O. Hälker

eROSITA on SRG

eROSITA (extended ROentgen Survey with an Imaging Telescope Array) is the core instrument on the Russian Spektrum-Roentgen-Gamma (SRG) mission which is scheduled for launch in late 2012. eROSITA is fully approved and funded by the German Space Agency DLR and the Max-Planck-Society. The instrument development is in phase C/D since fall 2009. The design driving science is the detection 100.000 Clusters of Galaxies up to redshift z ~1.3 in order to study the large scale structure in the Universe and test cosmological models, especially Dark Energy. This will be accomplished by an all-sky survey lasting for four years plus a phase of pointed observations. eROSITA consists of seven Wolter-I telescope modules, each equipped with 54 Wolter-I shells having an outer diameter of 360 mm. This would provide an effective area of ~1500 cm2 at 1.5 keV and an on axis PSF HEW of 15 arcsec resulting in an effective angular resolution of 28 - 30 arcsec, averaged over the field of view. In the focus of each mirror module, a fast frame-store pn-CCD provides a field of view of 1° in diameter.

First results of DEPFET based Active Pixel Sensor prototypes for the XEUS Wide Field Imager

The concept of an Active Pixel Sensor (APS) based on the integrated detector/amplifier structure DEPFET (DEpleted P-channel Field Effect Transistor) has been developed to cope with the challenging requirements of the XEUS Wide Field Imager. The DEPFET-APS combines high energy resolution, fast readout, and random accessible pixels allowing the application of flexible readout modes. First prototypes of DEPFET-based Active Pixel Sensors with a 64 x 64 pixel format and 75 μm x 75 μm pixel area have been produced at the MPI semiconductor laboratory. The APS is read out row by row, i.e. the pixel signals of one row are processed in parallel by a 64 channel CMOS amplifier/multiplexer chip of the CAMEX type. The addressing of one row of pixels for readout and reset is done by two control chips of the SWITCHER type fabricated in a high-voltage CMOS technology. The processing time for one row is of the order of a few micro-seconds. APS operation, the control and data acquisition system are described, and first experimental results are presented.

CCD detectors for spectroscopy and imaging of x-rays with the eROSITA space telescope

A special type of CCD, the so-called PNCCD, was originally developed for the focal plane camera of the XMMNewton space telescope. After the satellite launch in 1999, the MPI Halbleiterlabor continued the detector development for various ground-based applications. Finally, a new X-ray PNCCD was designed again for a space telescope named eROSITA. The space telescope will be equipped with an array of seven parallel oriented X-ray mirror systems of Wolter-I type and seven cameras, placed in their foci. This instrumentation will permit the exploration of the X-ray universe in the energy band from 0.3 keV up to 10 keV with a time resolution of 50 ms for a full image comprising 384 x 384 pixels. eROSITA will be accommodated on the new Russian Spectrum-RG satellite. The mission was already approved by the responsible German and Russian space agencies. The detector development is focussed to fulfil the scientific specifications for detector performance under the constraints of all the mechanical, power, thermal and radiation hardness issues for space instrumentation. This considers also the recent change of the satellites orbit. The Lagrange point L2 was decided as new destination of the satellite instead of a low-Earth orbit (LEO). We present a detailed description of the detector system and the current development status. The most recent test results are reported here. Essential steps for completion of the seven focal plane detectors until satellite launch in 2012 will be itemized.

Advancements in DEPMOSFET device developments for XEUS

DEPMOSFET based Active Pixel Sensor (APS) matrices are a new detector concept for X-ray imaging spectroscopy missions. They can cope with the challenging requirements of the XEUS Wide Field Imager and combine excellent energy resolution, high speed readout and low power consumption with the attractive feature of random accessibility of pixels. From the evaluation of first prototypes, new concepts have been developed to overcome the minor drawbacks and problems encountered for the older devices. The new devices will have a pixel size of 75 μm × 75 μm. Besides 64 × 64 pixel arrays, prototypes with a sizes of 256 × 256 pixels and 128 × 512 pixels and an active area of about 3.6 cm2 will be produced, a milestone on the way towards the fully grown XEUS WFI device. The production of these improved devices is currently on the way. At the same time, the development of the next generation of front-end electronics has been started, which will permit to operate the sensor devices with the readout speed required by XEUS. Here, a summary of the DEPFET capabilities, the concept of the sensors of the next generation and the new front-end electronics will be given. Additionally, prospects of new device developments using the DEPFET as a sensitive element are shown, e.g. so-called RNDR-pixels, which feature repetitive non-destructive readout to lower the readout noise below the 1 e- ENC limit.

