Using technology similar to that found in smartphone cameras, NASA scientists are developing improved sensors to reveal more details about exploding black holes and exploding stars—all while using less power and being easier to mass-produce than detectors used today.
“When you think about black holes that are actively destroying stars, or neutron stars that explode and create really high-energy flashes of light, you’re looking at the most extreme events in the universe,” said research astrophysicist Dr. Regina Caputo. “To observe these events, you have to look at the highest-energy form of light: gamma rays.”
Caputo leads an instrument development effort called AstroPix at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. The silicon pixel sensors in AstroPix—still under development and testing—resemble the solid-state sensors that allow smartphone cameras to be so small.
“Gamma rays are notoriously difficult to measure because of the way the incoming particles interact with your detector,” said Dr. Amanda Steinhebel, a NASA postdoctoral fellow working with Caputo.
Gamma rays are wavelengths of light more energetic than ultraviolet and X-rays, and their photons act more like particles than waves. “Instead of just being absorbed by the sensor like visible light,” Steinhebel said, “the gamma rays are reflected all around.”
The Fermi Gamma-ray Space Telescope, which has been studying the gamma-ray sky since 2008, solved the “reflection” problem in its main instrument by using belt-shaped sensor towers. This desk-sized cube, the Fermi Large Field Telescope, was itself a breakthrough technology when the mission launched.
Each strip maps the incidence of gamma radiation in one dimension, while layers of strips oriented perpendicular to each other record the other dimension. Gamma rays generate a cascade of energy shocks through multiple layers and provide a map pointing back to the source.
A space telescope instrument using the golf bag-sized AstroPix sensors would require half as many layers as the Fermi strip detector technology for black holes, Caputo said.
“It’s easier to tell exactly where the particles are interacting,” Steinhebel said, “because you just need to identify the point on the grid that they’ve interacted with.” Then you use multiple layers to literally trace back the paths the particles took.”
AstroPix could detect lower-energy gamma rays than current technology, Steinhebel explained, because those photons tend to be lost by filtering through multiple layers of the fringe detector. Capturing them would provide more information about what happens during short-term, energetic events. “These low-energy gamma rays are most common during peak brightness,” she explained.
The pixel detectors also use less electricity to operate, Caputo said, a major advantage for future missions planning their energy use.
Pixelated silicon detectors have proven themselves in particle accelerator experiments, she said, and their common use and mass production for cell phones and digital cameras make them easier and cheaper to obtain.
Developing various prototypes over several years and seeing AstroPix produce accurate gamma light graphs was exciting and extremely satisfying, Steinhebel said.
While the team continues to work on developing and refining its technology, Caputo said the next step will be to launch the technology on a short sounding rocket flight for further testing above Earth’s atmosphere and black holes.
They hope to benefit a future gamma-ray mission to support the study of high-energy cosmic events.
“We can do such great science with it,” Caputo said. “I just want to see it happen.
