133 research outputs found
Radiation Hardness of X-ray PN-CCD Detectors
No full textDie Strahlenhärte von pn-CCD-Bauelementen, eines neuartigen Siliziumdetektortyps zur orts- und zeitaufgelösten Spektroskopie von Röntgenphotonen, wurde in verschiedenen Bestrahlungsexperimenten mit Protonen, Alpha-Teilchen und Röntgenphotonen untersucht. Als empfindlichste Größe erwies sich dabei der Signalladungstransferverlust, dessen Zunahme verursacht ist durch bestrahlungserzeugte Störstellen im Siliziumkristallgitter. Seine detaillierte experimentelle Analyse, unterstützt von einem Monte-Carlo-Simulationsmodell, führte zu wesentlichen Optimierungen und eröffnete zugleich eine neue Untersuchungsmethode für Störstellen im Kristallgitter. Insgesamt weist der pn-CCD eine außergewöhnlich hohe Strahlenhärte auf. Für den zehnjährigen Einsatz als Fokaldetektor bei der Röntgenastronomiemission XMM-Newton der ESA ist als Resultat der Untersuchungen eine mit der Zeit kontinuierlich ansteigende, aber geringfügige Beeinträchtigung der Energieauflösung zu erwarten. Dies konnte in den ersten beiden Betriebsjahren der pn-CCD-Kamera auf dem Satelliten bestätigt werden.The radiation hardness of pn-CCDs, a novel silicon detector type for X-ray spectroscopy with position and time resolution, was studied in various irradiation experiments with protons, alpha-particles and X-rays. The increase of charge transfer loss turned out to be the most sensitive performance parameter. It is caused by the generation of traps in the silicon lattice as a result of irradiation. The detailed experimental analysis, supported by a Monte Carlo simulation model, led to substantial optimizations of the degraded performance and offered a new method for analysis of lattice defects. Altogether, the pn-CCD shows an outstanding radiation hardness. During ten years of operation aboard ESA's X-ray astronomy satellite mission XMM-Newton, only a small degradation of the energy resolution is expected for the pn-CCD focal plane detector. This result could be validated in the first two years of pn-CCD camera operation on the satellite
Magnetic Shielding of Soft Protons in Future X-Ray Telescopes: The Case of the ATHENA Wide Field Imager
No full textBoth interplanetary space and Earth’s magnetosphere are populated by low-energy (≤300 keV) protons that are potentially able to scatter on the reflecting surface of the Wolter-I optics of X-ray focusing telescopes and reach the focal plane. This phenomenon, depending on the X-ray instrumentation, can dramatically increase the background level, reducing the sensitivity or, in the most extreme cases, compromising the observation itself. The use of a magnetic diverter, deflecting protons away from the field of view, requires a detailed characterization of their angular and energy distribution when exiting the mirror. We present the first end-to-end Geant4 simulation of proton scattering by X-ray optics and the consequent interaction with the diverter field and the X-ray detector assembly, selecting the ATHENA Wide Field Imager as a case study for the evaluation of the residual, soft-proton-induced background. We find that in the absence of a magnetic diverter, protons are indeed funneled toward the focal plane, with a focused non-X-ray background well above the level required by ATHENA science objectives (5 × 10‐4 counts cm‐2 s‐1 keV‐1), for all the plasma regimes encountered in both L1 and L2 orbits. These results set the proton diverter as a mandatory shielding system on board the ATHENA mission and all high throughput X-ray telescopes operating in the interplanetary space. For a magnetic field computed to deflect 99% of the protons that would otherwise reach the WFI, Geant4 simulations show that this configuration, in the assumption of a uniform field, would efficiently shield the focal plane, yielding a residual background level of the order or below the requirement
The wide field imager instrument for Athena
