A High Resolution "Color" X-Ray Camera with Thousands of Pixels
This blog post originated in the 2018 Science Mission Directorate Science and Technology Report.
Demonstration model of the Athena X-ray Integral Field Unit (X-IFU)
An X-ray camera is being developed with 3168 pixels and ground-breaking capability to resolve different “colors” of X-rays. This technology will allow observations of the dynamics and composition of hot energetic material in galaxy clusters and around black holes. Scientists can use these images to learn about the formation and evolution of these complex cosmic objects.
Some of the most fascinating objects in the universe are made up of hot energetic material that emit X-rays. NASA is part of an international collaboration developing a camera capable of imaging these systems using microcalorimeter pixels. This camera will be part of the European Space Agency’s (ESA) Advanced Telescope for High ENergy Astrophysics (ATHENA). Spatial resolution will be enabled by the presence of 1000s of these pixels laid out in an array, and the exquisite energy resolution of each pixel will give the images their “color.”
In an X-ray microcalorimeter, an X-ray absorber is connected to a very sensitive thermometer. When an X-ray is absorbed in the device it causes a small change in the temperature that is read by the thermometer. The greater the energy of the X-ray absorbed, the larger the change in temperature, and therefore, it is possible to determine the energy or “color” of the incident X-ray.
To achieve the best energy (color) resolution, the microcalorimeters are cooled to less than 0.1K and the temperature is monitored by measuring the current flowing through a superconductor that is held just at the point of transition from a normal metal to a superconductor. These transition-edge sensor (TES) microcalorimeters are then able to measure the energy of 6 KeV X-rays with a resolution of less than 2.5 eV, or 1 part in 2400.
X-ray microcalorimeter technology has been steadily improving since its inception three decades ago. The NASA/Japan Aerospace Exploration Agency (JAXA) Soft X-ray Spectrometer microcalorimeter instrument onboard the Hitomi satellite flew with 36 pixels. The X-ray integral field unit (X-IFU) instrument on ATHENA is baselined to have an array of 3168 TES microcalorimeter pixels fabricated at NASA and developed as part of a longstanding collaboration between NASA, the National Institute of Standards and Technology (NIST), and Stanford University. Dr. Caroline Kilbourne notes, “Combining imaging with spectroscopy, the X-IFU instrument will probe dynamics and composition within spatially extended cosmic objects such as supernova remnants and galaxy clusters with unprecedented sensitivity.”
The technology baselined by ESA to read out the X-ray detector pixels in the X-IFU uses an alternating-current (AC) bias. This process is fundamentally different from the direct-current (DC) readout that has been the focus of TES development at NASA historically. Over the last two years several changes have been made to the TES design to improve the resolution of the detectors with AC bias to enable the very best performance. NASA’s collaborators at the Netherlands Institute for Space Research (SRON) recently achieved, for the first time, a resolution of 2.6 eV in 9 pixels, using NASA-fabricated devices and their AC bias readout technology. NASA has
Dr. Antoine Miniussi at NASA Goddard Space Flight Center installing the assembly to test X-IFU prototype arrays. now delivered an array of 1000 pixels to SRON to demonstrate the performance on a larger scale.
“Combining imaging with spectroscopy, the X-IFU instrument will probe dynamics and composition within spatially extended cosmic objects such as supernova remnants and galaxy clusters with unprecedented sensitivity.” - Dr. Caroline Kilbourne, NASA GSFC
Meanwhile, the NASA/NIST/Stanford team has continued to develop the detector design and associated technology for DC readout. This option may serve as a backup for X-IFU, if necessary. This work has led to the demonstration of the energy resolution required for X-IFU simultaneously in 85 pixels. While this demonstration shows that the required energy resolution can be achieved on the scale of 10 to 100 pixels, the collaborators are now focusing on optimizations to allow the required performance simultaneously on the scale of 1000s of pixels required for X-IFU. The first prototype full-scale arrays, with 3168 pixels placed at the center of a hexagonal wafer, have been fabricated, and are being integrated into a new test apparatus, which will mimic the focal plane assembly of the final X-IFU instrument. Over the next year the team will be testing these full-scale wafers and continuing to improve the detector design to achieve the best possible performance with AC and DC bias.
Astrophysics Division’s PCOS Program
Dr. Caroline Kilbourne, NASA GSFC
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