Credit: Credits: NASA/Chris Gunn
NASA scientist William Zhang wasn’t always a mirror-making whiz. In fact, this year’s IRAD (Internal Research and Development) Innovator of the Year at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, started his career working underground — literally.
Zhang won the Goddard Office of the Chief Technologist’s top prize — bestowed annually on those who demonstrate the best in innovation — for his foresight, perseverance, and leadership advancing state-of-the-art X-ray optics.
In particular, he and his team created a new type of mirror made of mono-crystalline silicon, an abundantly available material commonly used to manufacture computer chips. The new mirror type is now baselined for use on the conceptual Lynx X-ray Observatory — one of four potential missions that scientists have vetted as worthy of consideration by the 2020 Decadal Survey for Astrophysics.
“Will saw a need and pursued the mirror-making concepts with tenacity,” said Goddard Chief Technologist Peter Hughes. “Silicon has never been used to make super-thin, lightweight, easily reproducible X-ray mirrors. His innovation could represent a paradigm shift in X-ray astronomy for decades to come. Certainly, Will and his team have reinvented the way NASA builds these highly specialized mirrors. He is the poster child for how to advance innovative new technologies.”
Though he has proven his mettle in mirror making, Zhang actually started his career as a particle physicist and detector scientist. As a graduate student at the University of Pennsylvania, he spent his daylight hours searching for neutrinos at the Kamioka Observatory located in the Mozumi mine two miles beneath a mountain peak in western Japan — a site chosen so that cosmic rays wouldn’t interfere with the measurements.
Although the experiment did not succeed in detecting proton decay, which was its original purpose, it successfully detected neutrinos from supernova 1987A and the Sun using state-of-the-art electronics and data-acquisition systems that Zhang helped build.
The experience served another useful purpose.
“I’d go down there before sunrise and wouldn’t come back up until nightfall,” Zhang recalled. After three years as a subterranean worker, “I discovered I would prefer having an office with windows. I had enough of mining.”
It’s a good thing for Goddard.
After a stint with Los Alamos National Laboratory in New Mexico, Zhang came to the center specifically to build and calibrate the Proportional Counter Array that flew on the Rossi X-ray Timing Explorer that launched in late 1995. With scientific papers published in technical journals, Zhang began casting about for a new challenge and got one when his former boss suggested he apply his talents to Constellation-X, a conceptual X-ray mission at the time.
“Optics turned out to be the most important technology for X-ray astronomy,” Zhang said, adding that these highly specialized mirror segments are curved and aligned inside a canister-type assembly to collect highly energetic X-ray photons emanating from hot objects, such as pulsars, galactic supernovae remnants, and the accretion disk of black holes.
Because the mirrors are curved, X-ray photons graze their surfaces — much like skipping stones — and deflect into an observatory’s instruments rather than passing through them. “They had a default technique for making these optics and I thought it wasn’t working very well.”
The key, he reasoned, was making these optics much thinner, lighter, and less expensive to manufacture. If the individual mirror segments were thick and heavy, like those employed by NASA’s Chandra X-ray Observatory, fewer mirrors could fly, limiting the observatory’s collecting area and therefore its sensitivity or ability to discern details of an astronomical target.
Before Zhang set out to tackle the challenge, optics developers traditionally used glass, ceramics, and metals. However, these materials suffer from high internal stress, especially when cut or exposed to changing temperatures. These stresses become increasingly more unpredictable as the mirror becomes thinner.
With support from Goddard’s IRAD program and other NASA research and development programs, he first experimented with glass slumping, a novel technique where he placed commercially available, super-thin glass segments on a mandrel, or mold, and heated the entire assembly in an oven. As the glass heated, it softened and folded over the mold to produce a cylindrically shaped optic that was then coated with layers of silicon and tungsten to maximize their X-ray reflectance.
Though Zhang proved the technique and produced 10,000 modest-resolution mirrors ideal for NASA’s Nuclear Spectroscopic Telescope Array, or NuSTAR, mission, Zhang realized he had taken this technique to the limit. He concluded that their performance was inadequate for achieving the desired sensitivity for future X-ray telescopes.
He turned to single-crystal silicon, a material never polished and figured for lightweight X-ray optics. The material itself intrigued him. Inexpensive and abundantly available because the semiconductor industry uses it to manufacture computer chips, crystalline silicon has little, if any, internal stresses, making it ideal for creating super-thin X-ray optics.
Leveraging his experience with glass slumping, Zhang started with blocks of silicon. With standard machining tools, he produced the approximate mirror shape and then used precision machining tools and chemicals to further grind and refine the blocks’ surfaces. Like slicing cheese, he then cut thin substrates measuring less than a millimeter in thickness and polished their surfaces. Any surface defects larger than several nanometers were removed with a special ion-beam polishing tool.
Zhang and his team prevailed.
As part of a study evaluating the conceptual Lynx telescope, a NASA-commissioned panel of 40 experts found that the optics could provide sub-arcsecond resolution, which is the same quality as the four pairs of larger, much heavier mirrors flying on Chandra. Furthermore, because the mirrors are 50 times lighter and less costly to build than Chandra’s, next-generation observatories can carry literally tens of thousands of mirror segments, improving sensitivity over even Chandra, the world’s most powerful X-ray observatory.
In addition to being baselined on Lynx, the mirror technology is now being investigated for potential use on the European Space Agency’s Athena X-ray Observatory, scheduled to launch in late 2031. Even if Lynx isn’t selected as NASA’s next flagship astrophysics mission, other NASA missions could benefit in the future, he said.
In the nearer term, Zhang will be flying his optics on a sounding rocket mission, called OGRE, in 2021. This flight opportunity will represent the technology’s first demonstration in space. For OGRE, which is short for the Off-plane Grating Rocket Experiment, Zhang is developing a 288-segment mirror assembly.
“I had many moments when I thought I had bitten off more than I could chew,” Zhang said, reflecting on his technology-development effort, which is continuing as he works with his engineering team to design an improved technique for aligning and bonding these fragile mirror segments inside a protective canister. “With technology development, you never know if you’ll achieve what you set out to achieve. But I’m fortunate that I work with a team of people who are really, really good. Teamwork isn’t an empty word. It’s precious and very, very important.”
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