Just Scratching the Surface
As a branch of science, physics embodies an almost head-scratching duality. It is at once cosmic, touching on the nature and properties of all matter and energy in the universe, and yet also incredibly intimate, right down to the scale of subatomic particles. Trying to visualize the structure of things at such a scale can be dizzying, but Rachel learned she had an aptitude for that kind of thinking in high school—thanks, in large part, to a great teacher her senior year.
Growing up around Menlo Park and Palo Alto in California, her first loves were art and writing, and she thought she might even want to be an architect (she’s still proud of a model bridge she made in trigonometry that withstood 240 pounds). Yet by the time she graduated high school, she had zeroed in on physics, going on to study it as an undergrad at the University of California, Santa Cruz, and then for her doctoral degree at the University of Wisconsin-Madison. “It turns out the ability to estimate things, to be able to understand the scale of things like you would in art and architecture, is really important on a day-to-day basis as a physicist in a lab.”
Through a two-year postdoc in Switzerland and then her five years at NIST, Rachel explored the dynamics of surfaces at the atomic scale, including an overlap with Fred on one particular material—porous silicon carbide—that would lead to their current work with the field emitter.
The discovery of X-rays in 1895 launched a new era of medicine with the ability to see inside the human body. Yet today, more than 120 years later, we are still using 20th-century technology that relies on hot filaments to convert electrical input into X-rays through X-ray tubes. Relying on a hot source of electrons, however, has always limited how scalable and versatile the applications of X-ray tubes can be. That’s why developing a cold electron source—one that generates X-rays without creating heat—could transform X-ray technology the same way LEDs revolutionized lighting, says Rachel.
Think of a computed tomography (CT) scan. “A CT machine is loud,” she says. “It’s just spinning around you like crazy, like recording a movie with one light source. You have to take frame by frame, move the light around. In order to do that they have to spin it as fast as possible, up to 30Gs, as fast they can without tearing the whole machine apart. And that’s all because it’s a hot filament, a hot source, and you can’t tile that because you have to manage all the heat it produces.”
A cold electron source fundamentally changes that equation, and the applications are enormous—especially for medical devices and security. If you don’t need a power supply for the filament, you’re that much closer to building a truly portable X-ray tool, she says. You can perform mammograms without releasing heat and requiring an air-conditioned room, which could have important implications in low-resource settings with unreliable power supplies. You can produce X-ray images in color and higher spatial resolution. You can do material identification using diffraction, meaning instead of going through security at an airport and having scanners look at shapes and densities, they would see exactly what the actual material is.
“It’s just so amazing how far it’s come,” says Rachel. “We’re there, basically, kind of at that cusp of where it could go in a number of directions.”
Above all, this project has captured everything Rachel loves about physics—the hands-on experimenting and doing—and why she feels so at home at IV. “That’s another thing I like about being here,” she says. “I’m actually at the bench, every day. You can’t really say that as a professor, or as a lab lead at a national lab, where there’s a lot of bureaucracy. It’s hard to be innovative in that kind of space; not so hard here.”