pnCCD detectors for eROSITA

Time-Resolved Spectroscopy and Imaging of X-Rays

We develop for the eROSITA X-ray space telescope an array of seven pnCCD focal plane cameras. The pnCCD detector system permits time-resolved spectroscopy and imaging of X-rays in the energy band from about 0.1 keV up to 15 keV. After production of the eROSITA pnCCD wafers in the MPG semiconductor laboratory, we measured the performance of the X-ray CCDs. Presently, we set up an engineering model of the eROSITA flight detector system.

The concept of the pnCCD was originally developed for the XMM-Newton mission [1]. The pnCCD detector performs continuously since 1999 excellent measurements onboard of the ESA satellite. The eROSITA pnCCDs are based on the same concept but various optimizations were implemented by means of design and technology improvements. The energy resolution was optimized for all energies and furthermore the new detectors permit spectroscopy even at energies down to 0.1 keV. A frame store was added to suppress out-of-time events and due to optimized manufacturing technology the occurrence of any pixel defects could be nearly eliminated [2].

A complete camera system was developed for performance characterization of the eROSITA CCD and its dedicated programmable analog signal processor chip CAMEX. This system serves furthermore for optimization of the supply and control flight electronics. Based on these results, the final design of the eROSITA CCD camera and its operating parameters are determined. Tests at external facilities were carried out for measurements of quantum efficiency and spectral response at the BESSY synchrotron and for radiation hardness studies at the TANDEM accelerator of the Maier-Leibnitz Laboratorium. The variety of experimental tests was completed by simulations, e.g. of the instrument background spectrum of eROSITA at L2 orbit with graded-Z-shield design of the cameras [3].

Fig.1: The first pnCCD camera is sucessfully in operation onboard of the XMM-Newton satellite for more than 10 years.

Fig.2: Detector wafer (150 mm), high purity, double-sided polished) with four eROSITA pnCCDs in its centre.

Fig. 6: Illumination of the pnCCD through a mask with Al-K X-rays (1.5 keV) in single photon counting mode. The image shows a superposition of 20,000 single frames.

Fig.3: CCD lab module for test of eROSITA CCDs, CAMEX ASICs and detector system concept.

pnCCD detector characteristics

• chip thickness (=450µm) fully sensitive
• back-illuminated CCD
• image: 384 ∙ 384 pixels of 75 x 75µm 2 size (≈ 3 cm ∙ 3 cm)
• analog signal processor CAMEX: low-noise 128 parallel channels (multiplexed to 1 output)
• readout time per image: 10 ms
• read noise: 2 electrons rms
• FWHM (6 keV) ≈ 135 eV
• FWHM ( 280 eV) ≈ 52 eV
• no pixel defects
• quantum efficiency still 90% @ 12 keV
• power consumption: 0.7 W per detector (20 images/s)

Energy resolution

Fig.7: Prototype of eROSITA flight CCD detector module: CCD chip (on the left), 3 CAMEX analog signal processors (center), RC filters and interface to flexible lead (on the right)

Fig.4: Noise histogram of eROSITA CCD pixels. Mean pixel noise: 2.2 electrons rms; noisiest pixel: 2.8 electrons rms.

Fig.5: Spectrum at an energy of 280 eV (at synchrotron) measured with an eROSITA pnCCD. FWHM@280eV = 52eV.

Outlook

The measurements with the eROSITA pnCCD devices and the associated CAMEX readout chips revealed excellent performance. The currently assembled prototype of the eROSITA flight detector serves for testing of the complete flight camera system including the supply, control and data acquisition electronics. After verification, we start with the eROSITA flight camera array.

References:

• [1] L. Strüder, U. Briel, K. Dennerl, et al., A&A, 365,1 (2001)
• [2] N. Meidinger, R. Andritschke, S. Ebermayer, et al.,
Proc. SPIE, 7435 (2009)
• [3] N. Meidinger, R. Andritschke, S. Ebermayer, et al.,
Nucl. Instr. and Meth. A (2010),
doi:10.1016/j.nima.2010.03.126