A 36 cm2 Large Monolythic pn-CCD Detector

L. Strüdera, H. Bräuningera, R. Hartmanna, D. Hauffc,
P. Hollb, J. Kemmerb, E. Kendziorrad, N. Krausea, P. Lechnerb, G. Lutzc,
B. Maiera, N. Meidingera, E. Pfeffermanna, M. Poppa, C. Reppina,
R. H. Richterc, J. Riedelc, H. Soltaub, D. Stöttera, U. Weberc, C. v. Zanthierb
aMax-Planck-Institut für extraterrestrische Physik, Giessenbachstraß e, D-85740 Garching
bKETEK GmbH, Am Isarbach 30, D-85764 Oberschleiß heim
cMax-Planck-Institut für Physik, Föhringer Ring 6, D-80805 München
dInstitut für Astronomie und Astrophysik der Universität Tübingen
Abteilung Astronomie, Waldhäuser Straß e 64, D-72076 Tübingen

Monolithic arrays of 12 CCDs, 3 × 1 cm2 each, have been developed and produced for the focal plane instrumentation of the European Photon Imaging Camera (EPIC) on XMM and the German ABRIXAS X-ray satellite mission. The design parameters have been optimized to match the properties of the X-ray imaging optics as well as the X-ray intensity, energy bandwidth and characteristic time constants of the objects to observe.

The pixel size is 150 × 150 µm2; readout is performed in parallel; low noise, spectroscopic performance is realized by on-chip integrated JFET electronics; highohmic, ultrapure bulk material allows full depletion and enhances the efficiency for higher energy X-ray detection. The fabrication process, the layout topology and the operating conditions guarantee for a ten year operation in space without performance degradation.

Flight devices have been fabricated and tested with unprecedented quality: Only 0.01% of the pixels are not usable for spectroscopy and imaging.

The pn-CCDs were tested under conditions which are close to the flight conditions. The most relevant results are:

  1. The energy resolution is as good as physically possible with silicon: above 1 keV the response of a monochromatic X-ray beam is dominated by the statistical fluctuations of the ionization process (Fano effect), e.g. @ 5.900 eV the FWHM is better than 140 eV (version 9).
  2. The quantum efficiency is high at low energies (e.g. 90% at 500 eV) because of the spatially homogeneous, thin, backside illuminated radiation entrance window. The sensitive thickness of 300 µm guarantees the high quantum efficiency, of e.g. 91% at 10 keV.
  3. The angular resolution of the pn-CCD system is about a factor of 4 better than the HEW of the XMM telescope system. Thus the imaging capabilities of the mirror system are fully exploited. The reduced number of pixels leads to a faster read-out.
  4. In the Full Frame mode the pn camera is able to perform single photon counting up to 1 mCrab surface brightness. In the fastest burst mode objects with a brightness up to 6 Crab can be observed.
  5. The use of pn-junctions instead of MOS gates, the transfer of signal charges deep in the bulk of the device and the rapid timing leads to a high tolerance with respect to damaging radiation. The calculated 10 year XMM dose (equivalent to 4×108 protons per cm2) decreases the energy resolution only about 5 eV. At a five times higher dose the increase of the FWHM is only 30 eV (e.g. 170 eV at 5.9 keV).

The demonstration model is ready for testing, the first flight model is actually being integrated. Two more cameras are waiting for their final assembly.

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