The Wide Field Imager (WFI), one of two instruments on ESA’s next large X-ray mission Athena, is designed for imaging spectroscopy of X-rays in the range of 0.2 to 15 keV with a large field of view and high count rate capability. The focal plane consists of back-illuminated DEPFET (Depleted p-channel field effect transistor) sensors that have a high radiation tolerance and provide a near Fano-limited energy resolution. To achieve this, a very low noise readout is required, ∼3 electrons ENC at beginning of life is foreseen. This makes the device very susceptible to any radiation induced worsening of the readout noise. The main mechanism of degradation will be the increase of dark current due to displacement damage caused primarily by high energy protons.
To study the expected performance degradation, a prototype detector module with fully representative pixel layout and fabrication technology was irradiated with 62.4 MeV protons at the accelerator facility MedAustron in Wiener Neustadt. A total dose equivalent to 3.3 · 109 10-MeV protons/cm2 was applied in two steps. During, in-between and after the irradiations the detector remained at the operating temperature of 213 K and was fully biased and operated. Data was recorded to analyze the signal of all incident particles.
We report on the increase of dark current after the irradiation and present the current related damage rate at 213 K. The effect of low temperature annealing at 213 , 236 K, 253 K, 273 K, and 289K is presented.
We present a silicon drift detector (SDD) system for the spectroscopy focusing array (SFA) of the enhanced X-ray timing and polarimetry (eXTP) mission. The SFA focuses on fast timing (time resolution below 10 μs) and good spectroscopy capabilities (energy resolution better than 180 eV @ 6 keV). The sensor, consisting of 19 hexagonally shaped pixels with a total sensitive area of 5.05 cm2, is connected to three high time resolution spectroscopy (HTRS) ASICs, allowing a fast readout of the detector signals. The detector works in a Charge-Sensitive Amplifier configuration. We assembled a prototype detector module and present here its mechanical design, describe the used sensor, and report about its performance.
The depleted p-channel field effect transistor is the chosen sensor type for the Wide Field Imager of the Athena mission. It will be used in two types of cameras. One will enable observations of a field of view of 40′ × 40′ by using an array of four 512 × 512 pixel sensors in a 2 × 2 configuration. A second, small one is designed to investigate bright, point-like sources with a time resolution of up to 40 μs. Sensors of final size, layout, and technology were fabricated, assembled and characterised. Also, first results from the flight production are available and confirm the excellent performance. In order to be able to estimate the future performance of degraded detectors, a simulation was developed that takes into account the non-analytical threshold effects on the basis of measurement results. We present the measurement analysis and the comparison of simulated and measured values as well as first attempts to use the Monte Carlo simulation to predict performance results based on noise measurements.
KEYWORDS: Equipment, Sensors, Scanning tunneling microscopy, Satellites, Interfaces, Space operations, Power supplies, Data acquisition, Vacuum chambers, Control systems
The Wide Field Imager (WFI) is one of the two instruments of the X-ray Advanced Telescope for High-Energy Astrophysics (NewAthena) mission selected by ESA. The WFI satellite telescope uses a camera fitted with DEPFET sensors, instrument electronics to control the camera and readout the sensors, as well as additional electronic units to communicate with the spacecraft on-board-computer. WFI successfully passed the instrument system requirement review (I-SRR) in March 2022. In 2023, NewAthena went through a redefinition phase in order to save cost, and on February 2024 the mission restarted again. Currently, a WFI Engineering Model (EM) consisting of the Camera Head Sub-System (CHS) and Detector Electronics Sub-System (DES) is being developed. A Structural and Thermal Model (STM) has also been created as an intermediate development step to gain design insights with respect to thermal and structural properties. The objective of these developments is to gain knowledge on the road to develop the WFI Flight Model (FM). Mechanical and environmental tests will be performed at the X4 and PUMA test facilities of the Max Planck Institute for Extraterrestrial Physics. In this paper, we present the electrical ground support equipment (EGSE) for WFI and in particular for these tests. The WFI EGSE consist of hardware and software to test the WFI instrument. It provides power control, temperature sensing, time synchronization, an interface to program FPGAs and microcontrollers, and the software necessary to control the instrument and acquire the science data. Dedicated telemetry and telecommand software tools have been developed to communicate via the SpaceWire interface with the WFI instrument. The software architecture and EGSE design trade-offs are also presented.
The wide field imager for the Athena x-ray telescope is composed of two back side illuminated detectors using DEPFET sensors operated in rolling shutter readout mode: a large detector array featuring four sensors with 512×512 pixels each and a small detector that facilitates the high count rate capability of the WFI for the investigation of bright, point-like sources. Both sensors were fabricated in full size featuring the pixel layout, fabrication technology and readout mode chosen in a preceding prototyping phase. We present the spectroscopic performance of these flight-like detectors for different photon energies in the relevant part of the targeted energy range from 0.2 keV to 15 keV with respect to the timing requirements of the instrument. For 5.9 keV photons generated by an 55Fe source the spectral performance expressed as full width at half maximum of the emission peak in the spectrum is 126.0 eV for the large detector and 129.1 eV for the fast detector. A preliminary analysis of the camera’s signal chain also allows for a first prediction of the performance in space at the end of the nominal operation phase.
