Johannes Elbs

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.

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.

Quantum efficiency measurements of eROSITA pnCCDs

For the eROSITA X-ray telescope, which is planned to be launched in 2012, detectors were developed and fabricated at the MPI Semiconductor Laboratory. The fully depleted, back-illuminated pnCCDs have an ultrathin pn-junction to improve the low-energy X-ray response function and quantum efficiency. The device thickness of 450 μm is fully sensitive to X-ray photons yielding high quantum efficiency of more than 90% at photon energies of 10 keV. An on-chip filter is deposited on top of the entrance window to suppress visible and UV light which would interfere with the X-ray observations. The pnCCD type developed for the eROSITA telescope was characterized in terms of quantum efficiency and spectral response function. The described measurements were performed in 2009 at the synchrotron radiation sources BESSY II and MLS as cooperation between the MPI Semiconductor Laboratory and the Physikalisch-Technische Bundesanstalt (PTB). Quantum efficiency measurements over a wide range of photon energies from 3 eV to 11 keV as well as spectral response measurements are presented. For X-ray energies from 3 keV to 10 keV the quantum efficiency of the CCD including on-chip filter is shown to be above 90% with an attenuation of visible light of more than five orders of magnitude. A detector response model is described and compared to the measurements.

Characterization of eROSITA PNCCDs

The eROSITA (extended Roentgen Survey with an Imaging Telecope Array) instrument on the satellite Spektrum-Roentgen-Gamma will perform an all-sky survey as well as pointed observations in the low and medium X-ray energy regime. The seven PNCCD detectors for this instrument have been developed and manufactured at the Max Planck Institute (MPI) Semiconductor Laboratory. We summarize the characterization of this type of PNCCD regarding the spectral redistribution function in the photon energy range from 100 eV to 11 keV. We present the results of Geant4 simulations compared to our measurements. The observed spectral features and their physical origin inside the detector are explained. An overview on the different contributions to the energy resolution of the detector is given.

CCD detector development for the eROSITA space telescope

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 55Fe 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.

Design and performance of the eROSITA focal plane instrumentation

We developed and tested X-ray PNCCD focal plane detectors for the eROSITA (extended ROentgen Survey with an Imaging Telescope Array) space telescope. General scientific goal of the eROSITA project is the exploration of the X-ray universe in the energy band from about 0.2 keV up to 10 keV with excellent energy, time, and spatial resolution in combination with large effective telescope areas. The observational program divides into an all-sky survey and pointed observations. The mission duration is scheduled for 7.5 years. The German instrument will be launched in near future to the Lagrange point L2 on the Russian satellite SRG. The detection of single X-ray photons with precise information about their energy, angle of incidence and time is accomplished for eROSITA by an array of seven identical and independent PNCCD cameras. Each camera is assigned to a dedicated mirror system of Wolter-I type. The key component of the camera is a 5 cm • 3 cm large, back-illuminated, 450 μm thick and fully depleted frame store PNCCD chip. This chip is a further development of the sensor type that is in operation as focal plane detector on the XMMNewton satellite since launch in 1999 to date. Development and production of the CCDs for the eROSITA project were performed by the MPI Halbleiterlabor, as already in the past for the XMM-Newton project. According to the status of the project, a complete design of the seven flight cameras including the camera electronics and the filter wheel has been developed. Various functional and performance tests have been accomplished for a detailed characterization of the eROSITA camera system. We focus here especially on the focal plane detector design and the performance of the detectors, which are essential for the success of the X-ray astronomy space project.

The spectral redistribution function of eROSITA PNCCDs

The eROSITA instrument on the satellite Spektrum-Roentgen-Gamma will perform an all-sky survey as well as pointed observations in the low and medium X-ray energy regime. The 7 PNCCD detectors for this instrument are developed and manufactured at the MPI Semiconductor Laboratory. We summarize the characterization of this type of PNCCD regarding the spectral redistribution function in the photon energy range from 100 eV to 11 keV. We present the results of Geant4 simulations compared to our measurements. We explain the observed spectral features and their origin inside the detector.

Particle detection with PNCCDs

A PNCCD is successfully operating as one of the focal plane CCDs aboard the satellite XMM-Newton. An advanced version of this kind of CCDs will be the sensing devices for the eROSITA X-ray astronomy mission. These fully depleted CCDs are developed and manufactured at the MaxPlanck-Institute Semiconductor Lab together with the company PNSensor. Their performance features make them useful in a variety of measurement situations in addition to the astronomical ones. Applications range from photon detection (e.g. optical wave front sensors, cameras for X-ray free electron lasers) to charged particle detection of e.g. electrons, protons, and alpha particles. First tests have been performed for using a PNCCD in a transmission electron microscope (TEM). Alpha particles of an 241Am radioactive source are used for generating large signal electron clouds for diagnostic purposes. Finally, protons have been used in a radiation hardness test. These three applications will be described and discussed. The prospect of resolving space and energy of each particle in combination with ∼100% efficiency makes the PNCCD especially suitable for low flux applications e.g. examining sensitive samples in a TEM.

Electronic test system for the eROSITA x-ray PNCCDs

The new X-ray telescope eROSITA (extended ROentgen Survey with an Imaging Telescope Array) is the main instrument on the Russian new Spectrum-RG satellite, scheduled for launch in 2012. The primary scientific goal of eROSITA is the detection of about 100,000 clusters of galaxies in an all sky survey. This allows a systematic study on the large scale structures in the universe and will give new information about the nature of dark energy. The focal plane detector is a 5 cm × 3 cm framestore PNCCD, an advanced successor of the XMM-Newton PNCCD, designed and fabricated at the MPI Halbleiterlabor. It has 384 × 384 pixels of 75 μm × 75 μm in the image area and will provide high position, time and spectral resolution as well as a high quantum efficiency for X-ray photons in the energy range from 0.2 keV up to 10 keV. The first flight-like CCDs have been finished in 2008. In order to extensively test these new PNCCDs we developed an electronic test-setup. It is very versatile, allowing us to test the CCDs under many different conditions and is appropriate to show at the same time excellent performance of the detector. In this contribution we present in detail the electronic test-setup, some test results and the conclusions which can be drawn for the eROSITA flight modules.

eROSITA focal plane instrumentation design

The German X-ray instrument eROSITA (extended ROentgen Survey with an Imaging Telescope Array) is the main instrument of the new Spectrum-RG mission orbiting at L2. The launch of the Russian satellite is planned for the year 2012. The scientific goal of eROSITA is primarily an all sky survey with 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. The focal plane instrumentation is realized with a 384 × 384 pixel frame store pnCCD mounted on a ceramic PCB and cooled to -80 C. The ceramic PCB also houses three CAMEX readout ASICs to amplify and filter each of the 384 channels in parallel. The cooled ceramic is connected via a flex lead to the electronics box at room temperature. A copper housing shields for proton radiation whereas a graded-Z shield attenuates X-ray fluorescence.