Report on the eROSITA camera system

The eROSITA space telescope is currently developed for the determination of cosmological parameters and the equation of state of dark energy via evolution of clusters of galaxies. Furthermore, the instrument development was strongly motivated by the intention of a first imaging X-ray all-sky survey enabling measurements above 2 keV. eROSITA is a scientific payload on the Russian research satellite SRG. Its destination after launch is the Lagrangian point L2. The observational program of the observatory divides into an all-sky survey and pointed observations and takes in total about 7.5 years. The instrument comprises an array of 7 identical and parallel aligned telescopes. Each of the seven focal plane cameras is equipped with a PNCCD detector, an enhanced type of the XMM-Newton focal plane detector. This instrumentation permits spectroscopy and imaging of X-rays in the energy band from 0.3 keV to 10 keV with a field of view of 1.0 degree. The camera development is done at the Max-Planck-Institute for extraterrestrial physics. Key component of each camera is the PNCCD chip. This silicon sensor is a back-illuminated, fully depleted and column-parallel type of charge coupled device. The image area of the 450 micron thick frame-transfer CCD comprises an array of 384 x 384 pixels, each with a size of 75 micron x 75 micron. Readout of the signal charge that is generated by an incident X-ray photon in the CCD is accomplished by an ASIC, the so-called eROSITA CAMEX. It provides 128 parallel analog signal processing channels but multiplexes the signals finally to one output which feeds the detector signals to a fast 14-bit ADC. The read noise of this system is equivalent to a noise charge of about 2.5 electrons rms. We achieve an energy resolution close to the theoretical limit given by Fano noise (except for very low energies). For example, the FWHM at an energy of 5.9 keV is approximately 140 eV. The complete camera assembly comprises the camera head with the detector as key component, the electronics for detector operation as well as data acquisition and the filter wheel unit. In addition to the on-chip light blocking filter directly deposited on the photon entrance window of the PNCCD, an external filter can be moved in front of the sensor, which serves also for contamination protection. Furthermore, an on-board calibration source emitting several fluorescence lines is accommodated on the filter wheel mechanism for the purpose of in-orbit calibration. Since the spectroscopic silicon sensors need cooling down to -95°C to mitigate best radiation damage effects, an elaborate cooling system is necessary. It consists of two different types of heat pipes linking the seven detectors to two radiators. Based on the tests with an engineering model, a flight design was developed for the camera and a qualification model has been built. The tests and the performance of this camera is presented in the following. In conclusion an outlook on the flight cameras is given.

Systematic testing and results of X-ray CCDs developed for eROSITA and other applications

An advanced pnCCD type has been developed, based on the concept of the XMM-Newton detector, which has been performing spectroscopy and imaging since 2000. This new detector is designed according to the requirements of eROSITA, a new X-ray astronomy mission, to be launched in 2010. The focal plane for each of the seven individual Wolter telescopes will be equipped with one of these new-type X-ray pnCCDs. In addition to the eROSITA chips, we have developed CCDs for other applications, e.g. for projects which require smaller pixel sizes. The devices that have been produced in the semiconductor laboratory (MPI Halbleiterlabor) of the Max-Planck-Institut fur extraterrestrische Physik are currently subject of systematic quality checks and spectroscopic tests. These tests are performed under standardized conditions on a representative subset of the many devices we have produced. The aim of these tests is to extract the key performance parameters of the individual CCDs like readout noise, energy resolution and the occurrence of bad pixels. The analysis includes the CAMEX analog signal processor, which has been developed for the readout of the CCD signals. After an introduction, we present the motivation for the detector development and give an overview about our CCD design and production, as well as about the CAMEX ASIC. Then device tests, test setups and data analysis are described. We report in detail about the performance of the tested devices. Failures that occurred during device tests are subsequently discussed. Finally, we give a review of the results.

eROSITA camera design and first performance measurements with CCDs

The German X-ray observatory eROSITA (extended ROentgen Survey with an Imaging Telescope Array) is the prime instrument of the new Spectrum-RG mission. Launch of the Russian satellite is planned for the year 2011. The scientific goal of eROSITA is primarily the detection and analysis of 100 thousand clusters of galaxies in order to study the large scale structures in the Universe and to test cosmological models. The therefore required large effective area is obtained by an array of seven identical and parallel aligned Wolter-I telescopes. In the focus of each mirror module, there is a large frame store pnCCD detector, providing a field of view of 1° in diameter. The same X-ray detector type will also be applied for ART-XC, another grazing-incidence telescope system aboard Spectrum-RG, which permits the detection of heavily obscured X-ray sources. These scientific instruments allow the exploration of the X-ray Universe in the energy band from 0.3 keV to 11 keV. During a mission time of at least five years, an all-sky survey, wide as well as deep surveys and pointed observations will be performed. Approval and funding for eROSITA were granted by the German space agency DLR in April 2007. The conceptual design of the X-ray focal plane cameras is presented here comprising electrical, thermal, and mechanical aspects. Key part of the camera is the pnCCD detector chip, which is developed and produced in our semiconductor laboratory, the MPI Halbleiterlabor. The CCD was designed according to the specifications given by the scientific goals of eROSITA. The eROSITA CCD differs apparently from all previously produced frame store pnCCDs by its larger size and format. The CCD image area of the seven eROSITA cameras is in total 58 cm2 large and their number of pixels is about seven times higher than that of the XMM-Newton pnCCD camera. First pnCCD devices were recently produced and tested. Their performance measurements and results are of most importance for eROSITA because the tested CCDs are the control sample of the flight detector production.

