Iridium thin-film coatings for the BabyIAXO hybrid X-ray optic.

Abstract

Reflective coatings are an essential feature of X-ray telescopes. Their overall performance relies heavily on substrate compatibility and how well they conform to the optics assembly processes. We use X-ray reflectometry (XRR) to demonstrate the compatibility of shaping flat substrates coated with iridium, and show that specular and nonspecular reflectance before and after shaping is on par with traditional hot-slumped coated substrates. From 1.487 and 8.048keV measurements, we find that the substrates have rms roughness of 0.38nm and magnetron sputtered iridium deposits with rms surface roughness of 0.27-0.35nm. A hydrocarbon overlayer from atmospheric contamination is present with a thickness of 1.4-1.6nm and a density of 1.2-1.6g/cm3. Both the traditional hot slumped and the flat substrates undergoing post-coating shaping have a similar characteristic surface morphology and are equally well-suited for use with X-ray optics. Finally, we demonstrate by simulation the improved effective area achieved by using a low-Z overlayer, and illustrate the performance of a hybrid optic coated with optimized bilayers for a Primakoff axion spectrum emitted by the sun.