Athena Project

The Wide Field Imager (WFI)

Please take a look first at the paper below:

— The Wide Field Imager (WFI) for Athena+ (Rau et al.)

This instrument was proposed as part of the payload for IXO and ATHENA and has been re-designed for ATHENA+. The WFI is an imaging X-ray spectrometer with a large field of view. Its purpose is to provide X-ray images in the 0.1-15 keV energy range, simultaneously with spectrally and time resolved photon detection. High-count rate capabilities are enabled through a central 7.5x7.5 arcmin^2 detector with high read-out capacity. Four outer detectors extend the field of view to 40x40 arcmin^2, thus providing the essential survey power for ATHENA+.

WFI main characteristics

Detector

The Wide Field Imager uses active pixel sensors (APS) based on a special type of field effect transistor called DePFET (depleted P-channel field effect transistor). The DePFET is a combined detector-amplifier structure. Here, incident X-ray photons interact with the Si bulk material, and the resulting electron-hole pairs are separated. While holes drift to the most nearby p-contact, the electrons are collected in the internal gate of the adjacent pixels. There, their presence influences the conductivity of the field effect transistor (FET) channel. Sensing the increase of channel conductivity of the FET is therefore a measure of the quantity of collected charge and thus the energy of the recorded photons. Cutaway display of a circular active pixel sensor based on the DePFET technology. The bulk thickness is 450µm.

The DEPFET active pixel array employed by the WFI offers many advantages over the use of more conventional CCD imagers. These include: nearly Fano-limited energy resolution, greater flexibility in data modes arising from the ability to control individual pixels, and higher readout speed. The sideways depletion provides an unobstructed, homogenous entrance window with 100% filling factor and the sensor is radiation hard and power efficient, as the pixels are turned on only during readout.

Camera

The WFI baseline design foresees a central array of 65k pixels and four rectangular outer arrays of 287k pixels each. The central array will cover a field of view of 7.5 x 7.5 arcmin2 and will in particular allow high-time resolution observations of bright X-ray sources. With a readout time of 8µs in window mode and a count rate capability of >1 Crab with 80% throughput and less than 5% pile-up. The four outer arrays will cover the remainder of the 40x40 arcmin2 field of view and will provide an unprecedented survey capability. Simulated 1Ms WFI image of Chandra Deep Field South extended to the WFI field of view using a logN-logS source distribution.

The main structures of the camera assembly are the 1+4 DePFET arrays and their respective readout electronics, the pre-processing electronics, and a filter wheel with baffle structure. A graded-Z shield is implemented to minimize the occurrence of fluorescence lines generated by the passage of charge particles in the instrument. This concept reduces the instrumental background. Cut through the focal plane assembly of the Wide Field Imager.

The WFI is being developed by a consortium consisting of the MPE (Germany), a consortium of German universities (ECAP, IAAT), the company PNSensor GmbH, together with partners in the United Kingdom (Leicester) and France (IRAP Toulouse).


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