DEPFET based X-ray detectors for the MIXS focal plane on BepiColombo

DEPFET Macropixel detectors, based on the fusion of the combined Detector-Amplifier structure DEPFET with a silicon drift chamber (SDD) like drift ring structure, combine the excellent properties of the DEPFETs with the advantages of the drift detectors. As both device concepts rely on the principle of sideways depletion, a device entrance window with excellent properties is obtained at full depletion of the detector volume. DEPFET based focal plane arrays have been proposed for the Focal Plane Detectors for the MIXS (Mercury Imaging X-ray Spectrometer) instrument on BepiColombo, ESAs fifth cornerstone mission, with destination Mercury. MIXS uses a lightweight Wolter Type 1 mirror system to focus fluorescent radiation from the Mercury surface on the FPA detector, which yields the spatially resolved relative element abundance in Mercurys crust. In combination with the reference information from the Solar Intensity X-ray Spectrometer (SIXS), the element abundance can be measured quantitatively as well. The FPA needs to have an energy resolution better than 200 eV FWHM @ 1 keV and is required to cover an energy range from 0.5 keV to 10 keV, for a pixel size of 300 x 300 μm2. Main challenges for the instrument are the increase in leakage current due to a high level of radiation damage, and the limited cooling resources due to the difficult thermal environment in the mercury orbit. By applying an advanced cooling concept, using all available cooling power for the detector itself, and very high speed readout, the energy resolution requirement can be kept during the entire mission lifetime up to an end-of-life dose of ~ 3 × 1010 10 MeV p / cm2. The production of the first batch of flight devices has been finished at the MPI semiconductor laboratory, and first prototype modules have been built. The results of the first tests will be presented here.

Progress of the x-ray CCD camera development for the eROSITA telescope

The eROSITA space telescope is presently developed for the determination of cosmological parameters and the equation of state of dark energy via evolution of galaxy clusters. It will perform in addition a census of the obscured black hole growth in the Universe. The instrument development was also strongly motivated by the intention of a first imaging X-ray all-sky survey above an energy of 2 keV. eROSITA is scientific payload on the Russian research satellite SRG and the mission duration is scheduled for 7.5 years. The instrument comprises an array of seven identical and parallel-aligned telescopes. The mirror system is of Wolter-I type and the focal plane is equipped with a PNCCD camera for each of the telescopes. This instrumentation permits spectroscopy and imaging of X-rays in the energy band from 0.3 keV to 10 keV with a field of view of 1.0 degree. The camera development is done at the Max-Planck-Institute for Extraterrestrial Physics and in particular the key component, the PNCCD sensor, has been designed and fabricated at the semiconductor laboratory of the Max-Planck Society. All produced devices have been tested and the best selected for the eROSITA project. Based on calculations, simulations, and experimental testing of prototype systems, the flight cameras have been configured. We describe the detector and its performance, the camera design and electronics, the thermal system, and report on the latest estimates of the expected radiation damage taking into account the generation of secondary neutrons. The most recent test results will be presented as well as the status of the instrument development.

Noise and spectroscopic performance of DEPMOSFET matrix devices for XEUS

DEPMOSFET based Active Pixel Sensor (APS) matrix devices, originally developed to cope with the challenging requirements of the XEUS Wide Field Imager, have proven to be a promising new imager concept for a variety of future X-ray imaging and spectroscopy missions like Simbol-X. The devices combine excellent energy resolution, high speed readout and low power consumption with the attractive feature of random accessibility of pixels. A production of sensor prototypes with 64 x 64 pixels with a size of 75 μm x 75 μm each has recently been finished at the MPI semiconductor laboratory in Munich. The devices are built for row-wise readout and require dedicated control and signal processing electronics of the CAMEX type, which is integrated together with the sensor onto a readout hybrid. A number of hybrids incorporating the most promising sensor design variants has been built, and their performance has been studied in detail. A spectroscopic resolution of 131 eV has been measured, the readout noise is as low as 3.5 e- ENC. Here, the dependence of readout noise and spectroscopic resolution on the device temperature is presented.