The Wide Field Imager (WFI), one of two complementary instruments on board ESA’s next large X-ray mission Athena, combines state-of-the-art resolution spectroscopy with a large field of view and high count rate capability. Centerpiece of the WFI instrument is the large detector assembly consisting of four DEPFET (Depleted p-channel field effect transistor) sensors with a size of 512×512 pixels each, and one fast detector with a size of 64×64 pixels. They are planned to be operated in drain current readout mode, which enables fast readout rates but is sensitive to inhomogeneities of the drain currents. These inhomogeneities arise from the sheer size of the DEPFET sensor matrix and are originated in the spatial distribution of wafer properties and process parameters. We characterized the drain current distribution of a large detector lab module (one quadrant of the large detector assembly, 512×512 pixels) out of the pre-flight production of Athena’s WFI DEPFET detectors. In order to better understand the origin of the current spread we measured I-V characteristics of all pixels and extracted the channel length modulation parameters, threshold voltages and transconductance values of the external (gm) and internal (gq) gate of the individual pixels of the detector in operational conditions. This is enabled by features of the VERITAS readout ASIC.
The wide field imager (WFI) for the ATHENA mission will enable the spectroscopic investigation of solid angles up to 40’ in the x-ray regime between 0.2 keV to 15 keV. The required sensors are planed as DEPFET (DEpleted P-channel field-effect transistor) active pixel matrices. These are readout by custom-designed ASICs called VERITAS in a rolling shutter mode. After column parallel processing of the analog signals row-by-row, the resulting pulse heights are multiplexed onto a differential analog line pair. Thus a transmission undisturbed from external influences is achieved. But successive signals can be affected by the limited rise-time of the VERITAS output stage, depending on the signal height, the multiplexing time and the cable length. In order to investigate the impact on the measurements a dedicated cable harness is used during the characterization of a prototype device. The characterizations are performed using either test pulses applied to the VERITAS or with a 55Fe calibration source illuminating the whole sensor. Different cable lengths and multiplexing times are tested to determine the influences. In order to deal with this effect a crosstalk correction algorithm is implemented for the analysis of the photon data. The determined crosstalk factors range up to 2.8% for the longest measured cablings of 2 m with the shortest multiplexing time. Using shorter cables and longer multiplexing times these factors can be reduced to below 1%. In all cases the accompanying effects in the data analysis can be corrected using the developed algorithm.
Wolfgang Treberspurg, Johannes Müller-Seidlitz, Norbert Meidinger, Annika Behrens, Robert Andritschke, Michael Bonholzer, Valentin Emberger, Günter Hauser
The Wide Field Imager instrument of ESA’s next X-ray observatory Athena will consist of specifically developed DEPFET detectors which enable a low noise and fast readout operation. In order to confirm the required spectroscopic performance within the energy range of 0.2 keV to 15 keV, various emission lines were probed with prototype detectors on a 64×64 pixel scale. These detectors include the first sensors representative for flight with respect to the transistor layout and fabrication technology. Four different detectors were tested and exhibit a Fano noise limited spectral performance. The required energy resolution was achieved by all detectors even at readout times as fast as 2.5 μs/row. Additionally, one of the detectors achieved an outstanding performance of a FWHM below 45 eV at C Kα (277 eV) and 6.1 μs/row. Complementary to the measurements, the charge cloud size was determined as a function of the photon energy based on Monte Carlo simulations. These simulations enable a quantification of the effective charge loss at different energies due to the thresholds applied at the event recombination.
Annika Behrens, Robert Andritschke, Michael Bonholzer, Valentin Emberger, Günter Hauser, Norbert Meidinger, Johannes Müller-Seidlitz, Wolfgang Treberspurg
The Wide Field Imager (WFI) is one of two instruments for ESA’s Athena X-ray observatory. It will employ DEPFET (depleted p-channel field effect transistor) active pixel technology to provide unprecedented spectroscopic and imaging capabilities over a broad energy band from 0.2 keV to 15 keV, with a large field of view of 40′ × 40′ . Using prototype detectors of various sizes we have started first tests of window mode operation, where only a subset of the full sensor array is being read out, leading to a higher frame rate for this area. We also investigated the possibility of operation at room temperature for a basic functionality test when cooling is not possible, e.g. on ground after integration on the satellite.
The Wide Field Imager (WFI) of ESA’s next X-ray observatory Athena will combine a high count rate capability with a large field of view, both with state-of-the-art spectroscopic performance. To meet these demands, specific DEPFET active pixel detectors have been developed and operated. Due to the intrinsic amplification of detected signals they are best suited to achieve a high speed and low noise performance. Different fabrication technologies and transistor geometries have been implemented on a dedicated prototype production in the course of the development of the DEPFET sensors. The main modifications between the sensors concern the shape of the transistor gate – regarding the layout – and the thickness of the gate oxide – regarding the technology. To facilitate the fabrication and testing of the resulting variety of sensors the presented studies were carried out with 64×64 pixel detectors. The detector comprises a control ASIC (Switcher-A), a readout ASIC (VERITAS- 2) and the sensor. In this paper we give an overview on the evaluation of different prototype sensors. The most important results, which have been decisive for the identification of the optimal fabrication technology and transistor layout for subsequent sensor productions are summarized. It will be shown that the developments result in an excellent performance of spectroscopic X-ray DEPFETs with typical noise values below 2.5 ENC at 2.5 μs/row.
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