Stress Distortion Free Coating for High Resolution X-ray Mirrors

Most of X-ray optics research and development in the US is to build a high resolution, large collecting area and light-weight optic, namely an soft X-ray mirror for the X-ray Surveyor, as identified in the NASA PCOS astrophysics roadmap. One of the promising technologies is a segmented silicon X-ray mirror being developed at GSFC. The silicon mirror substrate is a thin (~400 µm thick or less) curved substrate.  This substrate requires a heavy metal coating, such as iridium, for X-ray reflection. The coating layer has a residual stress which distorts the thin mirror substrates. The distortion amount increases as the coating layer thickness increases. This distortion greatly degrades the mirror angular resolution and prevents us from achieving the high resolution (~5”) that is required by X-ray Surveyor.

After the successful hard X-ray imaging observations by NuSTAR, the X-ray astronomy community has been wishing a next generation hard X-ray observatory with a greatly improved angular resolution < 10”, ~10 times better than NuSTAR. Such mission concepts have been discussed in many places and white papers were submitted for a Probe mission concept in the past. While such the Probe mission is being discussed in the US, the Japanese community also wants to have a new hard X-ray astronomy mission with the high angular resolution following up their ASTRO-H (Hitomi) mission and the mission called FORCE is being discussed. Now Japanese space program includes FORCE in their road map. Since the multilayer coating is a lot thicker (total thickness) than the soft X-ray coating, the mirror is expected to be severely affected by the distortion due to the residual stress. In fact, NuSTAR mirror was ended up with about 60” resolution while the substrate itself had <30” resolution performance before the coating.

For achieving the high resolution and the large collecting area with light weight discussed above, the segmented approach is the most promising one as a whole shell mirror will not likely be able to keep its shape, and its cost can be quite expensive. On the other hand, the segmented mirror is more susceptible to the residual stress than the whole shell. However, there is no clear path forward in research & development for removing the residual stress distortion, while the stress-free coating actually is the enabling technology for the high resolution mirror. We will study residual stress in the soft/hard X-ray reflection coating, i.e. single layer and multilayer coating, and develop a method to eliminate or minimize the residual stress distortion and keep the substrate figure intact after the coating.