NASA’s new detectors could improve views of gamma-ray events

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In a practical application such as the design of this sounding rocket test instrument, a gamma-ray observatory would use multiple layers of Astropix sensors, which could then trace a trajectory of three-dimensional particles through an array of pixelated two-dimensional detectors. Credit: Regina Caputo

Using technology similar to that found in smartphone cameras, NASA scientists are developing updated sensors to reveal more details about exploding black holes and exploding stars, all while being less power hungry and easier to detect. mass-produce than the detectors used today.

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“When you think of black holes actively destroying stars, or neutron stars exploding and creating bursts of high-energy light, you are looking at the most extreme events in the universe,” said research astrophysicist Dr. Regina Caputo. “To observe these events, you need 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 the AstroPix, still in development and testing, are reminiscent of the semiconductor sensors that allow smartphone cameras to be so small.

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“Gamma rays are notoriously difficult to measure because of the way the incoming particle interacts with your detector,” said Dr. Amanda Steinhebel, a NASA postdoctoral fellow working with Caputo.

Gamma rays are wavelengths of light that are more energetic than ultraviolet and X-rays, and their photons act more like particles than waves. “Instead of just being absorbed by a sensor like visible light,” Steinhebel said, “gamma rays just bounce around.”

NASA’s Fermi Gamma-ray Space Telescope, which has been studying the gamma-ray sky since 2008, solved the “bounce” problem in its main instrument by using strip-shaped sensor towers. This table-sized cube, Fermi’s Large Area Telescope, was itself groundbreaking technology when the mission launched.

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Each stripe maps a gamma ray in a single dimension, while layers of strips oriented perpendicular to each other record the second dimension. Gamma rays cascade energy blasts through multiple layers, providing a map pointing to the source.

About the size of a golf bag, a space telescope instrument using AstroPix sensors would require half as many layers as Fermi strip detector technology, Caputo said.


Gamma-ray bursts are the brightest explosions in the cosmos. Astronomers think most occur when the core of a massive star runs out of nuclear fuel, collapses under its own weight, and forms a black hole, as illustrated in this animation. The black hole then pushes jets of particles that pierce all over the collapsing star at nearly the speed of light. These jets streak across the star, emitting X-rays and gamma (magenta) rays as they pour out into space. They then penetrate the material surrounding the doomed star and produce a multi-wavelength afterglow that gradually fades away. The closer we approach one of these jets head-on, the brighter it appears. Credit: NASA Goddard Space Flight Center

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“It’s easier to tell exactly where the particles are interacting,” Steinhebel said, “because you simply identify the point in the grid that it has interacted with. Then you use multiple layers to literally plot the paths the particles have taken through it.”

The AstroPix could record lower-energy gamma rays than current technology, Steinhebel explained, because these photons tend to get lost as they filter through the multiple layers of a strip detector. Capturing them would provide more insight into what happens during short-lived energy events. “These low-energy gamma rays are most common during the peak brightness of the burst,” she explained.

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The pixel detectors also use less electricity to operate, Caputo said, a big boon for future missions planning their power consumption.

Pixelated silicon detectors have been tried in particle accelerator experiments, he said, and their common use and mass production for cell phones and digital cameras make them easier and cheaper to obtain.

Developing several prototypes over several years and seeing AstroPix create accurate graphs of gamma-ray light has been exhilarating and extremely satisfying, Steinhebel said.

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As the team continues to work on developing and improving its technology, Caputo said the next step would be to launch the technology on a short sounding rocket flight for further testing above Earth’s atmosphere.

They hope to benefit from a future gamma-ray mission intended to further the study of high-energy universe events.

“We can do such an interesting science with this,” Caputo said. “I just want to see it happen.”

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