X-ray light catcher for a much lighter space

Tokyo, Japan – A team led by scientists from Tokyo Metropolitan University has created unprecedented light optics for X-ray space telescopes, breaking the traditional trade-off between angular resolution and weight. They used Micro Electro-Mechanical System (MEMS) technology, creating intricate patterns into silicon wafers that can direct and collect X-rays. By annealing and polishing, they realized ultra-sharp features that could rival the performance of existing telescopes for a fraction of their weight, at significantly lower launch costs.

X-ray astronomy provides a vital tool that helps scientists study and classify various celestial bodies that emit and interact with X-rays, including our planet. But there’s something interesting: most of the X-ray radiation is absorbed in our atmosphere, which means that telescopes and detectors have to be launched into space. With this comes various limitations, in particular, how heavy the device can be.

One of the main features of all astronomical observation optics is their angular resolution, or the angle that two light sources with a detector can make and still be distinguishable. The problem with conventional X-ray optics is that, in order to achieve higher resolutions, devices become increasingly heavy. This makes their launch into space very expensive. Even for the Hitomi telescope launched in 2016, it is considered very light, its effective weight is 600 kg per square meter of effective area.

Now, a team led by Associate Professors Yuichiro Ezoe and Aoto Fukushima have solved this trade-off by engineering a high-performance unit weighing only 10 kg per square meter. They used Micro Electro-Mechanical Systems (MEMS) technology, a technique designed to fabricate microscopic mechanical actuators, to create sharp and intricate design patterns into silicon wafers that can direct and collect X-rays. The design itself follows the Wolter I geometry of existing X-ray telescopes, a concentric arrangement of tree ring-like slits that can push X-rays through a narrow range of angles and collect them to a point. What’s unique about teamwork is how they perfect the pattern themselves. After etching the gap using a technique called deep reactive ion etching (DRIE), they found that there is surface roughness in the pattern that can smear the X-ray collection, effectively lowering the resolution. So, they “annealed” the pattern, applying heat in a special device for an unprecedented time. With a longer annealing, the silicon atoms on the surface of the pattern can move more, rounding out any roughness and increasing the angular resolution of the telescope. This is followed by grinding and chemical polishing to straighten the rounded edges of the gap itself.

Importantly, the performance reported by the team matches that of telescopes already in action. Its weight makes it particularly suitable for the mission of GEO-X, a satellite designed to visualize Earth’s magnetosphere. The team is targeting a very low total weight of 50kg, a technological breakthrough that could see future missions sent into orbit at a much lower cost.

This work was supported by the Japan Society for the Promotion of Science (Grant Numbers 19J20910, 20H00177, 21H04972, 21J12023) and the Toray Science Foundation.

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