Introduction

Since several years, charge coupled devices (CCD) type detectors are used in X-ray satellite missions in the focus of imaging optics (e.g., ASCA, Chandra, XMM-Newton, Swift, Suzaku). They measure position, energy, and arrival time of individual X-rays from 100 eV to 15keV energy. The Japanese ASCA satellite was the first X-ray mission to employ CCDs as a focal plane detector in 1993. The US American Chandra and the European XMM-Newton missions followed with larger detector arrays and improved performance in 1999.

Since the launch of the European XMM-Newton satellite on December 10, 1999, reliably operating X-ray CCDs are delivering extraordinary images, recorded in a single photon counting mode, imaged through the largest X-ray telescope ever built. Behind two of the three X-ray mirror systems reflecting grating spectrometers are measuring high resolution X-ray spectra, recorded with 9 CCDs along the dispersion direction [26]. About 40% of the incident X-ray flux is transferred to a focal plane with a mosaic of 7 CCDs covering a field of view of approximately 30arcmin [5]. All of those CCDs are of the conventional MOS type.

The telescope without gratings getting 100% of the X-rays directed onto the focal plane is equipped with the novel pn-type CCD. The fully depleted, back side illuminated pn-junction CCD (pnCCD), operated in full frame mode, has been developed for applications in X-ray astronomy. A monolithic 6 x 6 cm2 large pnCCD is working since more than 7 years in orbit aboard the European X-ray satellite XMM-Newton.

Similar devices are working today in many different fields like hadron physics, synchrotron radiation research, quantum optics, X-ray microscopy, material analysis, and others. In the last years, the capabilities of those devices were largely extended for their use in high speed photography in the visible and near infrared.

For future missions, such as eROSITA, we have developed a new generation of frame store pnCCDs: As the depleted, sensitive thickness of the pnCCDs is increased up to 500 |im, their high energy response to X-rays is extended to 30keV with a quantum efficiency of still 20%. Frame store pnCCDs have been fabricated with up to 512 readout nodes, funneled into eight parallel output chains, enabling the detector with a format of 256 x 512 pixels and pixel sizes of 51 x 51 | m2 and 75 x 75 | m2 to be read out in 1 ms with a read noise of two electrons (rms).

A typical operation temperature is -60° C, where Fano-limited X-ray spectroscopy is reached.

The basic concept of CCDs, their charge transfer mechanisms, their limitations in energy resolution, their long-term stability, and the achieved results of measurements are presented in the following. The main focus is set on the concept of pnCCDs as they exhibit excellent energy and position resolution at high quantum efficiency from the near infrared up to 30 keV, high readout speed (resp. high time resolution), low noise, and high radiation hardness - key parameters for X-ray missions to date and in the future.

X-ray astronomy is pushing since several years the instrumentation for broadband imaging nondispersive X-ray spectrometers. A brief outlook will, therefore, be finally given on new concepts for imaging X-ray spectrometers based on the concept of the DEPFET active pixel sensors.

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