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eRosita pnCCDs make the X-ray sky visible

14.02.2024

Data from the first sky survey by the imaging X-ray telescope eRosita has now been released. The result: sources of around 710,000 supermassive black holes in distant galaxies, over 180,000 active stars in our own Milky Way up to 12,000 galaxy clusters and a small number of other exotic sources such as X-ray emitting binary stars, supernova remnants, pulsars and other objects. The basis for the novel detector system is the semiconductor laboratory's pnCCD technology.

The pnCCD detector enables precise spectroscopy of X-rays and imaging with high time resolution. It is based on the successful XMM-Newton pnCCD detector concept, but has been further improved in terms of design and technology. The pnCDDs have been redesigned and manufactured using innovative technology. State-of-the-art developments such as the self-aligning CCD register technology were used. The X-Ray Multi-Mirror (XMM-Newton) satellite was launched by the European Space Agency (ESA) in 1999. In particular, an image storage area was added to the imaging area to allow simultaneous imaging and readout in separate CCD areas. The overall pnCCD chip thickness of 450 μm is uniformly sensitive to X-rays from very low to very high energies. The X-ray photon detection efficiency is at least 90 % in the energy band from 0.3 keV to 10 keV. The image storage mode enables very high frame rates of up to 200 X-ray images per second without image smearing.

The eRASS1 observations (eROSITA All-Sky Survey Catalog) with the eROSITA telescope were carried out from December 12, 2019 to June 11, 2020. In the most sensitive energy range of the eROSITA detectors (0.2-2 keV), the telescope detected 170 million X-ray photons for which the cameras can accurately measure the incoming energy and arrival time.

A further development of the HLL's eRosita-pnCCDs has recently left Earth. The Einstein Probe spacecraft of the Chinese Academy of Sciences (CAS) lifted off on January 9, 2024 with a Chang Zheng 2C rocket from the Xichang Satellite Launch Center in China.

After the launch, the Einstein probe reached its orbit at an altitude of around 600 kilometers. The probe orbits the Earth every 96 minutes with an orbital inclination of 29 degrees and is able to observe almost the entire night sky in just three orbits.

pnCCDs, which are developed and produced at the Max Planck Semiconductor Laboratory, are sensors for light and particle detection. They are used in satellites for X-ray astronomy, for which new semiconductor sensors are required. They enable the detection threshold for soft X-rays. pnCCDs can be tailored to the needs of X-ray spectroscopy and X-ray photon counting. In them, the storage capacitors are constructed with pn junctions instead of MOS structures. This eliminates the sensitive silicon-silicon dioxide interface, making a pn-CCD intrinsically more radiation-hard. Over the past three decades, HLL has developed and produced many versions of pn-CCDs, mainly for applications in satellite-based X-ray spectroscopy cameras (XMM-Newton, eRosita, Einstein Probe) and materials science (CAMP, LAMP). The design and technology of pnCCDs is constantly being further developed. Most recently, following self-aligning CCD register technology, new low-impedance register bus connections with polysilicon have been used, which enable a much faster charge transfer from the sensor to the image storage area.