Thermal and temporal stability of W/B ₄ C multilayer mirrors for space-based astronomical applications

W / B₄C multilayer (ML) mirrors with varying periodicities (d) = 1.6 to 5.4 nm are tested for rapid thermal and temporal stability, which are required for space-based x-ray telescopes for astronomy. The aging effects on the structural parameters over a period of 2 years are assessed through hard x-ray reflectivity (HXR) measurements. Multiwavelength performance of ML mirrors is studied over thermal cycling from -40 ° C to +50 ° C for 1, 3, and 10 days, which simulate the expected temperature variation in the low-earth orbit. The structural parameters of all samples remained nearly constant over the first 2 years. It is observed that the short-period MLs develop a contamination layer over time. Rapid thermal cycling results indicate no change in HXR for all ML mirrors. However, at soft x-rays, there is a reduction in reflectivity after thermal cycling. The variations in optical performance at hard and soft x-ray energies after thermal cycling are due to variation in interface roughness at different spatial frequencies.