G. Lutz

The low energy detector of Simbol-X

Simbol-X is a French-Italian-German hard energy X-ray mission with a projected launch in 2014. Being sensitive in the energy range from 500 eV to 80 keV it will cover the sensitivity gap beyond the energy interval of todays telescopes XMM-Newton and Chandra. Simbol-X will use an imaging telescope of nested Wolter-I mirrors. To provide a focal length of 20 m it will be the first mission of two independent mirror and detector spacecrafts in autonomous formation flight. The detector spacecrafts payload is composed of an imaging silicon low energy detector in front of a pixelated cadmium-telluride hard energy detector. Both have a sensitive area of 8 × 8 cm2 to cover a 12 arcmin field of view and a pixel size of 625 × 625 μm2 adapted to the telescopes resolution of 20 arcsec. The additional LED specifications are: high energy resolution, high quantum efficiency, fast readout and optional window mode, monolithic device with 100 % fill factor and suspension mounting, and operation at warm temperature. To match these requirements the low energy detector is composed of active macro pixels, combining the large, scalable area of a Silicon Drift Detector and the low-noise, on-demand readout of an integrated DEPFET amplifier. Flight representative prototypes have been processed at the MPI semiconductor laboratory, and the prototypes measured performance demonstrates the technology readiness.

DEPFET active pixel sensor with non-linear amplification

One of the X-ray imaging detectors in development for the European XFEL (X-ray Free Electron Laser) is the DSSC (DEPFET Sensor with Signal Compression). The DSSC sensor array will have a format of 1024 × 1024 pixels with a pixel size in the order of 200 µm. It is optimized for the detection of low-energy X-ray photons and designed to operate at frame rates up to 4.5 MHz with a dynamic range of several thousand photons per pixel and frame and single photon resolution at small photon numbers. The DSSC systems core component is an active pixel sensor based on the DEPFET (DEpleted P-channel Field Effect Transistor) structure. A DEPFET is an integrated detector-amplifier combining internal amplification, full sensitivity over the whole bulk thickness, analog data storage, readout on demand, low serial noise, and absence of reset noise. For the DSSC-adaptation of the DEPFET principle the new feature of signal compression has been added, i.e. the device has a high gain at small signals and a reduced gain at large signals. With this strongly non-linear response the DSSC-DEPFET provides the required high charge handling capacity with simultaneous single photon resolution for low-energy X-rays. This paper presents the concept of the DSSC-DEPFET. Its functionality is demonstrated by measurements of a first series of prototypes.

Development and Characterization of New 256

DEPFET detectors are silicon (Si) active pixel sensors designed and manufactured in the Max-Planck-Institut semiconductor lab. Their high spatial resolution and high energy resolution in X-rays make them attractive for particle tracking in colliders and for X-ray astronomy. This technology is foreseen for the Wide Field Imager of the International X-ray Observatory currently in study with ESA, NASA, and JAXA. New DEPFET matrixes with 256 × 256 pixels of 75-μm pitch have been produced, mounted on ceramic boards with dedicated front-end electronics and integrated in a new setup able to acquire large-format images and spectra. Excellent homogeneity has been observed. Energy resolution as low as 127 eV FWHM at 5.9 keV has been obtained including all single events of the matrix back illuminated at -45<;°C and read out at a 300-frames/s rate. This paper presents experimental methods and results.

\,\times\,

Pixelated semiconductor detectors for X-ray imaging spectroscopy are foreseen as key components of the payload of various future space missions exploring the x-ray sky. Located on the platform of the new Spectrum-Roentgen-Gamma satellite, the eROSITA (extended Roentgen Survey with an Imaging Telescope Array) instrument will perform an imaging all-sky survey up to an X-ray energy of 10 keV with unprecedented spectral and angular resolution. The instrument will consist of seven parallel oriented mirror modules each having its own pnCCD camera in the focus. The satellite born X-ray observatory SIMBOL-X will be the first mission to use formation-flying techniques to implement an X-ray telescope with an unprecedented focal length of around 20 m. The detector instrumentation consists of separate high- and low energy detectors, a monolithic 128 × 128 DEPFET macropixel array and a pixellated CdZTe detector respectively, making energy band between 0.5 to 80 keV accessible. A similar concept is proposed for the next generation X-ray observatory IXO. Finally, the MIXS (Mercury Imaging X-ray Spectrometer) instrument on the European Mercury exploration mission BepiColombo will use DEPFET macropixel arrays together with a small X-ray telescope to perform a spatially resolved planetary XRF analysis of Mercurys crust. Here, the mission concepts and their scientific targets are briefly discussed, and the resulting requirements on the detector devices together with the implementation strategies are shown.

256 Pixel DEPFET Detectors for X-Ray Astronomy

The Mercury imaging X-ray spectrometer (MIXS) on board of ESAs fifth cornerstone mission BepiColombo will be the first space instrument using DEpleted P-channel FET (DEPFET) based detectors. The MIXS spectrometer comprises two channels with identical focal plane detectors and is dedicated to energy resolved imaging of X-ray fluorescence from the mercurial surface. We report on the characterization, integration, and spectroscopic qualification of MIXS flight detectors. Detector chips were precharacterized at die level in order to select the best dies for integration and to do homogeneity and yield studies. Then, the detector chips were integrated to MIXS Detector Plane Arrays (DPAs), a complicated process due to the sophisticated mechanical structure, which allows the thermal decoupling of the detector from its readout and control chips. After integration, spectroscopic qualification measurements were done in order to analyze the detector performance and to prove the excellent spectroscopic performance of the DEPFET Macropixel detectors over a wide temperature range. The integration and spectroscopic qualification of all flight grade modules is now successfully completed.