No full textThe "Hot and Energetic Universe" has been selected as the science theme for ESA's L2 mission, scheduled for launch in 2028. The proposed Athena X-ray observatory provides the necessary capabilities to achieve the ambitious goals of the science theme. The X-ray mirrors are based on silicon pore optics technology and will have a 12 m focal length. Two complementary camera systems are foreseen which can be moved in and out of the focal plane by an interchange mechanism. These instruments are the actively shielded micro-calorimeter spectrometer X-IFU and the Wide Field Imager (WFI).The WFI will combine an unprecedented survey power through its large field of view of 40 arcmin with a high count-rate capability (approx. 1 Crab). It permits a state-of-the-art energy resolution in the energy band of 0.1 keV to 15 keV during the entire mission lifetime (e.g. FWHM <= 150 eV at 6 keV). This performance is accomplished by a set of DEPFET active pixel sensor matrices with a pixel size matching the angular resolution of 5 arcsec (on-axis) of the mirror system. Each DEPFET pixel is a combined detector-amplifier structure with a MOSFET integrated onto a fully depleted 450 micron thick silicon bulk. The signal electrons generated by an X-ray photon are collected in a so-called internal gate below the transistor channel. The resulting change of the conductivity of the transistor channel is proportional to the number of electrons and thus a measure for the photon energy. DEPFETs have already been developed for the "Mercury Imaging X-ray Spectrometer" on-board of ESA's BepiColombo mission. For Athena we develop enhanced sensors with integrated electronic shutter and an additional analog storage area in each pixel. These features improve the peak-to-background ratio of the spectra and minimize dead time. The sensor will be read out with a new, fast, low-noise multi-channel analog signal processor with integrated sequencer and serial analog output. The architecture of sensor and readout ASIC allows readout in full frame mode and window mode as well by addressing selectively arbitrary sub-areas of the sensor allowing time resolution in the order of 10 mu s. The further detector electronics has mainly the following tasks: digitization, pre-processing and telemetry of event data as well as supply and control of the detector system. Although the sensor will already be equipped with an on-chip light blocking filter, a filter wheel is necessary to provide an additional external filter, an on-board calibration source, an open position for outgassing, and a closed position for protection of the sensor. The sensor concept provides high quantum efficiency over the entire energy band and we intend to keep the instrumental background as low as possible by designing a graded Z-shield around the sensor.All these properties make the WFI a very powerful survey instrument, significantly surpassing currently existing observatories and in addition allow high-time resolution of the brightest X-ray sources with low pile-up and high efficiency. This manuscript will summarize the current instrument concept and design, the status of the technology development, and the envisaged baseline performance
DEPFET Active Pixel Sensors
No full textAn array of DEPFET pixels is one of several concepts to implement an active
pixel sensor. Similar to PNCCD and SDD detectors, the typically 0.45 mm thick
silicon sensor is fully depleted by the principle of sideward depletion. They
have furthermore in common to be back-illuminated detectors, which allows for
ultra-thin and homogeneous photon entrance windows. This enables relatively
high quantum efficiencies at low energies and close to 100% for photon energies
between 1 keV and 10 keV. Steering of the DEPFET sensor is enabled by a
so-called Switcher ASIC and readout is performed by e.g. a VERITAS ASIC. The
configuration enables a readout time of a few microseconds per row. This
results in full frame readout times of a few milliseconds for a 512 x 512 pixel
array in a rolling shutter mode. The read noise is then typically three
electrons equivalent noise charge RMS. DEPFET detectors can be applied in
particular for spectroscopy in the energy band from 0.2 keV to 20 keV. For
example, an energy resolution of about 130 eV FWHM is achieved at an energy of
6 keV which is close to the theoretical limit given by Fano noise. Pixel sizes
of a few tens of microns up to a centimetre are feasible by the DEPFET concept.Comment: Invited chapter for the "Handbook of X-ray and Gamma-ray
Astrophysics" (Eds. C. Bambi and A. Santangelo, Springer Singapore, 2022
The Wide Field Imager instrument for Athena
