Until Astro E-2 — an X-ray mission performed jointly by NASA and Japan’s Institute of Space and Astronautical Science — no one had ever flown a microcalorimeter in space. Now Goddard scientist Richard Kelley hopes to advance the technology to the next level and build the “ultimate X-ray digital camera.”
The operating principle behind a microcalorimeter is simple. When an incoming X-ray hits the microcalorimeter’s absorber, the X-ray’s energy is converted to heat, which a thermometer then measures. The heat is directly proportional to the X-ray’s energy. Knowing a photon’s energy can reveal much about the physical properties of the object emitting the radiation. To gather as many X-ray photons as possible, scientists place an array of microcalorimeters at the focus of a large X-ray telescope and cool the instrument to about one-tenth of a degree above absolute zero.
The Astro E-2 spacecraft, now called Suzaku, carried a spectrometer equipped with a 32-pixel micorcalorimeter array that proved that the technology would work, Kelley said. Now, with funding from various NASA sources, including Goddard’s internal investment programs, he hopes to continue improving the technology so that he can ultimately build a 1,000-pixel microcalorimeter array — measuring only one-third of an inch wide — for NASA’s proposed next-generation X-ray mission, Constellation-X.
Building a 1,000-Pixel Array
More pixels mean higher-resolution images and spectroscopic measurements. “Our goal is to make the ultimate X-ray digital camera that has lots of pixels and colors,” said Kelley, Goddard’s calorimeter development team lead. He concedes that making the jump from a 32-pixel array used on Suzaku’s Goddard-built X-ray Spectrometer (XRS) to a proposed 1,000-pixel array presents many challenges. As a result, he and his team will be pursuing opportunities to use this technology on an intermediate mission. “We would like something in between to help us solve the hard problems and allow us to apply the technology to important problems in high-energy astrophysics,” he said.
To build a 1,000-pixel array, Goddard’s microcalorimeter group has had to advance the readiness of yet another new technology — the transition edge sensor (TES), a highly sensitive thermometer made of superconducting film. The advantage to using TES on a microcalorimeter array is that it offers higher spectral resolution than the Suzaku instrument and it recovers faster after measuring the temperature of each incoming X-ray photon. This would allow scientists to apply the device to a wider range of celestial sources.
Goddard has a long history in microcalorimetry. In the early 1980s, Goddard scientists first proposed the approach and within a year had developed a proof-of-concept device. Scientists originally conceived it to fly on the Advanced X-ray Astronomy Facility (AXAF), one of NASA’s proposed Great Observatories. Due to the downsizing of that mission, however, the technology was used instead on Suzaku.
This is technology where Goddard is very competitive,” Kelley said. However, without R&D investments, it would be difficult maintaining his core team and improving the performance of the technology, he said.
This Goddard-developed 32-pixel micorcalorimeter array — the first ever to fly in space — flew on Astro E-2 spacecraft, now called Suzaku.
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