eROSITA is the primary instrument on-board the Russian-German "Spectrum-Roentgen-Gamma" (SRG) mission which was successfully launched from Baikonur on July 13, 2019 and placed in a halo orbit around the L2 point. eROSITA will perform the first imaging all-sky survey in the medium energy X-ray range up to 10 keV with an unprecedented spectral and angular resolution.
The eROSITA telescope consists of seven identical Wolter-1 mirror modules. Each module contains 54 nested mirror shells in order to meet the required sensitivity. A novel detector system has been developed by the Max Planck Institute for extraterrestrial Physics on the basis of the successful XMM-Newton pn-CCD technology of MPG HLL.
The pnCCD detector permits accurate spectroscopy of X-rays as well as imaging with high time resolution. It is based on the successful XMM-Newton pnCCD detector concept but was further improved in terms of design and technology. In particular a frame store section is added to the image area for the purpose of simultaneous imaging and readout in separate CCD areas. The thickness of the whole pnCCD chip of 450 μm is uniformly sensitive to X-rays from very low up to very high energies. The Xray photon detection efficiency is at least 90 % in the energy band from 0.3 keV to 10 keV. Frame store operation allows very high frame rates up to 200 hundred X-ray images per second without smearing of the image. The pnCCD is tailored to the requirements of the eROSITA mission. These are in particular a 3 cm by 3 cm large image area (corresponding to a field of view of one degree in diameter on the satellite) with a pixel size of 75 μm by 75 μm.
The power consumption in the focal plane is about 4Watt for the seven pnCCD cameras in total. Interfering electron-hole pair generation due to optical and UV light has to be prevented in space to achieve best X-ray spectroscopy. Instead of a development of an external fragile filter, we deposit directly an on-chip light filter on the photon entrance window of the detector which is feasible with our device production technology. Another important requirement to the seven pnCCD detectors on the SRG satellite is operation during at least five years without any failure. This includes long-term stability of detector performance. With the existing prototype of our analog signal processor CAMEX for the readout of the CCD signals, we already obtain an excellent detector system readout noise of two electrons rms. The optimized photon entrance window allows straightforward spectroscopy of X-ray lines even at energies below 0.3 keV. For the best prototype devices we observed no pixel defects (neither noisy nor bright nor dark) at all on the entire detector area.
Prototype of the eROSITA pnCCD detector module with 2 cm by 2 cm large pnCCD. The prototype is read out by two 128-channel CAMEX analog signal processors. The chips are mounted and connected on a ceramic multi-layer printed circuit board.
Low energy response of frame store pnCCD demonstrated with a Mn-Kα spectrum (5.9 keV).
Low energy response of frame store pnCCD demonstrated with a C-Kα spectrum (277 eV).