Pixel detectors for x-ray imaging spectroscopy in space

An experiment was performed to measure the current related damage rate of silicon soon after a 10 MeV proton irradiation at −50° C, in a condition in which the effect of the leakage current annealing is negligible. This measurement is fundamental to predict the spectroscopic performance of the macropixel detectors which will be mounted on the Simbol-X and BepiColombo space missions. Macropixel detectors consist on matrices of Silicon Drift Detectors with an integrated DEPFET readout node on each pixel and offer an optimal solution when a large pixel area is needed but the noise should be kept at levels allowing X-ray spectroscopy. The most critical aspect of the operation of these detectors, in particular of the one which will be used in the BepiColombo mission, is whether the leakage current increase due to the proton irradiation would still allow the required energy resolution. This leakage current increase cannot be predicted with the available models because, during the whole mission, the sensor will be kept at temperatures below −40° C, and the existing empirical parameterizations are valid only at higher annealing temperatures. The irradiation was performed with diodes at the tandem accelerator of the Meier-Leibnitz Laboratorium in Garching with 10-MeV protons and fluences below 1011 protons/cm2, the interesting range for the missions. The diodes were cooled at a temperature of −50° C during the experiment and biased and read out with a charge sensitive preamplifier to perform a dosimetry based on proton counting. The leakage current was measured at the end of every exposure, before warming up and replacing the samples. Its time evolution after several steps of annealing at 60° C was then studied in the laboratory to check the agreement with the NIEL hypothesis predictions and thus, to validate the experiment. The experimental setup, the measurements of current induced damage rate at −50° C and its annealing are discussed in detail. The consequences on the Simbol-X and BepiColombo experiments are also examined.

DEPFET Macropixel Detectors for MIXS: Integration and Qualification of the Flight Detectors

The DSSC (DEPFET Sensor with Signal Compression) is a new instrument with non-linear compression of the input signal in the sensor and with parallel signal processing (filtering, linear amplification, and digitization) for all pixels. The design goal is to achieve at the same time single photon detection and high dynamic range, both for photon energies down to 0.5 keV and read-out speeds up to 4.5 MHz. Realization of this goal requires an accurate calibration of the non-linear system gain (NLSG), i.e. of the non-linear dependence of the digital DSSC output signal on the input signal charge generated by incident photons, over the full dynamic range of the detector. We present an overview of our basic strategy for calibrating the NLSG. The feasibility of our calibration strategy is demonstrated using our system simulation package, which is briefly described. Finally, we demonstrate the DSSC capabilities by simulating the measurement of a T4 virus diffraction image as recorded at the Linac Coherent Light Source.

Measurement of the current related damage rate at −50° C and consequences on macropixel detector operation in space experiments

Two new DEPFET concepts are presented motivated by potential applications in adaptive optics and in synchrotron radiation experiments at the future Free Electron X-ray Laser (XFEL) in Hamburg. The gatable DEPFET structure allows the selection of signal charges arriving in a predefined time interval. Charges produced outside this gate interval are lead to a sink electrode while charge collected already is protected and kept for later delayed readout. In synchrotron radiation experiments one faces the challenge of being sensitive enough for single X-ray photons in some parts of the detector while on other regions a very large charge due to the superposition of many X-rays has to be measured. A DEPFET with strongly non-linear characteristics combines naturally excellent energy resolution with high dynamic range, large charge handling capability and high read out speed.

Strategy for calibrating the non-linear gain of the DSSC detector for the European XFEL

pnCCDs are a special type of charge coupled devices (CCD) which were originally developed for applications in X-ray astronomy. At X-ray Free Electron Lasers (XFEL) pnCCDs are used as imaging X-ray spectrometers due to their outstanding characteristics like high readout speed, high and homogeneous quantum efficiency, low readout noise, radiation hardness and high pixel charge handling capacity. They can be used both as single-photon counting detectors for X-ray spectroscopy and as integrating detectors for X-ray imaging with count rates up to 104 photons of 1 keV per pixel and frame. However, extremely high photon intensities can result in pixel saturation and charge spilling into neighboring pixels. Because of this charge blooming effect, spatial information is reduced. Based on our research concerning the internal potential distribution we can enhance the pixel full well capacity even more and improve the quality of the image. This paper describes the influence of the operation voltages and space charge distribution of the pnCCD on the electric potential profile by using 2D numerical device simulations. Experimental results with signal injection from an optical laser confirm the simulation models.

New DEPFET structures: concepts, simulations and experimental results

Future x-ray observatories, such as the proposed ATHENA+ mission, will investigate bright and rapidly evolving radiation sources. To reach the scientific goals, high speed, spatial resolving sensors with excellent spectroscopic performance are mandatory. Well suited for this task are matrices of Depleted P-channel Field Effect Transistors (DEPFETs). DEPFETs provide intrinsic signal amplification, 100 percent fill factor, charge storage capability and a low read noise. Previous studies of DEPFET matrices of 256 × 256 pixels demonstrated an excellent energy resolution of 126 eV FWHM at 5.9 keV (compared to the theoretical Fano limit 120 eV). Usually these matrices are read out on demand, using e.g. the ASTEROID ASIC. Because the DEPFET is always sensitive, charge collected during the readout, causes so called misfits, which increase the background. For low frame rates this can be neglected. However, for fast timings, as suggested for ATHENA+, this effect reduces the spectral performance. We will present measurements on DEPFET macropixel structures, read out using a semi-Gaussian shaper, which demonstrate the excellent spectroscopic performance of these devices. Furthermore we will investigate the effect of misfits on the spectral background of DEPFET devices read out on demand. These measurements show the necessity to suppress misfits when the devices are operated for fast timing modes. As will be shown this can be done using so called gateable DEPFETs. The general advantage of gateable DEPFETs at fast timings, in terms of peak-to-background ratio will be demonstrated.