CCD detector development for the eROSITA space telescope

Abstract

The German X-ray telescope eROSITA is the core instrument on the Russian satellite Spectrum-Roentgen-Gamma (SRG). Its scientific goal is the exploration of the X-ray Universe in the energy band from about 0.3 keV up to 10 keV with excellent energy, time and spatial resolution and large effective telescope area. The launch of the SRG satellite is scheduled for 2013. The observational program divides the planned mission duration of seven years into an all-sky survey and pointed observations. For detection of the single X-ray photons with high resolution, adequate frame transfer pnCCDs and the associated front-end electronics have been developed. The back-illuminated, 450 μm thick and fully depleted pnCCDs with a 3 cm × 3 cm large image area have been produced in the MPI Halbleiterlabor in the course of further development of the XMM-Newton X-ray pnCCDs. By means of the concept of back-illumination and full depletion of the chip thickness, high quantum efficiency is obtained over the entire energy band of interest. The performance of each eROSITA CCD was tested on chip level using a so-called `cold chuck probe station'. A special feature of this setup is that it allows spectroscopic measurements with a <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">55</sup> Fe source. Based on these results, we will select the seven best CCDs for the eROSITA focal plane cameras. An analog signal processor with 128 parallel channels has been developed for readout of the pnCCD signals. This ASIC permits fast and low-noise signal filtering. For a detailed characterization of the CCD detectors an appropriate control, supply and data acquisition electronics system was developed. We achieve a read noise of 2 electrons rms and an energy resolution of 135 eV FWHM for photons with energy of 5.9 keV. Even at the low X-ray energy of 280 eV, we measure a spectrum of Gaussian shape with a FWHM of 52 eV. However, the energy resolution will degrade during the seven years in space due to radiation damage caused by protons. The radiation damage effect was studied and quantified for the eROSITA CCDs in an experiment. After successful development and verification of the CCD and its signal processor chip, we have started to assemble a flight-like eROSITA camera.