilization of a state-of-the-art compact Low-Energy X-ray Reflectometer with the aim of enabling quick characterization of mirrors at 1.487 keV, in order to evaluate the performance of low-density overcoatings and the impact of the Athena mirror post-coating processing and assembly steps on the quality of the coatings. The instrument was successfully commissioned and validated, and data acquisition modes were developed to optimize its performance and quality of measurements. The operational reflectometer has excellent measurement repeatability, high angular resolution, good spectral purity, and large dynamic range.

With the use of the Low-Energy X-ray Reflectometer as well as 8.048 keV measurements and synchrotron measurements in the range 3.4-10 keV, characterization of carbon coatings were performed which indicated resistance of this material to the Athena mirror cleaning procedure, thus allowing carbon to be introduced as an Athena coating material candidate. Optimized iridium/carbon bilayers are shown to be stable over time, resistant to the cleaning procedure, and not adversely affected by post-coating annealing of the mirrors. Diamond-like carbon coatings were also studied but were not found to be significantly better than the regular sputtered carbon.

The last part of this project concerned the optics development for a future axion helioscope called BabyIAXO. Simulations and numerical optimizations were carried out to determine the optimal geometry in terms of focal length and distribution of mirrors in its optic, as well as optimization of coating recipes. A focal length of just over 5 m and coatings consisting of simple bilayers, similar to those for Athena, were found to be best for the source in question. Test mirrors were produced, validating the compatibility of a baseline iridium coating with the cold-slumped glass approach envisioned for BabyIAXO.