No full textESA's next large X-ray mission ATHENA is designed to address the Cosmic Vision science theme 'The Hot and Energetic Universe'. It will provide answers to the two key astrophysical questions how does ordinary matter assemble into the large-scale structures we see today and how do black holes grow and shape the Universe. The ATHENA spacecraft will be equipped with two focal plane cameras, a Wide Field Imager (WFI) and an X-ray Integral Field Unit (X-IFU). The WFI instrument is optimized for state-of-The-Art resolution spectroscopy over a large field of view of 40 amin x 40 amin and high count rates up to and beyond 1 Crab source intensity. The cryogenic X-IFU camera is designed for high-spectral resolution imaging. Both cameras share alternately a mirror system based on silicon pore optics with a focal length of 12 m and large effective area of about 2 m2at an energy of 1 keV. Although the mission is still in phase A, i.e. studying the feasibility and developing the necessary technology, the definition and development of the instrumentation made already significant progress. The herein described WFI focal plane camera covers the energy band from 0.2 keV to 15 keV with 450 Î1⁄4m thick fully depleted back-illuminated silicon active pixel sensors of DEPFET type. The spatial resolution will be provided by one million pixels, each with a size of 130 Î1⁄4m x 130 Î1⁄4m. The time resolution requirement for the WFI large detector array is 5 ms and for the WFI fast detector 80 Î1⁄4s. The large effective area of the mirror system will be completed by a high quantum efficiency above 90% for medium and higher energies. The status of the various WFI subsystems to achieve this performance will be described and recent changes will be explained here
Structural modelling and mechanical tests supporting the design of the ATHENA X-IFU thermal filters and WFI optical blocking filter
No full textATHENA is a Large high energy astrophysics space mission selected by ESA in the Cosmic Vision 2015-2025 Science Program. It will be equipped with two interchangeable focal plane detectors: the X-Ray Integral Field Unit (X-IFU) and the Wide Field Imager (WFI). Both detectors require x-ray transparent filters to fully exploit their sensitivity. In order to maximize the X-ray transparency, filters must be very thin, from a few tens to few hundreds of nm, on the other hand, they must be strong enough to survive the severe launch stresses. In particular, the WFI OBF, being launched in atmospheric pressure, shall also survive acoustic loads. In this paper, we present a review of the structural modeling performed to assist the ATHENA filters design, the preliminary results from vibration and acoustic tests, and we discuss future activities necessary to consolidate the filters design, before the preliminary requirement review of the ATHENA instruments, scheduled before the end of 2018
Studies of prototype DEPFET sensors for the wide field imager of Athena
No full textThe Wide Field Imager of the Athena telescope will combine an excellent spectroscopic performance and high count rate capability with a large field of view. For these purposes, its focal plane consists of two complementary detectors, using DEPFET active pixel sensors. One is the high count rate detector with a small field of view, which has to be operated with a readout speed of 80 mu s per frame. In contrast, the large area detector will cover a large field of view and has to be read out with a frame rate <= 5 ms. Its sensitive area is covered by four identical active pixel arrays, consisting of 512 x 512 pixels, each. Since a column parallel readout will be used, 512 pixels are connected to one single channel of a readout ASIC. The readout will be accomplished by either sensing a voltage step on the source node or a change of the transistor drain current. The former so-called source follower mode requires long settling times - proportional to the load capacitances - but can cope with local inhomogeneities. Alternatively, the latter so-called drain current mode provides a fast readout - independent to the load capacitance - but implicates a higher sensitivity on local variations of the DEPFETs bias currents. Both modes are implemented in the VERITAS 2.1 readout ASIC and were studied with 64 x 64 pixels arrays. Drain current devices could be operated with significantly smaller settling times but suffer from a slightly increased noise at similar shaping times in comparison to the source follower ones. By using an optimized timing with dedicated settling and shaping times, the devices of both modes feature a comparable spectral performance
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