The Athena mirror will use an innovative technology called Silicon Pore Optics (SPO). The technology, combined with appropriate over coating, holds great potential to meet the Athena performance requirements in terms of effective area.
The SPO have to match several top-level performance requirements. The most driving ones are the angular resolution and the effective area. Regarding the latter, folded through the instrument efficiency, the Athena science requirements states that the X-IFU should deliver more than 10,500 cm2 at 1 keV and more than 1,600 cm2 at 7 keV. At the moment, the X-IFU non-compliance to those requirements is more severe at 7 keV, approaching 40%. Options for improvements are being evaluated to reach the requirement at 7 keV without degrading it at 1 keV.
After a careful optimisation of the Transition Edge Sensor absorbers, the X-IFU instrument efficiency leaves essentially no room for improvement. Therefore, approaching the requirements implies to look at the optics, for which solutions exist. The solutions rely on multi-layer coatings of the SPO plates. They were studied in the past, and have been revamped recently, specifically to address the non-compliance to the 7 keV requirement.
The characteristics of this technology are the following: the multi-layer coating is applied underneath a bi-layer (e.g., Ir/B4C, Ir/SiC, Ir/C) and consists of a series of several thin alternating layers (e.g., Ir and B4C). Simulations performed at DTU Space (Denmark’s National Space Institute) of multi-layer coatings, involving 15 layers applied to the mirror modules of rows 5 to 15 of the Athena mirror, have shown that a broad bump between ~6 and 7 keV, of up to ~50% of effective area could be obtained. A detailed optimisation of the multi-layer recipe could tune the centroid of the bump to the desired energy, e.g. 6.5 keV. Overall, this means that with an existing technology, demonstrated through simulations, it is possible to make Athena compliant to the 7 keV requirements.
The ESA study team is planning on confronting the results of the simulations with actual data, as required to reach a proper level of demonstration for the mission adoption. For this purpose, ESA follows a step-by-step approach, first checking that the baseline layer of Iridium is robust, then moving onto the demonstration of the bi-layer (with B4C, SiC, or C), based on the latest plates. The demonstration includes: compatibility with the SPO cleaning process, stability, compatibility with lithography, compatibility with resist lift-off, thermal annealing… If the bi-layer is demonstrated in 2021, there should be no reason for multi-layer coating not to work, with some delta demonstration activities required at the interface between the bilayer and the multi-layers underneath.
Bringing Athena to compliance at 7 keV would be a major achievement of the SPO program, which would be very beneficial to a wealth of the X-IFU science, and Athena overall.
By Didier Barret
X-IFU Principal Investigator