Hard X-ray multilayer coatings for HEX-P and other future missions

OBJECTIVES: To enable construction of next-generation hard X-ray telescopes, this project aims to (a) develop Co-, Ni-, and Pt-based X-ray multilayer coatings having high reflectance up to ~200 keV, in order to maximize telescope effective area and extend energy coverage beyond NuSTAR, and (b) develop techniques to mitigate multilayer-coating-stress-induced deformation of thin-shell mirror substrates, in order to achieve hard X-ray telescope angular resolution of 15 arc-seconds or better. METHODS/TECHNIQUES: The proposed project comprises two parallel investigations: one focuses on the development of new hard X-ray multilayers, the other on the development of new deposition techniques that will work with these multilayer coatings to mitigate stress-induced deformations of thin-shell mirror substrates. In support of the first investigation, new multilayers will be designed, fabricated, and tested using established procedures: both depth-graded and aperiodic multilayers will be designed with our IMD software; prototype coatings will be deposited using magnetron sputtering; coating structure and properties will be analyzed using X-ray diffraction, atomic force microscopy, transmission electron microscopy, wafer curvature, and other techniques as required; and "at-wavelength" performance will be measured using our novel hard X-ray reflectometer. Film deposition conditions and techniques that yield optimum performance will be identified, and in particular a new ion source will be used for ion-assisted growth of light metal films in order to realize sputtered Ni- and Co-based multilayers having lower roughness, and thus higher reflectance, than can be otherwise achieved. In support of the second investigation, two complementary techniques will be explored to mitigate substrate deformation resulting from multilayer film stress, extending to hard X-ray multilayer coatings similar techniques that are now being developed for Ir-based coatings operating at lower energies for Lynx. The two mitigation techniques are: 1) stress-balancing using Cr adhesion layers; and 2) double-side coatings acting in opposition. Both techniques will be explored in order to identify the most effective approach. Surface figure of both glass and silicon thin-shell segments, provided by GSFC, will be measured before and after coating using optical interferometry. Two-dimensional control of coating thickness, using novel technology already under development, will be used to spatially control film stress and thereby achieve sufficient figure preservation in thin-shell substrates. SIGNIFICANCE: Coating performance drives both effective area and angular resolution in the case of light-weight X-ray telescopes constructed from thin-shell mirror substrates. Without the development of new multilayer coatings having higher X-ray reflectance, and operating at higher X-ray energies, it will not be possible to meet the sensitivity and energy coverage requirements of future missions such as HEX-P. Additionally, without new techniques that sufficiently mitigate coating-stress-induced substrate deformations in thin-shell substrates, it will not be possible to achieve the higher angular resolution required for HEX-P or other future missions enabled by multilayer-coated X-ray telescopes designed to address key science objectives of NASA's Physics of the Cosmos (PCOS) program.

The Astrophysics Research and Analysis program (APRA) supports suborbital and suborbital-class investigations, development of detectors and supporting technology, laboratory astrophysics, and limited ground based observing. Basic research proposals in these areas are solicited for investigations that are relevant to NASA's programs in astronomy and astrophysics, including the entire range of photons, gravitational waves, and particle astrophysics. The emphasis of this solicitation is on technologies and investigations that advance NASA astrophysics missions and goals.

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